4. BIND 9 Configuration Reference

4.1. Configuration File Elements

Following is a list of elements used throughout the BIND configuration file documentation:

acl_name

The name of an address_match_list as defined by the acl statement.

address_match_list

A list of one or more ip_addr, ip_prefix, key_id, or acl_name elements; see Address Match Lists.

remoteserver_list

A named list of one or more ip_addr with optional tls_id, key_id and/or ip_port. A remoteserver_list may include other remoteserver_list.

domain_name

A quoted string which is used as a DNS name; for example. my.test.domain.

namelist

A list of one or more domain_name elements.

dotted_decimal

One to four integers valued 0 through 255 separated by dots (.), such as 123.45.67 or 89.123.45.67.

ip4_addr

An IPv4 address with exactly four elements in dotted_decimal notation.

ip6_addr

An IPv6 address, such as 2001:db8::1234. IPv6-scoped addresses that have ambiguity on their scope zones must be disambiguated by an appropriate zone ID with the percent character (%) as a delimiter. It is strongly recommended to use string zone names rather than numeric identifiers, to be robust against system configuration changes. However, since there is no standard mapping for such names and identifier values, only interface names as link identifiers are supported, assuming one-to-one mapping between interfaces and links. For example, a link-local address fe80::1 on the link attached to the interface ne0 can be specified as fe80::1%ne0. Note that on most systems link-local addresses always have ambiguity and need to be disambiguated.

ip_addr

An ip4_addr or ip6_addr.

ip_dscp

A number between 0 and 63, used to select a differentiated services code point (DSCP) value for use with outgoing traffic on operating systems that support DSCP.

ip_port

An IP port number. The number is limited to 0 through 65535, with values below 1024 typically restricted to use by processes running as root. In some cases, an asterisk (*) character can be used as a placeholder to select a random high-numbered port.

ip_prefix

An IP network specified as an ip_addr, followed by a slash (/) and then the number of bits in the netmask. Trailing zeros in an``ip_addr`` may be omitted. For example, 127/8 is the network 127.0.0.0``with netmask ``255.0.0.0 and 1.2.3.0/28 is network 1.2.3.0 with netmask 255.255.255.240. When specifying a prefix involving a IPv6-scoped address, the scope may be omitted. In that case, the prefix matches packets from any scope.

key_id

A domain_name representing the name of a shared key, to be used for transaction security.

key_list

A list of one or more key_id, separated by semicolons and ending with a semicolon.

tls_id

A string representing a TLS configuration object, including a key and certificate.

number

A non-negative 32-bit integer (i.e., a number between 0 and 4294967295, inclusive). Its acceptable value might be further limited by the context in which it is used.

fixedpoint

A non-negative real number that can be specified to the nearest one-hundredth. Up to five digits can be specified before a decimal point, and up to two digits after, so the maximum value is 99999.99. Acceptable values might be further limited by the contexts in which they are used.

path_name

A quoted string which is used as a pathname, such as zones/master/my.test.domain.

port_list

A list of an ip_port or a port range. A port range is specified in the form of range followed by two ip_port``s, ``port_low and port_high, which represents port numbers from port_low through port_high, inclusive. port_low must not be larger than port_high. For example, range 1024 65535 represents ports from 1024 through 65535. In either case an asterisk (*) character is not allowed as a valid ip_port.

size_spec

A 64-bit unsigned integer, or the keywords unlimited or default. Integers may take values 0 <= value <= 18446744073709551615, though certain parameters (such as max-journal-size) may use a more limited range within these extremes. In most cases, setting a value to 0 does not literally mean zero; it means “undefined” or “as big as possible,” depending on the context. See the explanations of particular parameters that use size_spec for details on how they interpret its use. Numeric values can optionally be followed by a scaling factor: K or k for kilobytes, M or m for megabytes, and G or g for gigabytes, which scale by 1024, 1024*1024, and 1024*1024*1024 respectively. unlimited generally means “as big as possible,” and is usually the best way to safely set a very large number. default uses the limit that was in force when the server was started.

size_or_percent

A size_spec or integer value followed by % to represent percent. The behavior is exactly the same as size_spec, but size_or_percent also allows specifying a positive integer value followed by the % sign to represent percent.

yes_or_no

Either yes or no. The words true and false are also accepted, as are the numbers 1 and 0.

dialup_option

One of yes, no, notify, notify-passive, refresh, or passive. When used in a zone, notify-passive, refresh, and passive are restricted to secondary and stub zones.

4.1.1. Address Match Lists

4.1.1.1. Syntax

address_match_list = address_match_list_element ; ...

address_match_list_element = [ ! ] ( ip_address | ip_prefix |
     key key_id | acl_name | { address_match_list } )

4.1.1.2. Definition and Usage

Address match lists are primarily used to determine access control for various server operations. They are also used in the listen-on and sortlist statements. The elements which constitute an address match list can be any of the following:

• an IP address (IPv4 or IPv6)

• an IP prefix (in / notation)

• a key ID, as defined by the key statement

• the name of an address match list defined with the acl statement

• a nested address match list enclosed in braces

Elements can be negated with a leading exclamation mark (!), and the match list names “any”, “none”, “localhost”, and “localnets” are predefined. More information on those names can be found in the description of the acl statement.

The addition of the key clause made the name of this syntactic element something of a misnomer, since security keys can be used to validate access without regard to a host or network address. Nonetheless, the term “address match list” is still used throughout the documentation.

When a given IP address or prefix is compared to an address match list, the comparison takes place in approximately O(1) time. However, key comparisons require that the list of keys be traversed until a matching key is found, and therefore may be somewhat slower.

The interpretation of a match depends on whether the list is being used for access control, defining listen-on ports, or in a sortlist, and whether the element was negated.

When used as an access control list, a non-negated match allows access and a negated match denies access. If there is no match, access is denied. The clauses allow-notify, allow-recursion, allow-recursion-on, allow-query, allow-query-on, allow-query-cache, allow-query-cache-on, allow-transfer, allow-update, allow-update-forwarding, blackhole, and keep-response-order all use address match lists. Similarly, the listen-on option causes the server to refuse queries on any of the machine’s addresses which do not match the list.

Order of insertion is significant. If more than one element in an ACL is found to match a given IP address or prefix, preference is given to the one that came first in the ACL definition. Because of this first-match behavior, an element that defines a subset of another element in the list should come before the broader element, regardless of whether either is negated. For example, in 1.2.3/24; ! 1.2.3.13; the 1.2.3.13 element is completely useless because the algorithm matches any lookup for 1.2.3.13 to the 1.2.3/24 element. Using ! 1.2.3.13; 1.2.3/24 fixes that problem by blocking 1.2.3.13 via the negation, but all other 1.2.3.* hosts pass through.

4.1.2. Comment Syntax

The BIND 9 comment syntax allows comments to appear anywhere that whitespace may appear in a BIND configuration file. To appeal to programmers of all kinds, they can be written in the C, C++, or shell/perl style.

4.1.2.1. Syntax

/* This is a BIND comment as in C */

// This is a BIND comment as in C++

# This is a BIND comment as in common Unix shells
# and perl

4.1.2.2. Definition and Usage

Comments may appear anywhere that whitespace may appear in a BIND configuration file.

C-style comments start with the two characters /* (slash, star) and end with */ (star, slash). Because they are completely delimited with these characters, they can be used to comment only a portion of a line or to span multiple lines.

C-style comments cannot be nested. For example, the following is not valid because the entire comment ends with the first */:

/* This is the start of a comment.
   This is still part of the comment.
/* This is an incorrect attempt at nesting a comment. */
   This is no longer in any comment. */

C++-style comments start with the two characters // (slash, slash) and continue to the end of the physical line. They cannot be continued across multiple physical lines; to have one logical comment span multiple lines, each line must use the // pair. For example:

// This is the start of a comment.  The next line
// is a new comment, even though it is logically
// part of the previous comment.

Shell-style (or perl-style) comments start with the character # (number sign) and continue to the end of the physical line, as in C++ comments. For example:

# This is the start of a comment.  The next line
# is a new comment, even though it is logically
# part of the previous comment.

Warning

The semicolon (;) character cannot start a comment, unlike in a zone file. The semicolon indicates the end of a configuration statement.

4.2. Configuration File Grammar

A BIND 9 configuration consists of statements and comments. Statements end with a semicolon; statements and comments are the only elements that can appear without enclosing braces. Many statements contain a block of sub-statements, which are also terminated with a semicolon.

The following statements are supported:

acl

Defines a named IP address matching list, for access control and other uses.

controls

Declares control channels to be used by the rndc utility.

dnssec-policy

Describes a DNSSEC key and signing policy for zones. See dnssec-policy Grammar for details.

include

Includes a file.

key

Specifies key information for use in authentication and authorization using TSIG.

logging

Specifies what information the server logs and where the log messages are sent.

masters

Synonym for primaries.

options

Controls global server configuration options and sets defaults for other statements.

parental-agents

Defines a named list of servers for inclusion in primary and secondary zones’ parental-agents lists.

primaries

Defines a named list of servers for inclusion in stub and secondary zones’ primaries or also-notify lists. (Note: this is a synonym for the original keyword masters, which can still be used, but is no longer the preferred terminology.)

server

Sets certain configuration options on a per-server basis.

statistics-channels

Declares communication channels to get access to named statistics.

tls

Specifies configuration information for a TLS connection, including a key-file, cert-file, dhparam-file, ciphers, protocols, prefer-server-ciphers, and session-tickets.

http

Specifies configuration information for an HTTP connection, including endponts, listener-clients and streams-per-connection.

trust-anchors

Defines DNSSEC trust anchors: if used with the initial-key or initial-ds keyword, trust anchors are kept up-to-date using RFC 5011 trust anchor maintenance; if used with static-key or static-ds, keys are permanent.

managed-keys

Is identical to trust-anchors; this option is deprecated in favor of trust-anchors with the initial-key keyword, and may be removed in a future release.

trusted-keys

Defines permanent trusted DNSSEC keys; this option is deprecated in favor of trust-anchors with the static-key keyword, and may be removed in a future release.

view

Defines a view.

zone

Defines a zone.

The logging and options statements may only occur once per configuration.

4.2.1. acl Statement Grammar

4.2.2. acl Statement Definition and Usage

The acl statement assigns a symbolic name to an address match list. It gets its name from one of the primary uses of address match lists: Access Control Lists (ACLs).

The following ACLs are built-in:

any

Matches all hosts.

none

Matches no hosts.

localhost

Matches the IPv4 and IPv6 addresses of all network interfaces on the system. When addresses are added or removed, the localhost ACL element is updated to reflect the changes.

localnets

Matches any host on an IPv4 or IPv6 network for which the system has an interface. When addresses are added or removed, the localnets ACL element is updated to reflect the changes. Some systems do not provide a way to determine the prefix lengths of local IPv6 addresses; in such cases, localnets only matches the local IPv6 addresses, just like localhost.

4.2.3. controls Statement Grammar

4.2.4. controls Statement Definition and Usage

The controls statement declares control channels to be used by system administrators to manage the operation of the name server. These control channels are used by the rndc utility to send commands to and retrieve non-DNS results from a name server.

An inet control channel is a TCP socket listening at the specified ip_port on the specified ip_addr, which can be an IPv4 or IPv6 address. An ip_addr of * (asterisk) is interpreted as the IPv4 wildcard address; connections are accepted on any of the system’s IPv4 addresses. To listen on the IPv6 wildcard address, use an ip_addr of ::. If rndc is only used on the local host, using the loopback address (127.0.0.1 or ::1) is recommended for maximum security.

If no port is specified, port 953 is used. The asterisk * cannot be used for ip_port.

The ability to issue commands over the control channel is restricted by the allow and keys clauses. Connections to the control channel are permitted based on the address_match_list. This is for simple IP address-based filtering only; any key_id elements of the address_match_list are ignored.

A unix control channel is a Unix domain socket listening at the specified path in the file system. Access to the socket is specified by the perm, owner, and group clauses. Note that on some platforms (SunOS and Solaris), the permissions (perm) are applied to the parent directory as the permissions on the socket itself are ignored.

The primary authorization mechanism of the command channel is the key_list, which contains a list of key_id``s. Each ``key_id in the key_list is authorized to execute commands over the control channel. See Administrative Tools for information about configuring keys in rndc.

If the read-only clause is enabled, the control channel is limited to the following set of read-only commands: nta -dump, null, status, showzone, testgen, and zonestatus. By default, read-only is not enabled and the control channel allows read-write access.

If no controls statement is present, named sets up a default control channel listening on the loopback address 127.0.0.1 and its IPv6 counterpart, ::1. In this case, and also when the controls statement is present but does not have a keys clause, named attempts to load the command channel key from the file rndc.key in /etc (or whatever sysconfdir was specified when BIND was built). To create an rndc.key file, run rndc-confgen -a.

To disable the command channel, use an empty controls statement: controls { };.

4.2.5. include Statement Grammar

include filename;

4.2.6. include Statement Definition and Usage

The include statement inserts the specified file (or files if a valid glob expression is detected) at the point where the include statement is encountered. The include statement facilitates the administration of configuration files by permitting the reading or writing of some things but not others. For example, the statement could include private keys that are readable only by the name server.

4.2.7. key Statement Grammar

4.2.8. key Statement Definition and Usage

The key statement defines a shared secret key for use with TSIG (see TSIG) or the command channel (see controls Statement Definition and Usage).

The key statement can occur at the top level of the configuration file or inside a view statement. Keys defined in top-level key statements can be used in all views. Keys intended for use in a controls statement (see controls Statement Definition and Usage) must be defined at the top level.

The key_id, also known as the key name, is a domain name that uniquely identifies the key. It can be used in a server statement to cause requests sent to that server to be signed with this key, or in address match lists to verify that incoming requests have been signed with a key matching this name, algorithm, and secret.

The algorithm_id is a string that specifies a security/authentication algorithm. The named server supports hmac-md5, hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384, and hmac-sha512 TSIG authentication. Truncated hashes are supported by appending the minimum number of required bits preceded by a dash, e.g., hmac-sha1-80. The secret_string is the secret to be used by the algorithm, and is treated as a Base64-encoded string.

4.2.9. logging Statement Grammar

4.2.10. logging Statement Definition and Usage

The logging statement configures a wide variety of logging options for the name server. Its channel phrase associates output methods, format options, and severity levels with a name that can then be used with the category phrase to select how various classes of messages are logged.

Only one logging statement is used to define as many channels and categories as desired. If there is no logging statement, the logging configuration is:

logging {
     category default { default_syslog; default_debug; };
     category unmatched { null; };
};

If named is started with the -L option, it logs to the specified file at startup, instead of using syslog. In this case the logging configuration is:

logging {
     category default { default_logfile; default_debug; };
     category unmatched { null; };
};

The logging configuration is only established when the entire configuration file has been parsed. When the server starts up, all logging messages regarding syntax errors in the configuration file go to the default channels, or to standard error if the -g option was specified.

4.2.10.1. The channel Phrase

All log output goes to one or more channels; there is no limit to the number of channels that can be created.

Every channel definition must include a destination clause that says whether messages selected for the channel go to a file, go to a particular syslog facility, go to the standard error stream, or are discarded. The definition can optionally also limit the message severity level that is accepted by the channel (the default is info), and whether to include a named-generated time stamp, the category name, and/or the severity level (the default is not to include any).

The null destination clause causes all messages sent to the channel to be discarded; in that case, other options for the channel are meaningless.

The file destination clause directs the channel to a disk file. It can include additional arguments to specify how large the file is allowed to become before it is rolled to a backup file (size), how many backup versions of the file are saved each time this happens (versions), and the format to use for naming backup versions (suffix).

The size option is used to limit log file growth. If the file ever exceeds the specified size, then named stops writing to the file unless it has a versions option associated with it. If backup versions are kept, the files are rolled as described below. If there is no versions option, no more data is written to the log until some out-of-band mechanism removes or truncates the log to less than the maximum size. The default behavior is not to limit the size of the file.

File rolling only occurs when the file exceeds the size specified with the size option. No backup versions are kept by default; any existing log file is simply appended. The versions option specifies how many backup versions of the file should be kept. If set to unlimited, there is no limit.

The suffix option can be set to either increment or timestamp. If set to timestamp, then when a log file is rolled, it is saved with the current timestamp as a file suffix. If set to increment, then backup files are saved with incrementing numbers as suffixes; older files are renamed when rolling. For example, if versions is set to 3 and suffix to increment, then when filename.log reaches the size specified by size, filename.log.1 is renamed to filename.log.2, filename.log.0 is renamed to filename.log.1, and filename.log is renamed to filename.log.0, whereupon a new filename.log is opened.

Here is an example using the size, versions, and suffix options:

channel an_example_channel {
    file "example.log" versions 3 size 20m suffix increment;
    print-time yes;
    print-category yes;
};

The syslog destination clause directs the channel to the system log. Its argument is a syslog facility as described in the syslog man page. Known facilities are kern, user, mail, daemon, auth, syslog, lpr, news, uucp, cron, authpriv, ftp, local0, local1, local2, local3, local4, local5, local6, and local7; however, not all facilities are supported on all operating systems. How syslog handles messages sent to this facility is described in the syslog.conf man page. On a system which uses a very old version of syslog, which only uses two arguments to the openlog() function, this clause is silently ignored.

The severity clause works like syslog’s “priorities,” except that they can also be used when writing straight to a file rather than using syslog. Messages which are not at least of the severity level given are not selected for the channel; messages of higher severity levels are accepted.

When using syslog, the syslog.conf priorities also determine what eventually passes through. For example, defining a channel facility and severity as daemon and debug, but only logging daemon.warning via syslog.conf, causes messages of severity info and notice to be dropped. If the situation were reversed, with named writing messages of only warning or higher, then syslogd would print all messages it received from the channel.

The stderr destination clause directs the channel to the server’s standard error stream. This is intended for use when the server is running as a foreground process, as when debugging a configuration, for example.

The server can supply extensive debugging information when it is in debugging mode. If the server’s global debug level is greater than zero, debugging mode is active. The global debug level is set either by starting the named server with the -d flag followed by a positive integer, or by running rndc trace. The global debug level can be set to zero, and debugging mode turned off, by running rndc notrace. All debugging messages in the server have a debug level; higher debug levels give more detailed output. Channels that specify a specific debug severity, for example:

channel specific_debug_level {
    file "foo";
    severity debug 3;
};

get debugging output of level 3 or less any time the server is in debugging mode, regardless of the global debugging level. Channels with dynamic severity use the server’s global debug level to determine what messages to print.

print-time can be set to yes, no, or a time format specifier, which may be one of local, iso8601, or iso8601-utc. If set to no, the date and time are not logged. If set to yes or local, the date and time are logged in a human-readable format, using the local time zone. If set to iso8601, the local time is logged in ISO 8601 format. If set to iso8601-utc, the date and time are logged in ISO 8601 format, with time zone set to UTC. The default is no.

print-time may be specified for a syslog channel, but it is usually pointless since syslog also logs the date and time.

If print-category is requested, then the category of the message is logged as well. Finally, if print-severity is on, then the severity level of the message is logged. The print- options may be used in any combination, and are always printed in the following order: time, category, severity. Here is an example where all three print- options are on:

28-Feb-2000 15:05:32.863 general: notice: running

If buffered has been turned on, the output to files is not flushed after each log entry. By default all log messages are flushed.

There are four predefined channels that are used for named’s default logging, as follows. If named is started with the -L option, then a fifth channel, default_logfile, is added. How they are used is described in The category Phrase.

channel default_syslog {
    // send to syslog's daemon facility
    syslog daemon;
    // only send priority info and higher
    severity info;
};

channel default_debug {
    // write to named.run in the working directory
    // Note: stderr is used instead of "named.run" if
    // the server is started with the '-g' option.
    file "named.run";
    // log at the server's current debug level
    severity dynamic;
};

channel default_stderr {
    // writes to stderr
    stderr;
    // only send priority info and higher
    severity info;
};

channel null {
   // toss anything sent to this channel
   null;
};

channel default_logfile {
    // this channel is only present if named is
    // started with the -L option, whose argument
    // provides the file name
    file "...";
    // log at the server's current debug level
    severity dynamic;
};

The default_debug channel has the special property that it only produces output when the server’s debug level is non-zero. It normally writes to a file called named.run in the server’s working directory.

For security reasons, when the -u command-line option is used, the named.run file is created only after named has changed to the new UID, and any debug output generated while named is starting - and still running as root - is discarded. To capture this output, run the server with the -L option to specify a default logfile, or the -g option to log to standard error which can be redirected to a file.

Once a channel is defined, it cannot be redefined. The built-in channels cannot be altered directly, but the default logging can be modified by pointing categories at defined channels.

4.2.10.2. The category Phrase

There are many categories, so desired logs can be sent anywhere while unwanted logs are ignored. If a list of channels is not specified for a category, log messages in that category are sent to the default category instead. If no default category is specified, the following “default default” is used:

category default { default_syslog; default_debug; };

If named is started with the -L option, the default category is:

category default { default_logfile; default_debug; };

As an example, let’s say a user wants to log security events to a file, but also wants to keep the default logging behavior. They would specify the following:

channel my_security_channel {
    file "my_security_file";
    severity info;
};
category security {
    my_security_channel;
    default_syslog;
    default_debug;
};

To discard all messages in a category, specify the null channel:

category xfer-out { null; };
category notify { null; };

The following are the available categories and brief descriptions of the types of log information they contain. More categories may be added in future BIND releases.

client

Processing of client requests.

cname

Name servers that are skipped for being a CNAME rather than A/AAAA records.

config

Configuration file parsing and processing.

database

Messages relating to the databases used internally by the name server to store zone and cache data.

default

Logging options for those categories where no specific configuration has been defined.

delegation-only

Queries that have been forced to NXDOMAIN as the result of a delegation-only zone or a delegation-only in a forward, hint, or stub zone declaration.

dispatch

Dispatching of incoming packets to the server modules where they are to be processed.

dnssec

DNSSEC and TSIG protocol processing.

dnstap

The “dnstap” DNS traffic capture system.

edns-disabled

Log queries that have been forced to use plain DNS due to timeouts. This is often due to the remote servers not being RFC 1034-compliant (not always returning FORMERR or similar to EDNS queries and other extensions to the DNS when they are not understood). In other words, this is targeted at servers that fail to respond to DNS queries that they don’t understand.

Note: the log message can also be due to packet loss. Before reporting servers for non-RFC 1034 compliance they should be re-tested to determine the nature of the non-compliance. This testing should prevent or reduce the number of false-positive reports.

Note: eventually named will have to stop treating such timeouts as due to RFC 1034 non-compliance and start treating it as plain packet loss. Falsely classifying packet loss as due to RFC 1034 non-compliance impacts DNSSEC validation, which requires EDNS for the DNSSEC records to be returned.

general

A catch-all for many things that still are not classified into categories.

lame-servers

Misconfigurations in remote servers, discovered by BIND 9 when trying to query those servers during resolution.

network

Network operations.

notify

The NOTIFY protocol.

nsid

NSID options received from upstream servers.

queries

A location where queries should be logged.

At startup, specifying the category queries also enables query logging unless the querylog option has been specified.

The query log entry first reports a client object identifier in @0x<hexadecimal-number> format. Next, it reports the client’s IP address and port number, and the query name, class, and type. Next, it reports whether the Recursion Desired flag was set (+ if set, - if not set), whether the query was signed (S), whether EDNS was in use along with the EDNS version number (E(#)), whether TCP was used (T), whether DO (DNSSEC Ok) was set (D), whether CD (Checking Disabled) was set (C), whether a valid DNS Server COOKIE was received (V), and whether a DNS COOKIE option without a valid Server COOKIE was present (K). After this, the destination address the query was sent to is reported. Finally, if any CLIENT-SUBNET option was present in the client query, it is included in square brackets in the format [ECS address/source/scope].

client 127.0.0.1#62536 (www.example.com): query: www.example.com IN AAAA +SE client ::1#62537 (www.example.net): query: www.example.net IN AAAA -SE

The first part of this log message, showing the client address/port number and query name, is repeated in all subsequent log messages related to the same query.

query-errors

Information about queries that resulted in some failure.

rate-limit

Start, periodic, and final notices of the rate limiting of a stream of responses that are logged at info severity in this category. These messages include a hash value of the domain name of the response and the name itself, except when there is insufficient memory to record the name for the final notice. The final notice is normally delayed until about one minute after rate limiting stops. A lack of memory can hurry the final notice, which is indicated by an initial asterisk (*). Various internal events are logged at debug level 1 and higher.

Rate limiting of individual requests is logged in the query-errors category.

resolver

DNS resolution, such as the recursive lookups performed on behalf of clients by a caching name server.

rpz

Information about errors in response policy zone files, rewritten responses, and, at the highest debug levels, mere rewriting attempts.

rpz-passthru

Information about RPZ PASSTHRU policy activity. This category allows pre-approved policy activity to be logged into a dedicated channel.

security

Approval and denial of requests.

serve-stale

Indication of whether a stale answer is used following a resolver failure.

spill

Queries that have been terminated, either by dropping or responding with SERVFAIL, as a result of a fetchlimit quota being exceeded.

sslkeylog

TLS pre-master secrets (for debugging purposes).

trust-anchor-telemetry

Trust-anchor-telemetry requests received by named.

unmatched

Messages that named was unable to determine the class of, or for which there was no matching view. A one-line summary is also logged to the client category. This category is best sent to a file or stderr; by default it is sent to the null channel.

update

Dynamic updates.

update-security

Approval and denial of update requests.

xfer-in

Zone transfers the server is receiving.

xfer-out

Zone transfers the server is sending.

zoneload

Loading of zones and creation of automatic empty zones.

4.2.10.3. The query-errors Category

The query-errors category is used to indicate why and how specific queries resulted in responses which indicate an error. Normally, these messages are logged at debug logging levels; note, however, that if query logging is active, some are logged at info. The logging levels are described below:

At debug level 1 or higher - or at info when query logging is active - each response with the rcode of SERVFAIL is logged as follows:

client 127.0.0.1#61502: query failed (SERVFAIL) for www.example.com/IN/AAAA at query.c:3880

This means an error resulting in SERVFAIL was detected at line 3880 of source file query.c. Log messages of this level are particularly helpful in identifying the cause of SERVFAIL for an authoritative server.

At debug level 2 or higher, detailed context information about recursive resolutions that resulted in SERVFAIL is logged. The log message looks like this:

fetch completed at resolver.c:2970 for www.example.com/A
in 10.000183: timed out/success [domain:example.com,
referral:2,restart:7,qrysent:8,timeout:5,lame:0,quota:0,neterr:0,
badresp:1,adberr:0,findfail:0,valfail:0]

The first part before the colon shows that a recursive resolution for AAAA records of www.example.com completed in 10.000183 seconds, and the final result that led to the SERVFAIL was determined at line 2970 of source file resolver.c.

The next part shows the detected final result and the latest result of DNSSEC validation. The latter is always “success” when no validation attempt was made. In this example, this query probably resulted in SERVFAIL because all name servers are down or unreachable, leading to a timeout in 10 seconds. DNSSEC validation was probably not attempted.

The last part, enclosed in square brackets, shows statistics collected for this particular resolution attempt. The domain field shows the deepest zone that the resolver reached; it is the zone where the error was finally detected. The meaning of the other fields is summarized in the following list.

referral

The number of referrals the resolver received throughout the resolution process. In the above example.com there are two.

restart

The number of cycles that the resolver tried remote servers at the domain zone. In each cycle, the resolver sends one query (possibly resending it, depending on the response) to each known name server of the domain zone.

qrysent

The number of queries the resolver sent at the domain zone.

timeout

The number of timeouts the resolver received since the last response.

lame

The number of lame servers the resolver detected at the domain zone. A server is detected to be lame either by an invalid response or as a result of lookup in BIND 9’s address database (ADB), where lame servers are cached.

quota

The number of times the resolver was unable to send a query because it had exceeded the permissible fetch quota for a server.

neterr

The number of erroneous results that the resolver encountered in sending queries at the domain zone. One common case is when the remote server is unreachable and the resolver receives an “ICMP unreachable” error message.

badresp

The number of unexpected responses (other than lame) to queries sent by the resolver at the domain zone.

adberr

Failures in finding remote server addresses of the``domain`` zone in the ADB. One common case of this is that the remote server’s name does not have any address records.

findfail

Failures to resolve remote server addresses. This is a total number of failures throughout the resolution process.

valfail

Failures of DNSSEC validation. Validation failures are counted throughout the resolution process (not limited to the domain zone), but should only happen in domain.

At debug level 3 or higher, the same messages as those at debug level 1 are logged for errors other than SERVFAIL. Note that negative responses such as NXDOMAIN are not errors, and are not logged at this debug level.

At debug level 4 or higher, the detailed context information logged at debug level 2 is logged for errors other than SERVFAIL and for negative responses such as NXDOMAIN.

4.2.11. parental-agents Statement Grammar

4.2.12. parental-agents Statement Definition and Usage

parental-agents lists allow for a common set of parental agents to be easily used by multiple primary and secondary zones in their parental-agents lists. A parental agent is the entity that the zone has a relationship with to change its delegation information (defined in RFC 7344).

4.2.13. primaries Statement Grammar

4.2.14. primaries Statement Definition and Usage

primaries lists allow for a common set of primary servers to be easily used by multiple stub and secondary zones in their primaries or also-notify lists. (Note: primaries is a synonym for the original keyword masters, which can still be used, but is no longer the preferred terminology.)

