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”.
`masters_list` | A named list of one or more `ip_addr` with optional `key_id` and/or `ip_port`. A `masters_list` may include other `masters_lists`.
`domain_name` | A quoted string which will be used as a DNS name, for example "`my.test.domain`".
`namelist` | A list of one or more `domain_name` elements.
`dotted_decimal` | One to four integers valued 0 through 255 separated by dots (`.'), such as 123, 45.67 or 89.123.45.67.
`ip4_addr` | An IPv4 address with exactly four elements in `dotted_decimal` notation.
`ip6_addr` | An IPv6 address, such as 2001:db8::1234. IPv6 scoped addresses that have ambiguity on their scope zones must be disambiguated by an appropriate zone ID with the percent character (`%') as 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`. The `number` is limited to 0 through 65535, with values below 1024 typically restricted to use by processes running as root. In some cases, an asterisk (`*') character can be used as a placeholder to select a random high-numbered port.
`ip_prefix` | An IP network specified as an `ip_addr`, followed by a slash (`/') and then the number of bits in the netmask. Trailing zeros in 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. When specifying a prefix involving a IPv6 scoped address the scope may be omitted. In that case the prefix will match packets from any scope.
`key_id` | A `domain_name` representing the name of a shared key, to be used for transaction security.
`key_list` | A list of one or more `key_id`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`.
`port_list` | A list of an `ip_port` or a port range. A port range is specified in the form of `range` followed by two `ip_port`s, `port_low` and `port_high`, which represents port numbers from `port_low` through `port_high`, inclusive. `port_low` must not be larger than `port_high`. For example, `range 1024 65535` represents ports from 1024 through 65535. In either case an asterisk (`*') character is not allowed as a valid `ip_port`.
`size_spec` | A 64-bit unsigned integer, or the keywords `unlimited` or `default`. Integers may take values 0 <= value <= 18446744073709551615, though certain parameters may use a more limited range within these extremes. In most cases, setting a value to 0 does not literally mean zero; it means "undefined" or "as big as psosible", depending on the context. See the expalantions of particular parameters that use `size_spec` for details on how they interpret its use. Numeric values can optionally be followed by a scaling factor: `K` or `k` for kilobytes, `M` or `m` for megabytes, and `G` or `g` for gigabytes, which scale by 1024, 1024*1024, and 1024*1024*1024 respectively. `unlimited` generally means "as big as possible", though in certain contexts, (including `max-cache-size`), it may mean the largest possible 32-bit unsigned integer (0xffffffff); this distinction can be important when dealing with larger quantities. `unlimited` is usually the best way to safely set a very large number. `default` uses the limit that was in force when the server was started.
`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 comparison takes place in approximately O(1) time. However, key comparisons require that the list of keys be traversed until a matching key is found, and therefore may be somewhat slower.

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

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

Order of insertion is significant. If more than one element in an ACL is found to match a given IP address or prefix, preference will be given to the one that came first in the ACL definition. Because of this first-match behavior, an element that defines a subset of another element in the list should come before the broader element, regardless of whether either is negated. For example, in 1.2.3/24; ! 1.2.3.13; the 1.2.3.13 element is completely useless because the algorithm 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 whitespace may appear in a BIND configuration file. To appeal to programmers of all kinds, they can be written in the C, C++, or shell/perl style.

#### 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 zones' masters or also-notify lists.
options | controls global server configuration options and sets defaults for other statements.
server | sets certain configuration options on a per-server basis.
statistics-channels | declares communication channels to get access to named statistics.
trusted-keys | defines trusted DNSSEC keys.
managed-keys | lists DNSSEC keys to be kept up to date using RFC 5011 trust anchor maintenance.
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 ...; ]
       [ unix path perm number owner number group number
         keys { key_list }; ]
       [ unix ...; ]
    };

### 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 `*` (asterisk) 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. The asterisk "`*`" cannot be used for ip_port.

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

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

The primary authorization mechanism of the command channel is the key_list, which contains a list of key_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 a `rndc.conf` file and make it group readable by a group that contains the users who should have access.

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. Named supports `hmac-md5`, `hmac-sha1`, `hmac-sha224`, `hmac-sha256`, `hmac-sha384` and `hmac-sha512` TSIG authentication. Truncated hashes are supported by appending the minimum number of required bits preceded by a dash, e.g. `hmac-sha1-80`. The `secret_string` is the secret to be used by the algorithm, and is treated as a 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 three old versions of the file `lamers.log`, then just before it is opened `lamers.log.1` is renamed to `lamers.log.2`, `lamers.log.0` is renamed to `lamers.log.1`, 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.

On Windows machines syslog messages are directed to the EventViewer.

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 rndc 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 logs 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 {
        // send to syslog's daemon facility
        syslog daemon;
        // only send priority info and higher
        severity info;

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

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

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

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 called `named.run` in the server's working directory.

For security reasons, when the "`-u`" command line option is used, the `named.run` file is created only after named has changed to the new UID, and any debug output generated while named is starting 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, specifying 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, and the query name, class and type. Next it reports whether the Recursion Desired flag was set (+ if set, - if not set), if the query was signed (S), EDNS was in use (E), if TCP was used (T), if DO (DNSSEC Ok) was set (D), or if CD (Checking Disabled) was set (C). After this the destination address the query was sent to is reported. `client 127.0.0.1#62536 (www.example.com): query: www.example.com IN AAAA +SE` `client ::1#62537 (www.example.net): query: www.example.net IN AAAA -SE` (The first part of this log message, showing the client address/port number and query name, is repeated in all subsequent log messages related to the same query.)
query-errors | Information about queries that resulted in some failure.
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 been forced to NXDOMAIN as the result of a delegation-only zone or a delegation-only in a hint or stub zone declaration.
edns-disabled | Log queries that have been forced to use plain DNS due to timeouts. This is often due to the remote servers not being RFC 1034 compliant (not always returning FORMERR or similar to EDNS queries and other extensions to the DNS when they are not understood). In other words, this is targeted at servers that fail to respond to DNS queries that they don't understand. Note: the log message can also be due to packet loss. Before reporting servers for non-RFC 1034 compliance they should be re-tested to determine the nature of the non-compliance. This testing should prevent or reduce the number of false-positive reports. Note: eventually named will have to stop treating such timeouts as due to RFC 1034 non compliance and start treating it as plain packet loss. Falsely classifying packet loss as due to RFC 1034 non compliance impacts on DNSSEC validation which requires EDNS for the DNSSEC records to be returned.
RPZ | Information about errors in response policy zone files, rewritten responses, and at the highest debug levels, mere rewriting attempts.
rate-limit | (Only available when BIND 9 is configured with the `--enable-rrl` option at compile time.) The start, periodic, and final notices of the rate limiting of a stream of responses are logged at info severity in this category. These messages include a hash value of the domain name of the response and the name itself, except when there is insufficient memory to record the name for the final notice The final notice is normally delayed until about one minute after rate limit stops. A lack of memory can hurry the final notice, in which case it starts with an asterisk (*). Various internal events are logged at debug 1 level and higher. Rate limiting of individual requests is logged in the query-errors category.

#### The query-errors Category

The query-errors category is specifically intended for debugging purposes: To identify why and how specific queries result in responses which indicate an error. Messages of this category are therefore only logged with debug levels.

At the debug levels of 1 or higher, each response with the rcode of SERVFAIL is logged as follows:

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

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

At the debug levels of 2 or higher, detailed context information of recursive resolutions that resulted in SERVFAIL is logged. The log message will look like as follows:

    fetch completed at resolver.c:2970 for www.example.com/A
    in 30.000183: timed out/success [domain:example.com,
    referral:2,restart:7,qrysent:8,timeout:5,lame:0,neterr:0,
    badresp:1,adberr:0,findfail:0,valfail:0]

The first part before the colon shows that a recursive resolution for AAAA records of www.example.com completed in 30.000183 seconds and the final result that led to the SERVFAIL was determined at line 2970 of source file `resolver.c`.

The following part shows the detected final result and the latest result of DNSSEC validation. The latter is always success when no validation attempt is made. In this example, this query resulted in SERVFAIL probably because all name servers are down or unreachable, leading to a timeout in 30 seconds. DNSSEC validation was probably not attempted.

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

`referral` | The number of referrals the resolver received throughout the resolution process. In the above example this is 2, which are most likely com and example.com.
`restart` | The number of cycles that the resolver tried remote servers at the `domain` zone. In each cycle the resolver sends one query (possibly resending it, depending on the response) to each known name server of the `domain` zone.
`qrysent` | The number of queries the resolver sent at the `domain` zone.
`timeout` | The number of timeouts since the resolver received the last response.
`lame` | The number of lame servers the resolver detected at the `domain` zone. A server is detected to be lame either by an invalid response or as a result of lookup in BIND9's address database (ADB), where lame servers are cached.
`neterr` | The number of erroneous results that the resolver encountered in sending queries at the `domain` zone. One common case is the remote server is unreachable and the resolver receives an ICMP unreachable error message.
`badresp` | The number of unexpected responses (other than `lame`) to queries sent by the resolver at the `domain` zone.
`adberr` | Failures in finding remote server addresses of the `domain` zone in the ADB. One common case of this is that the remote server's name does not have any address records.
`findfail` | Failures of resolving remote server addresses. This is a total number of failures throughout the resolution process.
`valfail` | Failures of DNSSEC validation. Validation failures are counted throughout the resolution process (not limited to the `domain` zone), but should only happen in `domain`.

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

At the debug levels of 4 or higher, the same messages as those at the debug 2 level are logged for other errors than SERVFAIL. Unlike the above case of level 3, messages are logged for negative responses. This is because any unexpected results can be difficult to debug in the recursion case.

### 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 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 in their masters or also-notify lists.

