named.conf File Format for TCP/IP

Purpose

Defines the configuration and behavior of the named daemon.

Description

The /etc/named.conf file is the default configuration file for the named8 and named9 server. If the named daemon is started without specifying an alternate file, the named daemon reads this file for information on how to set up the local name server.

The data in the named.conf file specifies general configuration characteristics for the name server, defines each zone for which the name server is responsible (its zones of authority), and provides further config information per zone, possibly including the source DOMAIN database file for the zone.

Any database files referenced in the named.conf file must be in the Standard Resource Record Format. These data files can have any name and any directory path. However, for convenience in maintaining the named database, they are given names in the following form: /etc/named.extension. The general format of named data files is described in DOMAIN Data File, DOMAIN Reverse Data File, DOMAIN Cache File, and DOMAIN Local File.

Format of the named.conf file when configuring named8

General

Comments in the named.conf file can begin with a # (pound sign) or // (two forward slashes), or can be enclosed in the C-style comment characters, for example, /* comment text */.

Configuration options are lines of text beginning with a keyword, possibly including some option text or a list, and ending in a ; (semicolon).

The named.conf file is organized into stanzas. Each stanza is an enclosed set of configuration options that define either general characteristics of the daemon or a zone configuration. Certain stanza definitions are allowed only at the top level, therefore nesting these stanzas is not allowed. The current top-level configuration stanza keywords are: acl, key, logging, options, server, and zone.

Further configuration information can be incorporated through the include keyword into the conf file. This keyword directs the daemon to insert the contents of the indicated file into the current position of the include directive.

Access Control List (ACL) Definition

acl acl-name {
    access-element;
    [ access-element; ... ]
};

Defines an access control list to be referenced throughout the configuration file byacl-name. Multiple acl definitions can exist within one configuration file if each acl-name is unique. Additionally, four default access control lists are defined:

• any Any host is allowed.
• none No host is allowed.
• localhost Only the localhost is allowed.
• localnets Only hosts on a network matching a name server interface is allowed.

acl hostlist1 {
    !9.53.150.239;
    9.3.149/24;
};

Key Definition

key key-name {
    algorithm alg-id;
    secret secret-string;
};

Defines an algorithm and shared secret key to be referenced in a server stanza and used for authentication by that name server. This feature is included for future use and is currently unused in the name server.

Logging Configuration

logging {

    [ channel channel-name {
       ( file file-name
             [ versions ( num-vers | unlimited ) ]
             [ size size-value ]
       | syslog ( kern | user | mail | daemon |
                  syslog | lpr | news | uucp )
       | null );
       [ print-category ( yes | no ); ]
       [ print-severity ( yes | no ); ]
       [ print-time ( yes | no ); ]
      }; ... ]
    [ category category-name {
          channel-reference;
          [ channel-reference; ... ]
      }; ... ]
};

In this newest version of the name server, the logging facility has been greatly improved to allow for much reconfiguration of the default logging mechanism. The logging stanza is used to define logging output channels and to associate the predefined logging categories with either the predefined or user-defined logging output channels.

When no logging stanza is included in the conf file, the name server still logs messages and errors just as it has in previous releases. Informational and some critical messages are logged through the syslog daemon facility, and debug and other esoteric information are logged to the named.run file when the global debug level (set with the -d command-line option) is non-zero.

Global Options

options {
    [ directory path-string; ]
    [ named-xfer path-string; ]
    [ dump-file path-string; ]
    [ pid-file path-string; ]
    [ statistics-file path-string; ]
    [ auth-nxdomain ( yes | no ); ]
    [ fake-iquery ( yes | no ); ]
    [ fetch-glue ( yes | no ); ]
    [ multiple-cnames ( yes | no ); ]
    [ notify ( yes | no ); ]
    [ recursion ( yes | no ); ]
    [ forward ( only | first ); ]
    [ forwarders { ipaddr; [...] }; ]
    [ check-names
       ( master|slave|response )
       ( warn|fail|ignore ); ]
    [ allow-query { access-element; [...] }; ]
    [ allow-transfer { access-element; [...] ); ]
    [ listen-on [ port port-num ] { access-element; [...] }; ... ]
    [ query-source [ address ( ipaddr|* ) ] [ port ( port|* ) ]; ]
    [ max-transfer-time-in seconds; ]
    [ transfer-format ( one-answer | many-answers ); ]
    [ transfers-in value; ]
    [ transfers-out value; ]
    [ transfers-per-ns value; ]
    [ coresize size-value; ]
    [ datasize size-value; ]
    [ files size-value; ]
    [ stacksize size-value; ]
    [ clean-interval value; ]
    [ interface-interval value; ]
    [ statistics-interval value; ]
    [ topology { access-element; [...] }; ]
};

Defines many globally available options to modify basic characteristics of the name server.

Because some of the options in this configuration stanza may modify the behavior in how the named daemon reads and interprets later sections of the named file, it is highly recommended that the options stanza be the first stanza listed in the configuration file.

Server Specific Options

server ipaddr
{
    [ bogus ( yes | no ); ]
    [ transfers value;
]
    [ transfer-format ( one-answer |
many-answers ); ]
}

Modifies the behavior in which the remote name server matching the specified ipaddr IP address should be treated.

