named.conf(4)

NAME

named.conf — configuration file for Internet domain name server

SYNOPSIS

/etc/named.conf

DESCRIPTION

named.conf is the configuration file for the named name server daemon. The default path name is /etc/named.conf.

BIND 9 configuration is broadly similar to BIND 8.x. However, there are a few new areas of configuration, such as views. BIND 8.x configuration files should work with few alterations in BIND 9.3, although more complex configurations need to be reviewed to see if they can be more efficiently implemented using the new features implemented in BIND 9.3. BIND 4.9.7 configuration files can be converted to the BIND 9.3 format using the shell script, /usr/bin/named-bootconf.sh.

Syntax Rules

In the syntax descriptions in this manpage, the following typographic rules apply:

literal: Characters in this font should be entered as is.
variable: Characters in this font should be replaced with appropriate values.
( ): Parentheses are metacharacters that enclose required content. (The brace characters ({ }) are used in the configuration syntax as block delimiters.)
[ ]: Brackets are metacharacters that enclose optional content.
|: Bars within parentheses and brackets are metacharacters that separate alternatives.
token ... [ ]... ( )...: Trailing ellipses are metacharacters that indicate that the previous token, parenthesized item, or bracketed item may be repeated.

Configuration File Elements

The following configuration elements are used in the BIND 9.3 configuration file grammar:

acl_name

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

address_match_list

A list of one or more ip_addr, ip_prefix, key_id, or acl_name elements.

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 slave and stub zones.

domain_name

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

dotted_decimal

One or more integers valued 0 through 255 separated only by periods (.), 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 fe80::200:f8ff:fe01:9742.

ip_addr

An ip4_addr or ip6_addr.

ip_port

An IP port number. This is limited to 0 through 65535, with values below 1024 typically restricted to root-owned processes. 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 zero elements in 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 the network 1.2.3.0 with netmask 255.255.255.240.

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_ids, separated by semicolons and ending with a semicolon.

number

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

path_name

A quoted string that is used as a path name, such as "zones/master/my.test.domain" .

size_spec

One of the following:

number: A decimal number, 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. The value must be representable as a 64-bit unsigned integer (0 to 18446744073709551615, inclusive).
default: Uses the limit that was in force when the server was started.
unlimited: Requests unlimited use, or the maximum available amount. This is the best way to set a really large number.

yes_or_no

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

Address Match List Syntax

An address_match_list has the format:

address_match_list_element ;
     [ address_match_list_element ; ]... 

An address_match_list_element has the format:

[ ! ] ( ip_addr
      | ip_prefix
      | key key_id
      | acl_name
      | { address_match_list } )

Address Match List Definition and Usage

Address match lists are primarily used to determine access control for various server operations. They are also used to define priorities for querying other name servers and to set the addresses on which named will listen for queries. The elements which constitute an address match list may be any of the following:

An IP address (IPv4 or IPv6).
An IP prefix (in the /-notation).
A key ID, as defined by the key statement.
The name of an address match list previously defined with an acl statement.
A nested address match list enclosed in braces.
Elements can be negated with a leading exclamation mark (!). The match list names of any, none, localhost, and localnets are predefined. For more information on these match list names, refer to The acl Statement section. 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. However, the term address match list is still being used.
When a given IP address or prefix is compared to an address match list, the list is traversed in order until an element matches. The interpretation of a match depends on whether the list is being used for access control, defining listen-on ports and whether the element was negated. When used as an access control list, a nonnegated match allows access and a negated match denies access. If there is no match, access is denied.
The clauses allow-notify, allow-query, allow-transfer, allow-update, allow-update-forwarding, and blackhole, which can be specified in the options and/or zone statements use the address match lists. Similarly, the listen-on option causes the server not to accept queries on any of the machine's addresses which do not match the list.
Because of the first-match aspect of the algorithm, 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 of no use because the algorithm will match 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 having 1.2.3.13 blocked by the negation but all other 1.2.3.* hosts fall through.

Comment Syntax

Comments in the BIND 9.3 configuration file can be written in the following styles:

C:: /* comment */
C++:: // to end of line
UNIX:: # to end of line

Note: Unlike a zone file, you cannot use a semicolon (;) character to start a comment in the BIND 9.3 configuration file. The semicolon indicates the end of a configuration statement.

CONFIGURATION FILE GRAMMAR

A BIND 9.3 configuration file 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 substatements, which is 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.
include: Includes a file.
key: Specifies key information for use in authentication and authorization using TSIG.
logging: Specifies what data the server logs, and where the log messages are sent.
lwres: Configures the name server to also act as a lightweight resolver server.
masters: Defines a masters list for inclusion in masters clauses of stub and slave zone statements
options: Controls global server configuration options and sets defaults for other statements.
server: Sets certain configuration options on a per-server basis.
trusted-keys: Defines trusted DNSSEC keys.
view: Defines a view.
zone: Defines a zone.

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

The acl Statement

acl Statement Grammar

acl acl-name {
     address_match_list
};

acl Statement Definition and Usage

The acl statement assigns a symbolic name to an address match list. It gets its name from the primary use of address match lists for Access Control Lists (ACLs). Note that an address match list's name must be defined with acl before it can be used elsewhere; no forward references are allowed. The following ACL names are built-in:

any: Matches all hosts.
none: Matches no hosts.
localhost: Matches the IPv4 addresses of all network interfaces on the system.
localnets: Matches any host on an IPv4 network for which the system has an interface.

The localhost and localnets ACLs do not currently support IPv6 (that is, localhost does not match the host's IPv6 addresses, and localnets does not match the host's attached IPv6 networks) due to the lack of a standard method of determining the complete set of local IPv6 addresses for a host.

The controls Statement

controls Statement Grammar

controls {
     ( inet ( ip_addr | * ) [ port ip_port ]
       allow { address_match_list }
       keys { key_list }; )...
};

controls Statement Definition and Usage

The controls statement declares control channels to be used by system administrators to control the operation of the local 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/IP socket accessible to the Internet, created at the specified ip_port on the specified ip_addr. If no port is specified, port 953 is used by default. * cannot be used for ip_port.

The allow and keys clauses restrict the ability to issue commands over the control channel. Connections to the control channel are permitted based on the address permissions in address_match_list. key members of the address_match_list are ignored, and instead are interpreted independently based on the key_list. Each key_id in the key_list is allowed to be used to authenticate commands and responses given over the control channel by digitally signing each message between the server and a command client. All commands to the control channel must be signed by one of its specified keys to be honored.

If no controls statement is present, named will set 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 will attempt to load the command channel key from the file /etc/rndc.key. To create a rndc.key file, run rndc-confgen -a. The rndc.key feature was implemented to ease the transition of systems from BIND 8, which did not have digital signatures on its command channel messages and thus did not have a keys clause.

Since the rndc.key feature is only intended to allow the backward-compatible usage of BIND 8 configuration files, this feature does not have a high degree of configurability. You cannot easily change the key name or the size of the secret, so you should make an rndc.conf with your own key if you wish to change them. The rndc.key file also has its permissions set such that only the owner of the file (the user that named is running as) can access it. If you desire greater flexibility in allowing other users to access rndc commands, then you need to create an rndc.conf and make it group-readable by a group that contains the users who should have access.

The UNIX control channel type of BIND 8 is not supported in BIND 9.3, and is not expected to be added in future releases. If it is present in the controls statement from a BIND 8 configuration file, it is ignored and a warning is logged.

As a special case, to disable the command channel, use an empty controls statement:

controls { };

The include Statement

include Statement Grammar

include filename ;

include Statement Definition and Usage

The include statement inserts the specified file 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 a name server.

The key Statement

key Statement Grammar

key key_id {
     algorithm algoname ;
     secret secretstring ;
};

key Statement Definition and Usage

The key statement defines a shared secret key for use with TSIG. 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 must be defined at the top level.

key_id: A domain name uniquely identifying the key. Also known as the key name. It can be used in a server statement to sign requests 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.
algoname: A string that specifies a security/authentication algorithm. hmac-md5 is the only algorithm which is currently supported with TSIG authentication.
secretstring: A base-64-encoded secret string to be used by the algorithm.

The logging Statement

logging Statement Grammar

logging {
     [ channel channel_name {
          ( file path name
               [ versions ( number | unlimited ) ]
               [ size size spec ]
          | null
          | stderr
          | syslog syslog_facility
          ) ;
          [ severity ( critical | error | warning | notice
                       | info | debug [ level ] | dynamic ) ; ]
          [ print-category yes_or_no ; ]
          [ print-severity yes_or_no ; ]
          [ print-time yes_or_no ; ]
     }; ]...

     [ category category_name {
          ( channel_name ; )...
     }; ]...
};

The category and channel clauses may be repeated in any order.

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, channel_name, that can be used with the category phrase to select how various classes of messages are logged.

Only one logging statement is used to define any number of channels and categories. If there is no logging statement, the logging configuration defaults to:

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

In BIND 9.3, the logging configuration is established only when the entire configuration file has been parsed. In BIND 8, it was established as soon as the logging statement was parsed. When the server starts up, all logging messages related to syntax errors in the configuration file go to the default channels, or to standard error if the -g option is specified.