To force the zone transfer requests to be sent over TLS, use tls keyword, e.g. primaries { 192.0.2.1 tls tls-configuration-name; };, where tls-configuration-name refers to a previously defined tls statement.

Warning

Please note that TLS connections to primaries are currently not authenticated. This mode provides protection from passive observers but does not protect from man-in-the-middle attacks on zone transfers.

4.2.15. options Statement Grammar

This is the grammar of the options statement in the named.conf file:

4.2.16. options Statement Definition and Usage

The options statement sets up global options to be used by BIND. This statement may appear only once in a configuration file. If there is no options statement, an options block with each option set to its default is used.

attach-cache

This option allows multiple views to share a single cache database. Each view has its own cache database by default, but if multiple views have the same operational policy for name resolution and caching, those views can share a single cache to save memory, and possibly improve resolution efficiency, by using this option.

The attach-cache option may also be specified in view statements, in which case it overrides the global attach-cache option.

The cache_name specifies the cache to be shared. When the named server configures views which are supposed to share a cache, it creates a cache with the specified name for the first view of these sharing views. The rest of the views simply refer to the already-created cache.

One common configuration to share a cache is to allow all views to share a single cache. This can be done by specifying attach-cache as a global option with an arbitrary name.

Another possible operation is to allow a subset of all views to share a cache while the others retain their own caches. For example, if there are three views A, B, and C, and only A and B should share a cache, specify the attach-cache option as a view of A (or B)’s option, referring to the other view name:

view "A" {
  // this view has its own cache
  ...
};
view "B" {
  // this view refers to A's cache
  attach-cache "A";
};
view "C" {
  // this view has its own cache
  ...
};

Views that share a cache must have the same policy on configurable parameters that may affect caching. The current implementation requires the following configurable options be consistent among these views: check-names, dnssec-accept-expired, dnssec-validation, max-cache-ttl, max-ncache-ttl, max-stale-ttl, max-cache-size, min-cache-ttl, min-ncache-ttl, and zero-no-soa-ttl.

Note that there may be other parameters that may cause confusion if they are inconsistent for different views that share a single cache. For example, if these views define different sets of forwarders that can return different answers for the same question, sharing the answer does not make sense or could even be harmful. It is the administrator’s responsibility to ensure that configuration differences in different views do not cause disruption with a shared cache.

directory

This sets the working directory of the server. Any non-absolute pathnames in the configuration file are taken as relative to this directory. The default location for most server output files (e.g., named.run) is this directory. If a directory is not specified, the working directory defaults to ".", the directory from which the server was started. The directory specified should be an absolute path, and must be writable by the effective user ID of the named process.

dnstap

dnstap is a fast, flexible method for capturing and logging DNS traffic. Developed by Robert Edmonds at Farsight Security, Inc., and supported by multiple DNS implementations, dnstap uses libfstrm (a lightweight high-speed framing library; see https://github.com/farsightsec/fstrm) to send event payloads which are encoded using Protocol Buffers (libprotobuf-c, a mechanism for serializing structured data developed by Google, Inc.; see https://developers.google.com/protocol-buffers/).

To enable dnstap at compile time, the fstrm and protobuf-c libraries must be available, and BIND must be configured with --enable-dnstap.

The dnstap option is a bracketed list of message types to be logged. These may be set differently for each view. Supported types are client, auth, resolver, forwarder, and update. Specifying type all causes all dnstap messages to be logged, regardless of type.

Each type may take an additional argument to indicate whether to log query messages or response messages; if not specified, both queries and responses are logged.

Example: To log all authoritative queries and responses, recursive client responses, and upstream queries sent by the resolver, use:

dnstap {
  auth;
  client response;
  resolver query;
};

Logged dnstap messages can be parsed using the dnstap-read utility (see dnstap-read - print dnstap data in human-readable form for details).

For more information on dnstap, see http://dnstap.info.

The fstrm library has a number of tunables that are exposed in named.conf, and can be modified if necessary to improve performance or prevent loss of data. These are:

• fstrm-set-buffer-hint: The threshold number of bytes to accumulate in the output buffer before forcing a buffer flush. The minimum is 1024, the maximum is 65536, and the default is 8192.

• fstrm-set-flush-timeout: The number of seconds to allow unflushed data to remain in the output buffer. The minimum is 1 second, the maximum is 600 seconds (10 minutes), and the default is 1 second.

• fstrm-set-output-notify-threshold: The number of outstanding queue entries to allow on an input queue before waking the I/O thread. The minimum is 1 and the default is 32.

• fstrm-set-output-queue-model: The queuing semantics to use for queue objects. The default is mpsc (multiple producer, single consumer); the other option is spsc (single producer, single consumer).

• fstrm-set-input-queue-size: The number of queue entries to allocate for each input queue. This value must be a power of 2. The minimum is 2, the maximum is 16384, and the default is 512.

• fstrm-set-output-queue-size: The number of queue entries to allocate for each output queue. The minimum is 2, the maximum is system-dependent and based on IOV_MAX, and the default is 64.

• fstrm-set-reopen-interval: The number of seconds to wait between attempts to reopen a closed output stream. The minimum is 1 second, the maximum is 600 seconds (10 minutes), and the default is 5 seconds. For convenience, TTL-style time-unit suffixes may be used to specify the value.

Note that all of the above minimum, maximum, and default values are set by the libfstrm library, and may be subject to change in future versions of the library. See the libfstrm documentation for more information.

dnstap-output

This configures the path to which the dnstap frame stream is sent if dnstap is enabled at compile time and active.

The first argument is either file or unix, indicating whether the destination is a file or a Unix domain socket. The second argument is the path of the file or socket. (Note: when using a socket, dnstap messages are only sent if another process such as fstrm_capture (provided with libfstrm) is listening on the socket.)

If the first argument is file, then up to three additional options can be added: size indicates the size to which a dnstap log file can grow before being rolled to a new file; versions specifies the number of rolled log files to retain; and suffix indicates whether to retain rolled log files with an incrementing counter as the suffix (increment) or with the current timestamp (timestamp). These are similar to the size, versions, and suffix options in a logging channel. The default is to allow dnstap log files to grow to any size without rolling.

dnstap-output can only be set globally in options. Currently, it can only be set once while named is running; once set, it cannot be changed by rndc reload or rndc reconfig.

dnstap-identity

This specifies an identity string to send in dnstap messages. If set to hostname, which is the default, the server’s hostname is sent. If set to none, no identity string is sent.

dnstap-version

This specifies a version string to send in dnstap messages. The default is the version number of the BIND release. If set to none, no version string is sent.

geoip-directory

When named is compiled using the MaxMind GeoIP2 geolocation API, this specifies the directory containing GeoIP database files. By default, the option is set based on the prefix used to build the libmaxminddb module; for example, if the library is installed in /usr/local/lib, then the default geoip-directory is /usr/local/share/GeoIP. See acl Statement Definition and Usage for details about geoip ACLs.

key-directory

This is the directory where the public and private DNSSEC key files should be found when performing a dynamic update of secure zones, if different than the current working directory. (Note that this option has no effect on the paths for files containing non-DNSSEC keys such as bind.keys, rndc.key, or session.key.)

lmdb-mapsize

When named is built with liblmdb, this option sets a maximum size for the memory map of the new-zone database (NZD) in LMDB database format. This database is used to store configuration information for zones added using rndc addzone. Note that this is not the NZD database file size, but the largest size that the database may grow to.

Because the database file is memory-mapped, its size is limited by the address space of the named process. The default of 32 megabytes was chosen to be usable with 32-bit named builds. The largest permitted value is 1 terabyte. Given typical zone configurations without elaborate ACLs, a 32 MB NZD file ought to be able to hold configurations of about 100,000 zones.

managed-keys-directory

This specifies the directory in which to store the files that track managed DNSSEC keys (i.e., those configured using the initial-key or initial-ds keywords in a trust-anchors statement). By default, this is the working directory. The directory must be writable by the effective user ID of the named process.

If named is not configured to use views, managed keys for the server are tracked in a single file called managed-keys.bind. Otherwise, managed keys are tracked in separate files, one file per view; each file name is the view name (or, if it contains characters that are incompatible with use as a file name, the SHA256 hash of the view name), followed by the extension .mkeys.

(Note: in earlier releases, file names for views always used the SHA256 hash of the view name. To ensure compatibility after upgrading, if a file using the old name format is found to exist, it is used instead of the new format.)

max-ixfr-ratio

This sets the size threshold (expressed as a percentage of the size of the full zone) beyond which named chooses to use an AXFR response rather than IXFR when answering zone transfer requests. See Incremental Zone Transfers (IXFR).

The minimum value is 1%. The keyword unlimited disables ratio checking and allows IXFRs of any size. The default is 100%.

new-zones-directory

This specifies the directory in which to store the configuration parameters for zones added via rndc addzone. By default, this is the working directory. If set to a relative path, it is relative to the working directory. The directory must be writable by the effective user ID of the named process.

qname-minimization

This option controls QNAME minimization behavior in the BIND resolver. When set to strict, BIND follows the QNAME minimization algorithm to the letter, as specified in RFC 7816. Setting this option to relaxed causes BIND to fall back to normal (non-minimized) query mode when it receives either NXDOMAIN or other unexpected responses (e.g., SERVFAIL, improper zone cut, REFUSED) to a minimized query. disabled disables QNAME minimization completely. The current default is relaxed, but it may be changed to strict in a future release.

tkey-gssapi-keytab

This is the KRB5 keytab file to use for GSS-TSIG updates. If this option is set and tkey-gssapi-credential is not set, updates are allowed with any key matching a principal in the specified keytab.

tkey-gssapi-credential

This is the security credential with which the server should authenticate keys requested by the GSS-TSIG protocol. Currently only Kerberos 5 authentication is available; the credential is a Kerberos principal which the server can acquire through the default system key file, normally /etc/krb5.keytab. The location of the keytab file can be overridden using the tkey-gssapi-keytab option. Normally this principal is of the form DNS/server.domain. To use GSS-TSIG, tkey-domain must also be set if a specific keytab is not set with tkey-gssapi-keytab.

tkey-domain

This domain is appended to the names of all shared keys generated with TKEY. When a client requests a TKEY exchange, it may or may not specify the desired name for the key. If present, the name of the shared key is client-specified part + tkey-domain. Otherwise, the name of the shared key is random hex digits + tkey-domain. In most cases, the domainname should be the server’s domain name, or an otherwise nonexistent subdomain like _tkey.domainname. If using GSS-TSIG, this variable must be defined, unless a specific keytab is specified using tkey-gssapi-keytab.

tkey-dhkey

This is the Diffie-Hellman key used by the server to generate shared keys with clients using the Diffie-Hellman mode of TKEY. The server must be able to load the public and private keys from files in the working directory. In most cases, the key_name should be the server’s host name.

dump-file

This is the pathname of the file the server dumps the database to, when instructed to do so with rndc dumpdb. If not specified, the default is named_dump.db.

memstatistics-file

This is the pathname of the file the server writes memory usage statistics to on exit. If not specified, the default is named.memstats.

lock-file

This is the pathname of a file on which named attempts to acquire a file lock when starting for the first time; if unsuccessful, the server terminates, under the assumption that another server is already running. If not specified, the default is none.

Specifying lock-file none disables the use of a lock file. lock-file is ignored if named was run using the -X option, which overrides it. Changes to lock-file are ignored if named is being reloaded or reconfigured; it is only effective when the server is first started.

pid-file

This is the pathname of the file the server writes its process ID in. If not specified, the default is /var/run/named/named.pid. The PID file is used by programs that send signals to the running name server. Specifying pid-file none disables the use of a PID file; no file is written and any existing one is removed. Note that none is a keyword, not a filename, and therefore is not enclosed in double quotes.

recursing-file

This is the pathname of the file where the server dumps the queries that are currently recursing, when instructed to do so with rndc recursing. If not specified, the default is named.recursing.

statistics-file

This is the pathname of the file the server appends statistics to, when instructed to do so using rndc stats. If not specified, the default is named.stats in the server’s current directory. The format of the file is described in The Statistics File.

bindkeys-file

This is the pathname of a file to override the built-in trusted keys provided by named. See the discussion of dnssec-validation for details. If not specified, the default is /etc/bind.keys.

secroots-file

This is the pathname of the file the server dumps security roots to, when instructed to do so with rndc secroots. If not specified, the default is named.secroots.

session-keyfile

This is the pathname of the file into which to write a TSIG session key generated by named for use by nsupdate -l. If not specified, the default is /var/run/named/session.key. (See Dynamic Update Policies, and in particular the discussion of the update-policy statement’s local option, for more information about this feature.)

session-keyname

This is the key name to use for the TSIG session key. If not specified, the default is local-ddns.

session-keyalg

This is the algorithm to use for the TSIG session key. Valid values are hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384, hmac-sha512, and hmac-md5. If not specified, the default is hmac-sha256.

port

This is the UDP/TCP port number the server uses to receive and send DNS protocol traffic. The default is 53. This option is mainly intended for server testing; a server using a port other than 53 is not able to communicate with the global DNS.

tls-port

This is the TCP port number the server uses to receive and send DNS-over-TLS protocol traffic. The default is 853.

https-port

This is the TCP port number the server uses to receive and send DNS-over-HTTPS protocol traffic. The default is 443.

http-port

This is the TCP port number the server uses to receive and send unencrypted DNS traffic via HTTP (a configuration that may be useful when encryption is handled by third-party software or by a reverse proxy).

http-listener-clients

This sets the hard quota on the number of active concurrent connections on a per-listener basis. The default value is 300; setting it to 0 removes the quota.

http-streams-per-connection

This sets the hard limit on the number of active concurrent HTTP/2 streams on a per-connection basis. The default value is 100; setting it to 0 removes the limit. Once the limit is exceeded, the server finishes the HTTP session.

dscp

This is the global Differentiated Services Code Point (DSCP) value to classify outgoing DNS traffic, on operating systems that support DSCP. Valid values are 0 through 63. It is not configured by default.

random-device

This specifies a source of entropy to be used by the server; it is a device or file from which to read entropy. If it is a file, operations requiring entropy will fail when the file has been exhausted.

Entropy is needed for cryptographic operations such as TKEY transactions, dynamic update of signed zones, and generation of TSIG session keys. It is also used for seeding and stirring the pseudo-random number generator which is used for less critical functions requiring randomness, such as generation of DNS message transaction IDs.

If random-device is not specified, or if it is set to none, entropy is read from the random number generation function supplied by the cryptographic library with which BIND was linked (i.e. OpenSSL or a PKCS#11 provider).

The random-device option takes effect during the initial configuration load at server startup time and is ignored on subsequent reloads.

preferred-glue

If specified, the listed type (A or AAAA) is emitted before other glue in the additional section of a query response. The default is to prefer A records when responding to queries that arrived via IPv4 and AAAA when responding to queries that arrived via IPv6.

root-delegation-only

This turns on enforcement of delegation-only in TLDs (top-level domains) and root zones with an optional exclude list.

DS queries are expected to be made to and be answered by delegation-only zones. Such queries and responses are treated as an exception to delegation-only processing and are not converted to NXDOMAIN responses, provided a CNAME is not discovered at the query name.

If a delegation-only zone server also serves a child zone, it is not always possible to determine whether an answer comes from the delegation-only zone or the child zone. SOA NS and DNSKEY records are apex-only records and a matching response that contains these records or DS is treated as coming from a child zone. RRSIG records are also examined to see whether they are signed by a child zone, and the authority section is examined to see if there is evidence that the answer is from the child zone. Answers that are determined to be from a child zone are not converted to NXDOMAIN responses. Despite all these checks, there is still a possibility of false negatives when a child zone is being served.

Similarly, false positives can arise from empty nodes (no records at the name) in the delegation-only zone when the query type is not ANY.

Note that some TLDs are not delegation-only; e.g., “DE”, “LV”, “US”, and “MUSEUM”. This list is not exhaustive.

options {
    root-delegation-only exclude { "de"; "lv"; "us"; "museum"; };
};

disable-algorithms

This disables the specified DNSSEC algorithms at and below the specified name. Multiple disable-algorithms statements are allowed. Only the best-match disable-algorithms clause is used to determine the algorithms.

If all supported algorithms are disabled, the zones covered by the disable-algorithms setting are treated as insecure.

Configured trust anchors in trust-anchors (or managed-keys or trusted-keys) that match a disabled algorithm are ignored and treated as if they were not configured.

disable-ds-digests

This disables the specified DS digest types at and below the specified name. Multiple disable-ds-digests statements are allowed. Only the best-match disable-ds-digests clause is used to determine the digest types.

If all supported digest types are disabled, the zones covered by disable-ds-digests are treated as insecure.

dnssec-must-be-secure

This specifies hierarchies which must be or may not be secure (signed and validated). If yes, then named only accepts answers if they are secure. If no, then normal DNSSEC validation applies, allowing insecure answers to be accepted. The specified domain must be defined as a trust anchor, for instance in a trust-anchors statement, or dnssec-validation auto must be active.

dns64

This directive instructs named to return mapped IPv4 addresses to AAAA queries when there are no AAAA records. It is intended to be used in conjunction with a NAT64. Each dns64 defines one DNS64 prefix. Multiple DNS64 prefixes can be defined.

Compatible IPv6 prefixes have lengths of 32, 40, 48, 56, 64, and 96, per RFC 6052. Bits 64..71 inclusive must be zero, with the most significant bit of the prefix in position 0.

In addition, a reverse IP6.ARPA zone is created for the prefix to provide a mapping from the IP6.ARPA names to the corresponding IN-ADDR.ARPA names using synthesized CNAMEs. dns64-server and dns64-contact can be used to specify the name of the server and contact for the zones. These can be set at the view/options level but not on a per-prefix basis.

dns64 will also cause IPV4ONLY.ARPA to be created if not explicitly disabled using ipv4only-enable.

Each dns64 supports an optional clients ACL that determines which clients are affected by this directive. If not defined, it defaults to any;.

Each dns64 supports an optional mapped ACL that selects which IPv4 addresses are to be mapped in the corresponding A RRset. If not defined, it defaults to any;.

Normally, DNS64 does not apply to a domain name that owns one or more AAAA records; these records are simply returned. The optional exclude ACL allows specification of a list of IPv6 addresses that are ignored if they appear in a domain name’s AAAA records; DNS64 is applied to any A records the domain name owns. If not defined, exclude defaults to ::ffff:0.0.0.0/96.

An optional suffix can also be defined to set the bits trailing the mapped IPv4 address bits. By default these bits are set to ::. The bits matching the prefix and mapped IPv4 address must be zero.

If recursive-only is set to yes, the DNS64 synthesis only happens for recursive queries. The default is no.

If break-dnssec is set to yes, the DNS64 synthesis happens even if the result, if validated, would cause a DNSSEC validation failure. If this option is set to no (the default), the DO is set on the incoming query, and there are RRSIGs on the applicable records, then synthesis does not happen.

acl rfc1918 { 10/8; 192.168/16; 172.16/12; };

dns64 64:FF9B::/96 {
    clients { any; };
    mapped { !rfc1918; any; };
    exclude { 64:FF9B::/96; ::ffff:0000:0000/96; };
    suffix ::;
};

dnssec-loadkeys-interval

When a zone is configured with auto-dnssec maintain;, its key repository must be checked periodically to see if any new keys have been added or any existing keys’ timing metadata has been updated (see dnssec-keygen: DNSSEC key generation tool and dnssec-settime: set the key timing metadata for a DNSSEC key). The dnssec-loadkeys-interval option sets the frequency of automatic repository checks, in minutes. The default is 60 (1 hour), the minimum is 1 (1 minute), and the maximum is 1440 (24 hours); any higher value is silently reduced.

dnssec-policy

This specifies which key and signing policy (KASP) should be used for this zone. This is a string referring to a dnssec-policy statement. There are three built-in policies: default, which uses the default policy, insecure, to be used when you want to gracefully unsign your zone, and none, which means no DNSSEC policy. The default is none. See dnssec-policy Grammar for more details.

dnssec-update-mode

If this option is set to its default value of maintain in a zone of type primary which is DNSSEC-signed and configured to allow dynamic updates (see Dynamic Update Policies), and if named has access to the private signing key(s) for the zone, then named automatically signs all new or changed records and maintains signatures for the zone by regenerating RRSIG records whenever they approach their expiration date.

If the option is changed to no-resign, then named signs all new or changed records, but scheduled maintenance of signatures is disabled.

With either of these settings, named rejects updates to a DNSSEC-signed zone when the signing keys are inactive or unavailable to named. (A planned third option, external, will disable all automatic signing and allow DNSSEC data to be submitted into a zone via dynamic update; this is not yet implemented.)

nta-lifetime

This specifies the default lifetime, in seconds, for negative trust anchors added via rndc nta.

A negative trust anchor selectively disables DNSSEC validation for zones that are known to be failing because of misconfiguration, rather than an attack. When data to be validated is at or below an active NTA (and above any other configured trust anchors), named aborts the DNSSEC validation process and treats the data as insecure rather than bogus. This continues until the NTA’s lifetime has elapsed. NTAs persist across named restarts.

For convenience, TTL-style time-unit suffixes can be used to specify the NTA lifetime in seconds, minutes, or hours. It also accepts ISO 8601 duration formats.

nta-lifetime defaults to one hour; it cannot exceed one week.

nta-recheck

This specifies how often to check whether negative trust anchors added via rndc nta are still necessary.

A negative trust anchor is normally used when a domain has stopped validating due to operator error; it temporarily disables DNSSEC validation for that domain. In the interest of ensuring that DNSSEC validation is turned back on as soon as possible, named periodically sends a query to the domain, ignoring negative trust anchors, to find out whether it can now be validated. If so, the negative trust anchor is allowed to expire early.

Validity checks can be disabled for an individual NTA by using rndc nta -f, or for all NTAs by setting nta-recheck to zero.

For convenience, TTL-style time-unit suffixes can be used to specify the NTA recheck interval in seconds, minutes, or hours. It also accepts ISO 8601 duration formats.

The default is five minutes. It cannot be longer than nta-lifetime, which cannot be longer than a week.

max-zone-ttl

This specifies a maximum permissible TTL value in seconds. For convenience, TTL-style time-unit suffixes may be used to specify the maximum value. When loading a zone file using a masterfile-format of text or raw, any record encountered with a TTL higher than max-zone-ttl causes the zone to be rejected.

This is useful in DNSSEC-signed zones because when rolling to a new DNSKEY, the old key needs to remain available until RRSIG records have expired from caches. The max-zone-ttl option guarantees that the largest TTL in the zone is no higher than the set value.

The default value is unlimited. A max-zone-ttl of zero is treated as unlimited.

stale-answer-ttl

This specifies the TTL to be returned on stale answers. The default is 30 seconds. The minimum allowed is 1 second; a value of 0 is updated silently to 1 second.

For stale answers to be returned, they must be enabled, either in the configuration file using stale-answer-enable or via rndc serve-stale on.

serial-update-method

Zones configured for dynamic DNS may use this option to set the update method to be used for the zone serial number in the SOA record.

With the default setting of serial-update-method increment;, the SOA serial number is incremented by one each time the zone is updated.

When set to serial-update-method unixtime;, the SOA serial number is set to the number of seconds since the Unix epoch, unless the serial number is already greater than or equal to that value, in which case it is simply incremented by one.

When set to serial-update-method date;, the new SOA serial number is the current date in the form “YYYYMMDD”, followed by two zeroes, unless the existing serial number is already greater than or equal to that value, in which case it is incremented by one.

zone-statistics

If full, the server collects statistical data on all zones, unless specifically turned off on a per-zone basis by specifying zone-statistics terse or zone-statistics none in the zone statement. The statistical data includes, for example, DNSSEC signing operations and the number of authoritative answers per query type. The default is terse, providing minimal statistics on zones (including name and current serial number, but not query type counters).

These statistics may be accessed via the statistics-channel or using rndc stats, which dumps them to the file listed in the statistics-file. See also The Statistics File.

For backward compatibility with earlier versions of BIND 9, the zone-statistics option can also accept yes or no; yes has the same meaning as full. As of BIND 9.10, no has the same meaning as none; previously, it was the same as terse.

4.2.16.1. Boolean Options

automatic-interface-scan

If yes and supported by the operating system, this automatically rescans network interfaces when the interface addresses are added or removed. The default is yes. This configuration option does not affect the time-based interface-interval option; it is recommended to set the time-based interface-interval to 0 when the operator confirms that automatic interface scanning is supported by the operating system.

The automatic-interface-scan implementation uses routing sockets for the network interface discovery; therefore, the operating system must support the routing sockets for this feature to work.

allow-new-zones

If yes, then zones can be added at runtime via rndc addzone. The default is no.

Newly added zones’ configuration parameters are stored so that they can persist after the server is restarted. The configuration information is saved in a file called viewname.nzf (or, if named is compiled with liblmdb, in an LMDB database file called viewname.nzd). “viewname” is the name of the view, unless the view name contains characters that are incompatible with use as a file name, in which case a cryptographic hash of the view name is used instead.

Configurations for zones added at runtime are stored either in a new-zone file (NZF) or a new-zone database (NZD), depending on whether named was linked with liblmdb at compile time. See rndc - name server control utility for further details about rndc addzone.

auth-nxdomain

If yes, then the AA bit is always set on NXDOMAIN responses, even if the server is not actually authoritative. The default is no.

memstatistics

This writes memory statistics to the file specified by memstatistics-file at exit. The default is no unless -m record is specified on the command line, in which case it is yes.

dialup

If yes, then the server treats all zones as if they are doing zone transfers across a dial-on-demand dialup link, which can be brought up by traffic originating from this server. Although this setting has different effects according to zone type, it concentrates the zone maintenance so that everything happens quickly, once every heartbeat-interval, ideally during a single call. It also suppresses some normal zone maintenance traffic. The default is no.

If specified in the view and zone statements, the dialup option overrides the global dialup option.

If the zone is a primary zone, the server sends out a NOTIFY request to all the secondaries (default). This should trigger the zone serial number check in the secondary (providing it supports NOTIFY), allowing the secondary to verify the zone while the connection is active. The set of servers to which NOTIFY is sent can be controlled by notify and also-notify.

If the zone is a secondary or stub zone, the server suppresses the regular “zone up to date” (refresh) queries and only performs them when the heartbeat-interval expires, in addition to sending NOTIFY requests.

Finer control can be achieved by using notify, which only sends NOTIFY messages; notify-passive, which sends NOTIFY messages and suppresses the normal refresh queries; refresh, which suppresses normal refresh processing and sends refresh queries when the heartbeat-interval expires; and passive, which disables normal refresh processing.

dialup mode	normal refresh	heart-beat refresh	heart-beat notify
no (default)	yes	no	no
yes	no	yes	yes
notify	yes	no	yes
refresh	no	yes	no
passive	no	no	no
notify-passive	no	no	yes

Note that normal NOTIFY processing is not affected by dialup.

flush-zones-on-shutdown

When the name server exits upon receiving SIGTERM, flush or do not flush any pending zone writes. The default is flush-zones-on-shutdown no.

root-key-sentinel

If yes, respond to root key sentinel probes as described in draft-ietf-dnsop-kskroll-sentinel-08. The default is yes.

message-compression

If yes, DNS name compression is used in responses to regular queries (not including AXFR or IXFR, which always use compression). Setting this option to no reduces CPU usage on servers and may improve throughput. However, it increases response size, which may cause more queries to be processed using TCP; a server with compression disabled is out of compliance with RFC 1123 Section 6.1.3.2. The default is yes.

minimal-responses

This option controls the addition of records to the authority and additional sections of responses. Such records may be included in responses to be helpful to clients; for example, NS or MX records may have associated address records included in the additional section, obviating the need for a separate address lookup. However, adding these records to responses is not mandatory and requires additional database lookups, causing extra latency when marshalling responses. minimal-responses takes one of four values:

• no: the server is as complete as possible when generating responses.

• yes: the server only adds records to the authority and additional sections when such records are required by the DNS protocol (for example, when returning delegations or negative responses). This provides the best server performance but may result in more client queries.

• no-auth: the server omits records from the authority section except when they are required, but it may still add records to the additional section.

• no-auth-recursive: the same as no-auth when recursion is requested in the query (RD=1), or the same as no if recursion is not requested.

no-auth and no-auth-recursive are useful when answering stub clients, which usually ignore the authority section. no-auth-recursive is meant for use in mixed-mode servers that handle both authoritative and recursive queries.

The default is no-auth-recursive.

glue-cache

When set to yes, a cache is used to improve query performance when adding address-type (A and AAAA) glue records to the additional section of DNS response messages that delegate to a child zone.

The glue cache uses memory proportional to the number of delegations in the zone. The default setting is yes, which improves performance at the cost of increased memory usage for the zone. To avoid this, set it to no.

Note

This option is deprecated and its use is discouraged. The glue cache will be permanently enabled in a future release.

minimal-any

If set to yes, the server replies with only one of the RRsets for the query name, and its covering RRSIGs if any, when generating a positive response to a query of type ANY over UDP, instead of replying with all known RRsets for the name. Similarly, a query for type RRSIG is answered with the RRSIG records covering only one type. This can reduce the impact of some kinds of attack traffic, without harming legitimate clients. (Note, however, that the RRset returned is the first one found in the database; it is not necessarily the smallest available RRset.) Additionally, minimal-responses is turned on for these queries, so no unnecessary records are added to the authority or additional sections. The default is no.

notify

If set to yes (the default), DNS NOTIFY messages are sent when a zone the server is authoritative for changes; see Notify. The messages are sent to the servers listed in the zone’s NS records (except the primary server identified in the SOA MNAME field), and to any servers listed in the also-notify option.