Zone Definition

zone domain-string [ class ] {
    type ( hint | stub | slave | master );
    [ file path-string; ]
    [ masters { ipaddr; [...] }; ]
    [ check-names ( warn | fail | ignore ); ]
    [ allow-update { access-element; [...] }; ]
    [ update-security ( unsecured | presecured | controlled ); ]
    [ allow-query { access-element; [...] }; ]
    [ allow-transfer { access-element; [...] }; ]
    [ max-transfer-time-in seconds; ]
    [ notify ( yes | no ); ]
    [ also-notify { ipaddr; [...] }; ]
    [ dont-notify { ipaddr; [...] }; ]
    [ notify-delaytime seconds; ]
    [ notify-retrytime seconds; ]
    [ notify-retrycount value; ]
    [ dump-interval seconds; ]
    [ incr-interval seconds; ]
    [ deferupdcnt value; ]
    [ key-xfer ( yes | no ); ]
    [ timesync ( yes | no ); ]
    [ timesync-xfer ( yes | no ); ]
    [ save-backups ( yes | no ); ]
    [ ixfr-directory path-string; ]
    [ separate-dynamic ( yes | no ); ]
};

The zone stanza is used to define a zone, its type, possible location of data, and operating parameters. The domain-string is a quoted string specifying the zone, where “.” is used to specify the root zone. The class parameter specifies the class of the zone as either in, hs, hesiod, or chaos. By default, the class is assumed to be IN.

Examples

The following examples show some of the various methods to use to configure a simple named.conf file. In these examples, two networks are represented: abc and xyz.

Network abc consists of:

• gobi.abc, the master name server for the abc network, 192.9.201.2
• mojave.abc, a host machine, 192.9.201.6
• sandy.abc, a slave name server for the abc network and the gateway between abc and xyz, 192.9.201.3

Network xyz consists of:

• kalahari.xyz, master name server for the xyz network, 160.9.201.4
• lopnor.xyz, a host machine, 160.9.201.5
• sahara.xyz, a host machine and hint name server for the xyz network, 160.9.201.13
• sandy.xyz, a slave name server for the xyz network and gateway between abc and xyz, 160.9.201.3
  Note: sandy, a gateway host, is on both networks and also serves as a slave name server for both domains.

1. The /etc/named.conf file for gobi.abc, the master name server for network abc, contains these entries:
   # # conf file for abc master server - gobi.abc # server 192.9.201.3 { transfer-format many-answers; };
   zone “abc” in { type master; file “/etc/named.abcdata”; allow-update { localhost; }; };
   zone “201.9.192.in-addr.arpa” in { type master; file “/etc/named.abcrev”; allow-update { localhost; }; };
   zone “0.0.127.in-addr.arpa” in { type master; file “/etc/named.abclocal”; };
2. The /etc/named.conf file for kalahari.xyz, the master name server for network xyz, contains these entries:
   # # conf file for abc master server - kalahari.xyz # acl xyz-slaves { 160.9.201.3; };
   options { directory “/etc”; allow-transfer { xyz-slaves; localhost; }; };
   zone “xyz” in { type master; file “named.xyzdata”; };
   zone “9.160.in-addr.arpa” in { type master; file “named.xyxrev”; };
   zone “0.0.127.in-addr.arpa” in { type master; file “named.xyzlocal”; };
3. The /etc/named.conf file for sandy, the slave name server for networks abc and xyz, contains the following entries:
   # # conf file for slave server for abc and xyz - sandy # options { directory “/etc”; };
   zone “abc” in { type slave; masters { 192.9.201.2; }; file “named.abcdata.bak”; };
   zone “xyz” in { type slave; masters { 160.9.201.4; }; file “named.xyzdata.bak”; };
   zone “201.9.192.in-addr.arpa” in { type slave; masters { 192.9.201.2; }; };
   zone “9.160.in-addr.arpa” in { type slave; masters { 192.9.201.4; }; };
   zone “0.0.127.in-addr.arpa” in { type master; file “named.local”; };
4. The /etc/named.conf file for sahara, a hint name server for the network xyz, contains the following entries:
   # # conf file for hint server for xyz - sahara # zone “.” in { type hint; file “/etc/named.ca”; };
   zone “0.0.127.in-addr.arpa” in { type master; file “/etc/named.local”; };

Format of the named.conf file when configuring named9

General

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

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

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 a primary use of address match lists: 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.

controls Statement Grammar

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

controls Statement Definition and Usage

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

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 ability to issue commands over the control channel is restricted by the allow and keys clauses. Connections to the control channel are permitted based on the address permissions in address_match_list. key_id members of the address_match_list are ignored, and instead are interpreted independently based 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, named9 sets up a default control channel listening on the loopback address 127.0.0.1 and its IPv6 counterpart ::1. In this case, and also when the controls statement is present but does not have a keys clause, named9 attempts to load the command channel key from the /etc/rndc.key file in (or whatever sysconfdir was specified as when BIND was built). To create a rndc.key file, run rndc-confgen -a.