### options Statement Grammar

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

    options {
        [ attach-cache cache_name; ]
        [ version version_string; ]
        [ hostname hostname_string; ]
        [ server-id server_id_string; ]
        [ directory path_name; ]
        [ key-directory path_name; ]
        [ managed-keys-directory path_name; ]
        [ named-xfer path_name; ]
        [ tkey-gssapi-keytab path_name; ]
        [ tkey-gssapi-credential principal; ]
        [ tkey-domain domainname; ]
        [ tkey-dhkey key_name key_tag; ]
        [ cache-file path_name; ]
        [ dump-file path_name; ]
        [ bindkeys-file path_name; ]
        [ secroots-file path_name; ]
        [ session-keyfile path_name; ]
        [ session-keyname key_name; ]
        [ session-keyalg algorithm_id; ]
        [ memstatistics yes_or_no; ]
        [ memstatistics-file path_name; ]
        [ pid-file path_name; ]
        [ recursing-file path_name; ]
        [ statistics-file path_name; ]
        [ zone-statistics full | terse | none; ]
        [ 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 | master-only; ]
        [ recursion yes_or_no; ]
        [ request-nsid yes_or_no; ]
        [ rfc2308-type1 yes_or_no; ]
        [ use-id-pool yes_or_no; ]
        [ maintain-ixfr-base yes_or_no; ]
        [ ixfr-from-differences (yes_or_no | master | slave); ]
        [ dnssec-enable yes_or_no; ]
        [ dnssec-validation (yes_or_no | auto); ]
        [ dnssec-lookaside ( auto |
                            no |
                            domain trust-anchor domain ); ]
        [ dnssec-must-be-secure domain yes_or_no; ]
        [ dnssec-accept-expired 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 ); ]
        [ check-dup-records ( warn | fail | ignore ); ]
        [ check-mx ( warn | fail | ignore ); ]
        [ check-wildcard yes_or_no; ]
        [ check-integrity yes_or_no; ]
        [ check-mx-cname ( warn | fail | ignore ); ]
        [ check-srv-cname ( warn | fail | ignore ); ]
        [ check-sibling yes_or_no; ]
        [ check-spf ( warn | fail | ignore ); ]
        [ allow-new-zones { yes_or_no }; ]
        [ allow-notify { address_match_list }; ]
        [ allow-query { address_match_list }; ]
        [ allow-query-on { address_match_list }; ]
        [ allow-query-cache { address_match_list }; ]
        [ allow-query-cache-on { address_match_list }; ]
        [ allow-transfer { address_match_list }; ]
        [ allow-recursion { address_match_list }; ]
        [ allow-recursion-on { address_match_list }; ]
        [ allow-update { address_match_list }; ]
        [ allow-update-forwarding { address_match_list }; ]
        [ update-check-ksk yes_or_no; ]
        [ dnssec-update-mode ( maintain | no-resign ); ]
        [ dnssec-dnskey-kskonly yes_or_no; ]
        [ dnssec-loadkeys-interval number; ]
        [ dnssec-secure-to-insecure yes_or_no ;]
        [ try-tcp-refresh yes_or_no; ]
        [ allow-v6-synthesis { address_match_list }; ]
        [ blackhole { address_match_list }; ]
        [ use-v4-udp-ports { port_list }; ]
        [ avoid-v4-udp-ports { port_list }; ]
        [ use-v6-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 ( ( ip4_addr | * )
            [ port ( ip_port | * ) ] |
            [ address ( ip4_addr | * ) ]
            [ port ( ip_port | * ) ] ) ; ]
        [ query-source-v6 ( ( ip6_addr | * )
            [ port ( ip_port | * ) ] |
            [ address ( ip6_addr | * ) ]
            [ port ( ip_port | * ) ] ) ; ]
        [ use-queryport-pool yes_or_no; ]
        [ queryport-pool-ports number; ]
        [ queryport-pool-updateinterval number; ]
        [ max-transfer-time-in number; ]
        [ max-transfer-time-out number; ]
        [ max-transfer-idle-in number; ]
        [ max-transfer-idle-out number; ]
        [ tcp-clients number; ]
        [ reserved-sockets 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-delay seconds ; ]
        [ notify-source (ip4_addr | *) [port ip_port] ; ]
        [ notify-source-v6 (ip6_addr | *) [port ip_port] ; ]
        [ notify-to-soa yes_or_no ; ]
        [ also-notify { ip_addr
                        [port ip_port] [key keyname] ;
                        [ ip_addr [port ip_port] [key keyname] ; ... ] }; ]
        [ 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 [number] ; ]
        [ sig-signing-nodes number ; ]
        [ sig-signing-signatures number ; ]
        [ sig-signing-type 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; ]
        [ filter-aaaa-on-v4 ( yes_or_no | break-dnssec ); ]
        [ filter-aaaa { address_match_list }; ]
        [ dns64 ipv6-prefix {
            [ clients { address_match_list }; ]
            [ mapped { address_match_list }; ]
            [ exclude { address_match_list }; ]
            [ suffix IPv6-address; ]
            [ recursive-only yes_or_no; ]
            [ break-dnssec yes_or_no; ]
        }; ];
        [ dns64-server name ]
        [ dns64-contact name ]
        [ preferred-glue ( A | AAAA | NONE ); ]
        [ edns-udp-size number; ]
        [ max-udp-size number; ]
        [ max-rsa-exponent-size number; ]
        [ root-delegation-only [ exclude { namelist } ] ; ]
        [ querylog yes_or_no ; ]
        [ disable-algorithms domain { algorithm;
                                    [ algorithm; ] }; ]
        [ acache-enable yes_or_no ; ]
        [ acache-cleaning-interval number; ]
        [ max-acache-size size_spec ; ]
        [ clients-per-query number ; ]
        [ max-clients-per-query number ; ]
        [ masterfile-format (text|raw) ; ]
        [ empty-server name ; ]
        [ empty-contact name ; ]
        [ empty-zones-enable yes_or_no ; ]
        [ disable-empty-zone zone_name ; ]
        [ zero-no-soa-ttl yes_or_no ; ]
        [ zero-no-soa-ttl-cache yes_or_no ; ]
        [ resolver-query-timeout number ; ]
        [ deny-answer-addresses { address_match_list } [ except-from { namelist } ];]
        [ deny-answer-aliases { namelist } [ except-from { namelist } ];]
        [ rate-limit {
            [ responses-per-second number ; ]
            [ referrals-per-second number ; ]
            [ nodata-per-second number ; ]
            [ nxdomains-per-second number ; ]
            [ errors-per-second number ; ]
            [ all-per-second number ; ]
            [ window number ; ]
            [ log-only yes_or_no ; ]
            [ qps-scale number ; ]
            [ ipv4-prefix-length number ; ]
            [ ipv6-prefix-length number ; ]
            [ slip number ; ]
            [ exempt-clients  { address_match_list } ; ]
            [ max-table-size number ; ]
            [ min-table-size number ; ]
          } ; ]
        [ response-policy { zone_name
            [ policy given | disabled | passthru | nxdomain | nodata | cname domain ]
            [ recursive-only yes_or_no ] [ max-policy-ttl number ] ;
        } [ recursive-only yes_or_no ] [ max-policy-ttl number ]
            [ break-dnssec yes_or_no ] [ min-ns-dots number ] ; ]
    };

### 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.

attach-cache

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

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

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

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

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

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

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

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

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 DNSSEC key files should be found, if different than the current working directory. (Note that this option has no effect on the paths for files containing non-DNSSEC keys such as `bind.keys`, `rndc.key` or `session.key`.)

managed-keys-directory

  Specifies the directory in which to store the files that track managed DNSSEC keys. By default, this is the working directory.

  If named is not configured to use views, then managed keys for the server will be tracked in a single file called `managed-keys.bind`. Otherwise, managed keys will be tracked in separate files, one file per view; each file name will be the SHA256 hash of the view name, followed by the extension `.mkeys`.

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-gssapi-keytab

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

tkey-gssapi-credential

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

tkey-domain

  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, or an otherwise non-existent subdomain like "_tkey.`domainname`". If you are using GSS-TSIG, this variable must be defined, unless you specify a specific keytab using tkey-gssapi-keytab.

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.

cache-file

  This is for testing only. Do not use.

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/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 filename, and therefore is not enclosed in double quotes.

recursing-file

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

statistics-file

  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”.

bindkeys-file

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

secroots-file

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

session-keyfile

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

session-keyname

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

session-keyalg

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

port

  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 prefer any type (NONE).

root-delegation-only

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

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

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

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

  Note some TLDs are not delegation only (e.g. "DE", "LV", "US" and "MUSEUM"). This list is not exhaustive.

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

disable-algorithms

  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 appended to the key name and a DLV record will be looked up to see if it can validate the key. If the DLV record validates a DNSKEY (similarly to the way a DS record does) the DNSKEY RRset is deemed to be trusted.

  If dnssec-lookaside is set to `auto`, then built-in default values for the DLV domain and trust anchor will be used, along with a built-in key for validation.

  If dnssec-lookaside is set to `no`, then dnssec-lookaside is not used.

  The default DLV key is stored in the file `bind.keys`; named will load that key at startup if dnssec-lookaside is set to `auto`. A copy of the file is installed along with BIND 9, and is current as of the release date. If the DLV key expires, a new copy of `bind.keys` can be downloaded from https://www.isc.org/solutions/dlv/.

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

  NOTE: named only loads certain specific keys from `bind.keys`: those for the DLV zone and for the DNS root zone. The file cannot be used to store keys for other zones.

dnssec-must-be-secure

  Specify hierarchies which must be or 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-keys or managed-keys statement, or dnssec-lookaside must be active.

dns64

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

  Compatible IPv6 prefixes have lengths of 32, 40, 48, 56, 64 and 96 as per RFC 6052.

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

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

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

  Normally, DNS64 won't apply to a domain name that owns one or more AAAA records; these records will simply be returned. The optional exclude ACL allows specification of a list of IPv6 addresses that will be ignored if they appear in a domain name's AAAA records, and DNS64 will be applied to any A records the domain name owns. If not defined, exclude defaults to none.

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

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

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

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

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

dnssec-update-mode

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

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

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

zone-statistics

  If `full`, the server will collect statistical data on all zones (unless specifically turned off on a per-zone basis by specifying zone-statistics terse or zone-statistics none in the zone statement). The default is `terse`, providing minimal statistics on zones (including name and current serial number, but not query type counters).

  These statistics may be accessed via the statistics-channel or using rndc stats, which will dump them to the file listed in the statistics-file. See also the section called “The Statistics File”.

  For backward compatibility with earlier versions of BIND 9, the zone-statistics option can also accept `yes` or `no`, which have the same effect as `full` and `terse`, respectively.