If set to primary-only (or the older keyword master-only), notifies are only sent for primary zones. If set to explicit, notifies are sent only to servers explicitly listed using also-notify. If set to no, no notifies are sent.

The notify option may also be specified in the zone statement, in which case it overrides the options notify statement. It would only be necessary to turn off this option if it caused secondary zones to crash.

notify-to-soa

If yes, do not check the name servers in the NS RRset against the SOA MNAME. Normally a NOTIFY message is not sent to the SOA MNAME (SOA ORIGIN), as it is supposed to contain the name of the ultimate primary server. Sometimes, however, a secondary server is listed as the SOA MNAME in hidden primary configurations; in that case, the ultimate primary should be set to still send NOTIFY messages to all the name servers listed in the NS RRset.

recursion

If yes, and a DNS query requests recursion, then the server attempts to do all the work required to answer the query. If recursion is off and the server does not already know the answer, it returns a referral response. The default is yes. Note that setting recursion no does not prevent clients from getting data from the server’s cache; it only prevents new data from being cached as an effect of client queries. Caching may still occur as an effect of the server’s internal operation, such as NOTIFY address lookups.

request-nsid

If yes, then an empty EDNS(0) NSID (Name Server Identifier) option is sent with all queries to authoritative name servers during iterative resolution. If the authoritative server returns an NSID option in its response, then its contents are logged in the nsid category at level info. The default is no.

require-server-cookie

If yes, require a valid server cookie before sending a full response to a UDP request from a cookie-aware client. BADCOOKIE is sent if there is a bad or nonexistent server cookie.

The default is no.

Users wishing to test that DNS COOKIE clients correctly handle BADCOOKIE, or who are getting a lot of forged DNS requests with DNS COOKIES present, should set this to yes. Setting this to yes results in a reduced amplification effect in a reflection attack, as the BADCOOKIE response is smaller than a full response, while also requiring a legitimate client to follow up with a second query with the new, valid, cookie.

answer-cookie

When set to the default value of yes, COOKIE EDNS options are sent when applicable in replies to client queries. If set to no, COOKIE EDNS options are not sent in replies. This can only be set at the global options level, not per-view.

answer-cookie no is intended as a temporary measure, for use when named shares an IP address with other servers that do not yet support DNS COOKIE. A mismatch between servers on the same address is not expected to cause operational problems, but the option to disable COOKIE responses so that all servers have the same behavior is provided out of an abundance of caution. DNS COOKIE is an important security mechanism, and should not be disabled unless absolutely necessary.

send-cookie

If yes, then a COOKIE EDNS option is sent along with the query. If the resolver has previously communicated with the server, the COOKIE returned in the previous transaction is sent. This is used by the server to determine whether the resolver has talked to it before. A resolver sending the correct COOKIE is assumed not to be an off-path attacker sending a spoofed-source query; the query is therefore unlikely to be part of a reflection/amplification attack, so resolvers sending a correct COOKIE option are not subject to response rate limiting (RRL). Resolvers which do not send a correct COOKIE option may be limited to receiving smaller responses via the nocookie-udp-size option.

The default is yes.

stale-answer-enable

If yes, enable the returning of “stale” cached answers when the name servers for a zone are not answering and the stale-cache-enable option is also enabled. The default is not to return stale answers.

Stale answers can also be enabled or disabled at runtime via rndc serve-stale on or rndc serve-stale off; these override the configured setting. rndc serve-stale reset restores the setting to the one specified in named.conf. Note that if stale answers have been disabled by rndc, they cannot be re-enabled by reloading or reconfiguring named; they must be re-enabled with rndc serve-stale on, or the server must be restarted.

Information about stale answers is logged under the serve-stale log category.

stale-answer-client-timeout

This option defines the amount of time (in milliseconds) that named waits before attempting to answer the query with a stale RRset from cache. If a stale answer is found, named continues the ongoing fetches, attempting to refresh the RRset in cache until the resolver-query-timeout interval is reached.

This option is off by default, which is equivalent to setting it to off or disabled. It also has no effect if stale-answer-enable is disabled.

The maximum value for this option is resolver-query-timeout minus one second. The minimum value, 0, causes a cached (stale) RRset to be immediately returned if it is available while still attempting to refresh the data in cache. RFC 8767 recommends a value of 1800 (milliseconds).

stale-cache-enable

If yes, enable the retaining of “stale” cached answers. Default no.

stale-refresh-time

If the name servers for a given zone are not answering, this sets the time window for which named will promptly return “stale” cached answers for that RRSet being requested before a new attempt in contacting the servers is made. For convenience, TTL-style time-unit suffixes may be used to specify the value. It also accepts ISO 8601 duration formats.

The default stale-refresh-time is 30 seconds, as RFC 8767 recommends that attempts to refresh to be done no more frequently than every 30 seconds. A value of zero disables the feature, meaning that normal resolution will take place first, if that fails only then named will return “stale” cached answers.

nocookie-udp-size

This sets the maximum size of UDP responses that are sent to queries without a valid server COOKIE. A value below 128 is silently raised to 128. The default value is 4096, but the max-udp-size option may further limit the response size as the default for max-udp-size is 1232.

cookie-algorithm

This sets the algorithm to be used when generating the server cookie; the options are “aes” or “siphash24”. The default is “siphash24”. The “aes” option remains for legacy purposes.

cookie-secret

If set, this is a shared secret used for generating and verifying EDNS COOKIE options within an anycast cluster. If not set, the system generates a random secret at startup. The shared secret is encoded as a hex string and needs to be 128 bits for either “siphash24” or “aes”.

If there are multiple secrets specified, the first one listed in named.conf is used to generate new server cookies. The others are only used to verify returned cookies.

response-padding

The EDNS Padding option is intended to improve confidentiality when DNS queries are sent over an encrypted channel, by reducing the variability in packet sizes. If a query:

1. contains an EDNS Padding option,

2. includes a valid server cookie or uses TCP,

3. is not signed using TSIG or SIG(0), and

4. is from a client whose address matches the specified ACL,

then the response is padded with an EDNS Padding option to a multiple of block-size bytes. If these conditions are not met, the response is not padded.

If block-size is 0 or the ACL is none;, this feature is disabled and no padding occurs; this is the default. If block-size is greater than 512, a warning is logged and the value is truncated to 512. Block sizes are ordinarily expected to be powers of two (for instance, 128), but this is not mandatory.

trust-anchor-telemetry

This causes named to send specially formed queries once per day to domains for which trust anchors have been configured via, e.g., trust-anchors or dnssec-validation auto.

The query name used for these queries has the form _ta-xxxx(-xxxx)(...).<domain>, where each “xxxx” is a group of four hexadecimal digits representing the key ID of a trusted DNSSEC key. The key IDs for each domain are sorted smallest to largest prior to encoding. The query type is NULL.

By monitoring these queries, zone operators are able to see which resolvers have been updated to trust a new key; this may help them decide when it is safe to remove an old one.

The default is yes.

provide-ixfr

See the description of provide-ixfr in server Statement Definition and Usage.

request-ixfr

See the description of request-ixfr in server Statement Definition and Usage.

request-expire

See the description of request-expire in server Statement Definition and Usage.

match-mapped-addresses

If yes, then an IPv4-mapped IPv6 address matches any address-match list entries that match the corresponding IPv4 address.

This option was introduced to work around a kernel quirk in some operating systems that causes IPv4 TCP connections, such as zone transfers, to be accepted on an IPv6 socket using mapped addresses. This caused address-match lists designed for IPv4 to fail to match. However, named now solves this problem internally. The use of this option is discouraged.

ixfr-from-differences

When yes and the server loads a new version of a primary zone from its zone file or receives a new version of a secondary file via zone transfer, it compares the new version to the previous one and calculates a set of differences. The differences are then logged in the zone’s journal file so that the changes can be transmitted to downstream secondaries as an incremental zone transfer.

By allowing incremental zone transfers to be used for non-dynamic zones, this option saves bandwidth at the expense of increased CPU and memory consumption at the primary server. In particular, if the new version of a zone is completely different from the previous one, the set of differences is of a size comparable to the combined size of the old and new zone versions, and the server needs to temporarily allocate memory to hold this complete difference set.

ixfr-from-differences also accepts primary and secondary at the view and options levels, which causes ixfr-from-differences to be enabled for all primary or secondary zones, respectively. It is off for all zones by default.

Note: if inline signing is enabled for a zone, the user-provided ixfr-from-differences setting is ignored for that zone.

multi-master

This should be set when there are multiple primary servers for a zone and the addresses refer to different machines. If yes, named does not log when the serial number on the primary is less than what named currently has. The default is no.

auto-dnssec

Zones configured for dynamic DNS may use this option to allow varying levels of automatic DNSSEC key management. There are three possible settings:

auto-dnssec allow; permits keys to be updated and the zone fully re-signed whenever the user issues the command rndc sign zonename.

auto-dnssec maintain; includes the above, but also automatically adjusts the zone’s DNSSEC keys on a schedule, according to the keys’ timing metadata (see dnssec-keygen: DNSSEC key generation tool and dnssec-settime: set the key timing metadata for a DNSSEC key). The command rndc sign zonename causes named to load keys from the key repository and sign the zone with all keys that are active. rndc loadkeys zonename causes named to load keys from the key repository and schedule key maintenance events to occur in the future, but it does not sign the full zone immediately. Note: once keys have been loaded for a zone the first time, the repository is searched for changes periodically, regardless of whether rndc loadkeys is used. The recheck interval is defined by dnssec-loadkeys-interval.

auto-dnssec off; does not allow for DNSSEC key management. This is the default setting.

This option may only be activated at the zone level; if configured at the view or options level, it must be set to off.

dnssec-validation

This option enables DNSSEC validation in named.

If set to auto, DNSSEC validation is enabled and a default trust anchor for the DNS root zone is used.

If set to yes, DNSSEC validation is enabled, but a trust anchor must be manually configured using a trust-anchors statement (or the managed-keys or trusted-keys statements, both deprecated). If there is no configured trust anchor, validation does not take place.

If set to no, DNSSEC validation is disabled.

The default is auto, unless BIND is built with configure --disable-auto-validation, in which case the default is yes.

The default root trust anchor is stored in the file bind.keys. named loads that key at startup if dnssec-validation is set to auto. A copy of the file is installed along with BIND 9, and is current as of the release date. If the root key expires, a new copy of bind.keys can be downloaded from https://www.isc.org/bind-keys.

(To prevent problems if bind.keys is not found, the current trust anchor is also compiled in named. Relying on this is not recommended, however, as it requires named to be recompiled with a new key when the root key expires.)

Note

named loads only the root key from bind.keys. The file cannot be used to store keys for other zones. The root key in bind.keys is ignored if dnssec-validation auto is not in use.

Whenever the resolver sends out queries to an EDNS-compliant server, it always sets the DO bit indicating it can support DNSSEC responses, even if dnssec-validation is off.

validate-except

This specifies a list of domain names at and beneath which DNSSEC validation should not be performed, regardless of the presence of a trust anchor at or above those names. This may be used, for example, when configuring a top-level domain intended only for local use, so that the lack of a secure delegation for that domain in the root zone does not cause validation failures. (This is similar to setting a negative trust anchor except that it is a permanent configuration, whereas negative trust anchors expire and are removed after a set period of time.)

dnssec-accept-expired

This accepts expired signatures when verifying DNSSEC signatures. The default is no. Setting this option to yes leaves named vulnerable to replay attacks.

querylog

Query logging provides a complete log of all incoming queries and all query errors. This provides more insight into the server’s activity, but with a cost to performance which may be significant on heavily loaded servers.

The querylog option specifies whether query logging should be active when named first starts. If querylog is not specified, then query logging is determined by the presence of the logging category queries. Query logging can also be activated at runtime using the command rndc querylog on, or deactivated with rndc querylog off.

check-names

This option is used to restrict the character set and syntax of certain domain names in primary files and/or DNS responses received from the network. The default varies according to usage area. For primary zones the default is fail. For secondary zones the default is warn. For answers received from the network (response), the default is ignore.

The rules for legal hostnames and mail domains are derived from RFC 952 and RFC 821 as modified by RFC 1123.

check-names applies to the owner names of A, AAAA, and MX records. It also applies to the domain names in the RDATA of NS, SOA, MX, and SRV records. It further applies to the RDATA of PTR records where the owner name indicates that it is a reverse lookup of a hostname (the owner name ends in IN-ADDR.ARPA, IP6.ARPA, or IP6.INT).

check-dup-records

This checks primary zones for records that are treated as different by DNSSEC but are semantically equal in plain DNS. The default is to warn. Other possible values are fail and ignore.

check-mx

This checks whether the MX record appears to refer to an IP address. The default is to warn. Other possible values are fail and ignore.

check-wildcard

This option is used to check for non-terminal wildcards. The use of non-terminal wildcards is almost always as a result of a lack of understanding of the wildcard matching algorithm (RFC 1034). This option affects primary zones. The default (yes) is to check for non-terminal wildcards and issue a warning.

check-integrity

This performs post-load zone integrity checks on primary zones. It checks that MX and SRV records refer to address (A or AAAA) records and that glue address records exist for delegated zones. For MX and SRV records, only in-zone hostnames are checked (for out-of-zone hostnames, use named-checkzone). For NS records, only names below top-of-zone are checked (for out-of-zone names and glue consistency checks, use named-checkzone). The default is yes.

The use of the SPF record to publish Sender Policy Framework is deprecated, as the migration from using TXT records to SPF records was abandoned. Enabling this option also checks that a TXT Sender Policy Framework record exists (starts with “v=spf1”) if there is an SPF record. Warnings are emitted if the TXT record does not exist; they can be suppressed with check-spf.

check-mx-cname

If check-integrity is set, then fail, warn, or ignore MX records that refer to CNAMES. The default is to warn.

check-srv-cname

If check-integrity is set, then fail, warn, or ignore SRV records that refer to CNAMES. The default is to warn.

check-sibling

When performing integrity checks, also check that sibling glue exists. The default is yes.

check-spf

If check-integrity is set, check that there is a TXT Sender Policy Framework record present (starts with “v=spf1”) if there is an SPF record present. The default is warn.

zero-no-soa-ttl

If yes, when returning authoritative negative responses to SOA queries, set the TTL of the SOA record returned in the authority section to zero. The default is yes.

zero-no-soa-ttl-cache

If yes, when caching a negative response to an SOA query set the TTL to zero. The default is no.

update-check-ksk

When set to the default value of yes, check the KSK bit in each key to determine how the key should be used when generating RRSIGs for a secure zone.

Ordinarily, zone-signing keys (that is, keys without the KSK bit set) are used to sign the entire zone, while key-signing keys (keys with the KSK bit set) are only used to sign the DNSKEY RRset at the zone apex. However, if this option is set to no, then the KSK bit is ignored; KSKs are treated as if they were ZSKs and are used to sign the entire zone. This is similar to the dnssec-signzone -z command-line option.

When this option is set to yes, there must be at least two active keys for every algorithm represented in the DNSKEY RRset: at least one KSK and one ZSK per algorithm. If there is any algorithm for which this requirement is not met, this option is ignored for that algorithm.

dnssec-dnskey-kskonly

When this option and update-check-ksk are both set to yes, only key-signing keys (that is, keys with the KSK bit set) are used to sign the DNSKEY, CDNSKEY, and CDS RRsets at the zone apex. Zone-signing keys (keys without the KSK bit set) are used to sign the remainder of the zone, but not the DNSKEY RRset. This is similar to the dnssec-signzone -x command-line option.

The default is yes. If update-check-ksk is set to no, this option is ignored.

try-tcp-refresh

If yes, try to refresh the zone using TCP if UDP queries fail. The default is yes.

dnssec-secure-to-insecure

This allows a dynamic zone to transition from secure to insecure (i.e., signed to unsigned) by deleting all of the DNSKEY records. The default is no. If set to yes, and if the DNSKEY RRset at the zone apex is deleted, all RRSIG and NSEC records are removed from the zone as well.

If the zone uses NSEC3, it is also necessary to delete the NSEC3PARAM RRset from the zone apex; this causes the removal of all corresponding NSEC3 records. (It is expected that this requirement will be eliminated in a future release.)

Note that if a zone has been configured with auto-dnssec maintain and the private keys remain accessible in the key repository, the zone will be automatically signed again the next time named is started.

synth-from-dnssec

This option enables support for RFC 8198, Aggressive Use of DNSSEC-Validated Cache. It allows the resolver to send a smaller number of queries when resolving queries for DNSSEC-signed domains by synthesizing answers from cached NSEC and other RRsets that have been proved to be correct using DNSSEC. The default is yes.

Note

DNSSEC validation must be enabled for this option to be effective. This initial implementation only covers synthesis of answers from NSEC records; synthesis from NSEC3 is planned for the future. This will also be controlled by synth-from-dnssec.

4.2.16.2. Forwarding

The forwarding facility can be used to create a large site-wide cache on a few servers, reducing traffic over links to external name servers. It can also be used to allow queries by servers that do not have direct access to the Internet, but wish to look up exterior names anyway. Forwarding occurs only on those queries for which the server is not authoritative and does not have the answer in its cache.

forward

This option is only meaningful if the forwarders list is not empty. A value of first is the default and causes the server to query the forwarders first; if that does not answer the question, the server then looks for the answer itself. If only is specified, the server only queries the forwarders.

forwarders

This specifies a list of IP addresses to which queries are forwarded. The default is the empty list (no forwarding). Each address in the list can be associated with an optional port number and/or DSCP value, and a default port number and DSCP value can be set for the entire list.

Forwarding can also be configured on a per-domain basis, allowing for the global forwarding options to be overridden in a variety of ways. Particular domains can be set to use different forwarders, or have a different forward only/first behavior, or not forward at all; see zone Statement Grammar.

4.2.16.3. Dual-stack Servers

Dual-stack servers are used as servers of last resort, to work around problems in reachability due to the lack of support for either IPv4 or IPv6 on the host machine.

dual-stack-servers

This specifies host names or addresses of machines with access to both IPv4 and IPv6 transports. If a hostname is used, the server must be able to resolve the name using only the transport it has. If the machine is dual-stacked, the dual-stack-servers parameter has no effect unless access to a transport has been disabled on the command line (e.g., named -4).

4.2.16.4. Access Control

Access to the server can be restricted based on the IP address of the requesting system. See Address Match Lists for details on how to specify IP address lists.

allow-notify

This ACL specifies which hosts may send NOTIFY messages to inform this server of changes to zones for which it is acting as a secondary server. This is only applicable for secondary zones (i.e., type secondary or slave).

If this option is set in view or options, it is globally applied to all secondary zones. If set in the zone statement, the global value is overridden.

If not specified, the default is to process NOTIFY messages only from the configured primaries for the zone. allow-notify can be used to expand the list of permitted hosts, not to reduce it.

allow-query

This specifies which hosts are allowed to ask ordinary DNS questions. allow-query may also be specified in the zone statement, in which case it overrides the options allow-query statement. If not specified, the default is to allow queries from all hosts.

Note

allow-query-cache is used to specify access to the cache.

allow-query-on

This specifies which local addresses can accept ordinary DNS questions. This makes it possible, for instance, to allow queries on internal-facing interfaces but disallow them on external-facing ones, without necessarily knowing the internal network’s addresses.

Note that allow-query-on is only checked for queries that are permitted by allow-query. A query must be allowed by both ACLs, or it is refused.

allow-query-on may also be specified in the zone statement, in which case it overrides the options allow-query-on statement.

If not specified, the default is to allow queries on all addresses.

Note

allow-query-cache is used to specify access to the cache.

allow-query-cache

This specifies which hosts are allowed to get answers from the cache. If allow-recursion is not set, BIND checks to see if the following parameters are set, in order: allow-query-cache and allow-query (unless recursion no; is set). If neither of those parameters is set, the default (localnets; localhost;) is used.

allow-query-cache-on

This specifies which local addresses can send answers from the cache. If allow-query-cache-on is not set, then allow-recursion-on is used if set. Otherwise, the default is to allow cache responses to be sent from any address. Note: both allow-query-cache and allow-query-cache-on must be satisfied before a cache response can be sent; a client that is blocked by one cannot be allowed by the other.

allow-recursion

This specifies which hosts are allowed to make recursive queries through this server. BIND checks to see if the following parameters are set, in order: allow-query-cache and allow-query. If neither of those parameters is set, the default (localnets; localhost;) is used.

allow-recursion-on

This specifies which local addresses can accept recursive queries. If allow-recursion-on is not set, then allow-query-cache-on is used if set; otherwise, the default is to allow recursive queries on all addresses. Any client permitted to send recursive queries can send them to any address on which named is listening. Note: both allow-recursion and allow-recursion-on must be satisfied before recursion is allowed; a client that is blocked by one cannot be allowed by the other.

allow-update

When set in the zone statement for a primary zone, this specifies which hosts are allowed to submit Dynamic DNS updates to that zone. The default is to deny updates from all hosts.

Note that allowing updates based on the requestor’s IP address is insecure; see Dynamic Update Security for details.

In general, this option should only be set at the zone level. While a default value can be set at the options or view level and inherited by zones, this could lead to some zones unintentionally allowing updates.

allow-update-forwarding

When set in the zone statement for a secondary zone, this specifies which hosts are allowed to submit Dynamic DNS updates and have them be forwarded to the primary. The default is { none; }, which means that no update forwarding is performed.

To enable update forwarding, specify allow-update-forwarding { any; }; in the zone statement. Specifying values other than { none; } or { any; } is usually counterproductive; the responsibility for update access control should rest with the primary server, not the secondary.

Note that enabling the update forwarding feature on a secondary server may expose primary servers to attacks if they rely on insecure IP-address-based access control; see Dynamic Update Security for more details.

In general this option should only be set at the zone level. While a default value can be set at the options or view level and inherited by zones, this can lead to some zones unintentionally forwarding updates.

allow-transfer

This specifies which hosts are allowed to receive zone transfers from the server. allow-transfer may also be specified in the zone statement, in which case it overrides the allow-transfer statement set in options or view. If not specified, the default is to allow transfers to all hosts.

The transport level limitations can also be specified. In particular, zone transfers can be restricted to a specific port and/or DNS transport protocol by using the options port and transport. Either option can be specified; if both are used, both constraints must be satisfied in order for the transfer to be allowed. Zone transfers are currently only possible via the TCP and TLS transports.

For example: allow-transfer port 853 transport tls { any; }; allows outgoing zone transfers to any host using the TLS transport over port 853.

Warning

Please note that incoming TLS connections are currently not authenticated at the TLS level. Please use TSIG to authenticate requestors.

blackhole

This specifies a list of addresses which the server does not accept queries from or use to resolve a query. Queries from these addresses are not responded to. The default is none.

keep-response-order

This specifies a list of addresses to which the server sends responses to TCP queries, in the same order in which they were received. This disables the processing of TCP queries in parallel. The default is none.

no-case-compress

This specifies a list of addresses which require responses to use case-insensitive compression. This ACL can be used when named needs to work with clients that do not comply with the requirement in RFC 1034 to use case-insensitive name comparisons when checking for matching domain names.

If left undefined, the ACL defaults to none: case-insensitive compression is used for all clients. If the ACL is defined and matches a client, case is ignored when compressing domain names in DNS responses sent to that client.

This can result in slightly smaller responses; if a response contains the names “example.com” and “example.COM”, case-insensitive compression treats the second one as a duplicate. It also ensures that the case of the query name exactly matches the case of the owner names of returned records, rather than matches the case of the records entered in the zone file. This allows responses to exactly match the query, which is required by some clients due to incorrect use of case-sensitive comparisons.

Case-insensitive compression is always used in AXFR and IXFR responses, regardless of whether the client matches this ACL.

There are circumstances in which named does not preserve the case of owner names of records: if a zone file defines records of different types with the same name, but the capitalization of the name is different (e.g., “www.example.com/A” and “WWW.EXAMPLE.COM/AAAA”), then all responses for that name use the first version of the name that was used in the zone file. This limitation may be addressed in a future release. However, domain names specified in the rdata of resource records (i.e., records of type NS, MX, CNAME, etc.) always have their case preserved unless the client matches this ACL.

resolver-query-timeout

This is the amount of time in milliseconds that the resolver spends attempting to resolve a recursive query before failing. The default and minimum is 10000 and the maximum is 30000. Setting it to 0 results in the default being used.

This value was originally specified in seconds. Values less than or equal to 300 are treated as seconds and converted to milliseconds before applying the above limits.

4.2.16.5. Interfaces

The interfaces, ports, and protocols that the server can use to answer queries may be specified using the listen-on and listen-on-v6 options.

listen-on and listen-on-v6 statements can each take an optional port, TLS configuration identifier, and/or HTTP configuration identifier, in addition to an address_match_list.

The address_match_list in listen-on specifies the IPv4 addresses on which the server will listen. (IPv6 addresses are ignored, with a logged warning.) The server listens on all interfaces allowed by the address match list. If no listen-on is specified, the default is to listen for standard DNS queries on port 53 of all IPv4 interfaces.

listen-on-v6 takes an address_match_list of IPv6 addresses. The server listens on all interfaces allowed by the address match list. If no listen-on-v6 is specified, the default is to listen for standard DNS queries on port 53 of all IPv6 interfaces.

If a TLS configuration is specified, named will listen for DNS-over-TLS (DoT) connections, using the key and certificate specified in the referenced tls statement. If the name ephemeral is used, an ephemeral key and certificate created for the currently running named process will be used.

If an HTTP configuration is specified, named will listen for DNS-over-HTTPS (DoH) connections using the HTTP endpoint specified in the referenced http statement. If the name default is used, then named will listen for connections at the default endpoint, /dns-query.

Use of an http specification requires tls to be specified as well. If an unencrypted connection is desired (for example, on load-sharing servers behind a reverse proxy), tls none may be used.

If a port number is not specified, the default is 53 for standard DNS, 853 for DNS over TLS, 443 for DNS over HTTPS, and 80 for DNS over HTTP (unencrypted). These defaults may be overridden using the port, tls-port, https-port and http-port options.

Multiple listen-on statements are allowed. For example:

listen-on { 5.6.7.8; };
listen-on port 1234 { !1.2.3.4; 1.2/16; };
listen-on port 8853 tls ephemeral { 4.3.2.1; };
listen-on port 8453 tls ephemeral http myserver { 8.7.6.5; };

The first two lines instruct the name server to listen for standard DNS queries on port 53 of the IP address 5.6.7.8 and on port 1234 of an address on the machine in net 1.2 that is not 1.2.3.4. The third line instructs the server to listen for DNS-over-TLS connections on port 8853 of the IP address 4.3.2.1 using the ephemeral key and certifcate. The fourth line enables DNS-over-HTTPS connections on port 8453 of address 8.7.6.5, using the ephemeral key and certificate, and the HTTP endpoint or endpoints configured in an http statement with the name myserver.

Multiple listen-on-v6 options can be used. For example:

listen-on-v6 { any; };
listen-on-v6 port 1234 { !2001:db8::/32; any; };
listen-on port 8853 tls example-tls { 2001:db8::100; };
listen-on port 8453 tls example-tls http default { 2001:db8::100; };
listen-on port 8000 tls none http myserver { 2001:db8::100; };

The first two lines instruct the name server to listen for standard DNS queries on port 53 of any IPv6 addresses, and on port 1234 of IPv6 addresses that are not in the prefix 2001:db8::/32. The third line instructs the server to listen for for DNS-over-TLS connections on port 8853 of the address 2001:db8::100, using a TLS key and certificate specified in the a tls statement with the name example-tls. The fourth instructs the server to listen for DNS-over-HTTPS connections, again using example-tls, on the default HTTP endpoint. The fifth line, in which the tls parameter is set to none, instructs the server to listen for unencrypted DNS queries over HTTP at the endpoint specified in myserver..

To instruct the server not to listen on any IPv6 addresses, use:

listen-on-v6 { none; };

4.2.16.6. Query Address

If the server does not know the answer to a question, it queries other name servers. query-source specifies the address and port used for such queries. For queries sent over IPv6, there is a separate query-source-v6 option. If address is * (asterisk) or is omitted, a wildcard IP address (INADDR_ANY) is used.

If port is * or is omitted, a random port number from a pre-configured range is picked up and used for each query. The port range(s) is specified in the use-v4-udp-ports (for IPv4) and use-v6-udp-ports (for IPv6) options, excluding the ranges specified in the avoid-v4-udp-ports and avoid-v6-udp-ports options, respectively.

The defaults of the query-source and query-source-v6 options are:

query-source address * port *;
query-source-v6 address * port *;

If use-v4-udp-ports or use-v6-udp-ports is unspecified, named checks whether the operating system provides a programming interface to retrieve the system’s default range for ephemeral ports. If such an interface is available, named uses the corresponding system default range; otherwise, it uses its own defaults:

use-v4-udp-ports { range 1024 65535; };
use-v6-udp-ports { range 1024 65535; };

The defaults of the avoid-v4-udp-ports and avoid-v6-udp-ports options are:

avoid-v4-udp-ports {};
avoid-v6-udp-ports {};

Note

Make sure the ranges are sufficiently large for security. A desirable size depends on several parameters, but we generally recommend it contain at least 16384 ports (14 bits of entropy). Note also that the system’s default range when used may be too small for this purpose, and that the range may even be changed while named is running; the new range is automatically applied when named is reloaded. Explicit configuration of use-v4-udp-ports and use-v6-udp-ports is encouraged, so that the ranges are sufficiently large and are reasonably independent from the ranges used by other applications.

