Chapter 6. BIND 9 Configuration Reference
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## Chapter 6. BIND 9 Configuration Reference

BIND 9 configuration is broadly similar to BIND 8; however, there are a few new areas of configuration, such as views. BIND 8 configuration files should work with few alterations in BIND 9, although more complex configurations should be reviewed to check if they can be more efficiently implemented using the new features found in BIND 9.

BIND 4 configuration files can be converted to the new format using the shell script `contrib/named-bootconf/named-bootconf.sh`.

## Configuration File Elements

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

`acl_name` | The name of an `address_match_list` as defined by the acl statement.
`address_match_list` | A list of one or more `ip_addr`, `ip_prefix`, `key_id`, or `acl_name` elements, see the section called “Address Match Lists”.
`domain_name` | A quoted string which will be used as a DNS name, for example "`my.test.domain`".
`dotted_decimal` | One to four integers valued 0 through 255 separated by dots (`.'), such as 123, 45.67 or 89.123.45.67.
`ip4_addr` | An IPv4 address with exactly four elements in `dotted_decimal` notation.
`ip6_addr` | An IPv6 address, such as 2001:db8::1234. IPv6 scoped addresses that have ambiguity on their scope zones must be disambiguated by an appropriate zone ID with the percent character (`%') as delimiter. It is strongly recommended to use string zone names rather than numeric identifiers, in order to be robust against system configuration changes. However, since there is no standard mapping for such names and identifier values, currently only interface names as link identifiers are supported, assuming one-to-one mapping between interfaces and links. For example, a link-local address fe80::1 on the link attached to the interface ne0 can be specified as fe80::1%ne0. Note that on most systems link-local addresses always have the ambiguity, and need to be disambiguated.
`ip_addr` | An `ip4_addr` or `ip6_addr`.
`ip_port` | An IP port `number`. `number` is limited to 0 through 65535, with values below 1024 typically restricted to use by processes running as root. In some cases an asterisk (`*') character can be used as a placeholder to select a random high-numbered port.
`ip_prefix` | An IP network specified as an `ip_addr`, followed by a slash (`/') and then the number of bits in the netmask. Trailing zeros in a `ip_addr` may omitted. For example, 127/8 is the network 127.0.0.0 with netmask 255.0.0.0 and 1.2.3.0/28 is network 1.2.3.0 with netmask 255.255.255.240.
`key_id` | A `domain_name` representing the name of a shared key, to be used for transaction security.
`key_list` | A list of one or more `key_id`s, separated by semicolons and ending with a semicolon.
`number` | A non-negative 32 bit integer (i.e., 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 which will be used as a pathname, such as `zones/master/my.test.domain`.
`size_spec` | A number, the word `unlimited`, or the word `default`. An `unlimited` `size_spec` requests unlimited use, or the maximum available amount. A `default size_spec` uses the limit that was in force when the server was started. A `number` can optionally be followed by a scaling factor: `K` or `k` for kilobytes, `M` or `m` for megabytes, and `G` or `g` for gigabytes, which scale by 1024, 1024*1024, and 1024*1024*1024 respectively. The value must be representable as a 64-bit unsigned integer (0 to 18446744073709551615, inclusive). Using `unlimited` is the best way to safely set a really large number.
`yes_or_no` | Either `yes` or `no`. The words `true` and `false` are also accepted, as are the numbers `1` and `0`.
`dialup_option` | One of `yes`, `no`, `notify`, `notify-passive`, `refresh` or `passive`. When used in a zone, `notify-passive`, `refresh`, and `passive` are restricted to slave and stub zones.

### Address Match Lists

#### Syntax

    address_match_list = address_match_list_element ;
      [ address_match_list_element; ... ]
    address_match_list_element = [ ! ] (ip_address [/length] |
       key key_id | acl_name | { address_match_list } )

#### Definition and Usage

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

- an IP address (IPv4 or IPv6)
- an IP prefix (in `/' notation)
- a key ID, as defined by the key statement
- the name of an address match list defined with the acl statement
- a nested address match list enclosed in braces

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

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

When a given IP address or prefix is compared to an address match list, the 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, or in a sortlist, and whether the element was negated.

When used as an access control list, a non-negated match allows access and a negated match denies access. If there is no match, access is denied. The clauses allow-notify, allow-query, allow-transfer, allow-update, allow-update-forwarding, and blackhole all use address match lists this. Similarly, the listen-on option will cause the server to not 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 completely useless 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

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

#### Syntax

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

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

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

#### Definition and Usage

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

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

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

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

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

For example:

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

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

For example:

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

### Warning

You cannot use the semicolon (`;') character to start a comment such as you would in a zone file. The semicolon indicates the end of a configuration statement.