#### Boolean Options

allow-new-zones

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

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.

memstatistics

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

dialup

  If `yes`, then the server treats all zones as if they are doing zone transfers across a dial-on-demand dialup link, which can be brought up by traffic originating from this server. 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 or 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 onwards 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 `master-only`, notifies are only sent for master zones. 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.

notify-to-soa

  If `yes` do not check the nameservers in the NS RRset against the SOA MNAME. Normally a NOTIFY message is not sent to the SOA MNAME (SOA ORIGIN) as it is supposed to contain the name of the ultimate master. Sometimes, however, a slave is listed as the SOA MNAME in hidden master configurations and in that case you would want the ultimate master to still send NOTIFY messages to all the nameservers listed in the NS RRset.

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.

request-nsid

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

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.

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.

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

filter-aaaa-on-v4

  This option is only available when BIND 9 is compiled with the `--enable-filter-aaaa` option on the "configure" command line. It is intended to help the transition from IPv4 to IPv6 by not giving IPv6 addresses to DNS clients unless they have connections to the IPv6 Internet. This is not recommended unless absolutely necessary. The default is `no`. The filter-aaaa-on-v4 option may also be specified in view statements to override the global filter-aaaa-on-v4 option.

  If `yes`, the DNS client is at an IPv4 address, in filter-aaaa, and if the response does not include DNSSEC signatures, then all AAAA records are deleted from the response. This filtering applies to all responses and not only authoritative responses.

  If `break-dnssec`, then AAAA records are deleted even when dnssec is enabled. As suggested by the name, this makes the response not verify, because the DNSSEC protocol is designed detect deletions.

  This mechanism can erroneously cause other servers to not give AAAA records to their clients. A recursing server with both IPv6 and IPv4 network connections that queries an authoritative server using this mechanism via IPv4 will be denied AAAA records even if its client is using IPv6.

  This mechanism is applied to authoritative as well as non-authoritative records. A client using IPv4 that is not allowed recursion can erroneously be given AAAA records because the server is not allowed to check for A records.

  Some AAAA records are given to IPv4 clients in glue records. IPv4 clients that are servers can then erroneously answer requests for AAAA records received via IPv4.

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 via 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.

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

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 `yes`.

dnssec-validation

  Enable DNSSEC validation in named. Note dnssec-enable also needs to be set to `yes` to be effective. If set to `no`, DNSSEC validation is disabled. If set to `auto`, DNSSEC validation is enabled, and a default trust-anchor for the DNS root zone is used. If set to `yes`, DNSSEC validation is enabled, but a trust anchor must be manually configured using a trusted-keys or managed-keys statement. The default is `yes`.

dnssec-accept-expired

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

querylog

  Specify whether query logging should be started when named starts. 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 answers received from the network (response) the default is ignore.

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

  check-names applies to the owner names of A, AAAA and MX records. It also applies to the domain names in the RDATA of NS, SOA, MX, and SRV records. It 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, or IP6.INT).

check-dup-records

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

check-mx

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

check-wildcard

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

check-integrity

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

  Check that the two forms of Sender Policy Framework records (TXT records starting with "v=spf1" and SPF) either both exist or both don't exist. Warnings are emitted it they don't and be suppressed with check-spf.

check-mx-cname

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

check-srv-cname

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

check-sibling

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

check-spf

  When performing integrity checks, check that the two forms of Sender Policy Framwork records (TXT records starting with "v=spf1" and SPF) both exist or both don't exist and issue a warning if not met. The default is warn.

zero-no-soa-ttl

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

zero-no-soa-ttl-cache

  When caching a negative response to a SOA query set the TTL to zero. The default is no.

update-check-ksk

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

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

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

dnssec-dnskey-kskonly

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

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

dnssec-loadkeys-interval

  When a zone is configured with auto-dnssec maintain; its key repository must be checked periodically to see if any new keys have been added or any existing keys' timing metadata has been updated (see dnssec-keygen(8) and dnssec-settime(8)). The dnssec-loadkeys-interval option sets the frequency of automatic repository checks, in minutes. The default is `60` (1 hour), the minimum is `1` (1 minute), and the maximum is `1440` (24 hours); any higher value is silently reduced.

try-tcp-refresh

  Try to refresh the zone using TCP if UDP queries fail. For BIND 8 compatibility, the default is yes.

dnssec-secure-to-insecure

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

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

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

#### 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 or addresses of machines with access to both IPv4 and IPv6 transports. If a hostname is used, the server must be able to resolve the name using only the transport it has. If the machine is dual stacked, 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.

  ### Note

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

allow-query-on

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

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

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

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

  ### Note

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

allow-query-cache

  Specifies which hosts are allowed to get answers from the cache. If allow-query-cache is not set then allow-recursion is used if set, otherwise allow-query is used if set unless recursion no; is set in which case none; is used, otherwise the default (localnets; localhost;) is used.

allow-query-cache-on

  Specifies which local addresses can give answers from the cache. If not specified, the default is to allow cache queries on any address, localnets and localhost.

allow-recursion

  Specifies which hosts are allowed to make recursive queries through this server. If allow-recursion is not set then allow-query-cache is used if set, otherwise allow-query is used if set, otherwise the default (localnets; localhost;) is used.

allow-recursion-on

  Specifies which local addresses can accept recursive queries. If not specified, the default is to allow recursive queries on all addresses.

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.

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`.

filter-aaaa

  Specifies a list of addresses to which filter-aaaa-on-v4 is applies. The default is `any`.

resolver-query-timeout

  The amount of time the resolver will spend attempting to resolve a recursive query before failing. The default and minimum is `10` and the maximum is `30`. Setting it to `0` will result in the default being used.

#### 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 IPv4 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 unless -6 is specified when named is invoked. If -6 is specified then named will listen on port 53 on all IPv6 interfaces by default.

#### 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 * (asterisk) or is omitted, a wildcard IP address (INADDR_ANY) will be used.

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

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

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

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

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

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

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

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

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

Note: BIND 9.5.0 introduced the use-queryport-pool option to support a pool of such random ports, but this option is now obsolete because reusing the same ports in the pool may not be sufficiently secure. For the same reason, it is generally strongly discouraged to specify a particular port for the query-source or query-source-v6 options; it implicitly disables the use of randomized port numbers.

use-queryport-pool

  This option is obsolete.

queryport-pool-ports

  This option is obsolete.

queryport-pool-updateinterval

  This option is obsolete.

### Note

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

### Note

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

### 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. Optionally, a port may be specified with each also-notify address to send the notify messages to a port other than the default of 53. An optional TSIG key can also be specified with each address to cause the notify messages to be signed; this can be useful when sending notifies to multiple views. In place of explicit addresses, one or more named masters lists can be used.

  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.

  In addition to controlling the rate SOA refresh queries are issued at serial-query-rate also controls the rate at which NOTIFY messages are sent from both master and slave zones.

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 BIND 4.9.5 onwards. The many-answers format is also supported by recent Microsoft Windows nameservers. 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.

  ### Note

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

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

  ### Note

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

notify-source-v6

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

#### UDP Port Lists

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

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

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

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

#### 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 9.

max-journal-size

  Sets a maximum size for each journal file (see 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 largest permitted value is 2 gigabytes. The default is `unlimited`, which also means 2 gigabytes. This may also be set on a per-zone basis.

host-statistics-max

  In BIND 8, specifies the maximum number of host statistics 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`.

reserved-sockets

  The number of file descriptors reserved for TCP, stdio, etc. This needs to be big enough to cover the number of interfaces named listens on, tcp-clients as well as to provide room for outgoing TCP queries and incoming zone transfers. The default is `512`. The minimum value is `128` and the maximum value is `128` less than maxsockets (-S). This option may be removed in the future.

  This option has little effect on Windows.

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 based on an LRU based strategy so that the limit is not exceeded. A value of 0 is special, meaning that records are purged from the cache only when their TTLs expire. Another special keyword `unlimited` means the maximum value of 32-bit unsigned integers (0xffffffff), which may not have the same effect as 0 on machines that support more than 32 bits of memory space. Any positive values less than 2MB will be ignored reset to 2MB. In a server with multiple views, the limit applies separately to the cache of each view. The default is 0.

tcp-listen-queue

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

#### Periodic Task Intervals

cleaning-interval

  This interval is effectively obsolete. Previously, the server would remove expired resource records from the cache every cleaning-interval minutes. BIND 9 now manages cache memory in a more sophisticated manner and does not rely on the periodic cleaning any more. Specifying this option therefore has no effect on the server's behavior.

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 BIND 9.

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

The following example 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 "*" (asterisk).

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 cyclic round-robin order. If BIND is configured with the "--enable-fixed-rrset" option at compile time, then the initial ordering of the RRset will match the one specified in the zone file.

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.

By default, all records are returned in random order.

### Note

In this release of BIND 9, the rrset-order statement does not support "fixed" ordering by default. Fixed ordering can be enabled at compile time by specifying "--enable-fixed-rrset" on the "configure" command line.

#### Tuning

lame-ttl

  Sets the number of seconds to cache a lame server indication. 0 disables caching. (This is NOT recommended.) The default is `600` (10 minutes) and the maximum value is `1800` (30 minutes).

  Lame-ttl also controls the amount of time DNSSEC validation failures are cached. There is a minimum of 30 seconds applied to bad cache entries if the lame-ttl is set to less than 30 seconds.

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

  Sets the maximum time for which the server will cache ordinary (positive) answers. The default is one week (7 days). A value of zero may cause all queries to return SERVFAIL, because of lost caches of intermediate RRsets (such as NS and glue AAAA/A records) in the resolution process.

min-roots

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

  ### Note

  Not implemented in BIND 9.

sig-validity-interval

  Specifies the number of days into the future when DNSSEC signatures automatically generated as a result of dynamic updates (the section called “Dynamic Update”) will expire. There is an optional second field which specifies how long before expiry that the signatures will be regenerated. If not specified, the signatures will be regenerated at 1/4 of base interval. The second field is specified in days if the base interval is greater than 7 days otherwise it is specified in hours. The default base interval is `30` days giving a re-signing interval of 7 1/2 days. The maximum values are 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.

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

sig-signing-nodes

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

sig-signing-signatures

  Specify a threshold number of signatures that will terminate processing a quantum when signing a zone with a new DNSKEY. The default is `10`.

sig-signing-type

  Specify a private RDATA type to be used when generating key signing records. The default is `65534`.