The channel Phrase

All log output goes to one or more user-defined or predefined channels. Every channel definition must include a destination clause that says whether messages selected for the channel go to a file, or to a particular syslog facility, or to the standard error stream, or are discarded. It can optionally also limit the message severity level that will be accepted by the channel (the default is info), and whether to include a named-generated time stamp, the category name, and/or severity level (the default is not to include any).

The file destination clause directs the channel to a disk file. It can include limitations on both the file size and the number of versions of the file that are saved each time the file is opened.

If you use the versions log file option, then named will retain that many backup versions of the file by renaming them when opening.

For example, if you choose to keep three old versions of the file lamers.log, then, just before it is opened:

lamers.log.1 is renamed to lamers.log.2
lamers.log.0 is renamed to lamers.log.1
lamers.log   is renamed to lamers.log.0

Use versions unlimited; if you do not want to limit the number of versions. If a size option is associated with the log file, then renaming is only done when the file being opened exceeds the indicated size. No backup versions are kept, by default; any existing log file is simply appended.

The size option for file is used to limit log growth. If the file size exceeds the limit, then named will stop writing to the file unless it has a versions option associated with it. If backup versions are kept, the files are rolled as described above and a new file is opened. If there is no versions option, no more data will be written to the log until the log file is removed or truncated (by some external process) to less than the maximum size. The default behavior is not to limit the size of the file.

Example usage of the size and versions options:

channel "an_example_channel" {
     file "example.log" versions 3 size 20m;
     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(3C) manpage. The syslog(3C) manpage also describes how syslog will handle messages sent to this facility. If you have a system which uses a very old version of syslog that uses only two arguments to the openlog() function, then the syslog destination clause is ignored.

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, for example when debugging the configuration.

The null destination clause discards all message sent to the channel, the severity and print-* clauses irrelevant.

The severity clause works like the syslog() priority parameter except that it can also be used if you are writing straight to a file rather than using syslog. Messages that are not at least of the severity level given will not be selected for the channel; messages of higher severity levels will be accepted. If you are using the syslog option, then the syslog.conf priorities will also determine what eventually passes through (see syslogd(1M)).

For example, defining a channel facility and severity as daemon and debug but only logging daemon.warning via syslog.conf will cause 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 server can supply extensive debugging information when it is in debugging mode. If the server's global debug level is greater than zero, then debugging mode will be active. The global debug level is set either by starting the named server with the -d option 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, and higher debug levels give more detailed output. For example:

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

In this example, channels that specify a particular debug severity will 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 level to determine what messages to print.

If print-time is on, then the date and time will be logged. print-time may be specified for a syslog channel, but that is usually pointless, since syslog also prints the date and time.

If print-category is on, then the category of the message is logged as well.

If print-severity is on, then the severity level of the message will be logged.

The print-* options may be used in any combination, and will always be printed in the 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
Time:
28-Feb-2000 15:05:32.863
Category:
general
Severity:
notice
Message:
running

There are four predefined channels that are used for named's default logging, as follows:

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

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

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

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

The default_debug channel has the special property that it only produces output when the server's debug level is nonzero. It normally writes to a file, 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 that is generated while named is starting up and still running as root is discarded. If you need to capture this output, you must run the server with the -g option and redirect standard error to a file.

Once a channel is defined, it cannot be redefined. Thus you cannot alter the built-in channels directly, but you can modify the default logging by pointing categories at channels you have defined.

The category Phrase

Predefined categories allow you to fine-tune what messages you want to log and where you want to log those messages to. If you do not specify a list of channels for a category, then log messages in that category will be sent to the default category instead. If you do not specify a default category, the following category is used:

category "default" { "default_syslog"; "default_debug"; };

For example, if you want to log security events to a file and also want to keep the default logging behavior, you need to specify the following in the logging statement:

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, as in the following:

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.

default: Defines the logging options for categories where no specific configuration has been defined.
general: The catch-all. All unclassified categories belong to this category.
client: Processing of client requests
config: Configuration file parsing and processing.
database: Messages relating to the databases used internally by the name server to store zone and cache data.
delegation-only: Logs queries that have have been forced to NXDOMAIN as the result of a delegation-only zone or a delegation-only in a 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.
lame-servers: Lame servers are misconfigurations in remote servers, discovered by BIND 9 when trying to query those servers during resolution.
network: Network operations.
notify: The NOTIFY protocol.
queries: Enable query logging.
resolver: DNS resolution, such as recursive lookups performed on behalf of clients by a caching name server.
security: Approval and denial of requests.
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.

The lwres Statement

lwres Statement Grammar

lwres {
     [ listen-on { ( ip_addr [ port ip_port ] ; )... }; ]
     [ ndots number ; ]
     [ search { domain_name ; [ domain_name ; ]... }; ]
     [ view view_name ; ]
};

lwres Statement Definition and Usage

The lwres statement configures the name server to also act as a lightweight resolver server. There may be be multiple lwres statements configuring lightweight resolver servers with different properties.

The listen-on statement specifies a list of addresses and ports that a lightweight resolver daemon should accept requests on. If no port is specified, port 921 is used. If this statement is omitted, requests will be accepted on 127.0.0.1, port 921.

The ndots statement is equivalent to the ndots directive in /etc/resolv.conf. It indicates the minimum number of dots in a relative domain name that should result in an exact match lookup before search path elements are appended.

The search statement is equivalent to the search directive in /etc/resolv.conf. It provides a list of domains that are appended to relative names in queries.

The view statement binds this instance of a lightweight resolver daemon to a view in the DNS name space, so that the response will be constructed in the same manner as a normal DNS query matching this view. If this statement is omitted, the default view is used, and if there is no default view, an error is triggered.

The masters Statement

masters Statement Grammar

masters name [ port ip_port ] { (
     ( masters_list | ip_addr [ port ip_port ] [ key key ] ) ;
)... };

masters Statement Definition and Usage

A masters statement defines a masters list. This allows you to include sets of masters in the masters clauses of multiple stub and slave zone statements. See type slave and type stub in the zone Statement Definition and Usage section.

name: The name of the masters statement.
masters_list: The acl_name of an acl statement that specifies a list of masters.

The options Statement

options Statement Grammar

options {

// General Options
     [ directory path_name ; ]
     [ disable-algorithms domain { algorithm ; [ algorithm ; ] }; ]
     [ dnssec-lookaside domain trust-anchor domain ; ]
     [ dnssec-must-be-secure domain yes_or_no ; ]
     [ dump-file path_name ; ]
     [ key-directory path_name ; ]
     [ memstatistics-file path_name ; ]
     [ pid-file path_name ; ]
     [ port ip_port ; ]
     [ preferred-glue ( A | AAAA | NONE ) ; ]
     [ random-device path_name ; ]
     [ root-delegation-only [ exclude { namelist } ] ; ]
     [ statistics-file path_name ; ]
     [ tkey-dhkey key_name key_tag ; ]
     [ tkey-domain domainname ; ]

// Boolean Options
     [ additional-from-auth yes_or_no ; ]
     [ additional-from-cache yes_or_no ; ]
     [ auth-nxdomain yes_or_no ; ]
     [ check-names ( master | slave | response )
                   ( warn | fail | ignore ) ; ]
     [ dialup dialup_option ; ]
     [ dnssec-enable yes_or_no ; ]
     [ flush-zones-on-shutdown yes_or_no ; ]
     [ match-mapped-addresses yes_or_no ; ]
     [ minimal-responses yes_or_no ; ]
     [ notify ( yes_or_no | explicit ) ; ]
     [ provide-ixfr yes_or_no ; ]
     [ querylog yes_or_no ; ]
     [ recursion yes_or_no ; ]
     [ request-ixfr yes_or_no ; ]
     [ zone-statistics yes_or_no ; ]

// Access Control Options
     [ allow-notify { address_match_list }; ]
     [ allow-query { address_match_list }; ]
     [ allow-recursion { address_match_list }; ]
     [ allow-transfer { address_match_list }; ]
     [ allow-update-forwarding { address_match_list }; ]
     [ blackhole { address_match_list }; ]

// Bad UDP Port List Options
     [ avoid-v4-udp-ports { port_list }; ]
     [ avoid-v6-udp-ports { port_list }; ]

// Built-In Server Information Zone Options
     [ hostname hostname_string ; ]
     [ server-id server_id_string ; ]
     [ version version_string ; ]

// Dual-Stack Server Option
     [ dual-stack-servers [ port ip_port ] { (
          ( domain_name [ port ip_port ] | ip_addr [ port ip_port ] ) ;
     )... }; ]

// Forwarding Options
     [ forward ( only | first ) ; ]
     [ forwarders { ( ip_addr [ port ip_port ] ; )... }; ]

// Interface Options
     [ listen-on [ port ip_port ] { address_match_list }; ]
     [ listen-on-v6 [ port ip_port ] { address_match_list }; ]

// Obsolete Option
     [ allow-v6-synthesis yes_or_no ; ]

// Operating System Resource Limit Options
     [ coresize size_spec ; ]
     [ datasize size_spec ; ]
     [ files size_spec ; ]
     [ stacksize size_spec ; ]