## Configuration File Grammar

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

The following statements are supported:

acl | defines a named IP address matching list, for access control and other uses.
controls | declares control channels to be used by the rndc utility.
include | includes a file.
key | specifies key information for use in authentication and authorization using TSIG.
logging | specifies what the server logs, and where the log messages are sent.
lwres | configures named to also act as a light weight resolver daemon (lwresd).
masters | defines a named masters list for inclusion in stub and slave zone masters clauses.
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 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.

The following ACLs are built-in:

any | Matches all hosts.
none | Matches no hosts.
localhost | Matches the IPv4 and IPv6 addresses of all network interfaces on the system.
localnets | Matches any host on an IPv4 or IPv6 network for which the system has an interface. Some systems do not provide a way to determine the prefix lengths of local IPv6 addresses. In such a case, localnets only matches the local IPv6 addresses, just like localhost.

### 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 control the operation of the name server. These control channels are used by the rndc utility to send commands to and retrieve non-DNS results from a name server.

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

If no port is specified, port 953 is used. "`*`" cannot be used for ip_port.

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

The primary authorization mechanism of the command channel is the key_list, which contains a list of key_ids. Each key_id in the key_list is authorized to execute commands over the control channel. See Remote Name Daemon Control application in the section called “Administrative Tools”) for information about configuring keys in rndc.

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 `rndc.key` in `/etc` (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-confgen -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, 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.

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

### 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 the name server.

### 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 (see the section called “TSIG”) or the command channel (see the section called “controls Statement Definition and Usage”).

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

The `key_id`, also known as the key name, is a domain name 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 name server. Its channel phrase associates output methods, format options and severity levels with a name that can then be used with the category phrase to select how various classes of messages are logged.

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

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

In BIND 9, the logging configuration is only established 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 is starting up, all logging messages regarding syntax errors in the configuration file go to the default channels, or to standard error if the "`-g`" option was specified.

#### 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 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 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 will be 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 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 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 one begun. If there is no versions option, no more data will be 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.

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 man page. Known facilities are kern, user, mail, daemon, auth, syslog, lpr, news, uucp, cron, authpriv, ftp, local0, local1, local2, local3, local4, local5, local6 and local7, however not all facilities are supported on all operating systems. How syslog will handle 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 will not be selected for the channel; messages of higher severity levels will be accepted.

If you are using syslog, then the syslog.conf priorities will 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 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 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.

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` flag followed by a positive integer, or by running rndc trace. The global debug level can be set to zero, and debugging mode turned off, by running ndc notrace. All debugging messages in the server have a debug level, and higher debug levels give more detailed output. Channels that specify a specific debug severity, for example:

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

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 debug level to determine what messages to print.

If print-time has been turned on, then the date and time will be logged. print-time may be specified for a syslog channel, but is usually pointless since syslog also prints the date and time. If print-category is requested, then the category of the message will be logged as well. Finally, 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 following order: time, category, severity. Here is an example where all three print- options are on:

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

There are four predefined channels that are used for named's default logging as follows. How they are used is described in the section called “The category Phrase”.

    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 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

There are many categories, so you can send the logs you want to see wherever you want, without seeing logs you don't want. If you don't specify a list of channels for a category, then log messages in that category will be sent to the default category instead. If you don't specify a default category, the following "default default" is used:

    category default { default_syslog; default_debug; };

As an example, let's say you want to log security events to a file, but you also want keep the default logging behavior. You'd specify the following:

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

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

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

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 | The default category defines the logging options for those categories where no specific configuration has been defined.
general | The catch-all. Many things still aren't classified into categories, and they all end up here.
database | Messages relating to the databases used internally by the name server to store zone and cache data.
security | Approval and denial of requests.
config | Configuration file parsing and processing.
resolver | DNS resolution, such as the recursive lookups performed on behalf of clients by a caching name server.
xfer-in | Zone transfers the server is receiving.
xfer-out | Zone transfers the server is sending.
notify | The NOTIFY protocol.
client | Processing of client 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.
network | Network operations.
update | Dynamic updates.
update-security | Approval and denial of update requests.
queries | Specify where queries should be logged to. At startup, specifing the category queries will also enable query logging unless querylog option has been specified. The query log entry reports the client's IP address and port number. The query name, class and type. It also reports whether the Recursion Desired flag was set (+ if set, - if not set), EDNS was in use (E) or if the query was signed (S). `client 127.0.0.1#62536: query: www.example.com IN AAAA +SE` `client ::1#62537: query: www.example.net IN AAAA -SE`
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. These are misconfigurations in remote servers, discovered by BIND 9 when trying to query those servers during resolution.
delegation-only | 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.