  It is expected that this parameter may be removed in a future version once there is a standard type.

  These records can be removed from the zone once named has completed signing the zone with the matching key using nsupdate or rndc signing -clear. rndc signing -clear is the only supported way to remove these records from inline-signing zones.

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.

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

edns-udp-size

  Sets the advertised EDNS UDP buffer size in bytes to control the size of packets received. 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 is to get UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP packets that are greater than 512 bytes.

  named will fallback to using 512 bytes if it get a series of timeout at the initial value. 512 bytes is not being offered to encourage sites to fix their firewalls. Small EDNS UDP sizes will result in the excessive use of TCP.

max-udp-size

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

  Setting this to a low value will encourage additional TCP traffic to the nameserver.

masterfile-format

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

  Note that when a zone file in a different format than `text` is loaded, named may omit some of the checks which would be performed for a file in the `text` format. In particular, check-names checks do not apply for the `raw` format. This means a zone file in the `raw` format must be generated with the same check level as that specified in the named configuration file. This statement sets the masterfile-format for all zones, but can be overridden on a per-zone or per-view basis by including a masterfile-format statement within the zone or view block in the configuration file.

clients-per-query, max-clients-per-query

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

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

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

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

notify-delay

  The delay, in seconds, between sending sets of notify messages for a zone. The default is five (5) seconds.

  The overall rate that NOTIFY messages are sent for all zones is controlled by serial-query-rate.

max-rsa-exponent-size

  The maximum RSA exponent size, in bits, that will be accepted when validating. Valid values are 35 to 4096 bits. The default zero (0) is also accepted and is equivalent to 4096.

#### 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. Most global configuration options (allow-query, etc) will apply to this view, but some are locally overridden: notify, recursion and allow-new-zones are always set to `no`.

If you 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 the gethostname() function. The primary purpose of such queries is to identify which of a group of anycast servers is actually answering your queries. Specifying hostname none; disables processing of the queries.

server-id

  The ID the server should report when receiving a Name Server Identifier (NSID) query, or 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 the gethostname() function. The default server-id is none.

#### Built-in Empty Zones

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

Named will attempt to determine if a built-in zone already exists or is active (covered by a forward-only forwarding declaration) and will not create an empty zone in that case.

The current list of empty zones is:

- 10.IN-ADDR.ARPA
- 16.172.IN-ADDR.ARPA
- 17.172.IN-ADDR.ARPA
- 18.172.IN-ADDR.ARPA
- 19.172.IN-ADDR.ARPA
- 20.172.IN-ADDR.ARPA
- 21.172.IN-ADDR.ARPA
- 22.172.IN-ADDR.ARPA
- 23.172.IN-ADDR.ARPA
- 24.172.IN-ADDR.ARPA
- 25.172.IN-ADDR.ARPA
- 26.172.IN-ADDR.ARPA
- 27.172.IN-ADDR.ARPA
- 28.172.IN-ADDR.ARPA
- 29.172.IN-ADDR.ARPA
- 30.172.IN-ADDR.ARPA
- 31.172.IN-ADDR.ARPA
- 168.192.IN-ADDR.ARPA
- 64.100.IN-ADDR.ARPA
- 65.100.IN-ADDR.ARPA
- 66.100.IN-ADDR.ARPA
- 67.100.IN-ADDR.ARPA
- 68.100.IN-ADDR.ARPA
- 69.100.IN-ADDR.ARPA
- 70.100.IN-ADDR.ARPA
- 71.100.IN-ADDR.ARPA
- 72.100.IN-ADDR.ARPA
- 73.100.IN-ADDR.ARPA
- 74.100.IN-ADDR.ARPA
- 75.100.IN-ADDR.ARPA
- 76.100.IN-ADDR.ARPA
- 77.100.IN-ADDR.ARPA
- 78.100.IN-ADDR.ARPA
- 79.100.IN-ADDR.ARPA
- 80.100.IN-ADDR.ARPA
- 81.100.IN-ADDR.ARPA
- 82.100.IN-ADDR.ARPA
- 83.100.IN-ADDR.ARPA
- 84.100.IN-ADDR.ARPA
- 85.100.IN-ADDR.ARPA
- 86.100.IN-ADDR.ARPA
- 87.100.IN-ADDR.ARPA
- 88.100.IN-ADDR.ARPA
- 89.100.IN-ADDR.ARPA
- 90.100.IN-ADDR.ARPA
- 91.100.IN-ADDR.ARPA
- 92.100.IN-ADDR.ARPA
- 93.100.IN-ADDR.ARPA
- 94.100.IN-ADDR.ARPA
- 95.100.IN-ADDR.ARPA
- 96.100.IN-ADDR.ARPA
- 97.100.IN-ADDR.ARPA
- 98.100.IN-ADDR.ARPA
- 99.100.IN-ADDR.ARPA
- 100.100.IN-ADDR.ARPA
- 101.100.IN-ADDR.ARPA
- 102.100.IN-ADDR.ARPA
- 103.100.IN-ADDR.ARPA
- 104.100.IN-ADDR.ARPA
- 105.100.IN-ADDR.ARPA
- 106.100.IN-ADDR.ARPA
- 107.100.IN-ADDR.ARPA
- 108.100.IN-ADDR.ARPA
- 109.100.IN-ADDR.ARPA
- 110.100.IN-ADDR.ARPA
- 111.100.IN-ADDR.ARPA
- 112.100.IN-ADDR.ARPA
- 113.100.IN-ADDR.ARPA
- 114.100.IN-ADDR.ARPA
- 115.100.IN-ADDR.ARPA
- 116.100.IN-ADDR.ARPA
- 117.100.IN-ADDR.ARPA
- 118.100.IN-ADDR.ARPA
- 119.100.IN-ADDR.ARPA
- 120.100.IN-ADDR.ARPA
- 121.100.IN-ADDR.ARPA
- 122.100.IN-ADDR.ARPA
- 123.100.IN-ADDR.ARPA
- 124.100.IN-ADDR.ARPA
- 125.100.IN-ADDR.ARPA
- 126.100.IN-ADDR.ARPA
- 127.100.IN-ADDR.ARPA
- 0.IN-ADDR.ARPA
- 127.IN-ADDR.ARPA
- 254.169.IN-ADDR.ARPA
- 2.0.192.IN-ADDR.ARPA
- 100.51.198.IN-ADDR.ARPA
- 113.0.203.IN-ADDR.ARPA
- 255.255.255.255.IN-ADDR.ARPA
- 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
- 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
- 8.B.D.0.1.0.0.2.IP6.ARPA
- D.F.IP6.ARPA
- 8.E.F.IP6.ARPA
- 9.E.F.IP6.ARPA
- A.E.F.IP6.ARPA
- B.E.F.IP6.ARPA

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

                disable-empty-zone ".";

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

### Note

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

empty-server

  Specify what server name will appear in the returned SOA record for empty zones. If none is specified, then the zone's name will be used.

empty-contact

  Specify what contact name will appear in the returned SOA record for empty zones. If none is specified, then "." will be used.

empty-zones-enable

  Enable or disable all empty zones. By default, they are enabled.

disable-empty-zone

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

#### Additional Section Caching

The additional section cache, also called acache, is an internal cache to improve the response performance of BIND 9. When additional section caching is enabled, BIND 9 will cache an internal short-cut to the additional section content for each answer RR. Note that acache is an internal caching mechanism of BIND 9, and is not related to the DNS caching server function.

Additional section caching does not change the response content (except the RRsets ordering of the additional section, see below), but can improve the response performance significantly. It is particularly effective when BIND 9 acts as an authoritative server for a zone that has many delegations with many glue RRs.

In order to obtain the maximum performance improvement from additional section caching, setting additional-from-cache to no is recommended, since the current implementation of acache does not short-cut of additional section information from the DNS cache data.

One obvious disadvantage of acache is that it requires much more memory for the internal cached data. Thus, if the response performance does not matter and memory consumption is much more critical, the acache mechanism can be disabled by setting acache-enable to no. It is also possible to specify the upper limit of memory consumption for acache by using max-acache-size.

Additional section caching also has a minor effect on the RRset ordering in the additional section. Without acache, cyclic order is effective for the additional section as well as the answer and authority sections. However, additional section caching fixes the ordering when it first caches an RRset for the additional section, and the same ordering will be kept in succeeding responses, regardless of the setting of rrset-order. The effect of this should be minor, however, since an RRset in the additional section typically only contains a small number of RRs (and in many cases it only contains a single RR), in which case the ordering does not matter much.

The following is a summary of options related to acache.

acache-enable

  If yes, additional section caching is enabled. The default value is no.

acache-cleaning-interval

  The server will remove stale cache entries, based on an LRU based algorithm, every acache-cleaning-interval minutes. The default is 60 minutes. If set to 0, no periodic cleaning will occur.

max-acache-size

  The maximum amount of memory in bytes to use for the server's acache. When the amount of data in the acache reaches this limit, the server will clean more aggressively so that the limit is not exceeded. In a server with multiple views, the limit applies separately to the acache of each view. The default is `16M`.

#### Content Filtering

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

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

returned by an "example.com" server will be accepted.

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

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

This filtering is intended to prevent "DNS rebinding attacks," in which an attacker, in response to a query for a domain name the attacker controls, returns an IP address within your own network or an alias name within your own domain. A naive web browser or script could then serve as an unintended proxy, allowing the attacker to get access to an internal node of your local network that couldn't be externally accessed otherwise. See the paper available at http://portal.acm.org/citation.cfm?id=1315245.1315298 for more details about the attacks.

For example, if you own a domain named "example.net" and your internal network uses an IPv4 prefix 192.0.2.0/24, you might specify the following rules:

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

If an external attacker lets a web browser in your local network look up an IPv4 address of "attacker.example.com", the attacker's DNS server would return a response like this:

    attacker.example.com. A 192.0.2.1

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

On the other hand, if the browser looks up a legitimate internal web server "www.example.net" and the following response is returned to the BIND 9 server

    www.example.net. A 192.0.2.2

it will be accepted since the owner name "www.example.net" matches the except-from element, "example.net".

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

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

#### Response Policy Zone (RPZ) Rewriting

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

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

Four policy triggers are encoded in RPZ records, QNAME, IP, NSIP, and NSDNAME. QNAME RPZ records triggered by query names of requests and targets of CNAME records resolved to generate the response. The owner name of a QNAME RPZ record is the query name relativized to the RPZ.