The rndc.key feature was created 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. It makes it possible to use an existing BIND 8 configuration file in BIND 9 unchanged, and still have rndc work the same way ndc worked in BIND 8, simply by executing the command rndc-keygen -a after BIND 9 is installed.

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 a rndc.conf with your own key if you wish to change those things. 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. If it is present in the controls statement from a BIND 8 configuration file, it is ignored and a warning is logged.

include Statement Grammar

include filename;

include Statement Definition and Usage

The include statement inserts the specified file at the point that 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 nameserver.

key Statement Grammar

key key_id {
    algorithm string;
    secret string;
};

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.

The key_id, also known as the key name, is a domain name uniquely identifying the key. It can be used in a "server" statement to cause requests sent to that server to be signed with this key, or in address match lists to verify that incoming requests have been signed with a key matching this name, algorithm, and secret. The algorithm_id is a string that specifies a security/authentication algorithm. The only algorithm currently supported with TSIG authentication is hmac-md5. The secret_string is the secret to be used by the algorithm, and is treated as a base-64 encoded string.

logging Statement Grammar

logging {
   [ channel channel_name {
     ( file path name
         [ versions ( number | unlimited ) ]
         [ size size spec ]
       | syslog syslog_facility
       | stderr
       | null );
     [ 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 ; [ channel_name ; ... ]
   }; ]
   ...
};

logging Statement Definition and Usage

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

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

The channel Phrase

All log output goes to one or more channels; you can make as many of them as you want.

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

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

The file destination clause directs the channel to a disk file. It can include limitations both on how large the file is allowed to become, and how many versions of the file are saved each time the file is opened.

If you use the versions log file option, then named9 retains that many backup versions of the file by renaming them when opening. For example, if you choose to keep 3 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, and lamers.log is renamed to lamers.log.0. You can say versions unlimited; to not 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 files is used to limit log growth. If the file ever exceeds the size, then named9 stops writing to the file unless it has a versions option associated with it. If backup versions are kept, the files are rolled as described above and a new one begun. If there is no versions option, no more data is written to the log until some out-of-band mechanism removes or truncates the log to less than the maximum size. The default behavior is not to limit the size of the file.

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 man page. How syslog handles messages sent to this facility is described in the syslog.conf man page. If you have a system which uses a very old version of syslog that only uses two arguments to the openlog() function, then this clause is silently ignored. The severity clause works like syslog's "priorities," except that they can also be used if you are writing straight to a file rather than using syslog. Messages which are not at least of the severity level given are not selected for the channel; messages of higher severity levels are accepted.

If you are using syslog, then the syslog.conf priorities also determine what eventually passes through. For example, defining a channel facility and severity as daemon and debug but only logging daemon.warning via syslog.conf causes messages of severity info and notice to be dropped. If the situation is reversed, with named9 writing messages of only warning or higher, then syslogd prints all messages it received from the channel.

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

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

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

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" {              // writes to stderr
    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 named9run in the server's working directory.

For security reasons, when the -u command-line option is used, the named9run file is created only after named9 has changed to the new UID, and any debug output generated while named9 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