### lwres Statement Grammar

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

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

### lwres Statement Definition and Usage

The lwres statement configures the name server to also act as a lightweight resolver server, see the section called “Running a Resolver Daemon”. 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 this instance of 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 view statement binds this instance of a lightweight resolver daemon to a view in the DNS namespace, 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 search statement is equivalent to the search statement in `/etc/resolv.conf`. It provides a list of domains which are appended to relative names in queries.

The ndots statement is equivalent to the ndots statement 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.

### masters Statement Grammar

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

### masters Statement Definition and Usage

masters lists allow for a common set of masters to be easily used by multiple stub and slave zones.

### options Statement Grammar

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

    options {
        [ version version_string; ]
        [ hostname hostname_string; ]
        [ server-id server_id_string; ]
        [ directory path_name; ]
        [ key-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; ]
        [ flush-zones-on-shutdown yes_or_no; ]
        [ has-old-clients yes_or_no; ]
        [ host-statistics yes_or_no; ]
        [ host-statistics-max number; ]
        [ 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; ]
        [ dnssec-enable yes_or_no; ]
        [ dnssec-lookaside domain trust-anchor domain; ]
        [ dnssec-must-be-secure domain yes_or_no; ]
        [ forward ( only | first ); ]
        [ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
        [ dual-stack-servers [port ip_port] { ( domain_name [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-update-forwarding { address_match_list }; ]
        [ allow-v6-synthesis { address_match_list }; ]
        [ blackhole { address_match_list }; ]
        [ avoid-v4-udp-ports { port_list }; ]
        [ avoid-v6-udp-ports { port_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 | * ) ]; ]
        [ query-source-v6 [ 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; ]
        [ tcp-listen-queue 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] ; ]
        [ alt-transfer-source (ip4_addr | *) [port ip_port] ; ]
        [ alt-transfer-source-v6 (ip6_addr | *) [port ip_port] ; ]
        [ use-alt-transfer-source yes_or_no; ]
        [ 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; ]
        [ max-journal-size size_spec; ]
        [ 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; ]
        [ preferred-glue ( A | AAAA | NONE ); ]
        [ edns-udp-size number; ]
        [ root-delegation-only [ exclude { namelist } ] ; ]
        [ querylog yes_or_no ; ]
        [ disable-algorithms domain { algorithm; [ algorithm; ] }; ]
    };

### options Statement Definition and Usage

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

directory

  The working directory of the server. Any non-absolute pathnames in the configuration file will be taken as relative to this directory. The default location for most server output files (e.g. `named.run`) is this directory. If a directory is not specified, the working directory defaults to ``.`', the directory from which the server was started. The directory specified should be an absolute path.

key-directory

  When performing dynamic update of secure zones, the directory where the public and private key files should be found, if different than the current working directory. The directory specified must be an absolute path.

named-xfer

  This option is obsolete. It was used in BIND 8 to specify the pathname to the named-xfer program. In BIND 9, no separate named-xfer program is needed; its functionality is built into the name server.

tkey-domain

  The domain appended to the names of all shared keys generated with TKEY. When a client requests a TKEY exchange, it may or may not specify the desired name for the key. If present, the name of the shared key 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 domainname should be the server's domain name.

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 keyname should be the server's host name.

dump-file

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

memstatistics-file

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

pid-file

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

statistics-file

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

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.

random-device

  The source of entropy to be used by the server. Entropy is primarily needed for DNSSEC operations, such as TKEY transactions and dynamic update of signed zones. This options 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. If not specified, the default value is `/dev/random` (or 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.

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 preference any type (NONE).

root-delegation-only

  Turn on enforcement of delegation-only in TLDs and root zones with an optional exclude list.

  Note some TLDs are NOT delegation only (e.g. "DE", "LV", "US" and "MUSEUM").

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

disable-algorithms

  Disable the specified DNSSEC algorithms at and below the specified name. Multiple disable-algorithms statements are allowed. Only the most specific will be 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 append 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 (similarly to the way a DS record does) the DNSKEY RRset is deemed to be trusted.

dnssec-must-be-secure

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

#### 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`, 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 the 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 (default). This should trigger the zone serial number check in the slave (providing it supports NOTIFY) allowing the slave to verify the zone while the connection is active. The set of servers to which NOTIFY is sent can be controlled by notify and also-notify.