The second kind of RPZ trigger is an IP address in an A and AAAA record in the ANSWER section of a response. IP address triggers are encoded in records that have owner names that are subdomains of `rpz-ip` relativized to the RPZ origin name and encode an IP address or address block. IPv4 trigger addresses are represented as `prefixlength.B4.B3.B2.B1.rpz-ip`. The prefix length must be between 1 and 32. All four bytes, B4, B3, B2, and B1, must be present. B4 is the decimal value of the least significant byte of the IPv4 address as in IN-ADDR.ARPA. IPv6 addresses are encoded in a format similar to the standard IPv6 text representation, `prefixlength.W8.W7.W6.W5.W4.W3.W2.W1.rpz-ip`. Each of W8,...,W1 is a one to four digit hexadecimal number representing 16 bits of the IPv6 address as in the standard text representation of IPv6 addresses, but reversed as in IN-ADDR.ARPA. All 8 words must be present except when consecutive zero words are replaced with `.zz.` analogous to double colons (::) in standard IPv6 text encodings. The prefix length must be between 1 and 128.

NSDNAME triggers match names of authoritative servers for the query name, a parent of the query name, a CNAME for query name, or a parent of a CNAME. They are encoded as subdomains of `rpz-nsdomain` relativized to the RPZ origin name. NSIP triggers match IP addresses in A and AAAA RRsets for domains that can be checked against NSDNAME policy records. NSIP triggers are encoded like IP triggers except as subdomains of `rpz-nsip`. NSDNAME and NSIP triggers are checked only for names with at least min-ns-dots dots. The default value of min-ns-dots is 1 to exclude top level domains.

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

- Choose the triggered record in the zone that appears first in the response-policy option.
- Prefer QNAME to IP to NSDNAME to NSIP triggers in a single zone.
- Among NSDNAME triggers, prefer the trigger that matches the smallest name under the DNSSEC ordering.
- Among IP or NSIP triggers, prefer the trigger with the longest prefix.
- Among triggers with the same prefex length, prefer the IP or NSIP trigger that matches the smallest IP address.

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

RPZ record sets are sets of any types of DNS record except DNAME or DNSSEC that encode actions or responses to queries.

- The NXDOMAIN response is encoded by a CNAME whose target is the root domain (.)
- A CNAME whose target is the wildcard top-level domain (*.) specifies the NODATA action, which rewrites the response to NODATA or ANCOUNT=1.
- The Local Data action is represented by a set ordinary DNS records that are used to answer queries. Queries for record types not the set are answered with NODATA. A special form of local data is a CNAME whose target is a wildcard such as *.example.com. It is used as if were an ordinary CNAME after the astrisk (*) has been replaced with the query name. The purpose for this special form is query logging in the walled garden's authority DNS server.
- The PASSTHRU policy is specified by a CNAME whose target is rpz-passthru. It causes the response to not be rewritten and is most often used to "poke holes" in policies for CIDR blocks. (A CNAME whose target is the variable part of its owner name is an obsolete specification of the PASSTHRU policy.)

The actions specified in an RPZ can be overridden with a policy clause in the response-policy option. An organization using an RPZ provided by another organization might use this mechanism to redirect domains to its own walled garden.

- GIVEN says "do not override but perform the action specified in the zone."
- DISABLED causes policy records to do nothing but log what they might have done. The response to the DNS query will be written according to any triggered policy records that are not disabled. Disabled policy zones should appear first, because they will often not be logged if a higher precedence trigger is found first.
- PASSTHRU causes all policy records to act as if they were CNAME records with targets the variable part of their owner name. They protect the response from being changed.
- NXDOMAIN causes all RPZ records to specify NXDOMAIN policies.
- NODATA overrides with the NODATA policy
- CNAME domain causes all RPZ policy records to act as if they were "cname domain" records.

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

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

The TTL of a record modified by RPZ policies is set from the TTL of the relevant record in policy zone. It is then limited to a maximum value. The max-policy-ttl clause changes that maximum from its default of 5.

For example, you might use this option statement

        response-policy { zone "badlist"; };

and this zone statement

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

with this zone file

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

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

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

    bzone.domain.com        CNAME   garden.example.com.

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

    ; IP policy records that rewrite all answers for 127/8 except 127.0.0.1
    8.0.0.0.127.rpz-ip      CNAME   .
    32.1.0.0.127.rpz-ip     CNAME   rpz-passthru.

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

RPZ can affect server performance. Each configured response policy zone requires the server to perform one to four additional database lookups before a query can be answered. For example, a DNS server with four policy zones, each with all four kinds of response triggers, QNAME, IP, NSIP, and NSDNAME, requires a total of 17 times as many database lookups as a similar DNS server with no response policy zones. A BIND9 server with adequate memory and one response policy zone with QNAME and IP triggers might achieve a maximum queries-per-second rate about 20% lower. A server with four response policy zones with QNAME and IP triggers might have a maximum QPS rate about 50% lower.

Responses rewritten by RPZ are counted in the RPZRewrites statistics.

#### Response Rate Limiting

This feature is only available when BIND 9 is compiled with the `--enable-rrl` option on the "configure" command line.

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

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

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

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

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

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

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

Many attacks using DNS involve UDP requests with forged source addresses. Rate limiting prevents the use of BIND 9 to flood a network with responses to requests with forged source addresses, but could let a third party block responses to legitimate requests. There is a mechanism that can answer some legitimate requests from a client whose address is being forged in a flood. Setting slip to 2 (its default) causes every other UDP request to be answered with a small truncated (TC=1) response. The small size and reduced frequency, and so lack of amplification, of "slipped" responses make them unattractive for reflection DoS attacks. slip must be between 0 and 10. A value of 0 does not "slip": no truncated responses are sent due to rate limiting, all responses are dropped. A value of 1 causes every response to slip; values between 2 and 10 cause every n'th response to slip. Some error responses including REFUSED and SERVFAIL cannot be replaced with truncated responses and are instead leaked at the slip rate.

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

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

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

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

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

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

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

### server Statement Grammar

    server ip_addr[/prefixlen] {
        [ bogus yes_or_no ; ]
        [ provide-ixfr yes_or_no ; ]
        [ request-ixfr yes_or_no ; ]
        [ edns yes_or_no ; ]
        [ edns-udp-size number ; ]
        [ max-udp-size number ; ]
        [ 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] ; ]
        [ notify-source (ip4_addr | *) [port ip_port] ; ]
        [ notify-source-v6 (ip6_addr | *) [port ip_port] ; ]
        [ query-source [ address ( ip_addr | * ) ]
                      [ port ( ip_port | * ) ]; ]
        [ query-source-v6 [ address ( ip_addr | * ) ]
                         [ port ( ip_port | * ) ]; ]
        [ use-queryport-pool yes_or_no; ]
        [ queryport-pool-ports number; ]
        [ queryport-pool-updateinterval number; ]
    };

### server Statement Definition and Usage

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

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

If 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. It may also be set in the zone block and, if set there, it will override the global or view setting for that zone.

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 edns-udp-size option sets the EDNS UDP size that is advertised by named when querying the remote server. Valid values are 512 to 4096 bytes (values outside this range will be silently adjusted). This option is useful when you wish to advertises a different value to this server than the value you advertise globally, for example, when there is a firewall at the remote site that is blocking large replies.

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

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

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

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

### statistics-channels Statement Grammar

    statistics-channels {
       [ inet ( ip_addr | * ) [ port ip_port ]
       [ allow {  address_match_list  } ]; ]
       [ inet ...; ]
    };

### statistics-channels Statement Definition and Usage

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

This statement intends to be flexible to support multiple communication protocols in the future, but currently only HTTP access is supported. It requires that BIND 9 be compiled with libxml2; the statistics-channels statement is still accepted even if it is built without the library, but any HTTP access will fail with an error.

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

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

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

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

If the statistics channel is configured to listen on 127.0.0.1 port 8888, then the statistics are accessible in XML format at http://127.0.0.1:8888/ or http://127.0.0.1:8888/xml. A CSS file is included which can format the XML statistics into tables when viewed with a stylesheet-capable browser. When BIND 9 is configured with --enable-newstats, a new XML schema is used (version 3) which adds additional zone statistics and uses a flatter tree for more efficient parsing. The stylesheet included uses the Google Charts API to render data into into charts and graphs when using a javascript-capable browser.

Applications that depend on a particular XML schema can request http://127.0.0.1:8888/xml/v2 for version 2 of the statistics XML schema or http://127.0.0.1:8888/xml/v3 for version 3. If the requested schema is supported by the server, then it will respond; if not, it will return a "page not found" error.

### 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.

All keys (and corresponding zones) listed in trusted-keys are deemed to exist regardless of what parent zones say. Similarly for all keys listed in trusted-keys only those keys are used to validate the DNSKEY RRset. The parent's DS RRset will not be used.

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. Spaces, tabs, newlines and carriage returns are ignored in the key data, so the configuration may be split up into multiple lines.

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

### managed-keys Statement Grammar

    managed-keys {
        name initial-key flags protocol algorithm key-data ;
        [ name initial-key flags protocol algorithm key-data ; [...]]
    };

### managed-keys Statement Definition and Usage

The managed-keys statement, like trusted-keys, defines DNSSEC security roots. The difference is that managed-keys can be kept up to date automatically, without intervention from the resolver operator.

Suppose, for example, that a zone's key-signing key was compromised, and the zone owner had to revoke and replace the key. A resolver which had the old key in a trusted-keys statement would be unable to validate this zone any longer; it would reply with a SERVFAIL response code. This would continue until the resolver operator had updated the trusted-keys statement with the new key.

If, however, the zone were listed in a managed-keys statement instead, then the zone owner could add a "stand-by" key to the zone in advance. named would store the stand-by key, and when the original key was revoked, named would be able to transition smoothly to the new key. It would also recognize that the old key had been revoked, and cease using that key to validate answers, minimizing the damage that the compromised key could do.

A managed-keys statement contains a list of the keys to be managed, along with information about how the keys are to be initialized for the first time. The only initialization method currently supported (as of BIND 9.7.0) is `initial-key`. This means the managed-keys statement must contain a copy of the initializing key. (Future releases may allow keys to be initialized by other methods, eliminating this requirement.)