// Periodic Task Interval Options
     [ cleaning-interval number ; ]
     [ heartbeat-interval number ; ]
     [ interface-interval number ; ]

// Query Address Options
     [ query-source [ address ( ip_addr | * ) ]
                    [ port ( ip_port | * ) ] ; ]
     [ query-source-v6 [ address ( ip_addr | * ) ]
                       [ port ( ip_port | * ) ] ; ]

// RRset Ordering Option
     [ rrset-order { order_spec ; [ order_spec ; ]... }; ]

// Server Resource Limit Options
     [ max-cache-size size_spec ; ]
     [ max-journal-size size_spec ; ]
     [ recursive-clients number ; ]
     [ tcp-clients number ; ]
     [ tcp-listen-queue number ; ]

// Sorting Option
     [ sortlist { address_match_list }; ]

// Tuning Options
     [ edns-udp-size number ; ]
     [ lame-ttl number ; ]
     [ max-cache-ttl number ; ]
     [ max-ncache-ttl number ; ]
     [ max-refresh-time number ; ]
     [ max-retry-time number ; ]
     [ min-refresh-time number ; ]
     [ min-retry-time number ; ]
     [ sig-validity-interval number ; ]

// Zone Transfer Options
     [ also-notify { ( ip_addr [ port ip_port ] ; )... }; ]
     [ alt-transfer-source ( ip4_addr | * ) [ port ip_port ] ; ]
     [ alt-transfer-source-v6 ( ip6_addr | * ) [ port ip_port ] ; ]
     [ max-transfer-idle-in number ; ]
     [ max-transfer-idle-out number ; ]
     [ max-transfer-time-in number ; ]
     [ max-transfer-time-out number ; ]
     [ notify-source ( ip4_addr | * ) [ port ip_port ] ; ]
     [ notify-source-v6 ( ip6_addr | * ) [ port ip_port ] ; ]
     [ serial-query-rate number ; ]
     [ transfer-format ( one-answer | many-answers ) ; ]
     [ transfer-source ( ip4_addr | * ) [ port ip_port ] ; ]
     [ transfer-source-v6 ( ip6_addr | * ) [ port ip_port ] ; ]
     [ transfers-in number ; ]
     [ transfers-out number ; ]
     [ transfers-per-ns number ; ]
     [ use-alt-transfer-source yes_or_no ; ]
};

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 more than one occurrence is found, the first occurrence determines the actual options used, and a warning is generated. If there is no options statement, an options block with each option set to its default will be used.

General Options

directory

The working directory of the server. Any nonabsolute path names in the configuration file will be taken as relative to this directory. The default location for most server output files (for example, 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.

disable-algorithms

Disable the specified DNSSEC algorithms at and below the specified name. Multiple disable-algorithms statements are allowed. Only the most specific is applied.

dnssec-lookaside

When set, dnssec-lookaside provides the validator with an alternate method to validate DNSKEY records at the top of a zone. When a DNSKEY is at or below a domain specified by the deepest dnssec-lookaside, and the normal DNSSEC validation has left the key untrusted, the trust-anchor will be appended to the key name and a DLV record will be looked up to see if it can validate the key. If the DLV record validates a DNSKEY (similar to the way a DS record does it), the DNSKEY RRset is deemed to be trusted.

dnssec-must-be-secure

Specify hierarchies which must be or may not be secure (signed and validated). If yes, named will only accept answers if they are secure. If no, normal DNSSEC validation applies and insecure answers are accepted. The specified domain must be under a trusted key, or dnssec-lookaside must be active.

dump-file

The path name of the file to which the server dumps the database with rndc dumpdb. The default is named_dump.db.

key-directory

The directory where the public and private key files should be found, if it is not the working directory. The specified directory must be an absolute path.

memstatistics-file

The path name of the file to which the server writes the memory usage statistics. The default is named.memstats.

pid-file

The path name of the file in which the server writes its process ID. The default path name is /var/run/named.pid. The pid-file is used by programs that need to 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 file is removed. Note that none is a keyword, not a file name, and therefore is not enclosed in quotation marks.

port

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

preferred-glue

If specified, the listed type (A or AAAA) will be emitted before other glue in the additional section of a query response. The default is not to prefer any type (NONE). ("Glue" is a record that is created as part of a delegation.)

random-device

The source of entropy (random data) to be used by the server. Entropy is primarily needed for DNSSEC operations, This option specifies the device (or file) from which to read entropy. If this is a file, operations requiring entropy will fail when the file has been exhausted. The default value is /dev/random (or the equivalent) when present, and none otherwise. The random-device option takes effect during the initial configuration load at server startup time and is ignored on subsequent reloads.

root-delegation-only

Turn on enforcement of delegation-only in top level domains (TLD) and root zones, with an optional exclude list.

Note: Some TLDs are not delegation-only (for example, DE, LV, US and MUSEUM).

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

statistics-file

The path name of the file in which the server appends statistics using rndc stats. The default is named.stats in the server's current directory. The file format is described in The Statistics File section.

tkey-dhkey

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. The key_tag is an integer that is part of the key.

tkey-domain

The domain appended to the names of all shared keys generated with TKEY. When a client requests a TKEY exchange, it can specify a preferred name for the key. If the name is present, the name of the shared key will be client_specified_part+tkey_domain. Otherwise, the name of the shared key will be random_hex_digits+tkey-domain. In most cases, the domain name should be the server's domain name.

Boolean Options

additional-from-auth, additional-from-cache

These options control the behavior of an authoritative server when answering queries which have additional data, or when following CNAME and DNAME chains.

When both of these options are set to yes (the default) and a query is being answered from authoritative data (a zone configured into the server), the additional data section of the reply will be filled in using data from other authoritative zones and from the cache. In some situations this is undesirable, such as when there is concern over the correctness of the cache, or in servers where slave zones may be added and modified by untrusted third parties. Also, avoiding the search for this additional data will speed up server operations at the possible expense of additional queries to resolve what would otherwise be provided in the additional section.

For example, if a query asks for an MX record for host foo.example.com, and the record found is MX 10 mail.example.net, normally the address records (A, A6, and AAAA) for mail.example.net will be provided as well, if known. Set these options to no to disable this behavior.

These options are intended for use in authoritative-only servers, or in authoritative-only views. Attempts to set them to no without also specifying recursion no will cause the server to ignore the options and log a warning message.

Specifying additional-from-cache no actually disables the use of the cache not only for additional data lookups but also when looking up the answer. This is usually the desired behavior in an authoritative-only server where the correctness of the cached data is an issue.

When a name server is nonrecursively queried for a name that is not below the apex of any served zone, it normally answers with an "upwards referral" to the root servers or the servers of some other known parent of the query name. Since the data in an upwards referral comes from the cache, the server will not be able to provide upwards referrals when additional-from-cache no has been specified. Instead, it will respond to such queries with REFUSED. This should not cause any problems since upwards referrals are not required for the resolution process.

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. If you are using an old version of BIND, you might need to set this option to yes.

check-names

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

The rules for legal host names 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 rrdata of NS, SOA, and MX, records. It also applies to the rrdata of PTR records where the owner name indicated that it is a reverse lookup of a host name (the owner name ends in IN-ADDR.ARPA, IP6.ARPA, IP6.INT).

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. This has different effects according to zone type and concentrates the zone maintenance so that it all happens in a short interval, once every heartbeat-interval and hopefully during the one call. It also suppresses some of the normal zone maintenance traffic. The default is no.

The dialup option may also be specified in view and zone statements, in which case, it overrides the global dialup option.

If the zone is a master zone, then the server will send out a NOTIFY request to all the slaves. This will trigger the zone serial number check in the slave (provided it supports NOTIFY), allowing the slave to verify the zone while the connection is active.

If the zone is a slave or stub zone, then the server will suppress the regular "zone up to date" (refresh) queries and only perform 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 just disables normal refresh processing.

dnssec-enable

Enable DNSSEC support in named. Unless set to yes, named behaves as if it does not support DNSSEC. The default is no.

flush-zones-on-shutdown

If yes, flush any pending zone writes when the name server exits due to receiving a SIGTERM. The default is no, do not flush on SIGTERM.

match-mapped-addresses

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

minimal-responses

If yes, the server will only add records to the authority when generating responses and additional data sections when they are required (for example, delegations, negative responses). This may improve the performance of the server. The default is no.

notify

If yes (the default), DNS NOTIFY messages are sent when a zone for which the server is authoritative, changes. The messages are sent to the servers listed in the zone's NS records (except the master server identified in the SOA MNAME field), and to any servers listed in the also-notify option. If explicit is specified, NOTIFY messages are sent only to servers explicitly listed using also-notify. If no, no NOTIFY messages are sent.

The notify option may also be specified in the zone statement, in which case it overrides the notify specified in the options statement. It needs to be turned off only when the slaves crash.

provide-ixfr

Determines whether the local server, acting as master, will respond with an incremental zone transfer when the given remote server, a slave, requests it. If yes, an incremental transfer will be provided whenever possible. If no, all transfers to the remote server will be nonincremental. If not set in a server statement, the value of the provide-ixfr option in the view or global options statement is used as a default.

querylog

If yes, start query logging when named starts. If no, do not start query logging when named starts. If querylog is not specified, query logging is determined from the presence of the logging category queries.

recursion

If yes and a DNS query requests recursion, then the server will attempt to answer the query. If no and the server does not know the answer, it will return a referral response. The default is yes.