  If the zone is a 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.

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

  Note that normal NOTIFY processing is not affected by dialup.

fake-iquery

  In BIND 8, this option enabled 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 didn't have when constructing the additional data section of a response. This is now considered a bad idea and BIND 9 never does it.

flush-zones-on-shutdown

  When the nameserver exits due receiving SIGTERM, flush / do not flush any pending zone writes. The default is flush-zones-on-shutdown `no`.

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 name server interacts with. 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 will only add records to the authority and additional data sections when they are required (e.g. delegations, negative responses). 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 have multiple CNAME records in violation of the DNS standards. BIND 9.2 always strictly enforces the CNAME rules both in master files and dynamic updates.

notify

  If `yes` (the default), DNS NOTIFY messages are sent when a zone the server is authoritative for changes, see the section called “Notify”. 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 will attempt to do all the work required to answer the query. If recursion is off and the server does not already know the answer, it will return a referral response. The default is `yes`. Note that setting recursion no does not prevent clients from getting data from the server's cache; it only prevents new data from being cached as an effect of client queries. Caching may still occur as an effect the server's internal operation, such as NOTIFY address lookups. See also fetch-glue above.

rfc2308-type1

  Setting this to `yes` will cause 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 will collect statistical data on all zones (unless specifically turned off on a per-zone basis by specifying zone-statistics no in the zone statement). These statistics may be accessed using rndc stats, which will dump them to the file listed in the statistics-file. See also the section called “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 the section called “server Statement Definition and Usage”. See also the section called “Incremental Zone Transfers (IXFR)”.

provide-ixfr

  See the description of provide-ixfr in the section called “server Statement Definition and Usage”

request-ixfr

  See the description of request-ixfr in the section called “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 an 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` (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 and AAAA) for `mail.example.net` will be provided as well, if known, even though they are not in the example.com zone. Setting these options to no disables this behavior and makes the server only search for additional data in the zone it answers from.

  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 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 will respond 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 will match 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.

ixfr-from-differences

  When '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 non-incremental 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 non-dynamic 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.

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`.

dnssec-enable

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

querylog

  Specify whether query logging should be started when named start. If querylog is not specified then the query logging is determined by the presence of the logging category queries.

check-names

  This option is used to 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 answer received from the network (response) the default is ignore.

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

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

#### Forwarding

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

forward

  This option is only meaningful if the forwarders list is not empty. A value of `first`, the default, causes the server to query the forwarders first, and if that doesn't 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 particular domains to use different forwarders, or have a different forward only/first behavior, or not forward at all, see the section called “zone Statement Grammar”.

#### Dual-stack Servers

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

dual-stack-servers

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

#### Access Control

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

allow-notify

  Specifies which hosts are allowed to notify this server, a slave, of zone changes 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 DNS questions. allow-query may also be specified in the zone statement, in which case it overrides the options allow-query statement. If not specified, the default is to allow queries from all hosts.

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; see the section called “Dynamic Update Security” for more details.

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.

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 to 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`.

#### Interfaces

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

The listen-on-v6 option is used to specify the interfaces and the ports on which the server will listen for incoming queries sent using IPv6.

When

    { any; }

is specified as the `address_match_list` for the listen-on-v6 option, the server does not bind a separate socket to each IPv6 interface address as it does for IPv4 if the operating system has enough API support for IPv6 (specifically if it conforms to RFC 3493 and RFC 3542). Instead, it listens on the IPv6 wildcard address. If the system only has incomplete API support for IPv6, however, the behavior is the same as that for IPv4.

A list of particular IPv6 addresses can also be specified, in which case the server listens on a separate socket for each specified address, regardless of whether the desired API is supported by the system.

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

    listen-on-v6 { any; };
    listen-on-v6 port 1234 { !2001:db8::/32; any; };

will enable the name server on port 53 for any IPv6 addresses (with a single wildcard socket), and on port 1234 of IPv6 addresses that is not in the prefix 2001:db8::/32 (with separate sockets for each matched address.)

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

    listen-on-v6 { none; };

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

#### Query Address

If the server doesn't know the answer to a question, it will query other name servers. query-source specifies the address and port used for such queries. For queries sent over IPv6, there is a separate query-source-v6 option. If address is * or is omitted, a wildcard IP address (INADDR_ANY) will be used. If port is * or is omitted, a random unprivileged port will be used, avoid-v4-udp-ports and avoid-v6-udp-ports can be used to prevent named from selecting certain ports. The defaults 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.