Note

The operational configuration where named runs may prohibit the use of some ports. For example, Unix systems do not allow named, if run without root privilege, to use ports less than 1024. If such ports are included in the specified (or detected) set of query ports, the corresponding query attempts will fail, resulting in resolution failures or delay. It is therefore important to configure the set of ports that can be safely used in the expected operational environment.

Note

The address specified in the query-source option is used for both UDP and TCP queries, but the port applies only to UDP queries. TCP queries always use a random unprivileged port.

Note

Solaris 2.5.1 and earlier does not support setting the source address for TCP sockets.

Warning

Specifying a single port is discouraged, as it removes a layer of protection against spoofing errors.

Warning

The configured port must not be same as the listening port.

Note

See also transfer-source, notify-source and parental-source.

4.2.16.7. Zone Transfers

BIND has mechanisms in place to facilitate zone transfers and set limits on the amount of load that transfers place on the system. The following options apply to zone transfers.

also-notify

This option defines a global list of IP addresses of name servers that are also sent NOTIFY messages whenever a fresh copy of the zone is loaded, in addition to the servers listed in the zone’s NS records. This helps to ensure that copies of the zones quickly converge on stealth servers. Optionally, a port may be specified with each also-notify address to send the notify messages to a port other than the default of 53. An optional TSIG key can also be specified with each address to cause the notify messages to be signed; this can be useful when sending notifies to multiple views. In place of explicit addresses, one or more named primaries lists can be used.

If an also-notify list is given in a zone statement, it overrides the options also-notify statement. When a zone notify statement is set to no, the IP addresses in the global also-notify list are not sent NOTIFY messages for that zone. The default is the empty list (no global notification list).

max-transfer-time-in

Inbound zone transfers running longer than this many minutes are terminated. The default is 120 minutes (2 hours). The maximum value is 28 days (40320 minutes).

max-transfer-idle-in

Inbound zone transfers making no progress in this many minutes are terminated. The default is 60 minutes (1 hour). The maximum value is 28 days (40320 minutes).

max-transfer-time-out

Outbound zone transfers running longer than this many minutes are terminated. The default is 120 minutes (2 hours). The maximum value is 28 days (40320 minutes).

max-transfer-idle-out

Outbound zone transfers making no progress in this many minutes are terminated. The default is 60 minutes (1 hour). The maximum value is 28 days (40320 minutes).

notify-rate

This specifies the rate at which NOTIFY requests are sent during normal zone maintenance operations. (NOTIFY requests due to initial zone loading are subject to a separate rate limit; see below.) The default is 20 per second. The lowest possible rate is one per second; when set to zero, it is silently raised to one.

startup-notify-rate

This is the rate at which NOTIFY requests are sent when the name server is first starting up, or when zones have been newly added to the name server. The default is 20 per second. The lowest possible rate is one per second; when set to zero, it is silently raised to one.

serial-query-rate

Secondary servers periodically query primary servers to find out if zone serial numbers have changed. Each such query uses a minute amount of the secondary server’s network bandwidth. To limit the amount of bandwidth used, BIND 9 limits the rate at which queries are sent. The value of the serial-query-rate option, an integer, is the maximum number of queries sent per second. The default is 20 per second. The lowest possible rate is one per second; when set to zero, it is silently raised to one.

transfer-format

Zone transfers can be sent using two different formats, one-answer and many-answers. The transfer-format option is used on the primary server to determine which format it sends. one-answer uses one DNS message per resource record transferred. many-answers packs as many resource records as possible into one message. many-answers is more efficient; the default is many-answers. transfer-format may be overridden on a per-server basis by using the server statement.

transfer-message-size

This is an upper bound on the uncompressed size of DNS messages used in zone transfers over TCP. If a message grows larger than this size, additional messages are used to complete the zone transfer. (Note, however, that this is a hint, not a hard limit; if a message contains a single resource record whose RDATA does not fit within the size limit, a larger message will be permitted so the record can be transferred.)

Valid values are between 512 and 65535 octets; any values outside that range are adjusted to the nearest value within it. The default is 20480, which was selected to improve message compression; most DNS messages of this size will compress to less than 16536 bytes. Larger messages cannot be compressed as effectively, because 16536 is the largest permissible compression offset pointer in a DNS message.

This option is mainly intended for server testing; there is rarely any benefit in setting a value other than the default.

transfers-in

This is the maximum number of inbound zone transfers that can run concurrently. The default value is 10. Increasing transfers-in may speed up the convergence of secondary zones, but it also may increase the load on the local system.

transfers-out

This is the maximum number of outbound zone transfers that can run concurrently. Zone transfer requests in excess of the limit are refused. The default value is 10.

transfers-per-ns

This is the maximum number of inbound zone transfers that can concurrently transfer from a given remote name server. The default value is 2. Increasing transfers-per-ns may speed up the convergence of secondary zones, but it also may increase the load on the remote name server. transfers-per-ns may be overridden on a per-server basis by using the transfers phrase of the server statement.

transfer-source

transfer-source determines which local address is bound to IPv4 TCP connections used to fetch zones transferred inbound by the server. It also determines the source IPv4 address, and optionally the UDP port, used for the refresh queries and forwarded dynamic updates. If not set, it defaults to a system-controlled value which is usually the address of the interface “closest to” the remote end. This address must appear in the remote end’s allow-transfer option for the zone being transferred, if one is specified. This statement sets the transfer-source for all zones, but can be overridden on a per-view or per-zone basis by including a transfer-source statement within the view or zone block in the configuration file.

Note

Solaris 2.5.1 and earlier does not support setting the source address for TCP sockets.

Warning

Specifying a single port is discouraged, as it removes a layer of protection against spoofing errors.

Warning

The configured port must not be same as the listening port.

transfer-source-v6

This option is the same as transfer-source, except zone transfers are performed using IPv6.

alt-transfer-source

This indicates an alternate transfer source if the one listed in transfer-source fails and use-alt-transfer-source is set.

Note

To avoid using the alternate transfer source, set use-alt-transfer-source appropriately and do not depend upon getting an answer back to the first refresh query.

alt-transfer-source-v6

This indicates an alternate transfer source if the one listed in transfer-source-v6 fails and use-alt-transfer-source is set.

use-alt-transfer-source

This indicates whether the alternate transfer sources should be used. If views are specified, this defaults to no; otherwise, it defaults to yes.

notify-source

notify-source determines which local source address, and optionally UDP port, is used to send NOTIFY messages. This address must appear in the secondary server’s primaries zone clause or in an allow-notify clause. This statement sets the notify-source for all zones, but can be overridden on a per-zone or per-view basis by including a notify-source statement within the zone or view block in the configuration file.

Note

Solaris 2.5.1 and earlier does not support setting the source address for TCP sockets.

Warning

Specifying a single port is discouraged, as it removes a layer of protection against spoofing errors.

Warning

The configured port must not be same as the listening port.

notify-source-v6

This option acts like notify-source, but applies to notify messages sent to IPv6 addresses.

4.2.16.8. UDP Port Lists

use-v4-udp-ports, avoid-v4-udp-ports, use-v6-udp-ports, and avoid-v6-udp-ports specify a list of IPv4 and IPv6 UDP ports that are or are not used as source ports for UDP messages. See Query Address about how the available ports are determined. For example, with the following configuration:

use-v6-udp-ports { range 32768 65535; };
avoid-v6-udp-ports { 40000; range 50000 60000; };

UDP ports of IPv6 messages sent from named are in one of the following ranges: 32768 to 39999, 40001 to 49999, and 60001 to 65535.

avoid-v4-udp-ports and avoid-v6-udp-ports can be used to prevent named from choosing as its random source port a port that is blocked by a firewall or a port that is used by other applications; if a query went out with a source port blocked by a firewall, the answer would not pass through the firewall and the name server would have to query again. Note: the desired range can also be represented only with use-v4-udp-ports and use-v6-udp-ports, and the avoid- options are redundant in that sense; they are provided for backward compatibility and to possibly simplify the port specification.

4.2.16.9. Operating System Resource Limits

The server’s usage of many system resources can be limited. Scaled values are allowed when specifying resource limits. For example, 1G can be used instead of 1073741824 to specify a limit of one gigabyte. unlimited requests unlimited use, or the maximum available amount. default uses the limit that was in force when the server was started. See the description of size_spec in Configuration File Elements.

The following options set operating system resource limits for the name server process. Some operating systems do not support some or any of the limits; on such systems, a warning is issued if an unsupported limit is used.

coresize

This sets the maximum size of a core dump. The default is default.

datasize

This sets the maximum amount of data memory the server may use. The default is default. This is a hard limit on server memory usage; if the server attempts to allocate memory in excess of this limit, the allocation will fail, which may in turn leave the server unable to perform DNS service. Therefore, this option is rarely useful as a way to limit the amount of memory used by the server, but it can be used to raise an operating system data size limit that is too small by default. To limit the amount of memory used by the server, use the max-cache-size and recursive-clients options instead.

files

This sets the maximum number of files the server may have open concurrently. The default is unlimited.

stacksize

This sets the maximum amount of stack memory the server may use. The default is default.

4.2.16.10. Server Resource Limits

The following options set limits on the server’s resource consumption that are enforced internally by the server rather than by the operating system.

max-journal-size

This sets a maximum size for each journal file (see The Journal File), expressed in bytes or, if followed by an optional unit suffix (‘k’, ‘m’, or ‘g’), in kilobytes, megabytes, or gigabytes. When the journal file approaches the specified size, some of the oldest transactions in the journal are automatically removed. The largest permitted value is 2 gigabytes. Very small values are rounded up to 4096 bytes. It is possible to specify unlimited, which also means 2 gigabytes. If the limit is set to default or left unset, the journal is allowed to grow up to twice as large as the zone. (There is little benefit in storing larger journals.)

This option may also be set on a per-zone basis.

max-records

This sets the maximum number of records permitted in a zone. The default is zero, which means the maximum is unlimited.

recursive-clients

This sets the maximum number (a “hard quota”) of simultaneous recursive lookups the server performs on behalf of clients. The default is 1000. Because each recursing client uses a fair bit of memory (on the order of 20 kilobytes), the value of the recursive-clients option may have to be decreased on hosts with limited memory.

recursive-clients defines a “hard quota” limit for pending recursive clients; when more clients than this are pending, new incoming requests are not accepted, and for each incoming request a previous pending request is dropped.

A “soft quota” is also set. When this lower quota is exceeded, incoming requests are accepted, but for each one, a pending request is dropped. If recursive-clients is greater than 1000, the soft quota is set to recursive-clients minus 100; otherwise it is set to 90% of recursive-clients.

tcp-clients

This is the maximum number of simultaneous client TCP connections that the server accepts. The default is 150.

clients-per-query; max-clients-per-query

These set the initial value (minimum) and maximum number of recursive simultaneous clients for any given query (<qname,qtype,qclass>) that the server accepts before dropping additional clients. named attempts to self-tune this value and changes are logged. The default values are 10 and 100.

This value should reflect how many queries come in for a given name in the time it takes to resolve that name. If the number of queries exceeds this value, named assumes that it is dealing with a non-responsive zone and drops additional queries. If it gets a response after dropping queries, it raises the estimate. The estimate is then lowered in 20 minutes if it has remained unchanged.

If clients-per-query is set to zero, there is no limit on the number of clients per query and no queries are dropped.

If max-clients-per-query is set to zero, there is no upper bound other than that imposed by recursive-clients.

fetches-per-zone

This sets the maximum number of simultaneous iterative queries to any one domain that the server permits before blocking new queries for data in or beneath that zone. This value should reflect how many fetches would normally be sent to any one zone in the time it would take to resolve them. It should be smaller than recursive-clients.

When many clients simultaneously query for the same name and type, the clients are all attached to the same fetch, up to the max-clients-per-query limit, and only one iterative query is sent. However, when clients are simultaneously querying for different names or types, multiple queries are sent and max-clients-per-query is not effective as a limit.

Optionally, this value may be followed by the keyword drop or fail, indicating whether queries which exceed the fetch quota for a zone are dropped with no response, or answered with SERVFAIL. The default is drop.

If fetches-per-zone is set to zero, there is no limit on the number of fetches per query and no queries are dropped. The default is zero.

The current list of active fetches can be dumped by running rndc recursing. The list includes the number of active fetches for each domain and the number of queries that have been passed (allowed) or dropped (spilled) as a result of the fetches-per-zone limit. (Note: these counters are not cumulative over time; whenever the number of active fetches for a domain drops to zero, the counter for that domain is deleted, and the next time a fetch is sent to that domain, it is recreated with the counters set to zero.)

fetches-per-server

This sets the maximum number of simultaneous iterative queries that the server allows to be sent to a single upstream name server before blocking additional queries. This value should reflect how many fetches would normally be sent to any one server in the time it would take to resolve them. It should be smaller than recursive-clients.

Optionally, this value may be followed by the keyword drop or fail, indicating whether queries are dropped with no response or answered with SERVFAIL, when all of the servers authoritative for a zone are found to have exceeded the per-server quota. The default is fail.

If fetches-per-server is set to zero, there is no limit on the number of fetches per query and no queries are dropped. The default is zero.

The fetches-per-server quota is dynamically adjusted in response to detected congestion. As queries are sent to a server and either are answered or time out, an exponentially weighted moving average is calculated of the ratio of timeouts to responses. If the current average timeout ratio rises above a “high” threshold, then fetches-per-server is reduced for that server. If the timeout ratio drops below a “low” threshold, then fetches-per-server is increased. The fetch-quota-params options can be used to adjust the parameters for this calculation.

fetch-quota-params

This sets the parameters to use for dynamic resizing of the fetches-per-server quota in response to detected congestion.

The first argument is an integer value indicating how frequently to recalculate the moving average of the ratio of timeouts to responses for each server. The default is 100, meaning that BIND recalculates the average ratio after every 100 queries have either been answered or timed out.

The remaining three arguments represent the “low” threshold (defaulting to a timeout ratio of 0.1), the “high” threshold (defaulting to a timeout ratio of 0.3), and the discount rate for the moving average (defaulting to 0.7). A higher discount rate causes recent events to weigh more heavily when calculating the moving average; a lower discount rate causes past events to weigh more heavily, smoothing out short-term blips in the timeout ratio. These arguments are all fixed-point numbers with precision of 1/100; at most two places after the decimal point are significant.

reserved-sockets

This option is deprecated and no longer has any effect.

max-cache-size

This sets the maximum amount of memory to use for an individual cache database and its associated metadata, in bytes or percentage of total physical memory. By default, each view has its own separate cache, which means the total amount of memory required for cache data is the sum of the cache database sizes for all views (unless the attach-cache option is used).

When the amount of data in a cache database reaches the configured limit, named starts purging non-expired records (following an LRU-based strategy).

The default size limit for each individual cache is:

• 90% of physical memory for views with recursion set to yes (the default), or

• 2 MB for views with recursion set to no.

Any positive value smaller than 2 MB is ignored and reset to 2 MB. The keyword unlimited, or the value 0, places no limit on the cache size; records are then purged from the cache only when they expire (according to their TTLs).

Note

For configurations which define multiple views with separate caches and recursion enabled, it is recommended to set max-cache-size appropriately for each view, as using the default value of that option (90% of physical memory for each individual cache) may lead to memory exhaustion over time.

Upon startup and reconfiguration, caches with a limited size preallocate a small amount of memory (less than 1% of max-cache-size for a given view). This preallocation serves as an optimization to eliminate extra latency introduced by resizing internal cache structures.

On systems where detection of the amount of physical memory is not supported, percentage-based values fall back to unlimited. Note that the amount of physical memory available is only detected on startup, so named does not adjust the cache size limits if the amount of physical memory is changed at runtime.

tcp-listen-queue

This sets the listen-queue depth. The default and minimum is 10. If the kernel supports the accept filter “dataready”, this also controls how many TCP connections are queued in kernel space waiting for some data before being passed to accept. Non-zero values less than 10 are silently raised. A value of 0 may also be used; on most platforms this sets the listen-queue length to a system-defined default value.

tcp-initial-timeout

This sets the amount of time (in units of 100 milliseconds) that the server waits on a new TCP connection for the first message from the client. The default is 300 (30 seconds), the minimum is 25 (2.5 seconds), and the maximum is 1200 (two minutes). Values above the maximum or below the minimum are adjusted with a logged warning. (Note: this value must be greater than the expected round-trip delay time; otherwise, no client will ever have enough time to submit a message.) This value can be updated at runtime by using rndc tcp-timeouts.

tcp-idle-timeout

This sets the amount of time (in units of 100 milliseconds) that the server waits on an idle TCP connection before closing it, when the client is not using the EDNS TCP keepalive option. The default is 300 (30 seconds), the maximum is 1200 (two minutes), and the minimum is 1 (one-tenth of a second). Values above the maximum or below the minimum are adjusted with a logged warning. See tcp-keepalive-timeout for clients using the EDNS TCP keepalive option. This value can be updated at runtime by using rndc tcp-timeouts.

tcp-keepalive-timeout

This sets the amount of time (in units of 100 milliseconds) that the server waits on an idle TCP connection before closing it, when the client is using the EDNS TCP keepalive option. The default is 300 (30 seconds), the maximum is 65535 (about 1.8 hours), and the minimum is 1 (one-tenth of a second). Values above the maximum or below the minimum are adjusted with a logged warning. This value may be greater than tcp-idle-timeout because clients using the EDNS TCP keepalive option are expected to use TCP connections for more than one message. This value can be updated at runtime by using rndc tcp-timeouts.

tcp-advertised-timeout

This sets the timeout value (in units of 100 milliseconds) that the server sends in responses containing the EDNS TCP keepalive option, which informs a client of the amount of time it may keep the session open. The default is 300 (30 seconds), the maximum is 65535 (about 1.8 hours), and the minimum is 0, which signals that the clients must close TCP connections immediately. Ordinarily this should be set to the same value as tcp-keepalive-timeout. This value can be updated at runtime by using rndc tcp-timeouts.

4.2.16.11. Periodic Task Intervals

heartbeat-interval

The server performs zone maintenance tasks for all zones marked as dialup whenever this interval expires. The default is 60 minutes. Reasonable values are up to 1 day (1440 minutes). The maximum value is 28 days (40320 minutes). If set to 0, no zone maintenance for these zones occurs.

interface-interval

The server scans the network interface list every interface-interval minutes. The default is 60 minutes; the maximum value is 28 days (40320 minutes). If set to 0, interface scanning only occurs when the configuration file is loaded, or when automatic-interface-scan is enabled and supported by the operating system. After the scan, the server begins listening for queries on any newly discovered interfaces (provided they are allowed by the listen-on configuration), and stops listening on interfaces that have gone away. For convenience, TTL-style time-unit suffixes may be used to specify the value. It also accepts ISO 8601 duration formats.

4.2.16.12. The sortlist Statement

The response to a DNS query may consist of multiple resource records (RRs) forming a resource record set (RRset). The name server normally returns the RRs within the RRset in an indeterminate order (but see the rrset-order statement in RRset Ordering). The client resolver code should rearrange the RRs as appropriate: that is, using any addresses on the local net in preference to other addresses. However, not all resolvers can do this or are correctly configured. When a client is using a local server, the sorting can be performed in the server, based on the client’s address. This only requires configuring the name servers, not all the clients.

The sortlist statement (see below) takes an address_match_list and interprets it in a special way. Each top-level statement in the sortlist must itself be an explicit address_match_list with one or two elements. The first element (which may be an IP address, an IP prefix, an ACL name, or a nested address_match_list) of each top-level list is checked against the source address of the query until a match is found. When the addresses in the first element overlap, the first rule to match is selected.

Once the source address of the query has been matched, if the top-level statement contains only one element, the actual primitive element that matched the source address is used to select the address in the response to move to the beginning of the response. If the statement is a list of two elements, then the second element is interpreted as a topology preference list. Each top-level element is assigned a distance, and the address in the response with the minimum distance is moved to the beginning of the response.

In the following example, any queries received from any of the addresses of the host itself get responses preferring addresses on any of the locally connected networks. Next most preferred are addresses on the 192.168.1/24 network, and after that either the 192.168.2/24 or 192.168.3/24 network, with no preference shown between these two networks. Queries received from a host on the 192.168.1/24 network prefer other addresses on that network to the 192.168.2/24 and 192.168.3/24 networks. Queries received from a host on the 192.168.4/24 or the 192.168.5/24 network only prefer other addresses on their directly connected networks.

sortlist {
    // IF the local host
    // THEN first fit on the following nets
    { localhost;
    { localnets;
        192.168.1/24;
        { 192.168.2/24; 192.168.3/24; }; }; };
    // IF on class C 192.168.1 THEN use .1, or .2 or .3
    { 192.168.1/24;
    { 192.168.1/24;
        { 192.168.2/24; 192.168.3/24; }; }; };
    // IF on class C 192.168.2 THEN use .2, or .1 or .3
    { 192.168.2/24;
    { 192.168.2/24;
        { 192.168.1/24; 192.168.3/24; }; }; };
    // IF on class C 192.168.3 THEN use .3, or .1 or .2
    { 192.168.3/24;
    { 192.168.3/24;
        { 192.168.1/24; 192.168.2/24; }; }; };
    // IF .4 or .5 THEN prefer that net
    { { 192.168.4/24; 192.168.5/24; };
    };
};

The following example illlustrates reasonable behavior for the local host and hosts on directly connected networks. Responses sent to queries from the local host favor any of the directly connected networks. Responses sent to queries from any other hosts on a directly connected network prefer addresses on that same network. Responses to other queries are not sorted.

sortlist {
       { localhost; localnets; };
       { localnets; };
};

4.2.16.13. RRset Ordering

Note

While alternating the order of records in a DNS response between subsequent queries is a known load distribution technique, certain caveats apply (mostly stemming from caching) which usually make it a suboptimal choice for load balancing purposes when used on its own.

The rrset-order statement permits configuration of the ordering of the records in a multiple-record response. See also: The sortlist Statement.

Each rule in an rrset-order statement is defined as follows:

[class <class_name>] [type <type_name>] [name "<domain_name>"] order <ordering>

The default qualifiers for each rule are:

• If no class is specified, the default is ANY.

• If no type is specified, the default is ANY.

• If no name is specified, the default is * (asterisk).

<domain_name> only matches the name itself, not any of its subdomains. To make a rule match all subdomains of a given name, a wildcard name (*.<domain_name>) must be used. Note that *.<domain_name> does not match <domain_name> itself; to specify RRset ordering for a name and all of its subdomains, two separate rules must be defined: one for <domain_name> and one for *.<domain_name>.

The legal values for <ordering> are:

fixed

Records are returned in the order they are defined in the zone file.

Note

The fixed option is only available if BIND is configured with --enable-fixed-rrset at compile time.

random

Records are returned in a random order.

cyclic

Records are returned in a cyclic round-robin order, rotating by one record per query.

none

Records are returned in the order they were retrieved from the database. This order is indeterminate, but remains consistent as long as the database is not modified.

The default RRset order used depends on whether any rrset-order statements are present in the configuration file used by named:

• If no rrset-order statement is present in the configuration file, the implicit default is to return all records in random order.

• If any rrset-order statements are present in the configuration file, but no ordering rule specified in these statements matches a given RRset, the default order for that RRset is none.

Note that if multiple rrset-order statements are present in the configuration file (at both the options and view levels), they are not combined; instead, the more-specific one (view) replaces the less-specific one (options).

If multiple rules within a single rrset-order statement match a given RRset, the first matching rule is applied.

Example:

rrset-order {
    type A name "foo.isc.org" order random;
    type AAAA name "foo.isc.org" order cyclic;
    name "bar.isc.org" order fixed;
    name "*.bar.isc.org" order random;
    name "*.baz.isc.org" order cyclic;
};

With the above configuration, the following RRset ordering is used:

QNAME	QTYPE	RRset Order
foo.isc.org	A	random
foo.isc.org	AAAA	cyclic
foo.isc.org	TXT	none
sub.foo.isc.org	all	none
bar.isc.org	all	fixed
sub.bar.isc.org	all	random
baz.isc.org	all	none
sub.baz.isc.org	all	cyclic

4.2.16.14. Tuning

lame-ttl

This is always set to 0. More information is available in the security advisory for CVE-2021-25219.

servfail-ttl

This sets the number of seconds to cache a SERVFAIL response due to DNSSEC validation failure or other general server failure. If set to 0, SERVFAIL caching is disabled. The SERVFAIL cache is not consulted if a query has the CD (Checking Disabled) bit set; this allows a query that failed due to DNSSEC validation to be retried without waiting for the SERVFAIL TTL to expire.

The maximum value is 30 seconds; any higher value is silently reduced. The default is 1 second.

min-ncache-ttl

To reduce network traffic and increase performance, the server stores negative answers. min-ncache-ttl is used to set a minimum retention time for these answers in the server, in seconds. For convenience, TTL-style time-unit suffixes may be used to specify the value. It also accepts ISO 8601 duration formats.

The default min-ncache-ttl is 0 seconds. min-ncache-ttl cannot exceed 90 seconds and is truncated to 90 seconds if set to a greater value.

min-cache-ttl

This sets the minimum time for which the server caches ordinary (positive) answers, in seconds. For convenience, TTL-style time-unit suffixes may be used to specify the value. It also accepts ISO 8601 duration formats.

The default min-cache-ttl is 0 seconds. min-cache-ttl cannot exceed 90 seconds and is truncated to 90 seconds if set to a greater value.

max-ncache-ttl

To reduce network traffic and increase performance, the server stores negative answers. max-ncache-ttl is used to set a maximum retention time for these answers in the server, in seconds. For convenience, TTL-style time-unit suffixes may be used to specify the value. It also accepts ISO 8601 duration formats.

The default max-ncache-ttl is 10800 seconds (3 hours). max-ncache-ttl cannot exceed 7 days and is silently truncated to 7 days if set to a greater value.

max-cache-ttl

This sets the maximum time for which the server caches ordinary (positive) answers, in seconds. For convenience, TTL-style time-unit suffixes may be used to specify the value. It also accepts ISO 8601 duration formats.

The default max-cache-ttl is 604800 (one week). A value of zero may cause all queries to return SERVFAIL, because of lost caches of intermediate RRsets (such as NS and glue AAAA/A records) in the resolution process.

max-stale-ttl

If retaining stale RRsets in cache is enabled, and returning of stale cached answers is also enabled, max-stale-ttl sets the maximum time for which the server retains records past their normal expiry to return them as stale records, when the servers for those records are not reachable. The default is 1 day. The minimum allowed is 1 second; a value of 0 is updated silently to 1 second.

For stale answers to be returned, the retaining of them in cache must be enabled via the configuration option stale-cache-enable, and returning cached answers must be enabled, either in the configuration file using the stale-answer-enable option or by calling rndc serve-stale on.

When stale-cache-enable is set to no, setting the max-stale-ttl has no effect, the value of max-cache-ttl will be 0 in such case.

resolver-nonbackoff-tries

This specifies how many retries occur before exponential backoff kicks in. The default is 3.

resolver-retry-interval

This sets the base retry interval in milliseconds. The default is 800.

sig-validity-interval

this specifies the upper bound of the number of days that RRSIGs generated by named are valid; the default is 30 days, with a maximum of 3660 days (10 years). The optional second value specifies the minimum bound on those RRSIGs and also determines how long before expiry named starts regenerating those RRSIGs. The default value for the lower bound is 1/4 of the upper bound; it is expressed in days if the upper bound is greater than 7, and hours if it is less than or equal to 7 days.

When new RRSIGs are generated, the length of time is randomly chosen between these two limits, to spread out the re-signing load. When RRSIGs are re-generated, the upper bound is used, with a small amount of jitter added. New RRSIGs are generated by a number of processes, including the processing of UPDATE requests (ref:dynamic_update), the addition and removal of records via in-line signing, and the initial signing of a zone.

The signature inception time is unconditionally set to one hour before the current time, to allow for a limited amount of clock skew.

The sig-validity-interval can be overridden for DNSKEY records by setting dnskey-sig-validity.

The sig-validity-interval should be at least several multiples of the SOA expire interval, to allow for reasonable interaction between the various timer and expiry dates.

dnskey-sig-validity

This specifies the number of days into the future when DNSSEC signatures that are automatically generated for DNSKEY RRsets as a result of dynamic updates (Dynamic Update) will expire. If set to a non-zero value, this overrides the value set by sig-validity-interval. The default is zero, meaning sig-validity-interval is used. The maximum value is 3660 days (10 years), and higher values are rejected.

sig-signing-nodes

This specifies the maximum number of nodes to be examined in each quantum, when signing a zone with a new DNSKEY. The default is 100.

sig-signing-signatures

This specifies a threshold number of signatures that terminates processing a quantum, when signing a zone with a new DNSKEY. The default is 10.

sig-signing-type

This specifies a private RDATA type to be used when generating signing-state records. The default is 65534.