Note that setting recursion to 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-ixfr

Determines whether the local server, acting as a slave, will request incremental zone transfers from the given remote server, a master. If not set in a server statement, the value of the request-ixfr option in the view or global options statement is used as a default.

zone-statistics

If yes, the server will, by default, collect statistical data on all zones in the server. These statistics may be accessed using the rndc stats command, which will dump them to the file listed in the statistics-file option.

Access Control Options

Access to the server can be restricted based on the IP address of the requesting system.

allow-notify

Specifies which hosts are allowed to notify slaves of a zone change in addition to the zone masters. allow-notify may also be specified in the zone statement, in which case it overrides the options allow-notify statement. It is only meaningful for a slave zone. If not specified, the default is to process notify messages only from a zone's master.

allow-query

Specifies which hosts are allowed to ask ordinary 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.

allow-recursion

Specifies which hosts are allowed to make recursive queries through this server. If not specified, the default is to allow recursive queries from all hosts. Note that disallowing recursive queries for a host does not prevent the host from retrieving data that is already in the server's cache.

allow-update-forwarding

Specifies which hosts are allowed to submit Dynamic DNS updates to slave zones to be forwarded to the master. The default is {none;}, which means that no update forwarding will be performed. To enable update forwarding, specify allow-update-forwarding {any;};. Specifying values other than {none;} or {any;} is usually counterproductive, since the responsibility for update access control should rest with the master server, not the slaves.

Note that enabling the update forwarding feature on a slave server may expose master servers relying on insecure IP-address-based access control to attacks.

allow-transfer

Specifies the hosts that 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 options allow-transfer statement. If not specified, the default is to allow transfers from all hosts.

blackhole

Specifies a list of addresses that the server will not accept queries from or use to resolve a query. Queries from these addresses will not be responded to. The default is none.

Bad UDP Port List Options

avoid-v4-udp-ports, avoid-v6-udp-ports: Specify a list of IPv4 and IPv6 UDP ports that will not be used as system assigned source ports for UDP sockets. These lists prevent named from choosing as its random source port a port that is blocked by your firewall. If a query went out with such a source port, the answer would not get by the firewall and the name server would have to query again.

Built-In Server Information Zone Options

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 of class CHAOS which is separate from the default view of class IN; therefore, any global server options such as allow-query do not apply the these zones. If you feel the need 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.

hostname: The host name the server should report via a query of the name hostname.bind with type TXT, class CHAOS. This defaults to the host name of the machine hosting the name server as found by gethostname() (see gethostname(2)). The primary purpose of such queries is to identify which of a group of anycast servers is actually answering your queries. Specifying hostname none; disables processing of the queries.
server-id: The ID the server should report via a query of the name ID.SERVER with type TXT, class CHAOS. The primary purpose of such queries is to identify which of a group of anycast servers is actually answering your queries. Specifying server-id none; disables processing of the queries. Specifying server-id hostname; causes named to use the host name as found by gethostname(). The default server-id is none.
version: 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.

Dual-Stack Server Option

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

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

Forwarding Options

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 useful only if the forwarders list is not empty. The default value first, causes the server to query the forwarders first, and if that is unable to answer the question, the server will then look for the answer itself. If only is specified, the server will only query the forwarders.

forwarders

Specifies the IP addresses to be used for forwarding. The default is the empty list (no forwarding).

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

Interface Options

The interfaces and ports that the server will answer queries from, may be specified using the listen-on option.

listen-on

The server listens on all interfaces allowed by the address match list. If a port is not specified, port 53 is used.

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; };

will enable the name server on port 53 for 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. If no listen-on is specified, the server will listen on port 53 on all interfaces.

listen-on-v6

Specifies the ports on which the server will listen for incoming queries sent using IPv6.

The server does not bind a separate socket to each IPv6 interface address as it does for IPv4. Instead, it always listens on the IPv6 wildcard address. Therefore, the only values allowed for the address_match_list argument of the listen-on-v6 statement are: {any;} and {none;}.

Multiple listen-on-v6 options can be used to listen on multiple ports:

listen-on-v6 port 53 { any; };
listen-on-v6 port 1234 { any; };

To make the server not to listen on any IPv6 address, use

listen-on-v6 { none; };

If no listen-on-v6 statement is specified, the server will not listen on any IPv6 address.

Obsolete Option

allow-v6-synthesis: This option was introduced for the smooth transition from AAAA to A6 and from "nibble labels" to binary labels. However, since both A6 and binary labels were then deprecated, this option was also deprecated. It is now ignored with some warning messages.

Operating System Resource Limit Options

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. An unlimited size_spec requests unlimited use, or the maximum available amount. default uses the limit that was in force when the server was started.

The following options set operating system resource limits for the name server process. A warning will be issued if an unsupported limit is used.

coresize: The maximum size of a core dump. The default is default.
datasize: 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 of limiting 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. If you wish to limit the amount of memory used by the server, use the max-cache-size and recursive-clients options instead; see the Server Resource Limit Options section.
files: The maximum number of files the server may have open concurrently. The default is unlimited.
stacksize: The maximum amount of stack memory the server may use. The default is default.

Periodic Task Interval Options

cleaning-interval: The server will remove expired resource records from the cache every cleaning-interval minutes. The default is 60 minutes. The maximum value is 28 days (40320 minutes). If set to 0, no periodic cleaning will occur.
heartbeat-interval: The server will perform zone maintenance tasks for all zones marked as dialup whenever this interval expires. The default is 60 minutes. The maximum value is 28 days (40320 minutes). Reasonable values are up to 1 day (1440 minutes). If set to 0, no zone maintenance for these zones will occur.
interface-interval: The server will scan 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 will only occur when the configuration file is loaded. After the scan, listeners will be started on any new interfaces (provided they are allowed by the listen-on configuration). Listeners on interfaces that have gone away will be cleaned up.

Query Address Options

If the server is unable to answer a question, it will query other name servers.

query-source: Specifies the address and port used for such queries.
query-source-v6: Specifies the address and port used for queries sent over IPv6.

If address is * or is omitted, a wildcard IP address (INADDR_ANY) is used. If port is * or is omitted, a random unprivileged port will be used. The default address and port are:

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

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.

RRset Ordering Option

When multiple records are returned in an answer, it may be useful to configure the order of the records placed into the response.

The rrset-order option permits the configuration of the ordering of the records in a multiple record response.

order_spec is defined as:

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

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 *.
The values for ordering are:
fixed
Records are returned in the order they are defined in the zone file.
random
Records are returned in some random order.
cyclic
Records are returned in a round-robin order.

In this example, any responses for type A records in class IN that have host.example.com as a suffix, are always returned in random order. All other records are returned in cyclic order.

rrset-order {
    class IN type A name "host.example.com" order random;
    order cyclic;
};

If multiple rrset-order statements appear, they are not combined; the last one applies.

Server Resource Limit Options

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

max-cache-size: The maximum amount of memory to use for the server's cache, in bytes. When the amount of data in the cache reaches this limit, the server will cause records to expire prematurely so that the limit is not exceeded. In a server with multiple views, the limit applies separately to the cache of each view. The default is unlimited, meaning that records are purged from the cache only when their TTLs expire.
max-journal-size: Sets a maximum size for each journal file. When the journal file approaches the specified size, some of the oldest transactions in the journal will be automatically removed. The default is unlimited.
recursive-clients: The maximum number of simultaneous recursive lookups the server will perform 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.
tcp-clients: The maximum number of simultaneous client TCP connections that the server will accept. The default is 100.
tcp-listen-queue: The listen queue depth. The default and minimum is 3. If the kernel supports the accept filter "dataready", this also controls how many TCP connections that will be queued in kernel space waiting for some data before being passed to accept. Values less than 3 are silently raised.

Sorting Option

The response to a DNS query may consist of multiple resource records (RRs) forming a resource records set (RRset). The name server will normally return the RRs within the RRset in an indeterminate order (but see the rrset-reorder statement in the RRset Reordering Option section). 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 option takes an address_match_list and interprets it. 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.

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 in a special way. 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 will get responses preferring addresses on any of the locally connected networks. Next will be 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 will 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 will only prefer other addresses on their directly connected networks.

sortlist {
     { localhost;                    // IF   the local host
          { localnets;               // THEN first fit on the
               192.168.1/24;         //      following nets
               { 192.168.2/24; 192.168.3/24; }; }; };

     { 192.168.1/24;                 // IF   on class C 192.168.1
          { 192.168.1/24;            // THEN use .1, or .2 or .3
               { 192.168.2/24; 192.168.3/24; }; }; };

     { 192.168.2/24;                 // IF   on class C 192.168.2
          { 192.168.2/24;            // THEN use .2, or .1 or .3
               { 192.168.1/24; 192.168.3/24; }; }; };

     { 192.168.3/24;                 // IF   on class C 192.168.3
          { 192.168.3/24;            // THEN use .3, or .1 or .2
               { 192.168.1/24; 192.168.2/24; }; }; };

     {                               // IF .4 or .5, prefer that net
                  { 192.168.4/24; 192.168.5/24; }; };
};

The following example gives reasonable behavior for the local host and hosts on directly connected networks. It is similar to the behavior of the address sort in BIND 4.9.x. Responses sent to queries from the local host will favor any of the directly connected networks. Responses sent to queries from any other hosts on a directly connected network will prefer addresses on that same network. Responses to other queries will not be sorted.