Consequently, a managed-keys statement appears similar to a trusted-keys, differing in the presence of the second field, containing the keyword `initial-key`. The difference is, whereas the keys listed in a trusted-keys continue to be trusted until they are removed from `named.conf`, an initializing key listed in a managed-keys statement is only trusted once: for as long as it takes to load the managed key database and start the RFC 5011 key maintenance process.

The first time named runs with a managed key configured in `named.conf`, it fetches the DNSKEY RRset directly from the zone apex, and validates it using the key specified in the managed-keys statement. If the DNSKEY RRset is validly signed, then it is used as the basis for a new managed keys database.

From that point on, whenever named runs, it sees the managed-keys statement, checks to make sure RFC 5011 key maintenance has already been initialized for the specified domain, and if so, it simply moves on. The key specified in the managed-keys is not used to validate answers; it has been superseded by the key or keys stored in the managed keys database.

The next time named runs after a name has been removed from the managed-keys statement, the corresponding zone will be removed from the managed keys database, and RFC 5011 key maintenance will no longer be used for that domain.

named only maintains a single managed keys database; consequently, unlike trusted-keys, managed-keys may only be set at the top level of `named.conf`, not within a view.

In the current implementation, the managed keys database is stored as a master-format zone file called `managed-keys.bind`. When the key database is changed, the zone is updated. As with any other dynamic zone, changes will be written into a journal file, `managed-keys.bind.jnl`. They are committed to the master file as soon as possible afterward; in the case of the managed key database, this will usually occur within 30 seconds. So, whenever named is using automatic key maintenance, those two files can be expected to exist in the working directory. (For this reason among others, the working directory should be always be writable by named.)

If the dnssec-validation option is set to `auto`, named will automatically initialize a managed key for the root zone. Similarly, if the dnssec-lookaside option is set to `auto`, named will automatically initialize a managed key for the zone `dlv.isc.org`. In both cases, the key that is used to initialize the key maintenance process is built into named, and can be overridden from bindkeys-file.

### 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 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 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;
        [ allow-query { address_match_list }; ]
        [ allow-query-on { address_match_list }; ]
        [ allow-transfer { address_match_list }; ]
        [ allow-update { address_match_list }; ]
        [ update-check-ksk yes_or_no; ]
        [ dnssec-dnskey-kskonly yes_or_no; ]
        [ dnssec-loadkeys-interval number; ]
        [ update-policy local | { update_policy_rule [...] }; ]
        [ also-notify { ip_addr [port ip_port] ;
                      [ ip_addr [port ip_port] ; ... ] }; ]
        [ check-names (warn|fail|ignore) ; ]
        [ check-mx (warn|fail|ignore) ; ]
        [ check-wildcard yes_or_no; ]
        [ check-spf ( warn | fail | ignore ); ]
        [ check-integrity yes_or_no ; ]
        [ dialup dialup_option ; ]
        [ file string ; ]
        [ masterfile-format (text|raw) ; ]
        [ journal string ; ]
        [ max-journal-size size_spec; ]
        [ forward (only|first) ; ]
        [ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
        [ ixfr-base string ; ]
        [ ixfr-from-differences yes_or_no; ]
        [ ixfr-tmp-file string ; ]
        [ request-ixfr yes_or_no ; ]
        [ maintain-ixfr-base yes_or_no ; ]
        [ max-ixfr-log-size number ; ]
        [ max-transfer-idle-out number ; ]
        [ max-transfer-time-out number ; ]
        [ notify yes_or_no | explicit | master-only ; ]
        [ notify-delay seconds ; ]
        [ notify-to-soa yes_or_no; ]
        [ pubkey number number number string ; ]
        [ notify-source (ip4_addr | *) [port ip_port] ; ]
        [ notify-source-v6 (ip6_addr | *) [port ip_port] ; ]
        [ zone-statistics full | terse | none; ]
        [ sig-validity-interval number [number] ; ]
        [ sig-signing-nodes number ; ]
        [ sig-signing-signatures number ; ]
        [ sig-signing-type number ; ]
        [ database string ; ]
        [ min-refresh-time number ; ]
        [ max-refresh-time number ; ]
        [ min-retry-time number ; ]
        [ max-retry-time number ; ]
        [ key-directory path_name; ]
        [ auto-dnssec allow|maintain|off; ]
        [ inline-signing yes_or_no; ]
        [ zero-no-soa-ttl yes_or_no ; ]
        [ serial-update-method increment|unixtime; ]
    };

    zone zone_name [class] {
        type slave;
        [ allow-notify { address_match_list }; ]
        [ allow-query { address_match_list }; ]
        [ allow-query-on { address_match_list }; ]
        [ allow-transfer { address_match_list }; ]
        [ allow-update-forwarding { address_match_list }; ]
        [ dnssec-update-mode ( maintain | no-resign ); ]
        [ update-check-ksk yes_or_no; ]
        [ dnssec-dnskey-kskonly yes_or_no; ]
        [ dnssec-loadkeys-interval number; ]
        [ dnssec-secure-to-insecure yes_or_no ; ]
        [ try-tcp-refresh yes_or_no; ]
        [ also-notify [port ip_port] { ( masters_list | ip_addr
                                  [port ip_port]
                                  [key key] ) ; [...] }; ]
        [ check-names (warn|fail|ignore) ; ]
        [ dialup dialup_option ; ]
        [ file string ; ]
        [ masterfile-format (text|raw) ; ]
        [ journal string ; ]
        [ max-journal-size size_spec; ]
        [ forward (only|first) ; ]
        [ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
        [ ixfr-base string ; ]
        [ ixfr-from-differences yes_or_no; ]
        [ 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 | master-only ; ]
        [ notify-delay seconds ; ]
        [ notify-to-soa yes_or_no; ]
        [ 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 full | terse | none; ]
        [ sig-validity-interval number [number] ; ]
        [ sig-signing-nodes number ; ]
        [ sig-signing-signatures number ; ]
        [ sig-signing-type number ; ]
        [ database string ; ]
        [ min-refresh-time number ; ]
        [ max-refresh-time number ; ]
        [ min-retry-time number ; ]
        [ max-retry-time number ; ]
        [ key-directory path_name; ]
        [ auto-dnssec allow|maintain|off; ]
        [ inline-signing yes_or_no; ]
        [ multi-master yes_or_no ; ]
        [ zero-no-soa-ttl yes_or_no ; ]
    };

    zone zone_name [class] {
        type hint;
        file string ;
        [ delegation-only yes_or_no ; ]
        [ check-names (warn|fail|ignore) ; ] // Not Implemented.
    };

    zone zone_name [class] {
        type stub;
        [ allow-query { address_match_list }; ]
        [ allow-query-on { address_match_list }; ]
        [ check-names (warn|fail|ignore) ; ]
        [ dialup dialup_option ; ]
        [ delegation-only yes_or_no ; ]
        [ file string ; ]
        [ masterfile-format (text|raw) ; ]
        [ forward (only|first) ; ]
        [ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
        [ masters [port ip_port] { ( masters_list | ip_addr
                                  [port ip_port]
                                  [key key] ) ; [...] }; ]
        [ max-transfer-idle-in number ; ]
        [ max-transfer-time-in number ; ]
        [ 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; ]
        [ zone-statistics yes_or_no ; ]
        [ database string ; ]
        [ min-refresh-time number ; ]
        [ max-refresh-time number ; ]
        [ min-retry-time number ; ]
        [ max-retry-time number ; ]
        [ multi-master yes_or_no ; ]
    };

    zone zone_name [class] {
        type static-stub;
        [ allow-query { address_match_list }; ]
        [ server-addresses { [ ip_addr ; ... ] }; ]
        [ server-names { [ namelist ] }; ]
        [ zone-statistics yes_or_no ; ]
    };

    zone zone_name [class] {
        type forward;
        [ forward (only|first) ; ]
        [ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
        [ delegation-only yes_or_no ; ]
    };

    zone "." [class] {
        type redirect;
        file string ;
        [ masterfile-format (text|raw) ; ]
        [ allow-query { address_match_list }; ]
    };

    zone zone_name [class] {
        type delegation-only;
    };

### 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 startup and eliminates a needless waste of bandwidth. Note that for large numbers (in the tens or hundreds of thousands) of zones per server, it is best to use a two-level naming scheme for zone filenames. For example, a 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 100000 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 RFC1918 addressing may be configured with stub zones for `10.in-addr.arpa` to use a set of internal name servers as the authoritative servers for that domain.
`static-stub` | A static-stub zone is similar to a stub zone with the following exceptions: the zone data is statically configured, rather than transferred from a master server; when recursion is necessary for a query that matches a static-stub zone, the locally configured data (nameserver names and glue addresses) is always used even if different authoritative information is cached. Zone data is configured via the server-addresses and server-names zone options. The zone data is maintained in the form of NS and (if necessary) glue A or AAAA RRs internally, which can be seen by dumping zone databases by rndc dumpdb -all. The configured RRs are considered local configuration parameters rather than public data. Non recursive queries (i.e., those with the RD bit off) to a static-stub zone are therefore prohibited and will be responded with REFUSED. Since the data is statically configured, no zone maintenance action takes place for a static-stub zone. For example, there is no periodic refresh attempt, and an incoming notify message will be rejected with an rcode of NOTAUTH. Each static-stub zone is configured with internally generated NS and (if necessary) glue A or AAAA RRs
`forward` | A "forward zone" is a way to configure forwarding on a per-domain basis. A zone statement of type forward can contain a forward and/or forwarders statement, which 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.
`redirect` | Redirect zones are used to provide answers to queries when normal resolution would result in NXDOMAIN being returned. Only one redirect zone is supported per view. allow-query can be used to restrict which clients see these answers. If the client has requested DNSSEC records (DO=1) and the NXDOMAIN response is signed then no substitution will occur. To redirect all NXDOMAIN responses to 100.100.100.2 and 2001:ffff:ffff::100.100.100.2, one would configure a type redirect zone named ".", with the zone file containing wildcard records that point to the desired addresses: `"*. IN A 100.100.100.2"` and `"*. IN AAAA 2001:ffff:ffff::100.100.100.2"`. To redirect all Spanish names (under .ES) one would use similar entries but with the names "*.ES." instead of "*.". To redirect all commercial Spanish names (under COM.ES) one would use wildcard entries called "*.COM.ES.". Note that the redirect zone supports all possible types; it is not limited to A and AAAA records. Because redirect zones are not referenced directly by name, they are not kept in the zone lookup table with normal master and slave zones. Consequently, it is not currently possible to use rndc reload `zonename` to reload a redirect zone. However, when using rndc reload without specifying a zone name, redirect zones will be reloaded along with other zones.
`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 an 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. See caveats in root-delegation-only.