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

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

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

options Statement Grammar

options {
    [ version version_string; ]
    [ directory path_name; ]
    [ named-xfer path_name; ]
    [ tkey-domain domainname; ]
    [ tkey-dhkey key_name key_tag; ]
    [ dump-file path_name; ]
    [ memstatistics-file path_name; ]
    [ pid-file path_name; ]
    [ statistics-file path_name; ]
    [ zone-statistics yes_or_no; ]
    [ auth-nxdomain yes_or_no; ]
    [ deallocate-on-exit yes_or_no; ]
    [ dialup dialup_option; ]
    [ fake-iquery yes_or_no; ]
    [ fetch-glue yes_or_no; ]
    [ has-old-clients yes_or_no; ]
    [ host-statistics yes_or_no; ]
    [ minimal-responses yes_or_no; ]
    [ multiple-cnames yes_or_no; ]
    [ notify yes_or_no | explicit; ]
    [ recursion yes_or_no; ]
    [ rfc2308-type1 yes_or_no; ]
    [ use-id-pool yes_or_no; ]
    [ maintain-ixfr-base yes_or_no; ]
    [ forward ( only | first ); ]
    [ forwarders { ip_addr [port ip_port] ; [ ip_addr [port ip_port] ; ... ] }; ]
    [ check-names ( master | slave |  response )( warn | fail | ignore ); ]
    [ allow-notify { address_match_list }; ]
    [ allow-query { address_match_list }; ]
    [ allow-transfer { address_match_list }; ]
    [ allow-recursion { address_match_list }; ]
    [ allow-v6-synthesis { address_match_list }; ]
    [ blackhole { address_match_list }; ]
    [ listen-on [ port ip_port ] { address_match_list }; ]
    [ listen-on-v6 [ port ip_port ] { address_match_list }; ]
    [ query-source [ address ( ip_addr | * ) ] [ port ( ip_port | * ) ]; ]
    [ max-transfer-time-in number; ]
    [ max-transfer-time-out number; ]
    [ max-transfer-idle-in number; ]
    [ max-transfer-idle-out number; ]
    [ tcp-clients number; ]
    [ recursive-clients number; ]
    [ serial-query-rate number; ]
    [ serial-queries number; ]
    [ transfer-format ( one-answer | many-answers ); ]
    [ transfers-in  number; ]
    [ transfers-out number; ]
    [ transfers-per-ns number; ]
    [ transfer-source (ip4_addr | *) [port ip_port] ; ]
    [ transfer-source-v6 (ip6_addr | *) [port ip_port] ; ]
    [ notify-source (ip4_addr | *) [port ip_port] ; ]
    [ notify-source-v6 (ip6_addr | *) [port ip_port] ; ]
    [ also-notify { ip_addr [port ip_port] ; [ ip_addr [port ip_port] ; ... ] }; ]
    [ max-ixfr-log-size number; ]
    [ coresize size_spec ; ]
    [ datasize size_spec ; ]
    [ files size_spec ; ]
    [ stacksize size_spec ; ]
    [ cleaning-interval number; ]
    [ heartbeat-interval number; ]
    [ interface-interval number; ]
    [ statistics-interval number; ]
    [ topology { address_match_list }];
    [ sortlist { address_match_list }];
    [ rrset-order { order_spec ; [ order_spec ; ... ] ] };
    [ lame-ttl number; ]
    [ max-ncache-ttl number; ]
    [ max-cache-ttl number; ]
    [ sig-validity-interval number ; ]
    [ min-roots number; ]
    [ use-ixfr yes_or_no ; ]
    [ provide-ixfr yes_or_no; ]
    [ request-ixfr yes_or_no; ]
    [ treat-cr-as-space yes_or_no ; ]
    [ min-refresh-time number ; ]
    [ max-refresh-time number ; ]
    [ min-retry-time number ; ]
    [ max-retry-time number ; ]
    [ port ip_port; ]
    [ additional-from-auth yes_or_no ; ]
    [ additional-from-cache yes_or_no ; ]
    [ random-device path_name ; ]
    [ max-cache-size size_spec ; ]
    [ match-mapped-addresses 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 is used.

Boolean Options

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; this is a change from BIND 8. If you are using very old DNS software, you may need to set it to yes.

deallocate-on-exit

This option was used in BIND 8 to enable checking for memory leaks on exit. BIND 9 ignores the option and always performs the checks.

dialup

If yes, 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-intervaland 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 the view and zone statements, in which case it overrides the global dialup option.

If the zone is a master zone then the server sends out a NOTIFY request to all the slaves. This triggers the zone serial number check in the slave (providing 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 suppresses 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; and refresh, which suppresses normal refresh processing and send refresh queries when the heartbeat-interval expires and passive which just disables normal refresh processing.

fake-iquery

In BIND 8, this option was used to enable simulating the obsolete DNS query type IQUERY. BIND 9 never does IQUERY simulation.

fetch-glue

This option is obsolete. In BIND 8, fetch-glue yes caused the server to attempt to fetch glue resource records it did not have when constructing the additional data section of a response. This is now considered bad practice, and BIND 9 never does it.

has-old-clients

This option was incorrectly implemented in BIND 8, and is ignored by BIND 9. To achieve the intended effect of has-old-clients yes, specify the two separate options auth-nxdomain yes and rfc2308-type1 no instead.

host-statistics

In BIND 8, this enables keeping of statistics for every host that the nameserver interacts with. It is not implemented in BIND 9.

maintain-ixfr-base

This option is obsolete. It was used in BIND 8 to determine whether a transaction log was kept for Incremental Zone Transfer. BIND 9 maintains a transaction log whenever possible. If you need to disable outgoing incremental zone transfers, use provide-ixfr no.

minimal-responses

If yes, then when generating responses, the server only adds records to the authority and additional data sections when they are required. This may improve the performance of the server. The default is no.

multiple-cnames

This option was used in BIND 8 to allow a domain name to allow multiple CNAME records in violation of the DNS standards. BIND 9.2 strictly enforces the CNAME rules both in master files and dynamic updates.

notify

If yes (default), DNS NOTIFY messages are sent when a zone the server is authoritative for 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, notifies are sent only to servers explicitly listed using also-notify. If no, no notifies are sent.

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

recursion

If yes, and a DNS query requests recursion, then the server attempts to do all the work required to answer the query. If recursion is off and the server does not already know the answer, it returns a referral response. The default is yes.

Note: Setting recursion no does not prevent clients from getting data from the cache on the server; it only prevents new data from being cached as an effect of client queries. Caching may still occur as an effect the server's internal operation, such as NOTIFY address lookups.

rfc2308-type1

Setting this to yes causes the server to send NS records along with the SOA record for negative answers. The default is no.

Note: Not yet implemented in BIND 9.

use-id-pool

This option is obsolete. BIND 9 always allocates query IDs from a pool.

zone-statistics

If yes, the server, by default, collects statistical data on all zones in the server. These statistics may be accessed using rndc stats, which dumps them to the file listed in the statistics-file. See The Statistics File.

use-ixfr

This option is obsolete. If you need to disable IXFR to a particular server or servers see the information on the provide-ixfr option in Server Statement Definition and Usage.

provide-ixfr

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

request-ixfr

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

treat-cr-as-space

This option was used in BIND 8 to make the server treat carriage return ("\r") characters the same way as a space or tab character to facilitate loading of zone files on a UNIX system that were generated on a Windows NT or DOS machine. In BIND 9, both UNIX "\n" and NT/DOS "\r\n" newlines are always accepted, and the option is ignored.