### Note

See also transfer-source and notify-source.

#### 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 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).

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-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-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).

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).

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 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 will be 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 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.

transfer-source

  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 zone transfers are performed using IPv6.

alt-transfer-source

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

  ### Note

  If you do not wish the alternate transfer source to be used you should set use-alt-transfer-source appropriately and you should not depend upon getting a answer back to the first refresh query.
alt-transfer-source-v6

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

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).

notify-source

  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 / per-view basis by including a notify-source statement within the zone or view block in the configuration file.

notify-source-v6

  Like notify-source, but applies to notify messages sent to IPv6 addresses.

#### Bad UDP Port Lists

avoid-v4-udp-ports and 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.

#### Operating System Resource Limits

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

The following options set operating system resource limits for the name server process. Some operating systems don't support some or any of the limits. On such systems, a warning will be 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 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.

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. The option max-journal-size performs a similar function in BIND 8.

max-journal-size

  Sets a maximum size for each journal file (the section called “The 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`.

host-statistics-max

  In BIND 8, specifies the maximum number of host statistic entries to be kept. Not implemented in BIND 9.

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`.

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.

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 will be silently raised.

#### Periodic Task Intervals

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. Reasonable values are up to 1 day (1440 minutes). The maximum value is 28 days (40320 minutes). If set to 0, no zone maintenance for these zones 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, the server will begin listening for queries on any newly discovered interfaces (provided they are allowed by the listen-on configuration), and will stop listening on interfaces that have gone away.

statistics-interval

  Name server statistics will be logged every statistics-interval minutes. The default is 60. The maximum value is 28 days (40320 minutes). If set to 0, no statistics will be logged.

  ### Note

  Not yet implemented in BIND9.

#### Topology

All other things being equal, when the server chooses a name server to query from a list of name servers, 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 will be assigned the maximum distance from the server. If there is no match, the address will get 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; };
    };

will prefer servers on network 10 the most, followed by hosts on network 1.2.0.0 (netmask 255.255.0.0) and network 3, with the exception of hosts on network 1.2.3 (netmask 255.255.255.0), which is preferred least of all.

The default topology is

        topology { localhost; localnets; };

### Note

The topology option is not implemented in BIND 9.

#### 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 will normally return the RRs within the RRset in an indeterminate order (but see the rrset-order statement in the section called “RRset Ordering”). The client resolver code should rearrange the RRs as appropriate, that is, using any addresses on the local net in preference to other addresses. However, not all resolvers can do this or are correctly configured. When a client is using a local server the sorting can be performed in the server, based on the client's address. This only requires configuring the name servers, not all the clients.

The sortlist statement (see below) takes an address_match_list and interprets it even more specifically than the topology statement does (the section called “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 will 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 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; }; }; };
        { { 192.168.4/24; 192.168.5/24; };             // if .4 or .5, prefer that net
        };
    };

The following example will give 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; };
    };

#### 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 section called “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:

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.

For example:

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

will cause 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.

### Note

The rrset-order statement is not yet fully implemented in BIND 9. BIND 9 currently does not support "fixed" ordering.

#### 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 will be 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 implemented in BIND9.

sig-validity-interval

  Specifies the number of days into the future when DNSSEC signatures automatically generated as a result of dynamic updates (the section called “Dynamic Update”) will expire. The default is `30` days. The maximum value 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.

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 globally. These options are valid for slave and stub zones, and clamp the SOA refresh and retry times to the specified values.

edns-udp-size

  edns-udp-size sets the advertised EDNS UDP buffer size. 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 non default value it to get UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP packets that are greater than 512 bytes.

#### Built-in server information zones

The server provides some helpful diagnostic information through a number of built-in zones under the pseudo-top-level-domain `bind` in the CHAOS class. These zones are part of a built-in view (see the section called “view Statement Grammar”) 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.

version

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

hostname

  The hostname the server should report via a query of the name `hostname.bind` with type TXT, class CHAOS. This defaults to the hostname of the machine hosting the name server as found by gethostname(). 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 of 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; will cause named to use the hostname as found by gethostname(). The default server-id is none.

#### 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:

success | The number of successful queries made to the server or zone. A successful query is defined as query which returns a NOERROR response with at least one answer RR.
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.
failure | The number of queries which resulted in a failure response other than those above.
recursion | The number of queries which caused the server to perform recursion in order to find the final answer.