#### 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-query-on

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

allow-transfer

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

allow-update

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

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. A TSIG key may also be specified to cause the `NOTIFY` to be signed by the given key. also-notify is not meaningful for stub zones. The default is the empty list.

check-names

  This option is used to restrict the character set and syntax of certain domain names in 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. It is not implemented for hint zones.

check-mx

  See the description of check-mx in the section called “Boolean Options”.

check-spf

  See the description of check-spf in the section called “Boolean Options”.

check-wildcard

  See the description of check-wildcard in the section called “Boolean Options”.

check-integrity

  See the description of check-integrity in the section called “Boolean Options”.

check-sibling

  See the description of check-sibling in the section called “Boolean Options”.

zero-no-soa-ttl

  See the description of zero-no-soa-ttl in the section called “Boolean Options”.

update-check-ksk

  See the description of update-check-ksk in the section called “Boolean Options”.

dnssec-update-mode

  See the description of dnssec-update-mode in the section called “options Statement Definition and Usage”.

dnssec-dnskey-kskonly

  See the description of dnssec-dnskey-kskonly in the section called “Boolean Options”.

try-tcp-refresh

  See the description of try-tcp-refresh in the section called “Boolean Options”.

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.

  See caveats in root-delegation-only.

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 and 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.

journal

  Allow the default journal's filename to be overridden. The default is the zone's filename with "`.jnl`" appended. This is applicable to master and slave zones.

max-journal-size

  See the description of max-journal-size in the section called “Server Resource Limits”.

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”.

notify-delay

  See the description of notify-delay in the section called “Tuning”.

notify-to-soa

  See the description of notify-to-soa 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.

server-addresses

  Only meaningful for static-stub zones. This is a list of IP addresses to which queries should be sent in recursive resolution for the zone. A non empty list for this option will internally configure the apex NS RR with associated glue A or AAAA RRs.

  For example, if "example.com" is configured as a static-stub zone with 192.0.2.1 and 2001:db8::1234 in a server-addresses option, the following RRs will be internally configured.

      example.com. NS example.com.
      example.com. A 192.0.2.1
      example.com. AAAA 2001:db8::1234

  These records are internally used to resolve names under the static-stub zone. For instance, if the server receives a query for "www.example.com" with the RD bit on, the server will initiate recursive resolution and send queries to 192.0.2.1 and/or 2001:db8::1234.

server-names

  Only meaningful for static-stub zones. This is a list of domain names of nameservers that act as authoritative servers of the static-stub zone. These names will be resolved to IP addresses when named needs to send queries to these servers. To make this supplemental resolution successful, these names must not be a subdomain of the origin name of static-stub zone. That is, when "example.net" is the origin of a static-stub zone, "ns.example" and "master.example.com" can be specified in the server-names option, but "ns.example.net" cannot, and will be rejected by the configuration parser.

  A non empty list for this option will internally configure the apex NS RR with the specified names. For example, if "example.com" is configured as a static-stub zone with "ns1.example.net" and "ns2.example.net" in a server-names option, the following RRs will be internally configured.

      example.com. NS ns1.example.net.
      example.com. NS ns2.example.net.

  These records are internally used to resolve names under the static-stub zone. For instance, if the server receives a query for "www.example.com" with the RD bit on, the server initiate recursive resolution, resolve "ns1.example.net" and/or "ns2.example.net" to IP addresses, and then send queries to (one or more of) these addresses.

sig-validity-interval

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

sig-signing-nodes

  See the description of sig-signing-nodes in the section called “Tuning”.

sig-signing-signatures

  See the description of sig-signing-signatures in the section called “Tuning”.

sig-signing-type

  See the description of sig-signing-type 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”. (Note that the ixfr-from-differences `master` and `slave` choices are not available at the zone level.)

key-directory

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

auto-dnssec

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

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

  auto-dnssec maintain; includes the above, but also automatically adjusts the zone's DNSSEC keys on schedule, according to the keys' timing metadata (see dnssec-keygen(8) and dnssec-settime(8)). The command rndc sign `zonename` causes named to load keys from the key repository and sign the zone with all keys that are active. rndc loadkeys `zonename` causes named to load keys from the key repository and schedule key maintenance events to occur in the future, but it does not sign the full zone immediately. Note: once keys have been loaded for a zone the first time, the repository will be searched for changes periodically, regardless of whether rndc loadkeys is used. The recheck interval is defined by dnssec-loadkeys-interval.)

  The default setting is auto-dnssec off.

serial-update-method

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

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

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

inline-signing

  If `yes`, this enables "bump in the wire" signing of a zone, where a unsigned zone is transferred in or loaded from disk and a signed version of the zone is served, with possibly, a different serial number. This behaviour is disabled by default.

multi-master

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

masterfile-format

  See the description of masterfile-format in the section called “Tuning”.

dnssec-secure-to-insecure

  See the description of dnssec-secure-to-insecure 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 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.

There is a pre-defined update-policy rule which can be switched on with the command update-policy local;. Switching on this rule in a zone causes named to generate a TSIG session key and place it in a file, and to allow that key to update the zone. (By default, the file is `/var/run/named/session.key`, the key name is "local-ddns" and the key algorithm is HMAC-SHA256, but these values are configurable with the session-keyfile, session-keyname and session-keyalg options, respectively).

A client running on the local system, and with appropriate permissions, may read that file and use the key to sign update requests. The zone's update policy will be set to allow that key to change any record within the zone. Assuming the key name is "local-ddns", this policy is equivalent to:

    update-policy { grant local-ddns zonesub any; };

The command nsupdate -l sends update requests to localhost, and signs them using the session key.

Other rule definitions look like this:

    ( 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.

No signer is required for `tcp-self` or `6to4-self` however the standard reverse mapping / prefix conversion must match the identity 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. TKEY is also the negotiation method used by GSS-TSIG, which establishes an identity that is the Kerberos principal of the client, such as `"user@host.domain"`. 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.

For nametypes `krb5-self`, `ms-self`, `krb5-subdomain`, and `ms-subdomain` the `identity` field specifies the Windows or Kerberos realm of the machine belongs to.

The `nametype` field has 13 values: `name`, `subdomain`, `wildcard`, `self`, `selfsub`, `selfwild`, `krb5-self`, `ms-self`, `krb5-subdomain`, `ms-subdomain`, `tcp-self`, `6to4-self`, `zonesub`, and `external`.

`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.
`zonesub` | This rule is similar to subdomain, except that it matches when the name being updated is a subdomain of the zone in which the update-policy statement appears. This obviates the need to type the zone name twice, and enables the use of a standard update-policy statement in multiple zones without modification. When this rule is used, the `name` field is omitted.
`wildcard` | The `name` field is subject to DNS wildcard expansion, and this rule matches when the name being updated 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 `*` (an asterisk) in this case.
`selfsub` | This rule is similar to `self` except that subdomains of `self` can also be updated.
`selfwild` | This rule is similar to `self` except that only subdomains of `self` can be updated.
`ms-self` | This rule takes a Windows machine principal (machine$@REALM) for machine in REALM and and converts it machine.realm allowing the machine to update machine.realm. The REALM to be matched is specified in the `identity` field.
`ms-subdomain` | This rule takes a Windows machine principal (machine$@REALM) for machine in REALM and converts it to machine.realm allowing the machine to update subdomains of machine.realm. The REALM to be matched is specified in the `identity` field.
`krb5-self` | This rule takes a Kerberos machine principal (host/machine@REALM) for machine in REALM and and converts it machine.realm allowing the machine to update machine.realm. The REALM to be matched is specified in the `identity` field.
`krb5-subdomain` | This rule takes a Kerberos machine principal (host/machine@REALM) for machine in REALM and converts it to machine.realm allowing the machine to update subdomains of machine.realm. The REALM to be matched is specified in the `identity` field.
`tcp-self` | Allow updates that have been sent via TCP and for which the standard mapping from the initiating IP address into the IN-ADDR.ARPA and IP6.ARPA namespaces match the name to be updated. ### Note It is theoretically possible to spoof these TCP sessions.
`6to4-self` | Allow the 6to4 prefix to be update by any TCP connection from the 6to4 network or from the corresponding IPv4 address. This is intended to allow NS or DNAME RRsets to be added to the reverse tree. ### Note It is theoretically possible to spoof these TCP sessions.
`external` | This rule allows named to defer the decision of whether to allow a given update to an external daemon. The method of communicating with the daemon is specified in the `identity` field, the format of which is "`local:``path`", where `path` is the location of a UNIX-domain socket. (Currently, "local" is the only supported mechanism.) Requests to the external daemon are sent over the UNIX-domain socket as datagrams with the following format:
       Protocol version number (4 bytes, network byte order, currently 1)
       Request length (4 bytes, network byte order)
       Signer (null-terminated string)
       Name (null-terminated string)
       TCP source address (null-terminated string)
       Rdata type (null-terminated string)
       Key (null-terminated string)
       TKEY token length (4 bytes, network byte order)
       TKEY token (remainder of packet)
The daemon replies with a four-byte value in network byte order, containing either 0 or 1; 0 indicates that the specified update is not permitted, and 1 indicates that it is.

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 RRSIG, NS, SOA, NSEC and NSEC3. Types may be specified by name, including "ANY" (ANY matches all types except NSEC and NSEC3, which can never be updated). Note that when an attempt is made to delete all records associated with a name, the rules are checked for each existing record type.