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 (default) and a query is being answered from authoritative data (a zone configured into the server), the additional data section of the reply is 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 speeds 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 for mail.example.net are provided as well, if known. Setting these options to no disables 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; causes 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 non-recursively 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 responds to such queries with REFUSED. This should not cause any problems since upwards referrals are not required for the resolution process.

match-mapped-addresses

If yes, then an IPv4-mapped IPv6 address matches any address match list entries that match the corresponding IPv4 address. Enabling this option is sometimes useful on IPv6-enabled Linux systems, to work around a kernel quirk that causes IPv4 TCP connections such as zone transfers to be accepted on an IPv6 socket using mapped addresses, causing address match lists designed for IPv4 to fail to match. The use of this option for any other purpose is discouraged.

Forwarding

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

forward

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

forwarders

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 various ways. You can set particular domains to use different forwarders, or have a different forward only/first behavior, or not forward at all. See Zone Statement Grammar.

Access Control

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. The allow-notify option 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. The allow-query option 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 cache on the server.

allow-transfer

Specifies which hosts are allowed to receive zone transfers from the server. allow-transfer may also be specified in the zone statement, in which case it overrides the 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 does not accept queries from or uses to resolve a query. Queries from these addresses are not responded to. The default is none.

Interfaces

The interfaces and ports that the server answers queries from may be specified using the listen-on option. listen-on takes an optional port, and an address_match_list. The server listens on all interfaces allowed by the address match list. If a port is not specified, port 53 is used.

listen-on { 5.6.7.8; };
listen-on port 1234 { !1.2.3.4; 1.2/16; };

If no listen-on is specified, the server listens on port 53 on all interfaces.

Query Address

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

Zone Transfers

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

also-notify

Defines a global list of IP addresses of name servers that are also sent NOTIFY messages whenever a fresh copy of the zone is loaded, in addition to the servers listed in the zone's NS records. This helps to ensure that copies of the zones quickly converge on stealth servers. If an also-notify list is given in a zone statement, it overrides the options also-notify statement. When a zone notify statement is set to no, the IP addresses in the global also-notify list are not sent NOTIFY messages for that zone. The default is the empty list (no global notification list).

max-transfer-time-in

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

max-transfer-idle-in

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

max-transfer-time-out

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

max-transfer-idle-out

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

serial-query-rate

Slave servers 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 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.

serial-queries

In BIND 8, the serial-queries option set the maximum number of concurrent serial number queries allowed to be outstanding at any given time. BIND 9 does not limit the number of outstanding serial queries and ignores the serial-queries option. Instead, it limits the rate at which the queries are sent as defined using the serial-query-rate option.

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, BIND 8.x and patched versions of BIND 4.9.5. The default is many-answers. transfer-format may be overridden on a per-server basis by using the server statement.

transfers-in

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

transfers-out

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

transfers-per-ns

The maximum number of inbound zone transfers that can be concurrently transferring from a given remote nameserver. 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 nameserver. transfers-per-ns may be overridden on a per-server basis by using the transfers phrase of the server statement.

transfer-source

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

notify-source

notify-source determines which local source address, and optionally UDP port, is used to send NOTIFY messages. This address must appear in the 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 / per-view basis by including a notify-source statement within the zone or view block in the configuration file.

Operating System Resource Limits

The server's usage of many system resources can be limited. Scaled values are allowed when specifying resource limits. For example, 1G can be used instead of 1073741824 to specify a limit of one gigabyte. The unlimited option requests unlimited use, or the maximum available amount. The default option 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. Some operating systems do not support some or any of the limits. On such systems, a warning is issued if the 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 fails, 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.

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.

Server Resource Limits

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

max-ixfr-log-size

This option is obsolete; it is accepted and ignored for BIND 8 compatibility.

recursive-clients

The maximum number of simultaneous recursive lookups the server performs on behalf of clients. The default is 1000. Because each recursing client uses a fair bit of memory, on the order of 20 KB, 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 accepts. The default is 100.

max-cache-size

The maximum amount of memory to use for the cache on the server, in bytes. When the amount of data in the cache reaches this limit, the server causes the 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 value is unlimited, indicating that records are purged from the cache only when their TTLs expire. The minimum value that you can set for this limit is 2 MB.

Periodic Task Intervals

cleaning-interval

The server removes expired resource records from the cache every cleaning-interval minutes. The default is 60 minutes. If set to 0, periodic cleaning does not occur.

heartbeat-interval

The server performs zone maintenance tasks for all zones marked as dialup whenever this interval expires. The default is 60 minutes. Reasonable values are up to 1 day (1440 minutes). If set to 0, zone maintenance for these zones does not occur.

interface-interval

The server scans the network interface list every interface-interval minutes. The default is 60 minutes. If set to 0, interface scanning only occurs when the configuration file is loaded. After the scan, listeners are started on any new interfaces (provided they are allowed by the listen-on configuration). Listeners on interfaces that have gone away are cleaned up.

statistics-interval

Nameserver statistics are logged every statistics-interval minutes. The default is 60. If set to 0, no statistics are logged.