Each query received by the server will cause exactly one of success, referral, nxrrset, nxdomain, or failure to be incremented, and may additionally cause the recursion counter to be incremented.

### 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 ; [...]] } ; ]
        [ transfer-source (ip4_addr | *) [port ip_port] ; ]
        [ transfer-source-v6 (ip6_addr | *) [port ip_port] ; ]
    };

### 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 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 will prevent further queries to it. The default value of bogus is no.

The provide-ixfr clause 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 non-incremental. If not set, the value of the provide-ixfr option in the view or global options block is used as a default.

The request-ixfr clause determines whether the local server, acting as a 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 block 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 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 will be used.

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

The keys clause identifies a key_id defined by the key statement, to be used for transaction security (TSIG, the section called “TSIG”) when talking to the remote server. 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.

The transfer-source and transfer-source-v6 clauses 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. Form more details, see the description of transfer-source and transfer-source-v6 in the section called “Zone Transfers”.

### 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. DNSSEC is described in the section called “DNSSEC”. 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 because 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_statement; ...]
    };

### 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 will be seen by a subset of clients. A client matches a view if its source IP address matches the `address_match_list` of the view's match-clients clause and its destination IP address matches the `address_match_list` of the view's match-destinations clause. If not specified, both match-clients and match-destinations default to matching all addresses. In addition to checking IP addresses match-clients and match-destinations can also take keys which provide an mechanism for the client to select the view. A view can also be specified as match-recursive-only, which means that only recursive requests from matching clients will match that view. The order of the view statements is significant — a client request will be resolved in the context of the first view that it matches.

Zones defined within a view statement will be 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.

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

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

If there are no view statements in the config file, a default view that matches any client is automatically created in class IN. Any zone statements specified on the top level of the configuration file are considered to be part of this default view, and the options statement will apply to the default view. If any explicit view statements are present, all zone statements must occur inside view statements.

Here is an example of a typical split DNS setup implemented using view statements.

    view "internal" {
          // This should match our internal networks.
          match-clients { 10.0.0.0/8; };

          // Provide recursive service to internal clients only.
          recursion yes;

          // Provide a complete view of the example.com zone
          // including addresses of internal hosts.
          zone "example.com" {
                type master;
                file "example-internal.db";
          };
    };

    view "external" {
          // Match all clients not matched by the previous view.
          match-clients { any; };

          // Refuse recursive service to external clients.
          recursion no;

          // Provide a restricted view of the example.com zone
          // containing only publicly accessible hosts.
          zone "example.com" {
               type master;
               file "example-external.db";
          };
    };

### 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 } ; ]
        [ 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 ; ]
        [ delegation-only yes_or_no ; ]
        [ file string ; ]
        [ forward (only|first) ; ]
        [ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
        [ ixfr-base string ; ]
        [ ixfr-tmp-file string ; ]
        [ maintain-ixfr-base yes_or_no ; ]
        [ masters [port ip_port] { ( masters_list | 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] ; ]
        [ alt-transfer-source (ip4_addr | *) [port ip_port] ; ]
        [ alt-transfer-source-v6 (ip6_addr | *) [port ip_port] ; ]
        [ use-alt-transfer-source yes_or_no; ]
        [ 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 ; ]
        [ multi-master yes_or_no ; ]
        [ key-directory path_name; ]

    }];

### zone Statement Definition and Usage

#### Zone Types

`master` | The server has a master copy of the data for the zone and will be able to provide authoritative answers for it.
`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. Masters list elements can also be names of other masters lists. By default, transfers are made from port 53 on the servers; this can be changed for all servers by specifying a port number before the list of IP addresses, or on a per-server basis after the IP address. Authentication to the 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. Note that for large numbers (in the tens or hundreds of thousands) of zones per server, it is best to use a two level naming scheme for zone file names. For example, a slave server for the zone `example.com` might place the zone contents into a file called `ex/example.com` where `ex/` is just the first two letters of the zone name. (Most operating systems behave very slowly if you put 100 000 files into a single directory.)
`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 record in a parent zone at the expense of maintaining a stub zone entry and a set of name server addresses in `named.conf`. This usage is not recommended for new configurations, and BIND 9 supports it only in a limited way. 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 as a way of forcing 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 RFC1981 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.
`forward` | A "forward zone" is a way to configure forwarding on a per-domain basis. A zone statement of type forward can contain a forward and/or forwarders statement, which will apply to queries within the domain given by the zone name. If no forwarders statement is present or an empty list for forwarders is given, then no forwarding 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 to", then "forward only", or vice versa, but want to use the same servers as set globally) you need to re-specify the global forwarders.
`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.
`delegation-only` | This is used to enforce the delegation only status of infrastructure zones (e.g. 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.