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

Tuning Options

edns-udp-size

Sets the advertised Extended DNS (EDNS) UDP buffer size in bytes. Valid values are 512 to 4096 (values outside this range will be silently adjusted). The default value is 4096. The usual reason for setting edns-udp-size to a nondefault value is to get UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP packets that are greater than 512 bytes.

lame-ttl

Sets the number of seconds to cache a lame server indication. 0 disables caching. (This is not recommended.) The default is 600 (10 minutes). The maximum value is 1800 (30 minutes). (See the lame-servers keyword in The Category Phrase section.)

max-cache-ttl

Sets the maximum time in seconds for which the server will cache ordinary (positive) answers. The default is one week (7 days).

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. The default is 10800 seconds (3 hours). The maximum is 7 days and will be truncated to 7 days if set to a greater value.

max-refresh-time, max-retry-time, min-refresh-time, min-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, but these values are set by the master, giving slave server administrators little control over their contents.

These options allow the administrator to set a minimum and maximum refresh and retry time either per-zone, per-view, or per-server. These options are valid for master, slave and stub zones, and clamp the SOA refresh and retry times to the specified values.

sig-validity-interval

Specifies the number of days into the future when DNSSEC signatures that were automatically generated as a result of dynamic updates will expire. The default is 30 days. The maximum is 10 years (3660 days). The signature inception time is unconditionally set to one hour before the current time to allow for a limited amount of clock skew.

Zone Transfer Options

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: 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 will quickly converge on stealth servers. If an also-notify list is given in a zone statement, it will override the options also-notify statement. When a zone notify statement is set to no, the IP addresses in the global also-notify list will not be sent NOTIFY messages for that zone. The default is the empty list (no global notification list).
alt-transfer-source: An alternate transfer source if the one listed in transfer-source fails and use-alt-transfer-source is set.
alt-transfer-source-v6: An alternate transfer source if the one listed in transfer-source-v6 fails and use-alt-transfer-source is set.
max-transfer-idle-in: Inbound zone transfers making no progress in this many minutes will be terminated. The default is 60 minutes (1 hour). The maximum value is 28 days (40320 minutes).
max-transfer-idle-out: Outbound zone transfers making no progress in this many minutes will be terminated. The default is 60 minutes (1 hour). The maximum value is 28 days (40320 minutes).
max-transfer-time-in: Inbound zone transfers running longer than this many minutes will be terminated. The default is 120 minutes (2 hours). The maximum value is 28 days (40320 minutes).
max-transfer-time-out: Outbound zone transfers running longer than this many minutes will be terminated. The default is 120 minutes (2 hours). The maximum value is 28 days (40320 minutes).
notify-source: Determines which local source address, and optionally UDP port, will be used to send NOTIFY messages. This address must appear in the slave server's masters 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 statement in the configuration file.
notify-source-v6: The same as notify-source, but applies to NOTIFY messages sent to IPv6 addresses.
serial-query-rate: Slave servers will periodically query master servers to find out if zone serial numbers have changed. Each such query uses a minute amount of the slave server's network bandwidth. To limit the amount of bandwidth used, BIND 9.3 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.
transfer-format: Zone transfers can be sent using two different formats, one-answer and many-answers. The transfer-format option is used on the master 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 a message. many-answers is more efficient, but is only supported by relatively new slave servers, such as BIND 9.3, BIND 8.x, and patched versions of BIND 4.9.x. The default is many-answers. transfer-format may be overridden on a per-server basis by using the server statement.
transfer-source: Determines which local address will be 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 will usually be 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.
transfer-source-v6: The same as transfer-source, except that zone transfers are performed using IPv6.
transfers-in: The maximum number of concurrently running inbound zone transfers. The default value is 10. The maximum value is 28 days (40320 minutes). Increasing transfers-in may speed up the convergence of slave zones, but it may also increase the load on the local system.
transfers-out: The maximum number of concurrently running outbound zone transfers. Zone transfer requests in excess of the limit will be refused. The default value is 10.
transfers-per-ns: The maximum number of concurrently running inbound zone transfers from a given remote name server. The default value is 2. Increasing transfers-per-ns may speed up the convergence of slave 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.
use-alt-transfer-source: Use the alternate transfer sources or not. If views are specified, this defaults to no; otherwise, it defaults to yes (for BIND 8 compatibility).

The server Statement

server Statement Grammar

server ip_addr {
     [ bogus yes_or_no ; ]
     [ edns yes_or_no ; ]
     [ keys { string ; [ string ; ]... }; ]
     [ provide-ixfr yes_or_no ; ]
     [ request-ixfr yes_or_no ; ]
     [ transfer-format ( one-answer | many-answers ) ; ]
     [ transfer-source ( ip4_addr | * ) [ port ip_port ] ; ]
     [ transfer-source-v6 ( ip6_addr | * ) [ port ip_port ] ; ]
     [ transfers number ; ]
};

server Statement Definition and Usage

The server statement defines characteristics to be associated with a remote name server. 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 statement contains no server statements, any top-level server statements are used as defaults.

bogus

If you discover that a remote server is giving out bad data, marking it as bogus yes will prevent further queries to it. The default value is bogus no.

edns

(Extended DNS) Determines whether the local server will attempt to use EDNS when communicating with the remote server. The default is yes.

keys

Identifies a key_id defined by a key statement, to be used for transaction security when talking to the remote server. The key statement must come before the server statement that references it. When a request is sent to the remote server, a request signature will be 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. Although the grammar of the keys clause allows for multiple keys, only a single key per server is currently supported.

provide-ixfr

Determines whether the local server, acting as master, will respond with an incremental zone transfer when the given remote server, a slave, requests it. If set to yes, incremental transfer will be provided whenever possible. If set to no, all transfers to the remote server will be nonincremental. If not set, the value of the provide-ixfr option in the view or global options statement is used as a default.

request-ixfr

Determines whether the local server, acting as a slave, will request incremental zone transfers from the given remote server, a master. If not set, the value of the request-ixfr option in the view or global options statement is used as a default.

IXFR requests to servers that do not support IXFR will 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 master and slave claim to support it; for example, if one of the servers is defective and crashes or corrupts data when IXFR is used.

transfer-format

The server supports two zone transfer methods. one-answer uses one DNS message per resource record transferred. many-answers packs as many resource records as possible into a message. many-answers is more efficient, but is only known to be understood by BIND 9, BIND 8.x, and patched versions of BIND 4.9.5. You can specify which method to use for a server with the transfer-format option. If transfer-format is not specified, the transfer-format specified by the options statement is used.

transfer-source, transfer-source-v6

Specify the IPv4 and IPv6 source address to be used for zone transfer with the remote server, respectively. For an IPv4 remote server, only transfer-source can be specified. Similarly, for an IPv6 remote server, only transfer-source-v6 can be specified.

transfers

Limits 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 trusted-keys Statement

trusted-keys Statement Grammar

trusted-keys {
     ( domain_name flags protocol algorithm key_data ; )...
};

trusted-keys Statement Definition and Usage

The trusted-keys statement defines DNSSEC security roots. A security root is defined when the public key for a nonauthoritative zone is known, but cannot be securely obtained through DNS, either because it is the DNS root zone or its parent zone is unsigned. Once a key has been configured as a trusted key, it is treated as if it had been validated and proven secure. The resolver attempts DNSSEC validation on all DNS data in subdomains of a security root.

The trusted-keys statement can contain multiple key entries, each consisting of the key's five parameters: domain_name (string), flags (number), protocol (number), algorithm (number), and the base-64 representation of the key_data (string).

The view Statement

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 ; ]...
};

view Statement Definition and Usage

The view statement 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 name space that will be seen by a subset of clients. The order of the view statements is significant; a client request will be resolved in the context of the first view that it matches.

view_name

A name for the view.

class

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.

match-clients, match-destinations

A client matches a view if its source IP address matches the address_match_list of the view statement's match-clients clause and its destination IP address matches the address_match_list of the view statement's match-destinations clause.

If not specified, match-clients and match-destinations each default to matching all addresses.

match-recursive-only

Means that only recursive requests from matching clients match that view.

view_option

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. See The options Statement section.

zone_statement

Zones defined within a view statement will only be 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. See The zone Statement section.