This parameter may be removed in a future version, once there is a standard type.

Signing-state records are used internally by named to track the current state of a zone-signing process, i.e., whether it is still active or has been completed. The records can be inspected using the command rndc signing -list zone. Once named has finished signing a zone with a particular key, the signing-state record associated with that key can be removed from the zone by running rndc signing -clear keyid/algorithm zone. To clear all of the completed signing-state records for a zone, use rndc signing -clear all zone.

min-refresh-time; max-refresh-time; min-retry-time; max-retry-time

These options control the server’s behavior on refreshing a zone (querying for SOA changes) or retrying failed transfers. Usually the SOA values for the zone are used, up to a hard-coded maximum expiry of 24 weeks. However, these values are set by the primary, giving secondary server administrators little control over their contents.

These options allow the administrator to set a minimum and maximum refresh and retry time in seconds per-zone, per-view, or globally. These options are valid for secondary and stub zones, and clamp the SOA refresh and retry times to the specified values.

The following defaults apply: min-refresh-time 300 seconds, max-refresh-time 2419200 seconds (4 weeks), min-retry-time 500 seconds, and max-retry-time 1209600 seconds (2 weeks).

edns-udp-size

This sets the maximum advertised EDNS UDP buffer size, in bytes, to control the size of packets received from authoritative servers in response to recursive queries. Valid values are 512 to 4096; values outside this range are silently adjusted to the nearest value within it. The default value is 1232.

The usual reason for setting edns-udp-size to a non-default value is to get UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP DNS packets that are greater than 512 bytes.

When named first queries a remote server, it advertises a UDP buffer size of 1232.

Query timeouts observed for any given server affect the buffer size advertised in queries sent to that server. Depending on observed packet dropping patterns, the query is retried over TCP. Per-server EDNS statistics are only retained in memory for the lifetime of a given server’s ADB entry.

The named now sets the DON’T FRAGMENT flag on outgoing UDP packets. According to the measurements done by multiple parties this should not be causing any operational problems as most of the Internet “core” is able to cope with IP message sizes between 1400-1500 bytes, the 1232 size was picked as a conservative minimal number that could be changed by the DNS operator to a estimated path MTU minus the estimated header space. In practice, the smallest MTU witnessed in the operational DNS community is 1500 octets, the Ethernet maximum payload size, so a a useful default for maximum DNS/UDP payload size on reliable networks would be 1432.

Any server-specific edns-udp-size setting has precedence over all the above rules.

max-udp-size

This sets the maximum EDNS UDP message size that named sends, in bytes. Valid values are 512 to 4096; values outside this range are silently adjusted to the nearest value within it. The default value is 1232.

This value applies to responses sent by a server; to set the advertised buffer size in queries, see edns-udp-size.

The usual reason for setting max-udp-size to a non-default value is to allow UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP packets that are greater than 512 bytes. This is independent of the advertised receive buffer (edns-udp-size).

Setting this to a low value encourages additional TCP traffic to the name server.

masterfile-format

This specifies the file format of zone files (see Additional File Formats for details). The default value is text, which is the standard textual representation, except for secondary zones, in which the default value is raw. Files in formats other than text are typically expected to be generated by the named-compilezone tool, or dumped by named.

Note that when a zone file in a format other than text is loaded, named may omit some of the checks which are performed for a file in text format. For example, check-names only applies when loading zones in text format, and max-zone-ttl only applies to text and raw. Zone files in binary formats should be generated with the same check level as that specified in the named configuration file.

When configured in options, this statement sets the masterfile-format for all zones, but it can be overridden on a per-zone or per-view basis by including a masterfile-format statement within the zone or view block in the configuration file.

masterfile-style

This specifies the formatting of zone files during dump, when the masterfile-format is text. This option is ignored with any other masterfile-format.

When set to relative, records are printed in a multi-line format, with owner names expressed relative to a shared origin. When set to full, records are printed in a single-line format with absolute owner names. The full format is most suitable when a zone file needs to be processed automatically by a script. The relative format is more human-readable, and is thus suitable when a zone is to be edited by hand. The default is relative.

max-recursion-depth

This sets the maximum number of levels of recursion that are permitted at any one time while servicing a recursive query. Resolving a name may require looking up a name server address, which in turn requires resolving another name, etc.; if the number of recursions exceeds this value, the recursive query is terminated and returns SERVFAIL. The default is 7.

max-recursion-queries

This sets the maximum number of iterative queries that may be sent while servicing a recursive query. If more queries are sent, the recursive query is terminated and returns SERVFAIL. The default is 100.

notify-delay

This sets the delay, in seconds, between sending sets of NOTIFY messages for a zone. Whenever a NOTIFY message is sent for a zone, a timer will be set for this duration. If the zone is updated again before the timer expires, the NOTIFY for that update will be postponed. The default is 5 seconds.

The overall rate at which NOTIFY messages are sent for all zones is controlled by notify-rate.

max-rsa-exponent-size

This sets the maximum RSA exponent size, in bits, that is accepted when validating. Valid values are 35 to 4096 bits. The default, zero, is also accepted and is equivalent to 4096.

prefetch

When a query is received for cached data which is to expire shortly, named can refresh the data from the authoritative server immediately, ensuring that the cache always has an answer available.

prefetch specifies the “trigger” TTL value at which prefetch of the current query takes place; when a cache record with a lower TTL value is encountered during query processing, it is refreshed. Valid trigger TTL values are 1 to 10 seconds. Values larger than 10 seconds are silently reduced to 10. Setting a trigger TTL to zero causes prefetch to be disabled. The default trigger TTL is 2.

An optional second argument specifies the “eligibility” TTL: the smallest original TTL value that is accepted for a record to be eligible for prefetching. The eligibility TTL must be at least six seconds longer than the trigger TTL; if not, named silently adjusts it upward. The default eligibility TTL is 9.

v6-bias

When determining the next name server to try, this indicates by how many milliseconds to prefer IPv6 name servers. The default is 50 milliseconds.

tcp-receive-buffer; udp-receive-buffer

These options control the operating system’s receive buffer sizes (SO_RCVBUF) for TCP and UDP sockets, respectively. Buffering at the operating system level can prevent packet drops during brief load spikes, but if the buffer size is set too high, a running server could get clogged with outstanding queries that have already timed out. The default is 0, which means the operating system’s default value should be used. The minimum configurable value is 4096; any nonzero value lower than that is silently raised. The maximum value is determined by the kernel, and values exceeding the maximum are silently reduced.

tcp-send-buffer; udp-send-buffer

These options control the operating system’s send buffer sizes (SO_SNDBUF) for TCP and UDP sockets, respectively. Buffering at the operating system level can prevent packet drops during brief load spikes, but if the buffer size is set too high, a running server could get clogged with outstanding queries that have already timed out. The default is 0, which means the operating system’s default value should be used. The minimum configurable value is 4096; any nonzero value lower than that is silently raised. The maximum value is determined by the kernel, and values exceeding the maximum are silently reduced.

4.2.16.15. Built-in Server Information Zones

The server provides some helpful diagnostic information through a number of built-in zones under the pseudo-top-level-domain bind in the CHAOS class. These zones are part of a built-in view (see view Statement Grammar) of class CHAOS, which is separate from the default view of class IN. Most global configuration options (allow-query, etc.) apply to this view, but some are locally overridden: notify, recursion, and allow-new-zones are always set to no, and rate-limit is set to allow three responses per second.

To disable these zones, use the options below or hide the built-in CHAOS view by defining an explicit view of class CHAOS that matches all clients.

version

This is the version the server should report via a query of the name version.bind with type TXT and class CHAOS. The default is the real version number of this server. Specifying version none disables processing of the queries.

Setting version to any value (including none) also disables queries for authors.bind TXT CH.

hostname

This is the hostname the server should report via a query of the name hostname.bind with type TXT and class CHAOS. This defaults to the hostname of the machine hosting the name server, as found by the gethostname() function. The primary purpose of such queries is to identify which of a group of anycast servers is actually answering the queries. Specifying hostname none; disables processing of the queries.

server-id

This is the ID the server should report when receiving a Name Server Identifier (NSID) query, or a query of the name ID.SERVER with type TXT and class CHAOS. The primary purpose of such queries is to identify which of a group of anycast servers is actually answering the queries. Specifying server-id none; disables processing of the queries. Specifying server-id hostname; causes named to use the hostname as found by the gethostname() function. The default server-id is none.

4.2.16.16. Built-in Empty Zones

The named server has some built-in empty zones, for SOA and NS records only. These are for zones that should normally be answered locally and for which queries should not be sent to the Internet’s root servers. The official servers that cover these namespaces return NXDOMAIN responses to these queries. In particular, these cover the reverse namespaces for addresses from RFC 1918, RFC 4193, RFC 5737, and RFC 6598. They also include the reverse namespace for the IPv6 local address (locally assigned), IPv6 link local addresses, the IPv6 loopback address, and the IPv6 unknown address.

The server attempts to determine if a built-in zone already exists or is active (covered by a forward-only forwarding declaration) and does not create an empty zone if either is true.

The current list of empty zones is:

• 10.IN-ADDR.ARPA

• 16.172.IN-ADDR.ARPA

• 17.172.IN-ADDR.ARPA

• 18.172.IN-ADDR.ARPA

• 19.172.IN-ADDR.ARPA

• 20.172.IN-ADDR.ARPA

• 21.172.IN-ADDR.ARPA

• 22.172.IN-ADDR.ARPA

• 23.172.IN-ADDR.ARPA

• 24.172.IN-ADDR.ARPA

• 25.172.IN-ADDR.ARPA

• 26.172.IN-ADDR.ARPA

• 27.172.IN-ADDR.ARPA

• 28.172.IN-ADDR.ARPA

• 29.172.IN-ADDR.ARPA

• 30.172.IN-ADDR.ARPA

• 31.172.IN-ADDR.ARPA

• 168.192.IN-ADDR.ARPA

• 64.100.IN-ADDR.ARPA

• 65.100.IN-ADDR.ARPA

• 66.100.IN-ADDR.ARPA

• 67.100.IN-ADDR.ARPA

• 68.100.IN-ADDR.ARPA

• 69.100.IN-ADDR.ARPA

• 70.100.IN-ADDR.ARPA

• 71.100.IN-ADDR.ARPA

• 72.100.IN-ADDR.ARPA

• 73.100.IN-ADDR.ARPA

• 74.100.IN-ADDR.ARPA

• 75.100.IN-ADDR.ARPA

• 76.100.IN-ADDR.ARPA

• 77.100.IN-ADDR.ARPA

• 78.100.IN-ADDR.ARPA

• 79.100.IN-ADDR.ARPA

• 80.100.IN-ADDR.ARPA

• 81.100.IN-ADDR.ARPA

• 82.100.IN-ADDR.ARPA

• 83.100.IN-ADDR.ARPA

• 84.100.IN-ADDR.ARPA

• 85.100.IN-ADDR.ARPA

• 86.100.IN-ADDR.ARPA

• 87.100.IN-ADDR.ARPA

• 88.100.IN-ADDR.ARPA

• 89.100.IN-ADDR.ARPA

• 90.100.IN-ADDR.ARPA

• 91.100.IN-ADDR.ARPA

• 92.100.IN-ADDR.ARPA

• 93.100.IN-ADDR.ARPA

• 94.100.IN-ADDR.ARPA

• 95.100.IN-ADDR.ARPA

• 96.100.IN-ADDR.ARPA

• 97.100.IN-ADDR.ARPA

• 98.100.IN-ADDR.ARPA

• 99.100.IN-ADDR.ARPA

• 100.100.IN-ADDR.ARPA

• 101.100.IN-ADDR.ARPA

• 102.100.IN-ADDR.ARPA

• 103.100.IN-ADDR.ARPA

• 104.100.IN-ADDR.ARPA

• 105.100.IN-ADDR.ARPA

• 106.100.IN-ADDR.ARPA

• 107.100.IN-ADDR.ARPA

• 108.100.IN-ADDR.ARPA

• 109.100.IN-ADDR.ARPA

• 110.100.IN-ADDR.ARPA

• 111.100.IN-ADDR.ARPA

• 112.100.IN-ADDR.ARPA

• 113.100.IN-ADDR.ARPA

• 114.100.IN-ADDR.ARPA

• 115.100.IN-ADDR.ARPA

• 116.100.IN-ADDR.ARPA

• 117.100.IN-ADDR.ARPA

• 118.100.IN-ADDR.ARPA

• 119.100.IN-ADDR.ARPA

• 120.100.IN-ADDR.ARPA

• 121.100.IN-ADDR.ARPA

• 122.100.IN-ADDR.ARPA

• 123.100.IN-ADDR.ARPA

• 124.100.IN-ADDR.ARPA

• 125.100.IN-ADDR.ARPA

• 126.100.IN-ADDR.ARPA

• 127.100.IN-ADDR.ARPA

• 0.IN-ADDR.ARPA

• 127.IN-ADDR.ARPA

• 254.169.IN-ADDR.ARPA

• 2.0.192.IN-ADDR.ARPA

• 100.51.198.IN-ADDR.ARPA

• 113.0.203.IN-ADDR.ARPA

• 255.255.255.255.IN-ADDR.ARPA

• 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA

• 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA

• 8.B.D.0.1.0.0.2.IP6.ARPA

• D.F.IP6.ARPA

• 8.E.F.IP6.ARPA

• 9.E.F.IP6.ARPA

• A.E.F.IP6.ARPA

• B.E.F.IP6.ARPA

• EMPTY.AS112.ARPA

• HOME.ARPA

Empty zones can be set at the view level and only apply to views of class IN. Disabled empty zones are only inherited from options if there are no disabled empty zones specified at the view level. To override the options list of disabled zones, disable the root zone at the view level. For example:

disable-empty-zone ".";

If using the address ranges covered here, reverse zones covering the addresses should already be in place. In practice this appears to not be the case, with many queries being made to the infrastructure servers for names in these spaces. So many, in fact, that sacrificial servers had to be deployed to channel the query load away from the infrastructure servers.

Note

The real parent servers for these zones should disable all empty zones under the parent zone they serve. For the real root servers, this is all built-in empty zones. This enables them to return referrals to deeper in the tree.

empty-server

This specifies the server name that appears in the returned SOA record for empty zones. If none is specified, the zone’s name is used.

empty-contact

This specifies the contact name that appears in the returned SOA record for empty zones. If none is specified, “.” is used.

empty-zones-enable

This enables or disables all empty zones. By default, they are enabled.

disable-empty-zone

This disables individual empty zones. By default, none are disabled. This option can be specified multiple times.

4.2.16.17. Content Filtering

BIND 9 provides the ability to filter out responses from external DNS servers containing certain types of data in the answer section. Specifically, it can reject address (A or AAAA) records if the corresponding IPv4 or IPv6 addresses match the given address_match_list of the deny-answer-addresses option. It can also reject CNAME or DNAME records if the “alias” name (i.e., the CNAME alias or the substituted query name due to DNAME) matches the given namelist of the deny-answer-aliases option, where “match” means the alias name is a subdomain of one of the name_list elements. If the optional namelist is specified with except-from, records whose query name matches the list are accepted regardless of the filter setting. Likewise, if the alias name is a subdomain of the corresponding zone, the deny-answer-aliases filter does not apply; for example, even if “example.com” is specified for deny-answer-aliases,

www.example.com. CNAME xxx.example.com.

returned by an “example.com” server is accepted.

In the address_match_list of the deny-answer-addresses option, only ip_addr and ip_prefix are meaningful; any key_id is silently ignored.

If a response message is rejected due to the filtering, the entire message is discarded without being cached, and a SERVFAIL error is returned to the client.

This filtering is intended to prevent “DNS rebinding attacks,” in which an attacker, in response to a query for a domain name the attacker controls, returns an IP address within the user’s own network or an alias name within the user’s own domain. A naive web browser or script could then serve as an unintended proxy, allowing the attacker to get access to an internal node of the local network that could not be externally accessed otherwise. See the paper available at https://dl.acm.org/doi/10.1145/1315245.1315298 for more details about these attacks.

For example, with a domain named “example.net” and an internal network using an IPv4 prefix 192.0.2.0/24, an administrator might specify the following rules:

deny-answer-addresses { 192.0.2.0/24; } except-from { "example.net"; };
deny-answer-aliases { "example.net"; };

If an external attacker let a web browser in the local network look up an IPv4 address of “attacker.example.com”, the attacker’s DNS server would return a response like this:

attacker.example.com. A 192.0.2.1

in the answer section. Since the rdata of this record (the IPv4 address) matches the specified prefix 192.0.2.0/24, this response would be ignored.

On the other hand, if the browser looked up a legitimate internal web server “www.example.net” and the following response were returned to the BIND 9 server:

www.example.net. A 192.0.2.2

it would be accepted, since the owner name “www.example.net” matches the except-from element, “example.net”.

Note that this is not really an attack on the DNS per se. In fact, there is nothing wrong with having an “external” name mapped to an “internal” IP address or domain name from the DNS point of view; it might actually be provided for a legitimate purpose, such as for debugging. As long as the mapping is provided by the correct owner, it either is not possible or does not make sense to detect whether the intent of the mapping is legitimate within the DNS. The “rebinding” attack must primarily be protected at the application that uses the DNS. For a large site, however, it may be difficult to protect all possible applications at once. This filtering feature is provided only to help such an operational environment; turning it on is generally discouraged unless there is no other choice and the attack is a real threat to applications.

Care should be particularly taken if using this option for addresses within 127.0.0.0/8. These addresses are obviously “internal,” but many applications conventionally rely on a DNS mapping from some name to such an address. Filtering out DNS records containing this address spuriously can break such applications.

4.2.16.18. Response Policy Zone (RPZ) Rewriting

BIND 9 includes a limited mechanism to modify DNS responses for requests analogous to email anti-spam DNS rejection lists. Responses can be changed to deny the existence of domains (NXDOMAIN), deny the existence of IP addresses for domains (NODATA), or contain other IP addresses or data.

Response policy zones are named in the response-policy option for the view, or among the global options if there is no response-policy option for the view. Response policy zones are ordinary DNS zones containing RRsets that can be queried normally if allowed. It is usually best to restrict those queries with something like allow-query { localhost; };.

A response-policy option can support multiple policy zones. To maximize performance, a radix tree is used to quickly identify response policy zones containing triggers that match the current query. This imposes an upper limit of 64 on the number of policy zones in a single response-policy option; more than that is a configuration error.

Rules encoded in response policy zones are processed after those defined in Access Control. All queries from clients which are not permitted access to the resolver are answered with a status code of REFUSED, regardless of configured RPZ rules.

Five policy triggers can be encoded in RPZ records.

RPZ-CLIENT-IP

IP records are triggered by the IP address of the DNS client. Client IP address triggers are encoded in records that have owner names that are subdomains of rpz-client-ip, relativized to the policy zone origin name, and that encode an address or address block. IPv4 addresses are represented as prefixlength.B4.B3.B2.B1.rpz-client-ip. The IPv4 prefix length must be between 1 and 32. All four bytes - B4, B3, B2, and B1 - must be present. B4 is the decimal value of the least significant byte of the IPv4 address as in IN-ADDR.ARPA.

IPv6 addresses are encoded in a format similar to the standard IPv6 text representation, prefixlength.W8.W7.W6.W5.W4.W3.W2.W1.rpz-client-ip. Each of W8,…,W1 is a one- to four-digit hexadecimal number representing 16 bits of the IPv6 address as in the standard text representation of IPv6 addresses, but reversed as in IP6.ARPA. (Note that this representation of IPv6 addresses is different from IP6.ARPA, where each hex digit occupies a label.) All 8 words must be present except when one set of consecutive zero words is replaced with .zz., analogous to double colons (::) in standard IPv6 text encodings. The IPv6 prefix length must be between 1 and 128.

QNAME

QNAME policy records are triggered by query names of requests and targets of CNAME records resolved to generate the response. The owner name of a QNAME policy record is the query name relativized to the policy zone.

RPZ-IP

IP triggers are IP addresses in an A or AAAA record in the ANSWER section of a response. They are encoded like client-IP triggers, except as subdomains of rpz-ip.

RPZ-NSDNAME

NSDNAME triggers match names of authoritative servers for the query name, a parent of the query name, a CNAME for the query name, or a parent of a CNAME. They are encoded as subdomains of rpz-nsdname, relativized to the RPZ origin name. NSIP triggers match IP addresses in A and AAAA RRsets for domains that can be checked against NSDNAME policy records. The nsdname-enable phrase turns NSDNAME triggers off or on for a single policy zone or for all zones.

If authoritative name servers for the query name are not yet known, named recursively looks up the authoritative servers for the query name before applying an RPZ-NSDNAME rule, which can cause a processing delay. To speed up processing at the cost of precision, the nsdname-wait-recurse option can be used; when set to no, RPZ-NSDNAME rules are only applied when authoritative servers for the query name have already been looked up and cached. If authoritative servers for the query name are not in the cache, the RPZ-NSDNAME rule is ignored, but the authoritative servers for the query name are looked up in the background and the rule is applied to subsequent queries. The default is yes, meaning RPZ-NSDNAME rules are always applied, even if authoritative servers for the query name need to be looked up first.

RPZ-NSIP

NSIP triggers match the IP addresses of authoritative servers. They are encoded like IP triggers, except as subdomains of rpz-nsip. NSDNAME and NSIP triggers are checked only for names with at least min-ns-dots dots. The default value of min-ns-dots is 1, to exclude top-level domains. The nsip-enable phrase turns NSIP triggers off or on for a single policy zone or for all zones.

If a name server’s IP address is not yet known, named recursively looks up the IP address before applying an RPZ-NSIP rule, which can cause a processing delay. To speed up processing at the cost of precision, the nsip-wait-recurse option can be used; when set to no, RPZ-NSIP rules are only applied when a name server’s IP address has already been looked up and cached. If a server’s IP address is not in the cache, the RPZ-NSIP rule is ignored, but the address is looked up in the background and the rule is applied to subsequent queries. The default is yes, meaning RPZ-NSIP rules are always applied, even if an address needs to be looked up first.

The query response is checked against all response policy zones, so two or more policy records can be triggered by a response. Because DNS responses are rewritten according to at most one policy record, a single record encoding an action (other than DISABLED actions) must be chosen. Triggers, or the records that encode them, are chosen for rewriting in the following order:

1. Choose the triggered record in the zone that appears first in the response-policy option.

2. Prefer CLIENT-IP to QNAME to IP to NSDNAME to NSIP triggers in a single zone.

3. Among NSDNAME triggers, prefer the trigger that matches the smallest name under the DNSSEC ordering.

4. Among IP or NSIP triggers, prefer the trigger with the longest prefix.

5. Among triggers with the same prefix length, prefer the IP or NSIP trigger that matches the smallest IP address.

When the processing of a response is restarted to resolve DNAME or CNAME records and a policy record set has not been triggered, all response policy zones are again consulted for the DNAME or CNAME names and addresses.

RPZ record sets are any types of DNS record, except DNAME or DNSSEC, that encode actions or responses to individual queries. Any of the policies can be used with any of the triggers. For example, while the TCP-only policy is commonly used with client-IP triggers, it can be used with any type of trigger to force the use of TCP for responses with owner names in a zone.

PASSTHRU

The auto-acceptance policy is specified by a CNAME whose target is rpz-passthru. It causes the response to not be rewritten and is most often used to “poke holes” in policies for CIDR blocks.

DROP

The auto-rejection policy is specified by a CNAME whose target is rpz-drop. It causes the response to be discarded. Nothing is sent to the DNS client.

TCP-Only

The “slip” policy is specified by a CNAME whose target is rpz-tcp-only. It changes UDP responses to short, truncated DNS responses that require the DNS client to try again with TCP. It is used to mitigate distributed DNS reflection attacks.

NXDOMAIN

The “domain undefined” response is encoded by a CNAME whose target is the root domain (.).

NODATA

The empty set of resource records is specified by a CNAME whose target is the wildcard top-level domain (*.). It rewrites the response to NODATA or ANCOUNT=0.

Local Data

A set of ordinary DNS records can be used to answer queries. Queries for record types not in the set are answered with NODATA.

A special form of local data is a CNAME whose target is a wildcard such as *.example.com. It is used as if an ordinary CNAME after the asterisk (*) has been replaced with the query name. This special form is useful for query logging in the walled garden’s authoritative DNS server.

All of the actions specified in all of the individual records in a policy zone can be overridden with a policy clause in the response-policy option. An organization using a policy zone provided by another organization might use this mechanism to redirect domains to its own walled garden.

GIVEN

The placeholder policy says “do not override but perform the action specified in the zone.”

DISABLED

The testing override policy causes policy zone records to do nothing but log what they would have done if the policy zone were not disabled. The response to the DNS query is written (or not) according to any triggered policy records that are not disabled. Disabled policy zones should appear first, because they are often not logged if a higher-precedence trigger is found first.

PASSTHRU; DROP; TCP-Only; NXDOMAIN; NODATA

These settings each override the corresponding per-record policy.

CNAME domain

This causes all RPZ policy records to act as if they were “cname domain” records.

By default, the actions encoded in a response policy zone are applied only to queries that ask for recursion (RD=1). That default can be changed for a single policy zone, or for all response policy zones in a view, with a recursive-only no clause. This feature is useful for serving the same zone files both inside and outside an RFC 1918 cloud and using RPZ to delete answers that would otherwise contain RFC 1918 values on the externally visible name server or view.

Also by default, RPZ actions are applied only to DNS requests that either do not request DNSSEC metadata (DO=0) or when no DNSSEC records are available for the requested name in the original zone (not the response policy zone). This default can be changed for all response policy zones in a view with a break-dnssec yes clause. In that case, RPZ actions are applied regardless of DNSSEC. The name of the clause option reflects the fact that results rewritten by RPZ actions cannot verify.

No DNS records are needed for a QNAME or Client-IP trigger; the name or IP address itself is sufficient, so in principle the query name need not be recursively resolved. However, not resolving the requested name can leak the fact that response policy rewriting is in use, and that the name is listed in a policy zone, to operators of servers for listed names. To prevent that information leak, by default any recursion needed for a request is done before any policy triggers are considered. Because listed domains often have slow authoritative servers, this behavior can cost significant time. The qname-wait-recurse yes option overrides the default and enables that behavior when recursion cannot change a non-error response. The option does not affect QNAME or client-IP triggers in policy zones listed after other zones containing IP, NSIP, and NSDNAME triggers, because those may depend on the A, AAAA, and NS records that would be found during recursive resolution. It also does not affect DNSSEC requests (DO=1) unless break-dnssec yes is in use, because the response would depend on whether RRSIG records were found during resolution. Using this option can cause error responses such as SERVFAIL to appear to be rewritten, since no recursion is being done to discover problems at the authoritative server.

The dnsrps-enable yes option turns on the DNS Response Policy Service (DNSRPS) interface, if it has been compiled in named using configure --enable-dnsrps.

The dnsrps-options block provides additional RPZ configuration settings, which are passed through to the DNSRPS provider library. Multiple DNSRPS settings in an dnsrps-options string should be separated with semi-colons (;). The DNSRPS provider, librpz, is passed a configuration string consisting of the dnsrps-options text, concatenated with settings derived from the response-policy statement.

Note: the dnsrps-options text should only include configuration settings that are specific to the DNSRPS provider. For example, the DNSRPS provider from Farsight Security takes options such as dnsrpzd-conf, dnsrpzd-sock, and dnzrpzd-args (for details of these options, see the librpz documentation). Other RPZ configuration settings could be included in dnsrps-options as well, but if named were switched back to traditional RPZ by setting dnsrps-enable to “no”, those options would be ignored.

The TTL of a record modified by RPZ policies is set from the TTL of the relevant record in the policy zone. It is then limited to a maximum value. The max-policy-ttl clause changes the maximum number of seconds from its default of 5. For convenience, TTL-style time-unit suffixes may be used to specify the value. It also accepts ISO 8601 duration formats.

For example, an administrator might use this option statement:

response-policy { zone "badlist"; };

and this zone statement:

zone "badlist" {type primary; file "primary/badlist"; allow-query {none;}; };

with this zone file:

$TTL 1H
@                       SOA LOCALHOST. named-mgr.example.com (1 1h 15m 30d 2h)
            NS  LOCALHOST.

; QNAME policy records.  There are no periods (.) after the owner names.
nxdomain.domain.com     CNAME   .               ; NXDOMAIN policy
*.nxdomain.domain.com   CNAME   .               ; NXDOMAIN policy
nodata.domain.com       CNAME   *.              ; NODATA policy
*.nodata.domain.com     CNAME   *.              ; NODATA policy
bad.domain.com          A       10.0.0.1        ; redirect to a walled garden
            AAAA    2001:2::1
bzone.domain.com        CNAME   garden.example.com.

; do not rewrite (PASSTHRU) OK.DOMAIN.COM
ok.domain.com           CNAME   rpz-passthru.

; redirect x.bzone.domain.com to x.bzone.domain.com.garden.example.com
*.bzone.domain.com      CNAME   *.garden.example.com.

; IP policy records that rewrite all responses containing A records in 127/8
;       except 127.0.0.1
8.0.0.0.127.rpz-ip      CNAME   .
32.1.0.0.127.rpz-ip     CNAME   rpz-passthru.

; NSDNAME and NSIP policy records
ns.domain.com.rpz-nsdname   CNAME   .
48.zz.2.2001.rpz-nsip       CNAME   .