#### 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 the section called “Access Control”

allow-query

  See the description of allow-query in the section called “Access Control”

allow-transfer

  See the description of allow-transfer in the section called “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. Note that allowing updates based on the requestor's IP address is insecure; see the section called “Dynamic Update Security” for details.

update-policy

  Specifies a "Simple Secure Update" policy. See the section called “Dynamic Update Policies”.

allow-update-forwarding

  See the description of allow-update-forwarding in the section called “Access Control”.

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.

check-names

  This option is used to 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. Some sample drivers are included with the distribution but none are linked in by default.

dialup

  See the description of dialup in the section called “Boolean Options”.

delegation-only

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

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 the section called “Zone Transfers”.

max-transfer-idle-in

  See the description of max-transfer-idle-in in the section called “Zone Transfers”.

max-transfer-time-out

  See the description of max-transfer-time-out in the section called “Zone Transfers”.

max-transfer-idle-out

  See the description of max-transfer-idle-out in the section called “Zone Transfers”.

notify

  See the description of notify in the section called “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 load and ignores the option.

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.

sig-validity-interval

  See the description of sig-validity-interval in the section called “Tuning”.

transfer-source

  See the description of transfer-source in the section called “Zone Transfers”

transfer-source-v6

  See the description of transfer-source-v6 in the section called “Zone Transfers”

alt-transfer-source

  See the description of alt-transfer-source in the section called “Zone Transfers”

alt-transfer-source-v6

  See the description of alt-transfer-source-v6 in the section called “Zone Transfers”

use-alt-transfer-source

  See the description of use-alt-transfer-source in the section called “Zone Transfers”

notify-source

  See the description of notify-source in the section called “Zone Transfers”

notify-source-v6

  See the description of notify-source-v6 in the section called “Zone Transfers”.

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

  See the description in the section called “Tuning”.

ixfr-from-differences

  See the description of ixfr-from-differences in the section called “Boolean Options”.

key-directory

  See the description of key-directory in the section called “options Statement Definition and Usage”

multi-master

  See the description of multi-master in the section called “Boolean Options”.

#### 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 in accordance with the nametype field, and the type matches the types specified in the type 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 4 values: `name`, `subdomain`, `wildcard`, and `self`.

`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 name 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 using 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 NXT. Types may be specified by name, including "ANY" (ANY 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.

## Zone File

### 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 name servers, resolvers, or other parts of the DNS. However, sorting of multiple RRs is permitted for optimization purposes, for example, to specify that a particular nearby server be tried first. See the section called “The sortlist Statement” and the section called “RRset Ordering”.

The components of a Resource Record are:

owner name | the domain name where the RR is found.
type | an encoded 16 bit value that specifies the type of the resource record.
TTL | the time to live of the RR. This field is a 32 bit integer in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be discarded.
class | an encoded 16 bit value that identifies a protocol family or instance of a protocol.
RDATA | the resource data. The format of the data is type (and sometimes class) specific.

The following are types of valid RRs:

A | a host address. In the IN class, this is a 32-bit IP address. Described in RFC 1035.
AAAA | IPv6 address. Described in RFC 1886.
A6 | IPv6 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. Described in RFC 2874.
AFSDB | location of AFS database servers. Experimental. Described in RFC 1183.
APL | address prefix list. Experimental. Described in RFC 3123.
CERT | holds a digital certificate. Described in RFC 2538.
CNAME | identifies the canonical name of an alias. Described in RFC 1035.
DNAME | Replaces the domain name specified with another name to be looked up, effectively aliasing an entire subtree of the domain name space rather than a single record as in the case of the CNAME RR. Described in RFC 2672.
GPOS | Specifies the global position. Superseded by LOC.
HINFO | identifies the CPU and OS used by a host. Described in RFC 1035.
ISDN | representation of ISDN addresses. Experimental. Described in RFC 1183.
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.
LOC | for storing GPS info. Described in RFC 1876. Experimental.
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. Described in RFC 974, RFC 1035.
NAPTR | name authority pointer. Described in RFC 2915.
NSAP | a network service access point. Described in RFC 1706.
NS | the 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 | a pointer to another part of the domain name space. Described in RFC 1035.
PX | provides mappings between RFC 822 and X.400 addresses. Described in RFC 2163.
RP | information on persons responsible for the domain. Experimental. Described in RFC 1183.
RT | route-through binding for hosts that do not have their own direct wide area network addresses. Experimental. Described in RFC 1183.
SIG | ("signature") contains data authenticated in the secure DNS. Described in RFC 2535.
SOA | identifies the start of a zone of authority. Described in RFC 1035.
SRV | information about well known network services (replaces WKS). Described in RFC 2782.
TXT | text records. Described in RFC 1035.
WKS | information about which well known network services, such as SMTP, that a domain supports. Historical.
X25 | representation of X.25 network addresses. Experimental. Described in RFC 1183.