If there are no view statements in the configuration file, a default view that matches any client is automatically created in class IN, and any zone statements specified on the top level of the configuration file are considered to be part of this 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 with 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 master;
          file "example-internal.db";
     };
};

view "external" {
     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 master;
          file "example-external.db";
     };
};

The zone Statement

zone Statement Grammar

zone zone_name [ class ] [ {
     type ( master | slave | hint
            | stub | forward | delegation-only ) ;
     [ allow-notify { address_match_list }; ]
     [ allow-query { address_match_list }; ]
     [ allow-transfer { address_match_list }; ]
     [ allow-update { address_match_list }; ]
     [ allow-update-forwarding { address_match_list }; ]
     [ also-notify { ( ip_addr [ port ip_port ] ; )... }; ]
     [ alt-transfer-source ( ip4_addr | * ) [ port ip_port ] ; ]
     [ alt-transfer-source-v6 ( ip6_addr | * ) [ port ip_port ] ; ]
     [ check-names ( warn | fail | ignore ) ; ]
     [ database string ; ]
     [ delegation-only yes_or_no ; ]
     [ dialup dialup_option ; ]
     [ file string ; ]
     [ forward ( only | first ) ; ]
     [ forwarders { ( ip_addr [ port ip_port ] ; )... }; ]
     [ ixfr-from-differences yes_or_no ; ]
     [ key-directory path_name ; ]
     [ masters [ port ip_port ] { (
          ( masters_name | ip_addr [ port ip_port ] [ key key ] ) ;
     )... }; ]
     [ max-refresh-time number ; ]
     [ max-retry-time number ; ]
     [ max-transfer-idle-in number ; ]
     [ max-transfer-idle-out number ; ]
     [ max-transfer-time-in number ; ]
     [ max-transfer-time-out number ; ]
     [ min-refresh-time number ; ]
     [ min-retry-time number ; ]
     [ multi-master yes_or_no ; ]
     [ notify yes_or_no | explicit ; ]
     [ notify-source ( ip4_addr | * ) [ port ip_port ] ; ]
     [ notify-source-v6 ( ip6_addr | * ) [ port ip_port ] ; ]
     [ sig-validity-interval number ; ]
     [ transfer-source ( ip4_addr | * ) [ port ip_port ] ; ]
     [ transfer-source-v6 ( ip6_addr | * ) [ port ip_port ] ; ]
     [ update-policy { update_policy_rule }; ]...
     [ use-alt-transfer-source yes_or_no ; ]
     [ zone-statistics yes_or_no ; ]
} ] ;

zone Statement Definition and Usage

zone_name

A name for the zone.

class

The class of the zone; one of the following:

IN: The Internet class. This is the default. IN is correct for the vast majority of cases.
HS, Hesiod: This class is named for an information service from MIT's Project Athena. It is used to share information about various systems databases, such as users, groups, printers and so on.
CHAOS: Another MIT development is Chaosnet, a LAN protocol created in the mid-1970s.

type

The type of the zone.

type master

The server has a master copy of the data for the zone and will be able to provide authoritative answers for it.

type slave

A slave zone is a replica of a master zone.

The masters list specifies one or more IP addresses of master servers that the slave contacts to update its copy of the zone.

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 master can also be done with per-server TSIG keys.

If a file is specified, then the replica will be 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 start-up and eliminates a needless waste of bandwidth. If the database files are very large, it is recommended to place them in different directories.

type stub

A stub zone is similar to a slave zone, except that it replicates only the NS records of a master 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 glue NS records 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 9.3 supports it only in a limited way. In BIND 4/8, zone transfers of a parent zone included the NS records from stub children of that zone. This meant that, in some cases, users could get away with configuring child stubs only in the master server for the parent zone. BIND 9 never mixes together zone data from different zones in this way. Therefore, if a BIND 9 master serving a parent zone has child stub zones configured, all the slave servers for the parent zone also need to have the same child stub zones configured.

Stub zones can also be used 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 2157 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.

type forward

A forward zone can be used to configure forwarding on a per-domain basis. A zone statement of type forward can contain a forward and/or forwarders statement, which will apply to queries within the domain given by the zone name. If no forwarders statement is present or an empty list of forwarders is given, then no forwarding will be done for the domain, canceling the effects of any forwarders in the options statement. Thus, if you want to use this type of zone to change the behavior of the global forward option (that is, forward first, then forward only, or vice versa, but want to use the same servers as set globally), you need to respecify the global forwarders.

type hint

The initial set of root name servers is specified using a hint zone. When the server starts up, 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 defaults hints.

type delegation-only

This is used to enforce the delegation-only status of infrastructure zones (for example, COM, NET, ORG). Any answer that is received without a explicit or implicit delegation in the authority section will be treated as NXDOMAIN. This does not apply to the zone apex. This should not be applied to leaf zones. delegation-only has no effect on answers received from forwarders.

allow-notify

See the description in The options Statement section.

allow-query

See the description in The options Statement section.

allow-transfer

See the description in The options Statement section.

allow-update

Specifies which hosts are allowed to submit Dynamic DNS updates for master zones. The default is to deny updates from all hosts. Please note that this option is not applicable for slave zones. See the Dynamic Update Policies section for more details.

allow-update-forwarding

also-notify

Only meaningful if notify is active for this zone. The set of machines that will receive a DNS NOTIFY message for this zone is made up of all the listed name servers (other than the primary master) 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. also-notify is not meaningful for stub zones. The default is the empty list.

alt-transfer-source

See the description in The options Statement section.

alt-transfer-source-v6

See the description in The options Statement section.

check-names

Restrict the character set and syntax of certain domain names in master files and/or DNS responses received from the network. The default varies according to zone type. For master zones, the default is fail. For slave zones, the default is warn.

database

Specify the type of database to be used for storing 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.

delegation-only

The flag only applies to hint and stub zones. If set to yes, then the zone is also treated as if it is also a delegation-only type zone.

dialup

See the description in The options Statement section.

file

A zone file designates a domain name with all of its associated subdomains, IP addresses, and mail server. A zone file contains resource records (A, CNAME, NS, SOA, and so on).

forward

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

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; the global options are not used.

ixfr-from-differences

If yes and the server loads a new version of a master zone from its zone file or receives a new version of a slave file by a nonincremental zone transfer, it will compare the new version to the previous one and calculate a set of differences. The differences are then logged in the zone's journal file such that the changes can be transmitted to downstream slaves as an incremental zone transfer.

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

key-directory

See the description in The options Statement section.

masters

See the type slave and type stub descriptions at the beginning of this section, and the description in The masters Statement section.

masters_name: The name of a masters statement.

max-refresh-time

See the description in The options Statement section.

max-retry-time

See the description in The options Statement section.

max-transfer-idle-in

See the description in The options Statement section.

max-transfer-idle-out

See the description in The options Statement section.

max-transfer-time-in

See the description in The options Statement section.

max-transfer-time-out

See the description in The options Statement section.

min-refresh-time

See the description in The options Statement section.

min-retry-time

See the description in The options Statement section.

multi-master

This should be set when you have multiple masters for a zone and the addresses refer to different machines. If yes, named will not log when the serial number on the master is less than what named currently has. The default is no.

notify

See the description in The options Statement section.

notify-source

See the description in The options Statement section.

notify-source-v6

See the description in The options Statement section.

sig-validity-interval

See the description in The options Statement section.

transfer-source

See the description in The options Statement section.

transfer-source-v6

See the description in The options Statement section.

update-policy

Specifies a "Simple Secure Update" policy. See the Dynamic Update Policies section for more details.

use-alt-transfer-source

See the description in The options Statement section.

zone-statistics

If yes, the server will keep statistical information for this zone, which can be dumped to the statistics-file defined in the server options.

Dynamic Update Policies

BIND 9.3 supports two alternative methods of granting clients the right to perform dynamic updates to a zone, configured by the allow-update and update-policy options, respectively.

The allow-update clause works the same way as in previous versions of BIND. It grants given clients the permission to update any record of any name in the zone.

The update-policy clause is new in BIND 9.3 and allows more fine-grained control over what updates are allowed. A set of rules is specified, where each rule either grants or denies permissions for one or more names to be updated by one or more identities. If the dynamic update request message is signed (that is, it includes either a TSIG or SIG(0) record), the identity of the signer can be determined.

Rules are specified in the update-policy zone option, and are only meaningful for master zones. When the update-policy statement is present, it is a configuration error for the allow-update statement to be present. The update-policy statement only examines the signer of a message; the source address is not relevant.

A sample rule definition is as shown below:

( grant | deny ) identity nametype name [ types ]

Each rule grants or denies privileges. Once a message has successfully matched a rule, the operation is immediately granted or denied and no further rules are examined. A rule is matched when the signer matches the identity field, the name matches the name field, and the type is specified in the list in the types field.

The identity field specifies a name or a wildcard name. Normally, this is the name of the TSIG or SIG(0) key used to sign the update request. When a TKEY exchange has been used to create a shared secret, the identity of the shared secret is the same as the identity of the key used to authenticate the TKEY exchange. When the identity field specifies a wildcard name, it is subject to DNS wildcard expansion, so the rule will apply to multiple identities. The identity field must contain a fully qualified domain name.

The nametype field has four possible values:

name: 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.
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 being updated matches the contents of the identity field. The name field is ignored, but should be the same as the identity field. The self nametype is most useful when allowing the use of one key per name to update, where the key has the same name as the name to be updated. The identity would be specified as * in this case.

In all cases, the name field must specify a fully qualified domain name.

If no types are explicitly specified, this rule matches all types except SIG, NS, SOA, and XT. A type may be specified by name, including ANY (which matches all types except NXT, 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.