Note: Not yet implemented in BIND 9.

Topology

All other things being equal, when the server chooses a nameserver to query from a list of nameservers, it prefers the one that is topologically closest to itself. The topology statement takes an address_match_list and interprets it in a special way. Each top-level list element is assigned a distance. Non-negated elements get a distance based on their position in the list, where the closer the match is to the start of the list, the shorter the distance is between it and the server. A negated match is assigned the maximum distance from the server. If there is no match, the address gets a distance which is further than any non-negated list element, and closer than any negated element. For example,

topology {
    10/8;
    !1.2.3/24;
    { 1.2/16; 3/8; };
};

prefers servers on network 10 the most, followed by hosts on network 1.2.0.0 (netmask 255.255.0.0) and network 3, except for hosts on network 1.2.3 (netmask 255.255.255.0), which is preferred least of all.

The default topology is

    topology { localhost; localnets; };

The sortlist Statement

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

The sortlist statement (see below) takes an address_match_list and interprets it even more specifically than the topology statement does (Topology). 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 treated the same as the address_match_list in a topology statement. Each top-level element is assigned a distance and the address in the response with the minimum distance is moved to the beginning of the response.

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

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

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 favor any of the directly connected networks. Responses sent to queries from any other hosts on a directly connected network prefer addresses on that same network. Responses to other queries are not sorted.

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

RRset Ordering

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 statement permits configuration of the ordering of the records in a multiple record response. See also the sortlist statement, The sortlist Statement.

An order_spec is defined as follows:

[ 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 legal values for ordering are:

For example:

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

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

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

Tuning

lame-ttl

Sets the number of seconds to cache a lame server indication. 0 disables caching. (This is not recommended.) Default is 600 (10 minutes). Maximum value is 1800 (30 minutes).

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

max-cache-ttl

max-cache-ttl sets the maximum time for which the server will cache ordinary (positive) answers. The default is one week (7 days).

min-roots

The minimum number of root servers that is required for a request for the root servers to be accepted. Default is 2.

Note: Not yet implemented in BIND 9.

sig-validity-interval

Specifies the number of days into the future when DNSSEC signatures automatically generated as a result of dynamic updates ((Section 4.1)) will expire. The default is 30 days. The signature inception time is unconditionally set to one hour before the current time to allow for a limited amount of clock skew.

min-refresh-time, max-refresh-time, min-retry-time, max-retry-time

These options control the server's behavior on refreshing a zone (querying for SOA changes) or retrying failed transfers. Usually the SOA values for the zone are used, 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.

The Statistics File

The statistics file generated by BIND 9 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 timestamp, 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 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:

server Statement Grammar

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

Server Statement Definition and Usage

The server statement defines characteristics to be associated with a remote nameserver.

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

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

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

The request-ixfr clause determines whether the local server, acting as a slave, requests 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 block is used as a default.

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

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

The server supports two zone transfer methods. The first, one-answer, uses one DNS message per resource record transferred. many-answers packs as many resource records as possible into a 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.

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

The keys clause is used to identify a key_id defined by the 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 is generated using the key specified here and appended to the message. A request originating from the remote server is not required to be signed by this key.

Although the grammar of the keys clause allows for multiple keys, only a single key per server is currently supported.

trusted-keys Statement Grammar

trusted-keys {
    string number number number string ;
    [ string number number number string ; [...]]
};

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 non-authoritative 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 domain name, flags, protocol, algorithm, and the base-64 representation of the key data.

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-statistics yes_or_no ; ]
      [ zone_statement; ...]
};

6.2.20. view Statement Definition and Usage

The view statement is a powerful new feature of BIND 9 that lets a name server answer a DNS query differently depending on who is asking. It is particularly useful for implementing split DNS setups without having to run multiple servers.

Each view statement defines a view of the DNS namespace that is seen by a subset of clients. A client matches a view if its source IP address matches the address_match_list of the view's match-clients clause and its destination IP address matches the address_match_list of the view's match-destinations clause. If not specified, both match-clients and match-destinations default to matching all addresses. A view can also be specified as match-recursive-only, which means that only recursive requests from matching clients match that view. The order of the view statements is significant — a client request is resolved in the context of the first view that it matches.

Zones defined within a view statement are only accessible to clients that match the view. By defining a zone of the same name in multiple views, different zone data can be given to different clients, for example, "internal" and "external" clients in a split DNS setup.

Many of the options given in the options statement can also be used within a view statement, and then apply only when resolving queries with that view. When no view-specific value is given, the value in the options statement is used as a default. Also, zone options can have default values specified in the view statement; these view-specific defaults take precedence over those in the options statement.

Views are class-specific. If no class is given, class IN is assumed. Note that all non-IN views must contain a hint zone, since only the IN class has compiled-in default hints.