; auto-reject and auto-accept some DNS clients
112.zz.2001.rpz-client-ip    CNAME   rpz-drop.
8.0.0.0.127.rpz-client-ip    CNAME   rpz-drop.

; force some DNS clients and responses in the example.com zone to TCP
16.0.0.1.10.rpz-client-ip   CNAME   rpz-tcp-only.
example.com                 CNAME   rpz-tcp-only.
*.example.com               CNAME   rpz-tcp-only.

RPZ can affect server performance. Each configured response policy zone requires the server to perform one to four additional database lookups before a query can be answered. For example, a DNS server with four policy zones, each with all four kinds of response triggers (QNAME, IP, NSIP, and NSDNAME), requires a total of 17 times as many database lookups as a similar DNS server with no response policy zones. A BIND 9 server with adequate memory and one response policy zone with QNAME and IP triggers might achieve a maximum queries-per-second (QPS) rate about 20% lower. A server with four response policy zones with QNAME and IP triggers might have a maximum QPS rate about 50% lower.

Responses rewritten by RPZ are counted in the RPZRewrites statistics.

The log clause can be used to optionally turn off rewrite logging for a particular response policy zone. By default, all rewrites are logged.

The add-soa option controls whether the RPZ’s SOA record is added to the section for traceback of changes from this zone. This can be set at the individual policy zone level or at the response-policy level. The default is yes.

Updates to RPZ zones are processed asynchronously; if there is more than one update pending they are bundled together. If an update to a RPZ zone (for example, via IXFR) happens less than min-update-interval seconds after the most recent update, the changes are not carried out until this interval has elapsed. The default is 60 seconds. For convenience, TTL-style time-unit suffixes may be used to specify the value. It also accepts ISO 8601 duration formats.

4.2.16.19. Response Rate Limiting

Excessive, almost-identical UDP responses can be controlled by configuring a rate-limit clause in an options or view statement. This mechanism keeps authoritative BIND 9 from being used to amplify reflection denial-of-service (DoS) attacks. Short BADCOOKIE errors or truncated (TC=1) responses can be sent to provide rate-limited responses to legitimate clients within a range of forged, attacked IP addresses. Legitimate clients react to dropped responses by retrying, to BADCOOKIE errors by including a server cookie when retrying, and to truncated responses by switching to TCP.

This mechanism is intended for authoritative DNS servers. It can be used on recursive servers, but can slow applications such as SMTP servers (mail receivers) and HTTP clients (web browsers) that repeatedly request the same domains. When possible, closing “open” recursive servers is better.

Response rate limiting uses a “credit” or “token bucket” scheme. Each combination of identical response and client has a conceptual “account” that earns a specified number of credits every second. A prospective response debits its account by one. Responses are dropped or truncated while the account is negative. Responses are tracked within a rolling window of time which defaults to 15 seconds, but which can be configured with the window option to any value from 1 to 3600 seconds (1 hour). The account cannot become more positive than the per-second limit or more negative than window times the per-second limit. When the specified number of credits for a class of responses is set to 0, those responses are not rate-limited.

The notions of “identical response” and “DNS client” for rate limiting are not simplistic. All responses to an address block are counted as if to a single client. The prefix lengths of address blocks are specified with ipv4-prefix-length (default 24) and ipv6-prefix-length (default 56).

All non-empty responses for a valid domain name (qname) and record type (qtype) are identical and have a limit specified with responses-per-second (default 0 or no limit). All empty (NODATA) responses for a valid domain, regardless of query type, are identical. Responses in the NODATA class are limited by nodata-per-second (default responses-per-second). Requests for any and all undefined subdomains of a given valid domain result in NXDOMAIN errors, and are identical regardless of query type. They are limited by nxdomains-per-second (default responses-per-second). This controls some attacks using random names, but can be relaxed or turned off (set to 0) on servers that expect many legitimate NXDOMAIN responses, such as from anti-spam rejection lists. Referrals or delegations to the server of a given domain are identical and are limited by referrals-per-second (default responses-per-second).

Responses generated from local wildcards are counted and limited as if they were for the parent domain name. This controls flooding using random.wild.example.com.

All requests that result in DNS errors other than NXDOMAIN, such as SERVFAIL and FORMERR, are identical regardless of requested name (qname) or record type (qtype). This controls attacks using invalid requests or distant, broken authoritative servers. By default the limit on errors is the same as the responses-per-second value, but it can be set separately with errors-per-second.

Many attacks using DNS involve UDP requests with forged source addresses. Rate limiting prevents the use of BIND 9 to flood a network with responses to requests with forged source addresses, but could let a third party block responses to legitimate requests. There is a mechanism that can answer some legitimate requests from a client whose address is being forged in a flood. Setting slip to 2 (its default) causes every other UDP request without a valid server cookie to be answered with a small response. The small size and reduced frequency, and resulting lack of amplification, of “slipped” responses make them unattractive for reflection DoS attacks. slip must be between 0 and 10. A value of 0 does not “slip”; no small responses are sent due to rate limiting. Rather, all responses are dropped. A value of 1 causes every response to slip; values between 2 and 10 cause every nth response to slip.

If the request included a client cookie, then a “slipped” response is a BADCOOKIE error with a server cookie, which allows a legitimate client to include the server cookie to be exempted from the rate limiting when it retries the request. If the request did not include a cookie, then a “slipped” response is a truncated (TC=1) response, which prompts a legitimate client to switch to TCP and thus be exempted from the rate limiting. Some error responses, including REFUSED and SERVFAIL, cannot be replaced with truncated responses and are instead leaked at the slip rate.

(Note: dropped responses from an authoritative server may reduce the difficulty of a third party successfully forging a response to a recursive resolver. The best security against forged responses is for authoritative operators to sign their zones using DNSSEC and for resolver operators to validate the responses. When this is not an option, operators who are more concerned with response integrity than with flood mitigation may consider setting slip to 1, causing all rate-limited responses to be truncated rather than dropped. This reduces the effectiveness of rate-limiting against reflection attacks.)

When the approximate query-per-second rate exceeds the qps-scale value, the responses-per-second, errors-per-second, nxdomains-per-second, and all-per-second values are reduced by the ratio of the current rate to the qps-scale value. This feature can tighten defenses during attacks. For example, with qps-scale 250; responses-per-second 20; and a total query rate of 1000 queries/second for all queries from all DNS clients including via TCP, then the effective responses/second limit changes to (250/1000)*20, or 5. Responses to requests that included a valid server cookie, and responses sent via TCP, are not limited but are counted to compute the query-per-second rate.

Communities of DNS clients can be given their own parameters or no rate limiting by putting rate-limit statements in view statements instead of in the global option statement. A rate-limit statement in a view replaces, rather than supplements, a rate-limit statement among the main options. DNS clients within a view can be exempted from rate limits with the exempt-clients clause.

UDP responses of all kinds can be limited with the all-per-second phrase. This rate limiting is unlike the rate limiting provided by responses-per-second, errors-per-second, and nxdomains-per-second on a DNS server, which are often invisible to the victim of a DNS reflection attack. Unless the forged requests of the attack are the same as the legitimate requests of the victim, the victim’s requests are not affected. Responses affected by an all-per-second limit are always dropped; the slip value has no effect. An all-per-second limit should be at least 4 times as large as the other limits, because single DNS clients often send bursts of legitimate requests. For example, the receipt of a single mail message can prompt requests from an SMTP server for NS, PTR, A, and AAAA records as the incoming SMTP/TCP/IP connection is considered. The SMTP server can need additional NS, A, AAAA, MX, TXT, and SPF records as it considers the SMTP Mail From command. Web browsers often repeatedly resolve the same names that are duplicated in HTML <IMG> tags in a page. all-per-second is similar to the rate limiting offered by firewalls but is often inferior. Attacks that justify ignoring the contents of DNS responses are likely to be attacks on the DNS server itself. They usually should be discarded before the DNS server spends resources making TCP connections or parsing DNS requests, but that rate limiting must be done before the DNS server sees the requests.

The maximum size of the table used to track requests and rate-limit responses is set with max-table-size. Each entry in the table is between 40 and 80 bytes. The table needs approximately as many entries as the number of requests received per second. The default is 20,000. To reduce the cold start of growing the table, min-table-size (default 500) can set the minimum table size. Enable rate-limit category logging to monitor expansions of the table and inform choices for the initial and maximum table size.

Use log-only yes to test rate-limiting parameters without actually dropping any requests.

Responses dropped by rate limits are included in the RateDropped and QryDropped statistics. Responses that are truncated by rate limits are included in RateSlipped and RespTruncated.

4.2.16.20. NXDOMAIN Redirection

named supports NXDOMAIN redirection via two methods:

• Redirect zone (zone Statement Grammar)

• Redirect namespace

With either method, when named gets an NXDOMAIN response it examines a separate namespace to see if the NXDOMAIN response should be replaced with an alternative response.

With a redirect zone (zone "." { type redirect; };), the data used to replace the NXDOMAIN is held in a single zone which is not part of the normal namespace. All the redirect information is contained in the zone; there are no delegations.

With a redirect namespace (option { nxdomain-redirect <suffix> };), the data used to replace the NXDOMAIN is part of the normal namespace and is looked up by appending the specified suffix to the original query name. This roughly doubles the cache required to process NXDOMAIN responses, as both the original NXDOMAIN response and the replacement data (or an NXDOMAIN indicating that there is no replacement) must be stored.

If both a redirect zone and a redirect namespace are configured, the redirect zone is tried first.

4.2.17. server Statement Grammar

4.2.18. server Statement Definition and Usage

The server statement defines characteristics to be associated with a remote name server. If a prefix length is specified, then a range of servers is covered. Only the most specific server clause applies, regardless of the order in named.conf.

The server statement can occur at the top level of the configuration file or inside a view statement. If a view statement contains one or more server statements, only those apply to the view and any top-level ones are ignored. If a view contains no server statements, any top-level server statements are used as defaults.

If a remote server is giving out bad data, marking it as bogus prevents further queries to it. The default value of bogus is no.

The provide-ixfr clause determines whether the local server, acting as primary, responds with an incremental zone transfer when the given remote server, a secondary, requests it. If set to yes, incremental transfer is provided whenever possible. If set to no, all transfers to the remote server are non-incremental. If not set, the value of the provide-ixfr option in the view or global options block is used as a default.

The request-ixfr clause determines whether the local server, acting as a secondary, requests incremental zone transfers from the given remote server, a primary. If not set, the value of the request-ixfr option in the view or global options block is used as a default. It may also be set in the zone block; if set there, it overrides the global or view setting for that zone.

IXFR requests to servers that do not support IXFR automatically fall back to AXFR. Therefore, there is no need to manually list which servers support IXFR and which ones do not; the global default of yes should always work. The purpose of the provide-ixfr and request-ixfr clauses is to make it possible to disable the use of IXFR even when both primary and secondary claim to support it: for example, if one of the servers is buggy and crashes or corrupts data when IXFR is used.

The request-expire clause determines whether the local server, when acting as a secondary, requests the EDNS EXPIRE value. The EDNS EXPIRE value indicates the remaining time before the zone data expires and needs to be refreshed. This is used when a secondary server transfers a zone from another secondary server; when transferring from the primary, the expiration timer is set from the EXPIRE field of the SOA record instead. The default is yes.

The edns clause determines whether the local server attempts to use EDNS when communicating with the remote server. The default is yes.

The edns-udp-size option sets the EDNS UDP size that is advertised by named when querying the remote server. Valid values are 512 to 4096 bytes; values outside this range are silently adjusted to the nearest value within it. This option is useful when advertising a different value to this server than the value advertised globally: for example, when there is a firewall at the remote site that is blocking large replies. Note: currently, this sets a single UDP size for all packets sent to the server; named does not deviate from this value. This differs from the behavior of edns-udp-size in options or view statements, where it specifies a maximum value. The server statement behavior may be brought into conformance with the options/view behavior in future releases.

The edns-version option sets the maximum EDNS VERSION that is sent to the server(s) by the resolver. The actual EDNS version sent is still subject to normal EDNS version-negotiation rules (see RFC 6891), the maximum EDNS version supported by the server, and any other heuristics that indicate that a lower version should be sent. This option is intended to be used when a remote server reacts badly to a given EDNS version or higher; it should be set to the highest version the remote server is known to support. Valid values are 0 to 255; higher values are silently adjusted. This option is not needed until higher EDNS versions than 0 are in use.

The max-udp-size option sets the maximum EDNS UDP message size named sends. Valid values are 512 to 4096 bytes; values outside this range are silently adjusted. This option is useful when there is a firewall that is blocking large replies from named.

The padding option adds EDNS Padding options to outgoing messages, increasing the packet size to a multiple of the specified block size. Valid block sizes range from 0 (the default, which disables the use of EDNS Padding) to 512 bytes. Larger values are reduced to 512, with a logged warning. Note: this option is not currently compatible with no TSIG or SIG(0), as the EDNS OPT record containing the padding would have to be added to the packet after it had already been signed.

The tcp-only option sets the transport protocol to TCP. The default is to use the UDP transport and to fallback on TCP only when a truncated response is received.

The tcp-keepalive option adds EDNS TCP keepalive to messages sent over TCP. Note that currently idle timeouts in responses are ignored.

The server supports two zone transfer methods. The first, one-answer, uses one DNS message per resource record transferred. many-answers packs as many resource records as possible into a single message, which is more efficient. It is possible to specify which method to use for a server via the transfer-format option; if not set there, the transfer-format specified by the options statement is used.

transfers is used to limit the number of concurrent inbound zone transfers from the specified server. If no transfers clause is specified, the limit is set according to the transfers-per-ns option.

The keys clause identifies a key_id defined by the key statement, to be used for transaction security (see TSIG) when talking to the remote server. When a request is sent to the remote server, a request signature is generated using the key specified here and appended to the message. A request originating from the remote server is not required to be signed by this key.

Only a single key per server is currently supported.

The transfer-source and transfer-source-v6 clauses specify the IPv4 and IPv6 source address, respectively, to be used for zone transfer with the remote server. For an IPv4 remote server, only transfer-source can be specified. Similarly, for an IPv6 remote server, only transfer-source-v6 can be specified. For more details, see the description of transfer-source and transfer-source-v6 in Zone Transfers.

The notify-source and notify-source-v6 clauses specify the IPv4 and IPv6 source address, respectively, to be used for notify messages sent to remote servers. For an IPv4 remote server, only notify-source can be specified. Similarly, for an IPv6 remote server, only notify-source-v6 can be specified.

The query-source and query-source-v6 clauses specify the IPv4 and IPv6 source address, respectively, to be used for queries sent to remote servers. For an IPv4 remote server, only query-source can be specified. Similarly, for an IPv6 remote server, only query-source-v6 can be specified.

The request-nsid clause determines whether the local server adds an NSID EDNS option to requests sent to the server. This overrides request-nsid set at the view or option level.

The send-cookie clause determines whether the local server adds a COOKIE EDNS option to requests sent to the server. This overrides send-cookie set at the view or option level. The named server may determine that COOKIE is not supported by the remote server and not add a COOKIE EDNS option to requests.

4.2.19. statistics-channels Statement Grammar

4.2.20. statistics-channels Statement Definition and Usage

The statistics-channels statement declares communication channels to be used by system administrators to get access to statistics information on the name server.

This statement is intended to be flexible to support multiple communication protocols in the future, but currently only HTTP access is supported. It requires that BIND 9 be compiled with libxml2 and/or json-c (also known as libjson0); the statistics-channels statement is still accepted even if it is built without the library, but any HTTP access fails with an error.

An inet control channel is a TCP socket listening at the specified ip_port on the specified ip_addr, which can be an IPv4 or IPv6 address. An ip_addr of * (asterisk) is interpreted as the IPv4 wildcard address; connections are accepted on any of the system’s IPv4 addresses. To listen on the IPv6 wildcard address, use an ip_addr of ::.

If no port is specified, port 80 is used for HTTP channels. The asterisk (*) cannot be used for ip_port.

Attempts to open a statistics channel are restricted by the optional allow clause. Connections to the statistics channel are permitted based on the address_match_list. If no allow clause is present, named accepts connection attempts from any address; since the statistics may contain sensitive internal information, it is highly recommended to restrict the source of connection requests appropriately.

If no statistics-channels statement is present, named does not open any communication channels.

The statistics are available in various formats and views, depending on the URI used to access them. For example, if the statistics channel is configured to listen on 127.0.0.1 port 8888, then the statistics are accessible in XML format at http://127.0.0.1:8888/ or http://127.0.0.1:8888/xml. A CSS file is included, which can format the XML statistics into tables when viewed with a stylesheet-capable browser, and into charts and graphs using the Google Charts API when using a JavaScript-capable browser.

Broken-out subsets of the statistics can be viewed at http://127.0.0.1:8888/xml/v3/status (server uptime and last reconfiguration time), http://127.0.0.1:8888/xml/v3/server (server and resolver statistics), http://127.0.0.1:8888/xml/v3/zones (zone statistics), http://127.0.0.1:8888/xml/v3/net (network status and socket statistics), http://127.0.0.1:8888/xml/v3/mem (memory manager statistics), http://127.0.0.1:8888/xml/v3/tasks (task manager statistics), and http://127.0.0.1:8888/xml/v3/traffic (traffic sizes).

The full set of statistics can also be read in JSON format at http://127.0.0.1:8888/json, with the broken-out subsets at http://127.0.0.1:8888/json/v1/status (server uptime and last reconfiguration time), http://127.0.0.1:8888/json/v1/server (server and resolver statistics), http://127.0.0.1:8888/json/v1/zones (zone statistics), http://127.0.0.1:8888/json/v1/net (network status and socket statistics), http://127.0.0.1:8888/json/v1/mem (memory manager statistics), http://127.0.0.1:8888/json/v1/tasks (task manager statistics), and http://127.0.0.1:8888/json/v1/traffic (traffic sizes).

4.2.21. tls Statement Grammar

4.2.22. tls Statement Definition and Usage

The tls statement is used to configure a TLS connection; this configuration can then be referenced by a listen-on or listen-on-v6 statement to cause named to listen for incoming requests via TLS, or in the primaries statement for a zone of type secondary to cause zone transfer requests to be sent via TLS.

tls can only be set at the top level of named.conf.

The following options can be specified in a tls statement:

key-file

Path to a file containing the private TLS key to be used for the connection.

cert-file

Path to a file containing the TLS certificate to be used for the connection.

dhparam-file

Path to a file containing Diffie-Hellman parameters, which is needed to enable the cipher suites depending on the Diffie-Hellman ephemeral key exchange (DHE). Having these parameters specified is essential for enabling perfect forward secrecy capable ciphers in TLSv1.2.

protocols

Allowed versions of the TLS protocol. TLS version 1.2 and higher are supported, depending on the cryptographic library in use. Multiple versions might be specified (e.g. protocols { TLSv1.2; TLSv1.3; };).

ciphers

Cipher list which defines allowed ciphers, such as HIGH:!aNULL:!MD5:!SHA1:!SHA256:!SHA384. The string must be formed according to the rules specified in the OpenSSL documentation (see https://www.openssl.org/docs/man1.1.1/man1/ciphers.html for details).

prefer-server-ciphers

Specifies that server ciphers should be preferred over client ones.

session-tickets

Enables or disables session resumption through TLS session tickets, as defined in RFC5077. Disabling the stateless session tickets might be required in the cases when forward secrecy is needed, or the TLS certificate and key pair is planned to be used across multiple BIND instances.

Warning

TLS configuration is subject to change and incompatible changes might be introduced in the future. Users of TLS are encouraged to carefully read release notes when upgrading.

The options described above are used to control different aspects of TLS functioning. Thus, most of them have no well-defined default values, as these depend on the cryptographic library version in use and system-wide cryptographic policy. On the other hand, by specifying the needed options one could have a uniform configuration deployable across a range of platforms.

An example of privacy-oriented, perfect forward secrecy enabled configuration can be found below. It can be used as a starting point.

tls local-tls {
    key-file "/path/to/key.pem";
    cert-file "/path/to/fullchain_cert.pem";
    dhparam-file "/path/to/dhparam.pem";
    ciphers "HIGH:!kRSA:!aNULL:!eNULL:!RC4:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!SHA1:!SHA256:!SHA384";
    prefer-server-ciphers yes;
    session-tickets no;
};

A Diffie-Hellman parameters file can be generated using e.g. OpenSSL, like follows:

openssl dhparam -out /path/to/dhparam.pem <3072_or_4096>

Ensure that it gets generated on a machine with enough entropy from external sources (e.g. the computer you work on should be fine, the remote virtual machine or server might be not). These files do not contain any sensitive data and can be shared if required.

There are two built-in TLS connection configurations: ephemeral, uses a temporary key and certificate created for the current named session only, and none, which can be used when setting up an HTTP listener with no encryption.

4.2.23. http Statement Grammar

4.2.24. http Statement Definition and Usage

The http statement is used to configure HTTP endpoints on which to listen for DNS-over-HTTPS (DoH) queries. This configuration can then be referenced by a listen-on or listen-on-v6 statement to cause named to listen for incoming requests over HTTPS.

http can only be set at the top level of named.conf.

The following options can be specified in an http statement:

endpoints

A list of HTTP query paths on which to listen. This is the portion of an RFC 3986-compliant URI following the hostname; it must be an absolute path, beginning with “/”. The default value is "/dns-query", if omitted.

listener-clients The option specifies a per-listener quota for active connections.

streams-per-connection

The option specifies the hard limit on the number of concurrent HTTP/2 streams over an HTTP/2 connection.

Any of the options above could be omitted. In such a case, a global value specified in the options statement is used (see http-listener-clients, http-streams-per-connection.

For example, the following configuration enables DNS-over-HTTPS queries on all local addresses:

http local {
    endpoints { "/dns-query"; };
};

options {
    ....
    listen-on tls ephemeral http local { any; };
    listen-on-v6 tls ephemeral http local { any; };
};

4.2.25. trust-anchors Statement Grammar

4.2.26. trust-anchors Statement Definition and Usage

The trust-anchors statement defines DNSSEC trust anchors. DNSSEC is described in DNSSEC.

A trust anchor is defined when the public key or public key digest for a non-authoritative zone is known but cannot be securely obtained through DNS, either because it is the DNS root zone or because its parent zone is unsigned. Once a key or digest has been configured as a trust anchor, it is treated as if it has been validated and proven secure.

The resolver attempts DNSSEC validation on all DNS data in subdomains of configured trust anchors. Validation below specified names can be temporarily disabled by using rndc nta, or permanently disabled with the validate-except option.

All keys listed in trust-anchors, and their corresponding zones, are deemed to exist regardless of what parent zones say. Only keys configured as trust anchors are used to validate the DNSKEY RRset for the corresponding name. The parent’s DS RRset is not used.

trust-anchors may be set at the top level of named.conf or within a view. If it is set in both places, the configurations are additive; keys defined at the top level are inherited by all views, but keys defined in a view are only used within that view.

The trust-anchors statement can contain multiple trust-anchor entries, each consisting of a domain name, followed by an “anchor type” keyword indicating the trust anchor’s format, followed by the key or digest data.

If the anchor type is static-key or initial-key, then it is followed with the key’s flags, protocol, and algorithm, plus the Base64 representation of the public key data. This is identical to the text representation of a DNSKEY record. Spaces, tabs, newlines, and carriage returns are ignored in the key data, so the configuration may be split into multiple lines.

If the anchor type is static-ds or initial-ds, it is followed with the key tag, algorithm, digest type, and the hexadecimal representation of the key digest. This is identical to the text representation of a DS record. Spaces, tabs, newlines, and carriage returns are ignored.

Trust anchors configured with the static-key or static-ds anchor types are immutable, while keys configured with initial-key or initial-ds can be kept up-to-date automatically, without intervention from the resolver operator. (static-key keys are identical to keys configured using the deprecated trusted-keys statement.)

Suppose, for example, that a zone’s key-signing key was compromised, and the zone owner had to revoke and replace the key. A resolver which had the original key configured using static-key or static-ds would be unable to validate this zone any longer; it would reply with a SERVFAIL response code. This would continue until the resolver operator had updated the trust-anchors statement with the new key.

If, however, the trust anchor had been configured using initial-key or initial-ds instead, the zone owner could add a “stand-by” key to the zone in advance. named would store the stand-by key, and when the original key was revoked, named would be able to transition smoothly to the new key. It would also recognize that the old key had been revoked and cease using that key to validate answers, minimizing the damage that the compromised key could do. This is the process used to keep the ICANN root DNSSEC key up-to-date.

Whereas static-key and static-ds trust anchors continue to be trusted until they are removed from named.conf, an initial-key or initial-ds is only trusted once: for as long as it takes to load the managed key database and start the RFC 5011 key maintenance process.

It is not possible to mix static with initial trust anchors for the same domain name.

The first time named runs with an initial-key or initial-ds configured in named.conf, it fetches the DNSKEY RRset directly from the zone apex, and validates it using the trust anchor specified in trust-anchors. If the DNSKEY RRset is validly signed by a key matching the trust anchor, then it is used as the basis for a new managed-keys database.

From that point on, whenever named runs, it sees the initial-key or initial-ds listed in trust-anchors, checks to make sure RFC 5011 key maintenance has already been initialized for the specified domain, and if so, simply moves on. The key specified in the trust-anchors statement is not used to validate answers; it is superseded by the key or keys stored in the managed-keys database.

The next time named runs after an initial-key or initial-ds has been removed from the trust-anchors statement (or changed to a static-key or static-ds), the corresponding zone is removed from the managed-keys database, and RFC 5011 key maintenance is no longer used for that domain.

In the current implementation, the managed-keys database is stored as a master-format zone file.

On servers which do not use views, this file is named managed-keys.bind. When views are in use, there is a separate managed-keys database for each view; the filename is the view name (or, if a view name contains characters which would make it illegal as a filename, a hash of the view name), followed by the suffix .mkeys.

When the key database is changed, the zone is updated. As with any other dynamic zone, changes are written into a journal file, e.g., managed-keys.bind.jnl or internal.mkeys.jnl. Changes are committed to the primary file as soon as possible afterward, usually within 30 seconds. Whenever named is using automatic key maintenance, the zone file and journal file can be expected to exist in the working directory. (For this reason, among others, the working directory should be always be writable by named.)

If the dnssec-validation option is set to auto, named automatically initializes an initial-key for the root zone. The key that is used to initialize the key-maintenance process is stored in bind.keys; the location of this file can be overridden with the bindkeys-file option. As a fallback in the event no bind.keys can be found, the initializing key is also compiled directly into named.

4.2.27. dnssec-policy Statement Grammar

4.2.28. dnssec-policy Statement Definition and Usage

The dnssec-policy statement defines a key and signing policy (KASP) for zones.

A KASP determines how one or more zones are signed with DNSSEC. For example, it specifies how often keys should roll, which cryptographic algorithms to use, and how often RRSIG records need to be refreshed.

Keys are not shared among zones, which means that one set of keys per zone is generated even if they have the same policy. If multiple views are configured with different versions of the same zone, each separate version uses the same set of signing keys.

Multiple key and signing policies can be configured. To attach a policy to a zone, add a dnssec-policy option to the zone statement, specifying the name of the policy that should be used.

Key rollover timing is computed for each key according to the key lifetime defined in the KASP. The lifetime may be modified by zone TTLs and propagation delays, to prevent validation failures. When a key reaches the end of its lifetime, named generates and publishes a new key automatically, then deactivates the old key and activates the new one; finally, the old key is retired according to a computed schedule.

Zone-signing key (ZSK) rollovers require no operator input. Key-signing key (KSK) and combined-signing key (CSK) rollovers require action to be taken to submit a DS record to the parent. Rollover timing for KSKs and CSKs is adjusted to take into account delays in processing and propagating DS updates.

There are two predefined dnssec-policy names: none and default. Setting a zone’s policy to none is the same as not setting dnssec-policy at all; the zone is not signed. Policy default causes the zone to be signed with a single combined-signing key (CSK) using algorithm ECDSAP256SHA256; this key has an unlimited lifetime. (A verbose copy of this policy may be found in the source tree, in the file doc/misc/dnssec-policy.default.conf.)

Note

The default signing policy may change in future releases. This could require changes to a signing policy when upgrading to a new version of BIND. Check the release notes carefully when upgrading to be informed of such changes. To prevent policy changes on upgrade, use an explicitly defined dnssec-policy, rather than default.

If a dnssec-policy statement is modified and the server restarted or reconfigured, named attempts to change the policy smoothly from the old one to the new. For example, if the key algorithm is changed, then a new key is generated with the new algorithm, and the old algorithm is retired when the existing key’s lifetime ends.

Note

Rolling to a new policy while another key rollover is already in progress is not yet supported, and may result in unexpected behavior.

The following options can be specified in a dnssec-policy statement:

dnskey-ttl

This indicates the TTL to use when generating DNSKEY resource records. The default is 1 hour (3600 seconds).

keys

This is a list specifying the algorithms and roles to use when generating keys and signing the zone. Entries in this list do not represent specific DNSSEC keys, which may be changed on a regular basis, but the roles that keys play in the signing policy. For example, configuring a KSK of algorithm RSASHA256 ensures that the DNSKEY RRset always includes a key-signing key for that algorithm.