The Statistics File

The statistics file generated by BIND 9.3 is similar, but not identical, to that generated by BIND 8. The statistics dump begins with the line

+++ Statistics Dump +++ (973798949) 

where the number in parentheses is a standard UNIX-style time stamp, measured as seconds since January 1, 1970. Following that line are a series of lines containing a counter type, the value of the counter, optionally a zone name, and optionally a view name. The lines without view and zone listed are global statistics for the entire server. Lines with a zone and view name are for the given view and zone (the view name is omitted for the default view). The statistics dump ends with the line

--- Statistics Dump --- (973798949) 

where the number is identical to the number in the beginning line.

The following statistics counters are maintained:

success: The number of successful queries made to the server or zone. A successful query is defined as query which returns a NOERROR response other than a referral response.
referral: The number of queries which resulted in referral responses.
nxrrset: The number of queries which resulted in NOERROR responses with no data.
nxdomain: The number of queries which resulted in NXDOMAIN responses.
recursion: The number of queries which caused the server to perform recursion in order to find the final answer.
failure: The number of queries which resulted in a failure response other than those above.

ZONE FILES

A zone file is a text file that defines a zone, designating a domain name, with all of its associated subdomains, IP addresses, and mail servers. It may contain directives, resource records, and comments. Blank lines may be included for readability. A zone definition starts with an SOA resource record. The zone file name is used in the file substatement of a zone statement in the named configuration file, /etc/named.conf.

A zone consists of those contiguous parts of the domain tree for which a domain server has complete information and over which it has authority. A domain server may be authoritative for more than one zone.

An absolute domain name or fully qualified domain name (FQDN) in a zone file is one that ends in a period (.). For example, example.com. is an absolute domain name.

A relative domain name in a zone file does not end in a period. For example, myhost.example is a relative domain name.

An origin in a zone file is an absolute domain name that is appended to a relative domain name to complete it. For example, if example.com. is the origin and myhost is a relative domain name, they would combine to form the absolute domain name, myhost.example.com..

A comment starts with a semicolon (;) and continues to the end of the line. A comment can appear on a line by itself or at the end of any directive or resource record line, including lines that are continued. For example, in the following record, ; Mail exchange is a comment.

hostname  IN  MX  10  mailhost.example.com. ; Mail exchange 

Records normally end at the end of a line. However, they may be continued across lines if the text is enclosed in parentheses, (...). See the example in the SOA Resource Records section.

Zone File Directives

Zone file directives help to simplify resource records. The directives include $ORIGIN, $INCLUDE, $TTL, and $GENERATE.

The $ORIGIN Directive

The $ORIGIN directive sets the origin that will be appended to any subsequent relative domain names. This provides a convenient shorthand for writing resource records.

Syntax

$ORIGIN origin

origin: A domain name that serves as the suffix for subsequent relative domain names.

When named starts, the default origin is the zone_name in the zone statement of the configuration file, /etc/named/conf.

If the new origin is not absolute (does not have a terminating period), the old origin is appended to it.

For example,

$ORIGIN    com.
$ORIGIN    example
WWW        CNAME    MAIN-SERVER

is equivalent to:

$ORIGIN    example.com.
WWW        CNAME    MAIN-SERVER

is equivalent to:

WWW.EXAMPLE.COM.    CNAME    MAIN-SERVER.EXAMPLE.COM.

The $INCLUDE Directive

The $INCLUDE directive reads and processes a file as if it were included into the file at this point.

Syntax

$INCLUDE filename [ origin ]

filename: The name of the file to be included.
origin: The origin for the data in the included file. The default is the current origin of the including (parent) file.

Neither the origin field nor $ORIGIN statements in the included file affect the origin of the parent file.

Once the included file has been processed, the origin and the current record owner name revert to the values they had prior to the $INCLUDE directive.

Note: RFC 1035 specifies that the current origin should be restored after an $INCLUDE, but it is silent on whether the current record owner 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.

The $TTL Directive

The $TTL directive sets the default Time to Live (TTL) value for subsequent RRs that have undefined TTLs.

Syntax

$TTL default-ttl

default-ttl: The default TTL value for subsequent RRs. See the Time to Live (TTL) and Time Specification sections for more detail.

The $GENERATE Directive

The $GENERATE directive creates a series of resource records that differ from each other only by an iterator value.

$GENERATE is a BIND extension and not part of the standard DNS zone file format.

Syntax

$GENERATE range lhs [ ttl ] [ class ] type rhs

(ttl and class may be entered in either order.)
range
The range of the iterator value. range can be in either of the forms: start-stop or start-stop/step. If the first form is used, then step is set to 1. All of start, stop, and step must be positive.
lhs
An expression that evaluates to the owner_name for each resource record that is created. If lhs is not an absolute domain name, the current origin is appended to it.
Any single $ symbols within lhs are replaced by the iterator value.
The $ may optionally be followed by modifiers that change the offset from the iterator, the field width, and the base. Modifiers are introduced by a { immediately following the $, in the format ${offset[,width[,base]]}. For example, ${-20,3,d}, which subtracts 20 from the current value and prints the result as a decimal in a zero-padded field of 3 characters. The available base values are d (decimal), o (octal), x (lowercase hexadecimal), and X (uppercase hexadecimal). The default modifier is ${0,0,d}.
To get a $ in the output, escape the $ with a backslash (\); for example, \$. For compatibility with earlier versions, $$ is still recognized as indicating a literal $ in the output.
ttl
The TTL for the generated records. If it is omitted, the normal TTL inheritance rules apply. See the Time to Live (TTL) and Time Specification sections for more detail.
class
The class of the generated records. This must match the zone class, if it is specified.
type
At present, the only supported types are A, AAAA, CNAME, DNAME, NS, and PTR.
rhs
An expression that evaluates to the rrdata for each resource record that is created. At present, this must be a domain name. It uses the same $ processing as lhs.

Example

$GENERATE easily generates the sets of records required to support the sub /24 reverse delegations described in RFC 2317, "Classless IN-ADDR.ARPA delegation".

$ORIGIN 0.0.192.IN-ADDR.ARPA.
$GENERATE 1-2 0 NS SERVER$.EXAMPLE.
$GENERATE 1-127 $ CNAME $.0

is equivalent to:

0.0.192.IN-ADDR.ARPA. NS SERVER1.EXAMPLE.
0.0.192.IN-ADDR.ARPA. NS SERVER2.EXAMPLE.
0.0.192.IN-ADDR.ARPA. CNAME 1.0.0.0.192.IN-ADDR.ARPA.
0.0.192.IN-ADDR.ARPA. CNAME 2.0.0.0.192.IN-ADDR.ARPA.
 ...
0.0.192.IN-ADDR.ARPA. CNAME 127.0.0.0.192.IN-ADDR.ARPA.

Resource Records (RRs)

This section, based on RFC 1034, describes the concept of a resource record, known as an RR, and when an RR is used. Since the publication of RFC 1034, several new RRs have been identified and implemented in the DNS. These are also included.

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, the sorting of multiple RRs is permitted for optimization purposes, for example, to specify that a particular nearby server be tried first.

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 a RR is as a typed pair of data, a domain name matched with relevant data, and stored with some additional type information to help systems determine when the RR is relevant.

Note:: 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. The binary format is defined in the RFCs that are listed with the RR type keywords in the following Syntax section. 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.