If there are no view statements in the config file, a default view that matches any client is automatically created in class IN, 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 using view statements.

view "internal" {
               // This should match our internal networks.
      match-clients { 10.0.0.0/8; };
               // Provide recursive service to internal clients only.
      recursion yes;
               // Provide a complete view of the example.com zone
               // including addresses of internal hosts.
      zone "example.com" {
            type 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";
      };
};

Zone Statement Grammar

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

}];

zone Statement Definition and Usage

Zone Types

Class

The zone's name may optionally be followed by a class. If a class is not specified, class IN (for Internet), is assumed. This is correct for the vast majority of cases.

The hesiod class is named for an information service from MIT's Project Athena. It is used to share information about various systems databases, such as users, groups, printers, and so on. The keyword HS is a synonym for hesiod.

Another MIT development is CHAOSnet, a LAN protocol created in the mid-1970s. Zone data for it can be specified with the CHAOS class.

Zone Options

allow-notify

See the description of allow-notify in Access Control.

allow-query

See the description of allow-query in Access Control.

allow-transfer

See the description of allow-transfer in Access Control.

allow-update

Specifies which hosts are allowed to submit Dynamic DNS updates for master zones. The default is to deny updates from all hosts.

update-policy

Specifies a "Simple Secure Update" policy. See Dynamic Update Policies.

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 is 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.

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 nameservers (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.

check-names

This option was used in BIND 8 to restrict the character set of domain names in master files or DNS responses received from the network. BIND 9 does not restrict the character set of domain names and does not implement the check-names option.

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. Some sample drivers are included with the distribution but none are linked in by default.

dialup

See the description of dialup in Boolean Options.

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 would allow 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-base

Was used in BIND 8 to specify the name of the transaction log (journal) file for dynamic update and IXFR. BIND 9 ignores the option and constructs the name of the journal file by appending " .jnl" to the name of the zone file.

ixfr-tmp-file

Was an undocumented option in BIND 8. Ignored in BIND 9.

max-transfer-time-in

See the description of max-transfer-time-in in Zone Transfers.

max-transfer-idle-in

See the description of max-transfer-idle-in in Zone Transfers.

max-transfer-time-out

See the description of max-transfer-time-out in Zone Transfers.

max-transfer-idle-out

See the description of max-transfer-idle-out in Zone Transfers.

notify

See the description of notify in Boolean Options.

pubkey

In BIND 8, this option was intended for specifying a public zone key for verification of signatures in DNSSEC signed zones when they are loaded from disk. BIND 9 does not verify signatures on loading and ignores the option.

zone-statistics

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

sig-validity-interval

See the description of sig-validity-interval in Tuning.

transfer-source

See the description of transfer-source in Zone Transfers

notify-source

See the description of notify-source in Zone Transfers

min-refresh-time, max-refresh-time, min-retry-time, max-retry-time

See the description in Tuning.

Dynamic Update Policies

BIND 9 supports two alternative methods of granting clients the right to perform dynamic updates to a zone, configured by the allow-update and update-policy option, 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 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.

This is how a rule definition looks:

( 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 type field.

The identity field specifies a name or a wildcard name. The nametype field has 4 values: name, subdomain, wildcard, and self

If no types are specified, the rule matches all types except SIG, NS, SOA, and NXT. Types may be specified by name, including "ANY" (ANY matches all types except NXT, which can never be updated).

Zone File

DOMAIN Data File, DOMAIN Reverse Data File, DOMAIN Cache File, and DOMAIN Local

Types of Resource Records and When to Use Them

This section, largely borrowed from RFC 1034, describes the concept of a Resource Record (RR) and explains when each is used. Since the publication of RFC 1034, several new RRs have been identified and implemented in the DNS. These are also included.

Resource Records

A domain name identifies a node. Each node has a set of resource information, which may be empty. The set of resource information associated with a particular name is composed of separate RRs. The order of RRs in a set is not significant and need not be preserved by nameservers, resolvers, or other parts of the DNS. However, sorting of multiple RRs is permitted for optimization purposes, for example, to specify that a particular nearby server be tried first. See The sortlist Statement and RRset Ordering.

The components of a Resource Record are:

The following are types of valid RRs (some of these listed, although not obsolete, are experimental (x) or historical (h) and no longer in general use):

The following classes of resource records are currently valid in the DNS:

RDATA is the type-dependent or class-dependent data that describes the resource:

The owner name is often implicit, rather than forming an integral part of the RR. For example, many nameservers internally form tree or hash structures for the name space, and chain RRs off nodes. The remaining RR parts are the fixed header (type, class, TTL) which is consistent for all RRs, and a variable part (RDATA) that fits the needs of the resource being described.

The meaning of the TTL field is a time limit on how long an RR can be kept in a cache. This limit does not apply to authoritative data in zones; 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 data in the RDATA section of RRs is carried as a combination of binary strings and domain names. The domain names are frequently used as "pointers" to other data in the DNS.

Textual expression of RRs

RRs are represented in binary form in the packets of the DNS protocol, and are usually represented in highly encoded form when stored in a nameserver or resolver. In the examples provided in RFC 1034, a style similar to that used in master files was employed in order to show the contents of RRs. In this format, most RRs are shown on a single line, although continuation lines are possible using parentheses.