Here is an example (for illustration purposes only) of some possible entries in a keys list:

keys {
    ksk key-directory lifetime unlimited algorithm rsasha1 2048;
    zsk lifetime P30D algorithm 8;
    csk lifetime P6MT12H3M15S algorithm ecdsa256;
};

This example specifies that three keys should be used in the zone. The first token determines which role the key plays in signing RRsets. If set to ksk, then this is a key-signing key; it has the KSK flag set and is only used to sign DNSKEY, CDS, and CDNSKEY RRsets. If set to zsk, this is a zone-signing key; the KSK flag is unset, and the key signs all RRsets except DNSKEY, CDS, and CDNSKEY. If set to csk, the key has the KSK flag set and is used to sign all RRsets.

An optional second token determines where the key is stored. Currently, keys can only be stored in the configured key-directory. This token may be used in the future to store keys in hardware security modules or separate directories.

The lifetime parameter specifies how long a key may be used before rolling over. In the example above, the first key has an unlimited lifetime, the second key may be used for 30 days, and the third key has a rather peculiar lifetime of 6 months, 12 hours, 3 minutes, and 15 seconds. A lifetime of 0 seconds is the same as unlimited.

Note that the lifetime of a key may be extended if retiring it too soon would cause validation failures. For example, if the key were configured to roll more frequently than its own TTL, its lifetime would automatically be extended to account for this.

The algorithm parameter specifies the key’s algorithm, expressed either as a string (“rsasha256”, “ecdsa384”, etc.) or as a decimal number. An optional second parameter specifies the key’s size in bits. If it is omitted, as shown in the example for the second and third keys, an appropriate default size for the algorithm is used.

purge-keys

This is the time after when DNSSEC keys that have been deleted from the zone can be removed from disk. If a key still determined to have presence (for example in some resolver cache), named will not remove the key files.

The default is P90D (90 days). Set this option to 0 to never purge deleted keys.

publish-safety

This is a margin that is added to the pre-publication interval in rollover timing calculations, to give some extra time to cover unforeseen events. This increases the time between when keys are published and when they become active. The default is PT1H (1 hour).

retire-safety

This is a margin that is added to the post-publication interval in rollover timing calculations, to give some extra time to cover unforeseen events. This increases the time a key remains published after it is no longer active. The default is PT1H (1 hour).

signatures-refresh

This determines how frequently an RRSIG record needs to be refreshed. The signature is renewed when the time until the expiration time is less than the specified interval. The default is P5D (5 days), meaning signatures that expire in 5 days or sooner are refreshed.

signatures-validity

This indicates the validity period of an RRSIG record (subject to inception offset and jitter). The default is P2W (2 weeks).

signatures-validity-dnskey

This is similar to signatures-validity, but for DNSKEY records. The default is P2W (2 weeks).

max-zone-ttl

Like the max-zone-ttl zone option, this specifies the maximum permissible TTL value, in seconds, for the zone.

This is needed in DNSSEC-maintained zones because when rolling to a new DNSKEY, the old key needs to remain available until RRSIG records have expired from caches. The max-zone-ttl option guarantees that the largest TTL in the zone is no higher than the set value.

The default value is PT24H (24 hours). A max-zone-ttl of zero is treated as if the default value were in use.

nsec3param

Use NSEC3 instead of NSEC, and optionally set the NSEC3 parameters.

Here is an example of an nsec3 configuration:

nsec3param iterations 0 optout no salt-length 0;

The default is to use NSEC. The iterations, optout and salt-length parts are optional, but if not set, the values in the example above are the default NSEC3 parameters. Note that you don’t specify a specific salt string, named will create a salt for you of the provided salt length.

zone-propagation-delay

This is the expected propagation delay from the time when a zone is first updated to the time when the new version of the zone is served by all secondary servers. The default is PT5M (5 minutes).

parent-ds-ttl

This is the TTL of the DS RRset that the parent zone uses. The default is P1D (1 day).

parent-propagation-delay

This is the expected propagation delay from the time when the parent zone is updated to the time when the new version is served by all of the parent zone’s name servers. The default is PT1H (1 hour).

4.2.28.1. Automated KSK Rollovers

BIND has mechanisms in place to facilitate automated KSK rollovers. It publishes CDS and CDNSKEY records that can be used by the parent zone to publish or withdraw the zone’s DS records. BIND will query the parental agents to see if the new DS is actually published before withdrawing the old DNSSEC key.

Note

The DS response is not validated so it is recommended to set up a trust relationship with the parental agent. For example, use TSIG to authenticate the parental agent, or point to a validating resolver.

The following options apply to DS queries sent to parental-agents:

parental-source

parental-source determines which local source address, and optionally UDP port, is used to send parental DS queries. This address must appear in the secondary server’s parental-agents zone clause. This statement sets the parental-source for all zones, but can be overridden on a per-zone or per-view basis by including a parental-source statement within the zone or view block in the configuration file.

Note

Solaris 2.5.1 and earlier does not support setting the source address for TCP sockets.

Warning

Specifying a single port is discouraged, as it removes a layer of protection against spoofing errors.

Warning

The configured port must not be same as the listening port.

parental-source-v6

This option acts like parental-source, but applies to parental DS queries sent to IPv6 addresses.

4.2.29. managed-keys Statement Grammar

4.2.30. managed-keys Statement Definition and Usage

The managed-keys statement has been deprecated in favor of trust-anchors Statement Grammar with the initial-key keyword.

4.2.31. trusted-keys Statement Grammar

4.2.32. trusted-keys Statement Definition and Usage

The trusted-keys statement has been deprecated in favor of trust-anchors Statement Grammar with the static-key keyword.

4.2.33. view Statement Grammar

view view_name [ class ] {
    match-clients { address_match_list } ;
    match-destinations { address_match_list } ;
    match-recursive-only yes_or_no ;
  [ view_option ; ... ]
  [ zone_statement ; ... ]
} ;

4.2.34. view Statement Definition and Usage

The view statement is a powerful feature of BIND 9 that lets a name server answer a DNS query differently depending on who is asking. It is particularly useful for implementing split DNS setups without having to run multiple servers.

Each view statement defines a view of the DNS namespace that is seen by a subset of clients. A client matches a view if its source IP address matches the address_match_list of the view’s match-clients clause, and its destination IP address matches the address_match_list of the view’s match-destinations clause. If not specified, both match-clients and match-destinations default to matching all addresses. In addition to checking IP addresses, match-clients and match-destinations can also take keys which provide an mechanism for the client to select the view. A view can also be specified as match-recursive-only, which means that only recursive requests from matching clients match that view. The order of the view statements is significant; a client request is resolved in the context of the first view that it matches.

Zones defined within a view statement are only accessible to clients that match the view. By defining a zone of the same name in multiple views, different zone data can be given to different clients: for example, “internal” and “external” clients in a split DNS setup.

Many of the options given in the options statement can also be used within a view statement, and then apply only when resolving queries with that view. When no view-specific value is given, the value in the options statement is used as a default. Also, zone options can have default values specified in the view statement; these view-specific defaults take precedence over those in the options statement.

Views are class-specific. If no class is given, class IN is assumed. Note that all non-IN views must contain a hint zone, since only the IN class has compiled-in default hints.

If there are no view statements in the config file, a default view that matches any client is automatically created in class IN. Any zone statements specified on the top level of the configuration file are considered to be part of this default view, and the options statement applies to the default view. If any explicit view statements are present, all zone statements must occur inside view statements.

Here is an example of a typical split DNS setup implemented using view statements:

view "internal" {
      // This should match our internal networks.
      match-clients { 10.0.0.0/8; };

      // Provide recursive service to internal
      // clients only.
      recursion yes;

      // Provide a complete view of the example.com
      // zone including addresses of internal hosts.
      zone "example.com" {
        type primary;
        file "example-internal.db";
      };
};

view "external" {
      // Match all clients not matched by the
      // previous view.
      match-clients { any; };

      // Refuse recursive service to external clients.
      recursion no;

      // Provide a restricted view of the example.com
      // zone containing only publicly accessible hosts.
      zone "example.com" {
       type primary;
       file "example-external.db";
      };
};

4.2.35. zone Statement Grammar

4.2.36. zone Statement Definition and Usage

4.2.36.1. Zone Types

The type keyword is required for the zone configuration unless it is an in-view configuration. Its acceptable values are: primary (or master), secondary (or slave), mirror, hint, stub, static-stub, forward, redirect, or delegation-only.

primary

A primary zone has a master copy of the data for the zone and is able to provide authoritative answers for it. Type master is a synonym for primary.

secondary

A secondary zone is a replica of a primary zone. Type slave is a synonym for secondary. The primaries list specifies one or more IP addresses of primary servers that the secondary contacts to update its copy of the zone. Primaries list elements can also be names of other primaries lists. By default, transfers are made from port 53 on the servers; this can be changed for all servers by specifying a port number before the list of IP addresses, or on a per-server basis after the IP address. Authentication to the primary can also be done with per-server TSIG keys. If a file is specified, then the replica is written to this file whenever the zone is changed, and reloaded from this file on a server restart. Use of a file is recommended, since it often speeds server startup and eliminates a needless waste of bandwidth. Note that for large numbers (in the tens or hundreds of thousands) of zones per server, it is best to use a two-level naming scheme for zone filenames. For example, a secondary server for the zone example.com might place the zone contents into a file called ex/example.com, where ex/ is just the first two letters of the zone name. (Most operating systems behave very slowly if there are 100000 files in a single directory.)

mirror

A mirror zone is similar to a zone of type secondary, except its data is subject to DNSSEC validation before being used in answers. Validation is applied to the entire zone during the zone transfer process, and again when the zone file is loaded from disk upon restarting named. If validation of a new version of a mirror zone fails, a retransfer is scheduled; in the meantime, the most recent correctly validated version of that zone is used until it either expires or a newer version validates correctly. If no usable zone data is available for a mirror zone, due to either transfer failure or expiration, traditional DNS recursion is used to look up the answers instead. Mirror zones cannot be used in a view that does not have recursion enabled.

Answers coming from a mirror zone look almost exactly like answers from a zone of type secondary, with the notable exceptions that the AA bit (“authoritative answer”) is not set, and the AD bit (“authenticated data”) is.

Mirror zones are intended to be used to set up a fast local copy of the root zone (see RFC 8806). A default list of primary servers for the IANA root zone is built into named, so its mirroring can be enabled using the following configuration:

zone "." {
    type mirror;
};

Mirror zone validation always happens for the entire zone contents. This ensures that each version of the zone used by the resolver is fully self-consistent with respect to DNSSEC. For incoming mirror zone IXFRs, every revision of the zone contained in the IXFR sequence is validated independently, in the order in which the zone revisions appear on the wire. For this reason, it might be useful to force use of AXFR for mirror zones by setting request-ixfr no; for the relevant zone (or view). Other, more efficient zone verification methods may be added in the future.

To make mirror zone contents persist between named restarts, use the file option.

Mirroring a zone other than root requires an explicit list of primary servers to be provided using the primaries option (see primaries Statement Grammar for details), and a key-signing key (KSK) for the specified zone to be explicitly configured as a trust anchor (see trust-anchors Statement Definition and Usage).

When configuring NOTIFY for a mirror zone, only notify no; and notify explicit; can be used at the zone level; any other notify setting at the zone level is a configuration error. Using any other notify setting at the options or view level causes that setting to be overridden with notify explicit; for the mirror zone. The global default for the notify option is yes, so mirror zones are by default configured with notify explicit;.

Outgoing transfers of mirror zones are disabled by default but may be enabled using allow-transfer.

Note

Use of this zone type with any zone other than the root should be considered experimental and may cause performance issues, especially for zones that are large and/or frequently updated.

hint

The initial set of root name servers is specified using a hint zone. When the server starts, it uses the root hints to find a root name server and get the most recent list of root name servers. If no hint zone is specified for class IN, the server uses a compiled-in default set of root servers hints. Classes other than IN have no built-in default hints.

stub

A stub zone is similar to a secondary zone, except that it replicates only the NS records of a primary zone instead of the entire zone. Stub zones are not a standard part of the DNS; they are a feature specific to the BIND implementation.

Stub zones can be used to eliminate the need for a glue NS record in a parent zone, at the expense of maintaining a stub zone entry and a set of name server addresses in named.conf. This usage is not recommended for new configurations, and BIND 9 supports it only in a limited way. If a BIND 9 primary, serving a parent zone, has child stub zones configured, all the secondary servers for the parent zone also need to have the same child stub zones configured.

Stub zones can also be used as a way to force the resolution of a given domain to use a particular set of authoritative servers. For example, the caching name servers on a private network using RFC 1918 addressing may be configured with stub zones for 10.in-addr.arpa to use a set of internal name servers as the authoritative servers for that domain.

static-stub

A static-stub zone is similar to a stub zone, with the following exceptions: the zone data is statically configured, rather than transferred from a primary server; and when recursion is necessary for a query that matches a static-stub zone, the locally configured data (name server names and glue addresses) is always used, even if different authoritative information is cached.

Zone data is configured via the server-addresses and server-names zone options.

The zone data is maintained in the form of NS and (if necessary) glue A or AAAA RRs internally, which can be seen by dumping zone databases with rndc dumpdb -all. The configured RRs are considered local configuration parameters rather than public data. Non-recursive queries (i.e., those with the RD bit off) to a static-stub zone are therefore prohibited and are responded to with REFUSED.

Since the data is statically configured, no zone maintenance action takes place for a static-stub zone. For example, there is no periodic refresh attempt, and an incoming notify message is rejected with an rcode of NOTAUTH.

Each static-stub zone is configured with internally generated NS and (if necessary) glue A or AAAA RRs.

forward

A forward zone is a way to configure forwarding on a per-domain basis. A zone statement of type forward can contain a forward and/or forwarders statement, which applies to queries within the domain given by the zone name. If no forwarders statement is present, or an empty list for forwarders is given, then no forwarding is done for the domain, canceling the effects of any forwarders in the options statement. Thus, to use this type of zone to change the behavior of the global forward option (that is, “forward first” to, then “forward only”, or vice versa), but use the same servers as set globally, re-specify the global forwarders.

redirect

Redirect zones are used to provide answers to queries when normal resolution would result in NXDOMAIN being returned. Only one redirect zone is supported per view. allow-query can be used to restrict which clients see these answers.

If the client has requested DNSSEC records (DO=1) and the NXDOMAIN response is signed, no substitution occurs.

To redirect all NXDOMAIN responses to 100.100.100.2 and 2001:ffff:ffff::100.100.100.2, configure a type redirect zone named “.”, with the zone file containing wildcard records that point to the desired addresses: *. IN A 100.100.100.2 and *. IN AAAA 2001:ffff:ffff::100.100.100.2.

As another example, to redirect all Spanish names (under .ES), use similar entries but with the names *.ES. instead of *.. To redirect all commercial Spanish names (under COM.ES), use wildcard entries called *.COM.ES..

Note that the redirect zone supports all possible types; it is not limited to A and AAAA records.

If a redirect zone is configured with a primaries option, then it is transferred in as if it were a secondary zone. Otherwise, it is loaded from a file as if it were a primary zone.

Because redirect zones are not referenced directly by name, they are not kept in the zone lookup table with normal primary and secondary zones. To reload a redirect zone, use rndc reload -redirect; to retransfer a redirect zone configured as a secondary, use rndc retransfer -redirect. When using rndc reload without specifying a zone name, redirect zones are reloaded along with other zones.

delegation-only

This zone type is used to enforce the delegation-only status of infrastructure zones (e.g., COM, NET, ORG). Any answer that is received without an explicit or implicit delegation in the authority section is treated as NXDOMAIN. This does not apply to the zone apex, and should not be applied to leaf zones.

delegation-only has no effect on answers received from forwarders.

See caveats in root-delegation-only.

in-view

When using multiple views, a primary or secondary zone configured in one view can be referenced in a subsequent view. This allows both views to use the same zone without the overhead of loading it more than once. This is configured using a zone statement, with an in-view option specifying the view in which the zone is defined. A zone statement containing in-view does not need to specify a type, since that is part of the zone definition in the other view.

See Multiple Views for more information.

4.2.36.2. Class

The zone’s name may optionally be followed by a class. If a class is not specified, class IN (for Internet) is assumed. This is correct for the vast majority of cases.

The hesiod class is named for an information service from MIT’s Project Athena. It was used to share information about various systems databases, such as users, groups, printers, and so on. The keyword HS is a synonym for hesiod.

Another MIT development is Chaosnet, a LAN protocol created in the mid-1970s. Zone data for it can be specified with the CHAOS class.

4.2.36.3. Zone Options

allow-notify

See the description of allow-notify in Access Control.

allow-query

See the description of allow-query in Access Control.

allow-query-on

See the description of allow-query-on in Access Control.

allow-transfer

See the description of allow-transfer in Access Control.

allow-update

See the description of allow-update in Access Control.

update-policy

This specifies a “Simple Secure Update” policy. See Dynamic Update Policies.

allow-update-forwarding

See the description of allow-update-forwarding in Access Control.

also-notify

This option is only meaningful if notify is active for this zone. The set of machines that receive a DNS NOTIFY message for this zone is made up of all the listed name servers (other than the primary) for the zone, plus any IP addresses specified with also-notify. A port may be specified with each also-notify address to send the notify messages to a port other than the default of 53. A TSIG key may also be specified to cause the NOTIFY to be signed by the given key. also-notify is not meaningful for stub zones. The default is the empty list.

check-names

This option is used to restrict the character set and syntax of certain domain names in primary files and/or DNS responses received from the network. The default varies according to zone type. For primary zones the default is fail; for secondary zones the default is warn. It is not implemented for hint zones.

check-mx

See the description of check-mx in Boolean Options.

check-spf

See the description of check-spf in Boolean Options.

check-wildcard

See the description of check-wildcard in Boolean Options.

check-integrity

See the description of check-integrity in Boolean Options.

check-sibling

See the description of check-sibling in Boolean Options.

zero-no-soa-ttl

See the description of zero-no-soa-ttl in Boolean Options.

update-check-ksk

See the description of update-check-ksk in Boolean Options.

dnssec-loadkeys-interval

See the description of dnssec-loadkeys-interval in options Statement Definition and Usage.

dnssec-update-mode

See the description of dnssec-update-mode in options Statement Definition and Usage.

dnssec-dnskey-kskonly

See the description of dnssec-dnskey-kskonly in Boolean Options.

try-tcp-refresh

See the description of try-tcp-refresh in Boolean Options.

database

This specifies the type of database to be used to store the zone data. The string following the database keyword is interpreted as a list of whitespace-delimited words. The first word identifies the database type, and any subsequent words are passed as arguments to the database to be interpreted in a way specific to the database type.

The default is rbt, BIND 9’s native in-memory red-black tree database. This database does not take arguments.

Other values are possible if additional database drivers have been linked into the server. Some sample drivers are included with the distribution but none are linked in by default.

dialup

See the description of dialup in Boolean Options.

delegation-only

This flag only applies to forward, hint, and stub zones. If set to yes, then the zone is treated as if it is also a delegation-only type zone.

See caveats in root-delegation-only.

file

This sets the zone’s filename. In primary, hint, and redirect zones which do not have primaries defined, zone data is loaded from this file. In secondary, mirror, stub, and redirect zones which do have primaries defined, zone data is retrieved from another server and saved in this file. This option is not applicable to other zone types.

forward

This option is only meaningful if the zone has a forwarders list. The only value causes the lookup to fail after trying the forwarders and getting no answer, while first allows a normal lookup to be tried.

forwarders

This is used to override the list of global forwarders. If it is not specified in a zone of type forward, no forwarding is done for the zone and the global options are not used.

journal

This allows the default journal’s filename to be overridden. The default is the zone’s filename with “.jnl” appended. This is applicable to primary and secondary zones.

max-ixfr-ratio

See the description of max-ixfr-ratio in options Statement Definition and Usage.

max-journal-size

See the description of max-journal-size in Server Resource Limits.

max-records

See the description of max-records in Server Resource Limits.

max-transfer-time-in

See the description of max-transfer-time-in in Zone Transfers.

max-transfer-idle-in

See the description of max-transfer-idle-in in Zone Transfers.

max-transfer-time-out

See the description of max-transfer-time-out in Zone Transfers.

max-transfer-idle-out

See the description of max-transfer-idle-out in Zone Transfers.

notify

See the description of notify in Boolean Options.

notify-delay

See the description of notify-delay in Tuning.

notify-to-soa

See the description of notify-to-soa in Boolean Options.

zone-statistics

See the description of zone-statistics in options Statement Definition and Usage.

server-addresses

This option is only meaningful for static-stub zones. This is a list of IP addresses to which queries should be sent in recursive resolution for the zone. A non-empty list for this option internally configures the apex NS RR with associated glue A or AAAA RRs.

For example, if “example.com” is configured as a static-stub zone with 192.0.2.1 and 2001:db8::1234 in a server-addresses option, the following RRs are internally configured:

example.com. NS example.com.
example.com. A 192.0.2.1
example.com. AAAA 2001:db8::1234

These records are used internally to resolve names under the static-stub zone. For instance, if the server receives a query for “www.example.com” with the RD bit on, the server initiates recursive resolution and sends queries to 192.0.2.1 and/or 2001:db8::1234.

server-names

This option is only meaningful for static-stub zones. This is a list of domain names of name servers that act as authoritative servers of the static-stub zone. These names are resolved to IP addresses when named needs to send queries to these servers. For this supplemental resolution to be successful, these names must not be a subdomain of the origin name of the static-stub zone. That is, when “example.net” is the origin of a static-stub zone, “ns.example” and “master.example.com” can be specified in the server-names option, but “ns.example.net” cannot; it is rejected by the configuration parser.

A non-empty list for this option internally configures the apex NS RR with the specified names. For example, if “example.com” is configured as a static-stub zone with “ns1.example.net” and “ns2.example.net” in a server-names option, the following RRs are internally configured:

example.com. NS ns1.example.net.
example.com. NS ns2.example.net.

These records are used internally to resolve names under the static-stub zone. For instance, if the server receives a query for “www.example.com” with the RD bit on, the server initiates recursive resolution, resolves “ns1.example.net” and/or “ns2.example.net” to IP addresses, and then sends queries to one or more of these addresses.

sig-validity-interval

See the description of sig-validity-interval in Tuning.

sig-signing-nodes

See the description of sig-signing-nodes in Tuning.

sig-signing-signatures

See the description of sig-signing-signatures in Tuning.

sig-signing-type

See the description of sig-signing-type in Tuning.

transfer-source

See the description of transfer-source in Zone Transfers.

transfer-source-v6

See the description of transfer-source-v6 in Zone Transfers.

alt-transfer-source

See the description of alt-transfer-source in Zone Transfers.

alt-transfer-source-v6

See the description of alt-transfer-source-v6 in Zone Transfers.

use-alt-transfer-source

See the description of use-alt-transfer-source in Zone Transfers.

notify-source

See the description of notify-source in Zone Transfers.

notify-source-v6

See the description of notify-source-v6 in Zone Transfers.

min-refresh-time; max-refresh-time; min-retry-time; max-retry-time

See the descriptions in Tuning.

ixfr-from-differences

See the description of ixfr-from-differences in Boolean Options. (Note that the ixfr-from-differences choices of primary and secondary are not available at the zone level.)

key-directory

See the description of key-directory in options Statement Definition and Usage.

auto-dnssec

See the description of auto-dnssec in options Statement Definition and Usage.

serial-update-method

See the description of serial-update-method in options Statement Definition and Usage.

inline-signing

If yes, this enables “bump in the wire” signing of a zone, where an unsigned zone is transferred in or loaded from disk and a signed version of the zone is served with, possibly, a different serial number. This behavior is disabled by default.

multi-master

See the description of multi-master in Boolean Options.

masterfile-format

See the description of masterfile-format in Tuning.

max-zone-ttl

See the description of max-zone-ttl in options Statement Definition and Usage.

dnssec-secure-to-insecure

See the description of dnssec-secure-to-insecure in Boolean Options.

4.2.36.4. Dynamic Update Policies

BIND 9 supports two methods of granting clients the right to perform dynamic updates to a zone, configured by the allow-update or update-policy options.

The allow-update clause is a simple access control list. Any client that matches the ACL is granted permission to update any record in the zone.

The update-policy clause allows more fine-grained control over which updates are allowed. It specifies a set of rules, in which each rule either grants or denies permission for one or more names in the zone to be updated by one or more identities. Identity is determined by the key that signed the update request, using either TSIG or SIG(0). In most cases, update-policy rules only apply to key-based identities. There is no way to specify update permissions based on the client source address.

update-policy rules are only meaningful for zones of type primary, and are not allowed in any other zone type. It is a configuration error to specify both allow-update and update-policy at the same time.

A pre-defined update-policy rule can be switched on with the command update-policy local;. named automatically generates a TSIG session key when starting and stores it in a file; this key can then be used by local clients to update the zone while named is running. By default, the session key is stored in the file /var/run/named/session.key, the key name is “local-ddns”, and the key algorithm is HMAC-SHA256. These values are configurable with the session-keyfile, session-keyname, and session-keyalg options, respectively. A client running on the local system, if run with appropriate permissions, may read the session key from the key file and use it to sign update requests. The zone’s update policy is set to allow that key to change any record within the zone. Assuming the key name is “local-ddns”, this policy is equivalent to:

update-policy { grant local-ddns zonesub any; };

with the additional restriction that only clients connecting from the local system are permitted to send updates.

Note that only one session key is generated by named; all zones configured to use update-policy local accept the same key.

The command nsupdate -l implements this feature, sending requests to localhost and signing them using the key retrieved from the session key file.

Other rule definitions look like this:

( grant | deny ) identity ruletype  name   types

Each rule grants or denies privileges. Rules are checked in the order in which they are specified in the update-policy statement. Once a message has successfully matched a rule, the operation is immediately granted or denied, and no further rules are examined. There are 16 types of rules; the rule type is specified by the ruletype field, and the interpretation of other fields varies depending on the rule type.

In general, a rule is matched when the key that signed an update request matches the identity field, the name of the record to be updated matches the name field (in the manner specified by the ruletype field), and the type of the record to be updated matches the types field. Details for each rule type are described below.

The identity field must be set to a fully qualified domain name. In most cases, this represents the name of the TSIG or SIG(0) key that must be used to sign the update request. If the specified name is a wildcard, it is subject to DNS wildcard expansion, and the rule may apply to multiple identities. When a TKEY exchange has been used to create a shared secret, the identity of the key used to authenticate the TKEY exchange is used as the identity of the shared secret. Some rule types use identities matching the client’s Kerberos principal (e.g, "host/machine@REALM") or Windows realm (machine$@REALM).

The name field also specifies a fully qualified domain name. This often represents the name of the record to be updated. Interpretation of this field is dependent on rule type.

If no types are explicitly specified, then a rule matches all types except RRSIG, NS, SOA, NSEC, and NSEC3. Types may be specified by name, including ANY; ANY matches all types except NSEC and NSEC3, which can never be updated. Note that when an attempt is made to delete all records associated with a name, the rules are checked for each existing record type.

If the type is immediately followed by a number in parentheses, that number is the maximum number of records of that type permitted to exist in the RRset after an update has been applied. For example, PTR(1) indicates that only one PTR record is allowed. If an attempt is made to add two PTR records in an update, the second one is silently discarded. If a PTR record already exists, both new records are silently discarded.

If type ANY is specified with a limit, then that limit applies to all types that are not otherwise specified. For example, A PTR(1) ANY(2) indicates that an unlimited number of A records can exist, but only one PTR record, and no more than two of any other type.

Typical use with a rule grant * tcp-self . PTR(1); in the zone 2.0.192.IN-ADDR.ARPA looks like this:

nsupdate -v <<EOF
local 192.0.2.1
del 1.2.0.192.IN-ADDR.ARPA PTR
add 1.2.0.192.IN-ADDR.ARPA 0 PTR EXAMPLE.COM
send
EOF

The ruletype field has 20 values: name, subdomain, zonesub, wildcard, self, selfsub, selfwild, ms-self, ms-selfsub, ms-subdomain, ms-subdomain-self-rhs, krb5-self, krb5-selfsub, krb5-subdomain, krb5-subdomain-self-rhs, tcp-self, 6to4-self, and external.

name

With exact-match semantics, this rule matches when the name being updated is identical to the contents of the name field.

subdomain

This rule matches when the name being updated is a subdomain of, or identical to, the contents of the name field.

zonesub

This rule is similar to subdomain, except that it matches when the name being updated is a subdomain of the zone in which the update-policy statement appears. This obviates the need to type the zone name twice, and enables the use of a standard update-policy statement in multiple zones without modification. When this rule is used, the name field is omitted.

wildcard

The name field is subject to DNS wildcard expansion, and this rule matches when the name being updated is a valid expansion of the wildcard.

self

This rule matches when the name of the record being updated matches the contents of the identity field. The name field is ignored. To avoid confusion, it is recommended that this field be set to the same value as the identity field or to “.” The self rule type is most useful when allowing one key per name to update, where the key has the same name as the record to be updated. In this case, the identity field can be specified as * (asterisk).

selfsub

This rule is similar to self, except that subdomains of self can also be updated.

selfwild

This rule is similar to self, except that only subdomains of self can be updated.

ms-self

When a client sends an UPDATE using a Windows machine principal (for example, machine$@REALM), this rule allows records with the absolute name of machine.REALM to be updated.

The realm to be matched is specified in the identity field.

The name field has no effect on this rule; it should be set to “.” as a placeholder.