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

IN | The Internet.
CH | CHAOSnet, a LAN protocol created at MIT in the mid-1970s. Rarely used for its historical purpose, but reused for BIND's built-in server information zones, e.g., `version.bind`.
HS | Hesiod, an information service developed by MIT's Project Athena. It is used to share information about various systems databases, such as users, groups, printers and so on.

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

The 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 name server 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:

`ISI.EDU.` | `MX` | `10 VENERA.ISI.EDU.`
| `MX` | `10 VAXA.ISI.EDU`
`VENERA.ISI.EDU` | `A` | `128.9.0.32`
| `A` | `10.1.0.52`
`VAXA.ISI.EDU` | `A` | `10.2.0.27`
| `A` | `128.9.0.33`

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

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

Similarly we might see:

`XX.LCS.MIT.EDU. IN` | `A` | `10.0.0.44`
`CH` | `A` | `MIT.EDU. 2420`

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

MX records are used to control delivery of email. The data specified in the record is a priority and a domain name. The priority controls the order in which email delivery is attempted, with the lowest number first. If two priorities are the same, a server is chosen randomly. If no servers at a given priority are responding, the mail transport agent 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. The domain name given is the machine to which the mail will be 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 will be ignored. Instead, the mail will be 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` | `20` | `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 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.

### 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 a RR can be cached before it should be discarded. The following three types of TTL are currently used in a zone file.

SOA | The last field in the SOA is the negative caching TTL. This controls how long other servers will cache no-such-domain (NXDOMAIN) responses from you. The maximum time for negative caching is 3 hours (3h).
$TTL | The $TTL directive at the top of the zone file (before the SOA) gives a default TTL for every RR without a specific TTL set.
RR TTLs | Each RR can have a TTL as the second field in the RR, which will control how long other servers can cache the it.

All of 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:

`$ORIGIN` | `2.1.10.in-addr.arpa`
`3` | `IN PTR foo.example.com.`

### Note

The $ORIGIN lines in the examples are for providing context to the examples only-they do not necessarily appear in the actual usage. They are only used here to indicate that the example is relative to the listed origin.

### 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 will be 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.

### Note

RFC 1035 specifies that the current origin should be restored after an $INCLUDE, but it is silent on whether the current domain name should also be restored. BIND 9 restores both of them. This could be construed as a deviation from RFC 1035, a feature, or both.

#### 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` [`ttl`] [`class`] `type` `rhs` [ `comment` ]

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

range | This can be one of two forms: start-stop or start-stop/step. If the first form is used then step is set to 1. All of start, stop and step must be positive.
lhs | lhs describes the owner name of the resource records to be created. Any single $ symbols within the lhs side are replaced by the iterator value. To get a $ in the output you need to escape the $ using a backslash \, e.g. \$. The $ may optionally be followed by modifiers which change the offset from the iterator, field width and base. Modifiers are introduced by a { immediately following the $ as ${offset[,width[,base]]}. e.g. ${-20,3,d} which subtracts 20 from the current value, prints the result as a decimal in a zero padded field of with 3. Available output forms are decimal (d), octal (o) and hexadecimal (x or X for uppercase). The default modifier is ${0,0,d}. If the lhs is not absolute, the current $ORIGIN is appended to the name. For compatibility with earlier versions $$ is still recognized a indicating a literal $ in the output.
ttl | ttl specifies the ttl of the generated records. If not specified this will be inherited using the normal ttl inheritance rules. class and ttl can be entered in either order.
class | class specifies the class of the generated records. This must match the zone class if it is specified. class and ttl can be entered in either order.
type | At present the only supported types are PTR, CNAME, DNAME, A, AAAA and NS.
rhs | rhs is a domain name. It is processed similarly to lhs.

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

BIND 8 does not support the optional TTL and CLASS fields.

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