The start of the line gives the owner of the RR. If a line begins with a blank, then the owner is assumed to be the same as that of the previous RR. Blank lines are often included for readability.

Following the owner, we list the TTL, type, and class of the RR. Class and type use the mnemonics defined above, and TTL is an integer before the type field. In order to avoid ambiguity in parsing, type and class mnemonics are disjoint, TTLs are integers, and the type mnemonic is always last. The IN class and TTL values are often omitted from examples in the interests of clarity.

The resource data or RDATA section of the RR are given using knowledge of the typical representation for the data.

For example, we might show the RRs carried in a message as:

The MX RRs have an RDATA section which consists of a 16 bit number followed by a domain name. The address RRs use a standard IP address format to contain a 32 bit internet address.

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

Similarly we might see:

This example shows two addresses for XX.LCS.MIT.EDU, each of a different class.

Discussion of MX Records

As described above, domain servers store information as a series of resource records, each of which contains a particular piece of information about a given domain name (which is usually, but not always, a host). The simplest way to think of an RR is as a typed pair of datum, a domain name matched with relevant data, and stored with some additional type information to help systems determine when the RR is relevant.

MX records are used to control delivery of email. The data specified in the record is a priority and a domain name. The priority controls the order in which email delivery is attempted, with the lowest number first. If two priorities are the same, a server is chosen randomly. If no servers at a given priority are responding, the mail transport agent falls back to the next largest priority. Priority numbers do not have any absolute meaning — they are relevant only respective to other MX records for that domain name. The domain name given is the machine to which the mail is delivered. It must have an associated A record — CNAME is not sufficient.

For a given domain, if there is both a CNAME record and an MX record, the MX record is in error, and is ignored. Instead, the mail is delivered to the server specified in the MX record pointed to by the CNAME.

example.com.			IN		MX		10				mail.example.com.
							IN		MX		10				mail2.example.com.
							IN		MX		10				mail.backup.org.
mail.example.com.		IN		A		10.0.0.1		
mail2.example.com.	IN		A		10.0.0.2		

For example:

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

Setting TTLs

The time to live of the RR field is a 32 bit integer represented in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long an RR can be cached before it should be discarded. The following three types of TTL are currently used in a zone file.

All these TTLs default to units of seconds, though units can be explicitly specified, for example, 1h30m.

Inverse Mapping in IPv4

Reverse name resolution (that is, translation from IP address to name) is achieved by means of the in-addr.arpa domain and PTR records. Entries in the in-addr.arpa domain are made in least-to-most significant order, read left to right. This is the opposite order to the way IP addresses are usually written. Thus, a machine with an IP address of 10.1.2.3 would have a corresponding in-addr.arpa name of 3.2.1.10.in-addr.arpa. This name should have a PTR resource record whose data field is the name of the machine or, optionally, multiple PTR records if the machine has more than one name. For example, in the [ example.com ] domain:

Other Zone File Directives

The Master File Format was initially defined in RFC 1035 and has subsequently been extended. While the Master File Format itself is class independent all records in a Master File must be of the same class.

Master File Directives include $ORIGIN, $INCLUDE, and $TTL.

The $ORIGIN Directive

Syntax: $ORIGIN domain-name [ comment ]

$ORIGIN sets the domain name that is appended to any unqualified records. When a zone is first read in there is an implicit $ORIGIN < zone-name> . The current $ORIGIN is appended to the domain specified in the $ORIGIN argument if it is not absolute.

$ORIGIN example.com.
WWW     CNAME   MAIN-SERVER

is equivalent to

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

The $INCLUDE Directive

Syntax: $INCLUDE filename [ origin ] [ comment ]

Read and process the file filename as if it were included into the file at this point. If origin is specified the file is processed with $ORIGIN set to that value, otherwise the current $ORIGIN is used.

The origin and the current domain name revert to the values they had prior to the $INCLUDE once the file has been read.

The $TTL Directive

Syntax: $TTL default-ttl [ comment ]

Set the default Time To Live (TTL) for subsequent records with undefined TTLs. Valid TTLs are of the range 0-2147483647 seconds.

$TTL is defined in RFC 2308.

BIND Master File Extension: the $GENERATE Directive

Syntax: $GENERATE range lhs type rhs [ comment ]

$GENERATE is used to create a series of resource records that only differ from each other by an iterator. $GENERATE can be used to easily generate the sets of records required to support sub /24 reverse delegations described in RFC 2317: 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

0.0.0.192.IN-ADDR.ARPA NS SERVER1.EXAMPLE.
0.0.0.192.IN-ADDR.ARPA NS SERVER2.EXAMPLE.
1.0.0.192.IN-ADDR.ARPA CNAME 1.0.0.0.192.IN-ADDR.ARPA
2.0.0.192.IN-ADDR.ARPA CNAME 2.0.0.0.192.IN-ADDR.ARPA
...
127.0.0.192.IN-ADDR.ARPA CNAME 127.0.0.0.192.IN-ADDR.ARPA
.

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

Files