For example, grant EXAMPLE.COM ms-self . A AAAA allows any machine with a valid principal in the realm EXAMPLE.COM to update its own address records.

ms-selfsub

This is similar to ms-self, except it also allows updates to any subdomain of the name specified in the Windows machine principal, not just to the name itself.

ms-subdomain

When a client sends an UPDATE using a Windows machine principal (for example, machine$@REALM), this rule allows any machine in the specified realm to update any record in the zone or in a specified subdomain of the zone.

The realm to be matched is specified in the identity field.

The name field specifies the subdomain that may be updated. If set to “.” or any other name at or above the zone apex, any name in the zone can be updated.

For example, if update-policy for the zone “example.com” includes grant EXAMPLE.COM ms-subdomain hosts.example.com. AA AAAA, any machine with a valid principal in the realm EXAMPLE.COM is able to update address records at or below hosts.example.com.

ms-subdomain-self-rhs

This rule is similar to ms-subdomain, with an additional restriction that PTR and SRV target names must match the name of the machine identified in the principal.

krb5-self

When a client sends an UPDATE using a Kerberos machine principal (for example, host/machine@REALM), this rule allows records with the absolute name of machine to be updated, provided it has been authenticated by REALM. This is similar but not identical to ms-self, due to the machine part of the Kerberos principal being an absolute name instead of an unqualified name.

The realm to be matched is specified in the identity field.

The name field has no effect on this rule; it should be set to “.” as a placeholder.

For example, grant EXAMPLE.COM krb5-self . A AAAA allows any machine with a valid principal in the realm EXAMPLE.COM to update its own address records.

krb5-selfsub

This is similar to krb5-self, except it also allows updates to any subdomain of the name specified in the machine part of the Kerberos principal, not just to the name itself.

krb5-subdomain

This rule is identical to ms-subdomain, except that it works with Kerberos machine principals (i.e., host/machine@REALM) rather than Windows machine principals.

krb5-subdomain-self-rhs

This rule is similar to krb5-subdomain, with an additional restriction that PTR and SRV target names must match the name of the machine identified in the principal.

tcp-self

This rule allows updates that have been sent via TCP and for which the standard mapping from the client’s IP address into the in-addr.arpa and ip6.arpa namespaces matches the name to be updated. The identity field must match that name. The name field should be set to “.”. Note that, since identity is based on the client’s IP address, it is not necessary for update request messages to be signed.

Note

It is theoretically possible to spoof these TCP sessions.

6to4-self

This allows the name matching a 6to4 IPv6 prefix, as specified in RFC 3056, to be updated by any TCP connection from either the 6to4 network or from the corresponding IPv4 address. This is intended to allow NS or DNAME RRsets to be added to the ip6.arpa reverse tree.

The identity field must match the 6to4 prefix in ip6.arpa. The name field should be set to “.”. Note that, since identity is based on the client’s IP address, it is not necessary for update request messages to be signed.

In addition, if specified for an ip6.arpa name outside of the 2.0.0.2.ip6.arpa namespace, the corresponding /48 reverse name can be updated. For example, TCP/IPv6 connections from 2001:DB8:ED0C::/48 can update records at C.0.D.E.8.B.D.0.1.0.0.2.ip6.arpa.

Note

It is theoretically possible to spoof these TCP sessions.

external

This rule allows named to defer the decision of whether to allow a given update to an external daemon.

The method of communicating with the daemon is specified in the identity field, the format of which is “local:path”, where “path” is the location of a Unix-domain socket. (Currently, “local” is the only supported mechanism.)

Requests to the external daemon are sent over the Unix-domain socket as datagrams with the following format:

Protocol version number (4 bytes, network byte order, currently 1)
Request length (4 bytes, network byte order)
Signer (null-terminated string)
Name (null-terminated string)
TCP source address (null-terminated string)
Rdata type (null-terminated string)
Key (null-terminated string)
TKEY token length (4 bytes, network byte order)
TKEY token (remainder of packet)

The daemon replies with a four-byte value in network byte order, containing either 0 or 1; 0 indicates that the specified update is not permitted, and 1 indicates that it is.

4.2.36.5. Multiple Views

When multiple views are in use, a zone may be referenced by more than one of them. Often, the views contain different zones with the same name, allowing different clients to receive different answers for the same queries. At times, however, it is desirable for multiple views to contain identical zones. The in-view zone option provides an efficient way to do this; it allows a view to reference a zone that was defined in a previously configured view. For example:

view internal {
    match-clients { 10/8; };

    zone example.com {
    type primary;
    file "example-external.db";
    };
};

view external {
    match-clients { any; };

    zone example.com {
    in-view internal;
    };
};

An in-view option cannot refer to a view that is configured later in the configuration file.

A zone statement which uses the in-view option may not use any other options, with the exception of forward and forwarders. (These options control the behavior of the containing view, rather than change the zone object itself.)

Zone-level ACLs (e.g., allow-query, allow-transfer), and other configuration details of the zone, are all set in the view the referenced zone is defined in. Be careful to ensure that ACLs are wide enough for all views referencing the zone.

An in-view zone cannot be used as a response policy zone.

An in-view zone is not intended to reference a forward zone.

4.3. Zone File

4.3.1. Types of Resource Records and When to Use Them

This section, largely borrowed from RFC 1034, describes the concept of a Resource Record (RR) and explains when each type is used. Since the publication of RFC 1034, several new RRs have been identified and implemented in the DNS. These are also included.

4.3.1.1. Resource Records

A domain name identifies a node. Each node has a set of resource information, which may be empty. The set of resource information associated with a particular name is composed of separate RRs. The order of RRs in a set is not significant and need not be preserved by name servers, resolvers, or other parts of the DNS. However, sorting of multiple RRs is permitted for optimization purposes: for example, to specify that a particular nearby server be tried first. See The sortlist Statement and RRset Ordering.

The components of a Resource Record are:

owner name

The domain name where the RR is found.

type

An encoded 16-bit value that specifies the type of the resource record.

TTL

The time-to-live of the RR. This field is a 32-bit integer in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be discarded.

class

An encoded 16-bit value that identifies a protocol family or an instance of a protocol.

RDATA

The resource data. The format of the data is type- and sometimes class-specific.

For a complete list of types of valid RRs, including those that have been obsoleted, please refer to https://en.wikipedia.org/wiki/List_of_DNS_record_types.

The following classes of resource records are currently valid in the DNS:

IN

The Internet.

CH

Chaosnet, a LAN protocol created at MIT in the mid-1970s. It was rarely used for its historical purpose, but was reused for BIND’s built-in server information zones, e.g., version.bind.

HS

Hesiod, an information service developed by MIT’s Project Athena. It was used to share information about various systems databases, such as users, groups, printers, etc.

The owner name is often implicit, rather than forming an integral part of the RR. For example, many name servers internally form tree or hash structures for the name space, and chain RRs off nodes. The remaining RR parts are the fixed header (type, class, TTL), which is consistent for all RRs, and a variable part (RDATA) that fits the needs of the resource being described.

The TTL field is a time limit on how long an RR can be kept in a cache. This limit does not apply to authoritative data in zones; that also times out, but follows the refreshing policies for the zone. The TTL is assigned by the administrator for the zone where the data originates. While short TTLs can be used to minimize caching, and a zero TTL prohibits caching, the realities of Internet performance suggest that these times should be on the order of days for the typical host. If a change is anticipated, the TTL can be reduced prior to the change to minimize inconsistency, and then increased back to its former value following the change.

The data in the RDATA section of RRs is carried as a combination of binary strings and domain names. The domain names are frequently used as “pointers” to other data in the DNS.

4.3.1.2. Textual Expression of RRs

RRs are represented in binary form in the packets of the DNS protocol, and are usually represented in highly encoded form when stored in a name server or resolver. In the examples provided in RFC 1034, a style similar to that used in primary files was employed in order to show the contents of RRs. In this format, most RRs are shown on a single line, although continuation lines are possible using parentheses.

The start of the line gives the owner of the RR. If a line begins with a blank, then the owner is assumed to be the same as that of the previous RR. Blank lines are often included for readability.

Following the owner are listed the TTL, type, and class of the RR. Class and type use the mnemonics defined above, and TTL is an integer before the type field. To avoid ambiguity in parsing, type and class mnemonics are disjoint, TTLs are integers, and the type mnemonic is always last. The IN class and TTL values are often omitted from examples in the interest of clarity.

The resource data or RDATA section of the RR is given using knowledge of the typical representation for the data.

For example, the RRs carried in a message might be shown as:

ISI.EDU.	MX	10 VENERA.ISI.EDU.
	MX	10 VAXA.ISI.EDU
VENERA.ISI.EDU	A	128.9.0.32
	A	10.1.0.52
VAXA.ISI.EDU	A	10.2.0.27
	A	128.9.0.33

The MX RRs have an RDATA section which consists of a 16-bit number followed by a domain name. The address RRs use a standard IP address format to contain a 32-bit Internet address.

The above example shows six RRs, with two RRs at each of three domain names.

Here is another possible example:

XX.LCS.MIT.EDU.	IN A	10.0.0.44
	CH A	MIT.EDU. 2420

This shows two addresses for XX.LCS.MIT.EDU, each of a different class.

4.3.2. Discussion of MX Records

As described above, domain servers store information as a series of resource records, each of which contains a particular piece of information about a given domain name (which is usually, but not always, a host). The simplest way to think of an RR is as a typed pair of data, a domain name matched with a relevant datum and stored with some additional type information, to help systems determine when the RR is relevant.

MX records are used to control delivery of email. The data specified in the record is a priority and a domain name. The priority controls the order in which email delivery is attempted, with the lowest number first. If two priorities are the same, a server is chosen randomly. If no servers at a given priority are responding, the mail transport agent falls back to the next largest priority. Priority numbers do not have any absolute meaning; they are relevant only respective to other MX records for that domain name. The domain name given is the machine to which the mail is delivered. It must have an associated address record (A or AAAA); CNAME is not sufficient.

For a given domain, if there is both a CNAME record and an MX record, the MX record is in error and is ignored. Instead, the mail is delivered to the server specified in the MX record pointed to by the CNAME. For example:

example.com.	IN	MX	10	mail.example.com.
	IN	MX	10	mail2.example.com.
	IN	MX	20	mail.backup.org.
mail.example.com.	IN	A	10.0.0.1
mail2.example.com.	IN	A	10.0.0.2

Mail delivery is attempted to mail.example.com and mail2.example.com (in any order); if neither of those succeeds, delivery to mail.backup.org is attempted.

4.3.3. Setting TTLs

The time-to-live (TTL) of the RR field is a 32-bit integer represented in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long an RR can be cached before it should be discarded. The following three types of TTLs are currently used in a zone file.

SOA

The last field in the SOA is the negative caching TTL. This controls how long other servers cache no-such-domain (NXDOMAIN) responses from this server.

The maximum time for negative caching is 3 hours (3h).

$TTL

The $TTL directive at the top of the zone file (before the SOA) gives a default TTL for every RR without a specific TTL set.

RR TTLs

Each RR can have a TTL as the second field in the RR, which controls how long other servers can cache it.

All of these TTLs default to units of seconds, though units can be explicitly specified: for example, 1h30m.

4.3.4. Inverse Mapping in IPv4

Reverse name resolution (that is, translation from IP address to name) is achieved by means of the in-addr.arpa domain and PTR records. Entries in the in-addr.arpa domain are made in least-to-most significant order, read left to right. This is the opposite order to the way IP addresses are usually written. Thus, a machine with an IP address of 10.1.2.3 would have a corresponding in-addr.arpa name of 3.2.1.10.in-addr.arpa. This name should have a PTR resource record whose data field is the name of the machine or, optionally, multiple PTR records if the machine has more than one name. For example, in the example.com domain:

$ORIGIN	2.1.10.in-addr.arpa
3	IN PTR foo.example.com.

Note

The $ORIGIN line in this example is only to provide context; it does not necessarily appear in the actual usage. It is only used here to indicate that the example is relative to the listed origin.

4.3.5. Other Zone File Directives

The DNS “master file” format was initially defined in RFC 1035 and has subsequently been extended. While the format itself is class-independent, all records in a zone file must be of the same class.

Master file directives include $ORIGIN, $INCLUDE, and $TTL.

4.3.5.1. The @ (at-sign)

When used in the label (or name) field, the asperand or at-sign (@) symbol represents the current origin. At the start of the zone file, it is the <zone_name>, followed by a trailing dot (.).

4.3.5.2. The $ORIGIN Directive

Syntax: $ORIGIN domain-name [comment]

$ORIGIN sets the domain name that is appended to any unqualified records. When a zone is first read, there is an implicit $ORIGIN <zone_name>``.``; note the trailing dot. The current $ORIGIN is appended to the domain specified in the $ORIGIN argument if it is not absolute.

$ORIGIN example.com.
WWW     CNAME   MAIN-SERVER

is equivalent to

WWW.EXAMPLE.COM. CNAME MAIN-SERVER.EXAMPLE.COM.

4.3.5.3. The $INCLUDE Directive

Syntax: $INCLUDE filename [origin] [comment]

This reads and processes the file filename as if it were included in the file at this point. The filename can be an absolute path, or a relative path. In the latter case it is read from named’s working directory. If origin is specified, the file is processed with $ORIGIN set to that value; otherwise, the current $ORIGIN is used.

The origin and the current domain name revert to the values they had prior to the $INCLUDE once the file has been read.

Note

RFC 1035 specifies that the current origin should be restored after an $INCLUDE, but it is silent on whether the current domain name should also be restored. BIND 9 restores both of them. This could be construed as a deviation from RFC 1035, a feature, or both.

4.3.5.4. The $TTL Directive

Syntax: $TTL default-ttl [comment]

This sets the default Time-To-Live (TTL) for subsequent records with undefined TTLs. Valid TTLs are of the range 0-2147483647 seconds.

$TTL is defined in RFC 2308.

4.3.6. BIND Primary File Extension: the $GENERATE Directive

Syntax: $GENERATE range lhs [ttl] [class] type rhs [comment]

$GENERATE is used to create a series of resource records that only differ from each other by an iterator. $GENERATE can be used to easily generate the sets of records required to support sub-/24 reverse delegations described in RFC 2317.

$ORIGIN 0.0.192.IN-ADDR.ARPA.
$GENERATE 1-2 @ NS SERVER$.EXAMPLE.
$GENERATE 1-127 $ CNAME $.0

is equivalent to

0.0.0.192.IN-ADDR.ARPA. NS SERVER1.EXAMPLE.
0.0.0.192.IN-ADDR.ARPA. NS SERVER2.EXAMPLE.
1.0.0.192.IN-ADDR.ARPA. CNAME 1.0.0.0.192.IN-ADDR.ARPA.
2.0.0.192.IN-ADDR.ARPA. CNAME 2.0.0.0.192.IN-ADDR.ARPA.
...
127.0.0.192.IN-ADDR.ARPA. CNAME 127.0.0.0.192.IN-ADDR.ARPA.

Both generate a set of A and MX records. Note the MX’s right-hand side is a quoted string. The quotes are stripped when the right-hand side is processed.

$ORIGIN EXAMPLE.
$GENERATE 1-127 HOST-$ A 1.2.3.$
$GENERATE 1-127 HOST-$ MX "0 ."

is equivalent to

HOST-1.EXAMPLE.   A  1.2.3.1
HOST-1.EXAMPLE.   MX 0 .
HOST-2.EXAMPLE.   A  1.2.3.2
HOST-2.EXAMPLE.   MX 0 .
HOST-3.EXAMPLE.   A  1.2.3.3
HOST-3.EXAMPLE.   MX 0 .
...
HOST-127.EXAMPLE. A  1.2.3.127
HOST-127.EXAMPLE. MX 0 .

range

This can be one of two forms: start-stop or start-stop/step. If the first form is used, then step is set to 1. “start”, “stop”, and “step” must be positive integers between 0 and (2^31)-1. “start” must not be larger than “stop”.

owner

This describes the owner name of the resource records to be created. Any single $ (dollar sign) symbols within the owner string are replaced by the iterator value. To get a $ in the output, escape the $ using a backslash \, e.g., \$. The $ may optionally be followed by modifiers which change the offset from the iterator, field width, and base.

Modifiers are introduced by a { (left brace) immediately following the $, as in ${offset[,width[,base]]}. For example, ${-20,3,d} subtracts 20 from the current value and prints the result as a decimal in a zero-padded field of width 3. Available output forms are decimal (d), octal (o), hexadecimal (x or X for uppercase), and nibble (n or N for uppercase).

The default modifier is ${0,0,d}. If the owner is not absolute, the current $ORIGIN is appended to the name.

In nibble mode, the value is treated as if it were a reversed hexadecimal string, with each hexadecimal digit as a separate label. The width field includes the label separator.

For compatibility with earlier versions, $$ is still recognized as indicating a literal $ in the output.

ttl

This specifies the time-to-live of the generated records. If not specified, this is inherited using the normal TTL inheritance rules.

class and ttl can be entered in either order.

class

This specifies the class of the generated records. This must match the zone class if it is specified.

class and ttl can be entered in either order.

type

This can be any valid type.

rdata

This is a string containing the RDATA of the resource record to be created. It may be quoted if there are spaces in the string; the quotation marks do not appear in the generated record.

The $GENERATE directive is a BIND extension and not part of the standard zone file format.

4.3.7. Additional File Formats

In addition to the standard text format, BIND 9 supports the ability to read or dump to zone files in other formats.

The raw format is a binary representation of zone data in a manner similar to that used in zone transfers. Since it does not require parsing text, load time is significantly reduced.

For a primary server, a zone file in raw format is expected to be generated from a text zone file by the named-compilezone command. For a secondary server or a dynamic zone, the zone file is automatically generated when named dumps the zone contents after zone transfer or when applying prior updates, if one of these formats is specified by the masterfile-format option.

If a zone file in raw format needs manual modification, it first must be converted to text format by the named-compilezone command, then converted back after editing. For example:

named-compilezone -f raw -F text -o zonefile.text <origin> zonefile.raw
[edit zonefile.text]
named-compilezone -f text -F raw -o zonefile.raw <origin> zonefile.text

4.4. BIND 9 Statistics

BIND 9 maintains lots of statistics information and provides several interfaces for users to access those statistics. The available statistics include all statistics counters that are meaningful in BIND 9, and other information that is considered useful.

The statistics information is categorized into the following sections:

Incoming Requests

The number of incoming DNS requests for each OPCODE.

Incoming Queries

The number of incoming queries for each RR type.

Outgoing Queries

The number of outgoing queries for each RR type sent from the internal resolver, maintained per view.

Name Server Statistics

Statistics counters for incoming request processing.

Zone Maintenance Statistics

Statistics counters regarding zone maintenance operations, such as zone transfers.

Resolver Statistics

Statistics counters for name resolutions performed in the internal resolver, maintained per view.

Cache DB RRsets

Statistics counters related to cache contents, maintained per view.

The “NXDOMAIN” counter is the number of names that have been cached as nonexistent. Counters named for RR types indicate the number of active RRsets for each type in the cache database.

If an RR type name is preceded by an exclamation point (!), it represents the number of records in the cache which indicate that the type does not exist for a particular name; this is also known as “NXRRSET”. If an RR type name is preceded by a hash mark (#), it represents the number of RRsets for this type that are present in the cache but whose TTLs have expired; these RRsets may only be used if stale answers are enabled. If an RR type name is preceded by a tilde (~), it represents the number of RRsets for this type that are present in the cache database but are marked for garbage collection; these RRsets cannot be used.

Socket I/O Statistics

Statistics counters for network-related events.

A subset of Name Server Statistics is collected and shown per zone for which the server has the authority, when zone-statistics is set to full (or yes), for backward compatibility. See the description of zone-statistics in options Statement Definition and Usage for further details.

These statistics counters are shown with their zone and view names. The view name is omitted when the server is not configured with explicit views.

There are currently two user interfaces to get access to the statistics. One is in plain-text format, dumped to the file specified by the statistics-file configuration option; the other is remotely accessible via a statistics channel when the statistics-channels statement is specified in the configuration file (see statistics-channels Statement Grammar.)

4.4.1. The Statistics File

The text format statistics dump begins with a line, like:

+++ Statistics Dump +++ (973798949)

The number in parentheses is a standard Unix-style timestamp, measured in seconds since January 1, 1970. Following that line is a set of statistics information, which is categorized as described above. Each section begins with a line, like:

++ Name Server Statistics ++

Each section consists of lines, each containing the statistics counter value followed by its textual description; see below for available counters. For brevity, counters that have a value of 0 are not shown in the statistics file.

The statistics dump ends with the line where the number is identical to the number in the beginning line; for example:

--- Statistics Dump --- (973798949)

4.4.2. Statistics Counters

The following lists summarize the statistics counters that BIND 9 provides. For each counter, the abbreviated symbol name is given; these symbols are shown in the statistics information accessed via an HTTP statistics channel. The description of the counter is also shown in the statistics file but, in this document, may be slightly modified for better readability.

4.4.2.1. Name Server Statistics Counters

Requestv4

This indicates the number of IPv4 requests received. Note: this also counts non-query requests.

Requestv6

This indicates the number of IPv6 requests received. Note: this also counts non-query requests.

ReqEdns0

This indicates the number of requests received with EDNS(0).

ReqBadEDN SVer

This indicates the number of requests received with an unsupported EDNS version.

ReqTSIG

This indicates the number of requests received with TSIG.

ReqSIG0

This indicates the number of requests received with SIG(0).

ReqBadSIG

This indicates the number of requests received with an invalid (TSIG or SIG(0)) signature.

ReqTCP

This indicates the number of TCP requests received.

AuthQryRej

This indicates the number of rejected authoritative (non-recursive) queries.

RecQryRej

This indicates the number of rejected recursive queries.

XfrRej

This indicates the number of rejected zone transfer requests.

UpdateRej

This indicates the number of rejected dynamic update requests.

Response

This indicates the number of responses sent.

RespTruncated

This indicates the number of truncated responses sent.

RespEDNS0

This indicates the number of responses sent with EDNS(0).

RespTSIG

This indicates the number of responses sent with TSIG.

RespSIG0

This indicates the number of responses sent with SIG(0).

QrySuccess

This indicates the number of queries that resulted in a successful answer, meaning queries which return a NOERROR response with at least one answer RR. This corresponds to the success counter of previous versions of BIND 9.

QryAuthAns

This indicates the number of queries that resulted in an authoritative answer.

QryNoauthAns

This indicates the number of queries that resulted in a non-authoritative answer.

QryReferral

This indicates the number of queries that resulted in a referral answer. This corresponds to the referral counter of previous versions of BIND 9.

QryNxrrset

This indicates the number of queries that resulted in NOERROR responses with no data. This corresponds to the nxrrset counter of previous versions of BIND 9.

QrySERVFAIL

This indicates the number of queries that resulted in SERVFAIL.

QryFORMERR

This indicates the number of queries that resulted in FORMERR.

QryNXDOMAIN

This indicates the number of queries that resulted in NXDOMAIN. This corresponds to the nxdomain counter of previous versions of BIND 9.

QryRecursion

This indicates the number of queries that caused the server to perform recursion in order to find the final answer. This corresponds to the recursion counter of previous versions of BIND 9.

QryDuplicate

This indicates the number of queries which the server attempted to recurse but for which it discovered an existing query with the same IP address, port, query ID, name, type, and class already being processed. This corresponds to the duplicate counter of previous versions of BIND 9.

QryDropped

This indicates the number of recursive queries for which the server discovered an excessive number of existing recursive queries for the same name, type, and class, and which were subsequently dropped. This is the number of dropped queries due to the reason explained with the clients-per-query and max-clients-per-query options (see clients-per-query). This corresponds to the dropped counter of previous versions of BIND 9.

QryFailure

This indicates the number of query failures. This corresponds to the failure counter of previous versions of BIND 9. Note: this counter is provided mainly for backward compatibility with previous versions; normally, more fine-grained counters such as AuthQryRej and RecQryRej that would also fall into this counter are provided, so this counter is not of much interest in practice.

QryNXRedir

This indicates the number of queries that resulted in NXDOMAIN that were redirected.

QryNXRedirRLookup

This indicates the number of queries that resulted in NXDOMAIN that were redirected and resulted in a successful remote lookup.

XfrReqDone

This indicates the number of requested and completed zone transfers.

UpdateReqFwd

This indicates the number of forwarded update requests.

UpdateRespFwd

This indicates the number of forwarded update responses.

UpdateFwdFail

This indicates the number of forwarded dynamic updates that failed.

UpdateDone

This indicates the number of completed dynamic updates.

UpdateFail

This indicates the number of failed dynamic updates.

UpdateBadPrereq

This indicates the number of dynamic updates rejected due to a prerequisite failure.

RateDropped

This indicates the number of responses dropped due to rate limits.

RateSlipped

This indicates the number of responses truncated by rate limits.

RPZRewrites

This indicates the number of response policy zone rewrites.

4.4.2.2. Zone Maintenance Statistics Counters

NotifyOutv4

This indicates the number of IPv4 notifies sent.

NotifyOutv6

This indicates the number of IPv6 notifies sent.

NotifyInv4

This indicates the number of IPv4 notifies received.

NotifyInv6

This indicates the number of IPv6 notifies received.

NotifyRej

This indicates the number of incoming notifies rejected.

SOAOutv4

This indicates the number of IPv4 SOA queries sent.

SOAOutv6

This indicates the number of IPv6 SOA queries sent.

AXFRReqv4

This indicates the number of requested IPv4 AXFRs.

AXFRReqv6

This indicates the number of requested IPv6 AXFRs.

IXFRReqv4

This indicates the number of requested IPv4 IXFRs.

IXFRReqv6

This indicates the number of requested IPv6 IXFRs.

XfrSuccess

This indicates the number of successful zone transfer requests.

XfrFail

This indicates the number of failed zone transfer requests.

4.4.2.3. Resolver Statistics Counters

Queryv4

This indicates the number of IPv4 queries sent.

Queryv6

This indicates the number of IPv6 queries sent.

Responsev4

This indicates the number of IPv4 responses received.

Responsev6

This indicates the number of IPv6 responses received.

NXDOMAIN

This indicates the number of NXDOMAINs received.

SERVFAIL

This indicates the number of SERVFAILs received.

FORMERR

This indicates the number of FORMERRs received.

OtherError

This indicates the number of other errors received.

EDNS0Fail

This indicates the number of EDNS(0) query failures.

Mismatch

This indicates the number of mismatched responses received, meaning the DNS ID, response’s source address, and/or the response’s source port does not match what was expected. (The port must be 53 or as defined by the port option.) This may be an indication of a cache poisoning attempt.

Truncated

This indicates the number of truncated responses received.

Lame

This indicates the number of lame delegations received.

Retry

This indicates the number of query retries performed.

QueryAbort

This indicates the number of queries aborted due to quota control.

QuerySockFail

This indicates the number of failures in opening query sockets. One common reason for such failures is due to a limitation on file descriptors.

QueryTimeout

This indicates the number of query timeouts.

GlueFetchv4

This indicates the number of IPv4 NS address fetches invoked.

GlueFetchv6

This indicates the number of IPv6 NS address fetches invoked.

GlueFetchv4Fail

This indicates the number of failed IPv4 NS address fetches.

GlueFetchv6Fail

This indicates the number of failed IPv6 NS address fetches.

ValAttempt

This indicates the number of attempted DNSSEC validations.

ValOk

This indicates the number of successful DNSSEC validations.

ValNegOk

This indicates the number of successful DNSSEC validations on negative information.

ValFail

This indicates the number of failed DNSSEC validations.

QryRTTnn

This provides a frequency table on query round-trip times (RTTs). Each nn specifies the corresponding frequency. In the sequence of nn_1, nn_2, …, nn_m, the value of nn_i is the number of queries whose RTTs are between nn_(i-1) (inclusive) and nn_i (exclusive) milliseconds. For the sake of convenience, we define nn_0 to be 0. The last entry should be represented as nn_m+, which means the number of queries whose RTTs are equal to or greater than nn_m milliseconds.

4.4.2.4. Socket I/O Statistics Counters

Socket I/O statistics counters are defined per socket type, which are UDP4 (UDP/IPv4), UDP6 (UDP/IPv6), TCP4 (TCP/IPv4), TCP6 (TCP/IPv6), Unix (Unix Domain), and FDwatch (sockets opened outside the socket module). In the following list, <TYPE> represents a socket type. Not all counters are available for all socket types; exceptions are noted in the descriptions.

<TYPE>Open

This indicates the number of sockets opened successfully. This counter does not apply to the FDwatch type.

<TYPE>OpenFail

This indicates the number of failures to open sockets. This counter does not apply to the FDwatch type.

<TYPE>Close

This indicates the number of closed sockets.

<TYPE>BindFail

This indicates the number of failures to bind sockets.

<TYPE>ConnFail

This indicates the number of failures to connect sockets.

<TYPE>Conn

This indicates the number of connections established successfully.

<TYPE>AcceptFail

This indicates the number of failures to accept incoming connection requests. This counter does not apply to the UDP and FDwatch types.

<TYPE>Accept

This indicates the number of incoming connections successfully accepted. This counter does not apply to the UDP and FDwatch types.

<TYPE>SendErr

This indicates the number of errors in socket send operations.

<TYPE>RecvErr

This indicates the number of errors in socket receive operations, including errors of send operations on a connected UDP socket, notified by an ICMP error message.