Syntax

owner_name [ ttl ] [ class ] type rrdata

(ttl and class may be entered in either order. The IN class and ttl values are often omitted from examples in the interests of clarity.)
owner_name
The domain name of the owner of the information in the RR. This can be one of:
. (period)
The domain name of the DNS root name server.
@
The current origin.
domain_name
A standard domain name. If domain_name does not end with a period (.), it is relative and the current origin is appended to it. If domain_name ends with a period (.), it is absolute.
blank
If the first character of the record is blank, the previous owner_name is used.
ttl
The Time to Live (TTL) 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 defines how long a RR can be cached before it should be discarded. See the Time to Live (TTL) and Time Specification sections for more detail.
class
A keyword, encoded as a 16-bit value, that identifies a protocol family or an instance of a protocol. The following keywords are supported:
IN
The Internet system, the default.
CH
Chaosnet, a LAN protocol created at MIT in the mid-1970s. Rarely used for its historical purpose, but reused for BIND's built-in server information zones, for example, version.bind.
HS
Hesiod, an information service developed by MIT's Project Athena. It is used to share information about various systems databases, such as users, groups, printers, and so on.
All records in a zone file must be of the same class.
type
A keyword, encoded as a 16-bit value, that specifies the type of the resource in this resource record. Types refer to abstract resources.
The following keywords are supported. Some of these listed, although not obsolete, are experimental or historical and not in general use.
A
Defines an IPv4 host address. In the IN class, this is a 32-bit IP address. Described in RFC 1035.
A6
Defines an IPv6 host address. This can be a partial address (a suffix) and an indirection to the name where the rest of the address (the prefix) can be found. Experimental. Obsoleted/deprecated. Use AAAA instead. Described in RFC 2874.
AAAA
Defines an IPv6 address. Described in RFC 1886.
CERT
Holds a digital certificate. Described in RFC 2538.
CNAME
The canonical name of an alias. Described in RFC 1035.
DNAME
Delegates reverse addresses. Replaces the domain name specified with another name to be looked up. Described in RFC 2672.
GPOS
Specifies the global position. Described in RFC 1712.
HINFO
Identifies the CPU and OS used by a host. Described in RFC 1035.
KEY
Stores a public key associated with a DNS name. Described in RFC 2535.
KX
Identifies a key exchanger for this DNS name. Described in RFC 2230.
MX
Identifies a mail exchange for the domain. A 16-bit preference value (lower is better) followed by the host name of the mail exchange. See the MX Resource Records section. Described in RFC 974 and RFC 1035.
NAPTR
Name authority pointer. Described in RFC 2915.
NSAP
A network service access point. Described in RFC 1706.
NS
An authoritative name server for the domain. Described in RFC 1035.
NXT
Used in DNSSEC to securely indicate that RRs with an owner name in a certain name interval do not exist in a zone and indicate what RR types are present for an existing name. Described in RFC 2535.
PTR
Domain name pointer. A pointer to another part of the domain name space. Often user to associate an IP address with a domain name. Described in RFC 1035.
PX
Provides mappings between RFC 822 and X.400 addresses. Described in RFC 2163.
SIG (signature)
Contains data authenticated in the secure DNS. Described in RFC 2535.
SOA
Identifies the start of a zone of authority in a zone file. See the SOA Resource Records section. Described in RFC 1035.
SRV
Information about the well-known network services, such as SMTP, that a domain supports. Supersedes WKS. Described in RFC 2282.
TXT
Text records. Described in RFC 1035.
WKS
Information about well known network services. Historical. Superseded by SRV. Described in RFC 1035.
rrdata
The type-dependent and sometimes class-dependent data that describes the resource. This data is defined in the RFCs that are specified with each type keyword.

MX Resource Records

MX records control the delivery of e-mail. Described in RFC 974 and RFC 1035.

Syntax

... MX priority host_domain_name

...: The owner_name, ttl, and class have been omitted for clarity.
priority: The priority controls the order in which e-mail 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 will fall 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.
host_domain_name: The domain name of the machine to which the mail should be delivered.

An MX record must have an associated A record; a 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 will be ignored. Instead, the mail will be delivered to the server specified in the MX record pointed to by the CNAME.

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 will be attempted to mail.example.com and mail2.example.com (in any order), and if neither of those succeed, delivery to mail.backup.org will be attempted.

SOA Resource Records

Each zone file begins with an SOA record for the zone. All records in a zone file must be of the same class. Described in RFC 1035.

Syntax

... SOA mname rname serial refresh retry expire minimum

...: The owner_name, ttl, and class have been omitted for clarity.
mname: The domain name of the name server that is the source of data for this zone.
rname: A domain name that specifies the mailbox of the person responsible for this zone. The first period represents the @ in the e-mail address. If the mailbox user name contains a period, you can escape it with a backslash (\). See the example.
serial: An arbitrary unsigned 32-bit integer serial number for the zone. The range is 0 to 4294967295 (2^32-1).
refresh: A 32-bit integer time interval in seconds to refresh the zone. See the Time Specification section for more detail.
retry: A 32-bit integer time to wait in seconds before retrying a failed refresh. See the Time Specification section for more detail.
expire: A 32-bit integer time interval in seconds after which the zone is no longer authoritative. See the Time Specification section for more detail.
minimum: The TTL in seconds for resolvers that cache negative responses. See the Time to Live (TTL) and Time Specification sections for more detail.

The SOA specifies a serial number, which should be changed each time the zone file is changed. Note that it is not advisable to give the serial number as a dotted number, since the translation to normal integers is via concatenation rather than multiplication and addition. You can represent the year, month, day of month, and a 0..99 version number (yyyymmddvv) and still fit inside the unsigned 32-bit size of this field. (It's true that we will have to rethink this strategy in the year 4294.)

Secondary servers check the serial number at intervals specified by the refresh time in seconds; if the serial number changes, a zone transfer will be done to load the new data. If a master server cannot be contacted when a refresh is due, the retry time specifies the interval at which refreshes should be attempted. If a master server cannot be contacted within the interval given by the expire time, all data from the zone is discarded by secondary servers.

Example

@  IN  SOA  ucbvax.Berkeley.EDU.  Jane\.Doe.ucbvax.Berkeley.EDU.  (
                              1989020501      ; serial
                              10800           ; refresh
                              3600            ; retry
                              3600000         ; expire
                              86400 )         ; minimum

Time to Live (TTL)

The TTL field of an RR is a 32-bit integer representing time in seconds. It is primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be discarded. This limit does not apply to authoritative data in zones; it is also timed out, but by 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 can be anticipated, the TTL can be reduced prior to the change to minimize inconsistency during the change, and then increased back to its former value following the change.

The following three types of TTL are currently used in a zone file.

SOA

The minimum field in an SOA RR is the negative-caching TTL. This controls how long other servers will cache no-such-domain (NXDOMAIN) responses from you. The maximum time for negative caching is 3 hours (3h).

Note: This use of the minimum field was implemented in RFC 2308, largely superseding the usage specified in RFC 1034 (but see the default calculation for the ttl field below).

$TTL

A $TTL directive at the top of the zone file (before the SOA) provides a default TTL for subsequent RRs.

Note: The $TTL directive, defined in RFC 2308, supersedes the original use of the SOA minimum field specified in RFC 1034.

ttl

The ttl field in an RR specifies the TTL for the record. If it is omitted, the value specified by the previous $TTL directive is used. If there is no previous $TTL directive, the minimum field in the SOA resource record is used.

Time Specification

All the TTLs and and the SOA time fields are specified in seconds, as a 32-bit integer value in the range 0 to 2147483647 (2^31-1).

Here's a table of convenient values:

Seconds in an Integer Value
minimum	0 seconds
1 minute	60 seconds
1 hour	3600 seconds
1 day	86400 seconds
7 days	604800 seconds
30 days	2592000 seconds
24855 days	2147472000 seconds
maximum	2147483647 seconds

For convenience, some units can be explicitly specified; you can use h for hours, m for minutes, and s for seconds. For example, 1h30m.

Inverse Mapping in IPv4

Reverse name resolution (that is, translation from an IP address to a domain name) is achieved with the in-addr.arpa domain and PTR records. Entries in the in-addr.arpa domain are made in least-to-most significant order, reading left to right. This is the reverse of 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 domain name of the machine. If the machine has more than one name. it will need multiple PTR records. For example, for IP address 10.1.2.3, corresponding to host name fred.example.com in the example.com domain:

$ORIGIN  2.1.10.in-addr.arpa
3  IN  PTR  fred.example.com.

Example

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 rrdata section that 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.

This example shows six RRs, with two RRs at each of three domain names.

AUTHOR

named.conf and the $GENERATE directive were developed by the Internet Systems Consortium (ISC).

Zone files were developed by the Internet Engineering Task Force (IETF).

FILES

/dev/random
/etc/named.conf: Default named configuration file.
/etc/resolv.conf
/etc/rndc.key
/usr/bin/named-bootconf.sh: Shell script to convert BIND 4.9.7 configuration files to the BIND 9.3 format.
/var/run/named.pid

named.conf(4)

Technical documentation

» Table of Contents

» Index

NAME

SYNOPSIS

DESCRIPTION

Syntax Rules

Configuration File Elements

Address Match List Syntax

Address Match List Definition and Usage

Comment Syntax

CONFIGURATION FILE GRAMMAR

The acl Statement

acl Statement Grammar

acl Statement Definition and Usage

The controls Statement

controls Statement Grammar

controls Statement Definition and Usage

The include Statement

include Statement Grammar

include Statement Definition and Usage

The key Statement

key Statement Grammar

key Statement Definition and Usage

The logging Statement

logging Statement Grammar

logging Statement Definition and Usage

The channel Phrase

The category Phrase

The lwres Statement

lwres Statement Grammar

lwres Statement Definition and Usage

The masters Statement

masters Statement Grammar

masters Statement Definition and Usage

The options Statement

options Statement Grammar

options Statement Definition and Usage

General Options

Boolean Options

Access Control Options

Bad UDP Port List Options

Built-In Server Information Zone Options

Dual-Stack Server Option

Forwarding Options

Interface Options

Obsolete Option

Operating System Resource Limit Options

Periodic Task Interval Options

Query Address Options

RRset Ordering Option

Server Resource Limit Options

Sorting Option

Tuning Options

Zone Transfer Options

The server Statement

server Statement Grammar

server Statement Definition and Usage

The trusted-keys Statement

trusted-keys Statement Grammar

trusted-keys Statement Definition and Usage

The view Statement

view Statement Grammar

view Statement Definition and Usage

The zone Statement

zone Statement Grammar

zone Statement Definition and Usage

Dynamic Update Policies

The Statistics File

ZONE FILES

Zone File Directives

The $ORIGIN Directive

The $INCLUDE Directive

The $TTL Directive

The $GENERATE Directive

Resource Records (RRs)

MX Resource Records

SOA Resource Records

Time to Live (TTL)