## 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.
DHCID | Is used for identifying which DHCP client is associated with this name. Described in RFC 4701.
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.
DNSKEY | Stores a public key associated with a signed DNS zone. Described in RFC 4034.
DS | Stores the hash of a public key associated with a signed DNS zone. Described in RFC 4034.
GPOS | Specifies the global position. Superseded by LOC.
HINFO | Identifies the CPU and OS used by a host. Described in RFC 1035.
IPSECKEY | Provides a method for storing IPsec keying material in DNS. Described in RFC 4025.
ISDN | Representation of ISDN addresses. Experimental. Described in RFC 1183.
KEY | Stores a public key associated with a DNS name. Used in original DNSSEC; replaced by DNSKEY in DNSSECbis, but still used with SIG(0). Described in RFCs 2535 and 2931.
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 with 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.
NSEC | Used in DNSSECbis 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 4034.
NSEC3 | Used in DNSSECbis 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. NSEC3 differs from NSEC in that it prevents zone enumeration but is more computationally expensive on both the server and the client than NSEC. Described in RFC 5155.
NSEC3PARAM | Used in DNSSECbis to tell the authoritative server which NSEC3 chains are available to use. Described in RFC 5155.
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. Used in original DNSSEC; replaced by NSEC in DNSSECbis. 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.
RRSIG | Contains DNSSECbis signature data. Described in RFC 4034.
RT | Route-through binding for hosts that do not have their own direct wide area network addresses. Experimental. Described in RFC 1183.
SIG | Contains DNSSEC signature data. Used in original DNSSEC; replaced by RRSIG in DNSSECbis, but still used for SIG(0). Described in RFCs 2535 and 2931.
SOA | Identifies the start of a zone of authority. Described in RFC 1035.
SPF | Contains the Sender Policy Framework information for a given email domain. Described in RFC 4408.
SRV | Information about well known network services (replaces WKS). Described in RFC 2782.
SSHFP | Provides a way to securely publish a secure shell key's fingerprint. Described in RFC 4255.
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.

The above 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 address record (A or AAAA) — CNAME is not sufficient.

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

`example.com.` | `IN` | `MX` | `10` | `mail.example.com.`
| `IN` | `MX` | `10` | `mail2.example.com.`
| `IN` | `MX` | `20` | `mail.backup.org.`
`mail.example.com.` | `IN` | `A` | `10.0.0.1` |
`mail2.example.com.` | `IN` | `A` | `10.0.0.2` |

Mail delivery 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 @ (at-sign)

When used in the label (or name) field, the asperand or at-sign (@) symbol represents the current origin. At the start of the zone file, it is the <`zone_name`> (followed by trailing dot).

#### 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`>. (followed by trailing dot). The current $ORIGIN is appended to the domain specified in the $ORIGIN argument if it is not absolute.

    $ORIGIN example.com.
    WWW     CNAME   MAIN-SERVER

is equivalent to

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

#### 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.
    $GENERATE 1-2 @ NS SERVER$.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.

Generate a set of A and MX records. Note the MX's right hand side is a quoted string. The quotes will be stripped when the right hand side is processed.

    $ORIGIN EXAMPLE.
    $GENERATE 1-127 HOST-$ A 1.2.3.$
    $GENERATE 1-127 HOST-$ MX "0 ."

is equivalent to

    HOST-1.EXAMPLE.   A  1.2.3.1
    HOST-1.EXAMPLE.   MX 0 .
    HOST-2.EXAMPLE.   A  1.2.3.2
    HOST-2.EXAMPLE.   MX 0 .
    HOST-3.EXAMPLE.   A  1.2.3.3
    HOST-3.EXAMPLE.   MX 0 .
    ...
    HOST-127.EXAMPLE. A  1.2.3.127
    HOST-127.EXAMPLE. MX 0 .

range | This can be one of two forms: start-stop or start-stop/step. If the first form is used, then step is set to 1. All of start, stop and step must be positive.
lhs | This describes the owner name of the resource records to be created. Any single $ (dollar sign) symbols within the lhs string 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 { (left brace) immediately following the $ as ${offset[,width[,base]]}. For example, ${-20,3,d} subtracts 20 from the current value, prints the result as a decimal in a zero-padded field of width 3. Available output forms are decimal (d), octal (o), hexadecimal (x or X for uppercase) and nibble (n or N\ for uppercase). The default modifier is ${0,0,d}. If the lhs is not absolute, the current $ORIGIN is appended to the name. In nibble mode the value will be treated as if it was a reversed hexadecimal string with each hexadecimal digit as a separate label. The width field includes the label separator. For compatibility with earlier versions, $$ is still recognized as indicating a literal $ in the output.
ttl | Specifies the time-to-live 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 | 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 | Any valid type.
rhs | rhs, optionally, quoted string.

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.

### Additional File Formats

In addition to the standard textual format, BIND 9 supports the ability to read or dump to zone files in other formats. The `raw` format is currently available as an additional format. It is a binary format representing BIND 9's internal data structure directly, thereby remarkably improving the loading time.

For a primary server, a zone file in the `raw` format is expected to be generated from a textual zone file by the named-compilezone command. For a secondary server or for a dynamic zone, it is automatically generated (if this format is specified by the masterfile-format option) when named dumps the zone contents after zone transfer or when applying prior updates.

If a zone file in a binary format needs manual modification, it first must be converted to a textual form by the named-compilezone command. All necessary modification should go to the text file, which should then be converted to the binary form by the named-compilezone command again.

Although the `raw` format uses the network byte order and avoids architecture-dependent data alignment so that it is as much portable as possible, it is primarily expected to be used inside the same single system. In order to export a zone file in the `raw` format or make a portable backup of the file, it is recommended to convert the file to the standard textual representation.

## BIND9 Statistics

BIND 9 maintains lots of statistics information and provides several interfaces for users to get access to the statistics. The available statistics include all statistics counters that were available in BIND 8 and are meaningful in BIND 9, and other information that is considered useful.

The statistics information is categorized into the following sections.

Incoming Requests | The number of incoming DNS requests for each OPCODE.
Incoming Queries | The number of incoming queries for each RR type.
Outgoing Queries | The number of outgoing queries for each RR type sent from the internal resolver. Maintained per view.
Name Server Statistics | Statistics counters about incoming request processing.
Zone Maintenance Statistics | Statistics counters regarding zone maintenance operations such as zone transfers.
Resolver Statistics | Statistics counters about name resolution performed in the internal resolver. Maintained per view.
Cache DB RRsets | The number of RRsets per RR type and nonexistent names stored in the cache database. If the exclamation mark (!) is printed for a RR type, it means that particular type of RRset is known to be nonexistent (this is also known as "NXRRSET"). Maintained per view.
Socket I/O Statistics | Statistics counters about network related events.

A subset of Name Server Statistics is collected and shown per zone for which the server has the authority when zone-statistics is set to `yes`. These statistics counters are shown with their zone and view names. In some cases the view names are omitted for the default view.

There are currently two user interfaces to get access to the statistics. One is in the plain text format dumped to the file specified by the statistics-file configuration option. The other is remotely accessible via a statistics channel when the statistics-channels statement is specified in the configuration file (see the section called “statistics-channels Statement Grammar”.)

#### The Statistics File

The text format statistics dump begins with a line, like:

+++ Statistics Dump +++ (973798949)

The number in parentheses is a standard Unix-style timestamp, measured as seconds since January 1, 1970. Following that line is a set of statistics information, which is categorized as described above. Each section begins with a line, like:

++ Name Server Statistics ++

Each section consists of lines, each containing the statistics counter value followed by its textual description. See below for available counters. For brevity, counters that have a value of 0 are not shown in the statistics file.

The statistics dump ends with the line where the number is identical to the number in the beginning line; for example:

--- Statistics Dump --- (973798949)

### Statistics Counters

The following tables summarize statistics counters that BIND 9 provides. For each row of the tables, the leftmost column is the abbreviated symbol name of that counter. These symbols are shown in the statistics information accessed via an HTTP statistics channel. The rightmost column gives the description of the counter, which is also shown in the statistics file (but, in this document, possibly with slight modification for better readability). Additional notes may also be provided in this column. When a middle column exists between these two columns, it gives the corresponding counter name of the BIND 8 statistics, if applicable.

#### Name Server Statistics Counters

Symbol | BIND8 Symbol | Description
Requestv4 | RQ | IPv4 requests received. Note: this also counts non query requests.
Requestv6 | RQ | IPv6 requests received. Note: this also counts non query requests.
ReqEdns0 |  | Requests with EDNS(0) received.
ReqBadEDNSVer |  | Requests with unsupported EDNS version received.
ReqTSIG |  | Requests with TSIG received.
ReqSIG0 |  | Requests with SIG(0) received.
ReqBadSIG |  | Requests with invalid (TSIG or SIG(0)) signature.
ReqTCP | RTCP | TCP requests received.
AuthQryRej | RUQ | Authoritative (non recursive) queries rejected.
RecQryRej | RURQ | Recursive queries rejected.
XfrRej | RUXFR | Zone transfer requests rejected.
UpdateRej | RUUpd | Dynamic update requests rejected.
Response | SAns | Responses sent.
RespTruncated |  | Truncated responses sent.
RespEDNS0 |  | Responses with EDNS(0) sent.
RespTSIG |  | Responses with TSIG sent.
RespSIG0 |  | Responses with SIG(0) sent.
QrySuccess |  | Queries resulted in a successful answer. This means the query which returns a NOERROR response with at least one answer RR. This corresponds to the success counter of previous versions of BIND 9.
QryAuthAns |  | Queries resulted in authoritative answer.
QryNoauthAns | SNaAns | Queries resulted in non authoritative answer.
QryReferral |  | Queries resulted in referral answer. This corresponds to the referral counter of previous versions of BIND 9.
QryNxrrset |  | Queries resulted in NOERROR responses with no data. This corresponds to the nxrrset counter of previous versions of BIND 9.
QrySERVFAIL | SFail | Queries resulted in SERVFAIL.
QryFORMERR | SFErr | Queries resulted in FORMERR.
QryNXDOMAIN | SNXD | Queries resulted in NXDOMAIN. This corresponds to the nxdomain counter of previous versions of BIND 9.
QryRecursion | RFwdQ | Queries which caused the server to perform recursion in order to find the final answer. This corresponds to the recursion counter of previous versions of BIND 9.
QryDuplicate | RDupQ | Queries which the server attempted to recurse but discovered an existing query with the same IP address, port, query ID, name, type and class already being processed. This corresponds to the duplicate counter of previous versions of BIND 9.
QryDropped |  | Recursive queries for which the server discovered an excessive number of existing recursive queries for the same name, type and class and were subsequently dropped. This is the number of dropped queries due to the reason explained with the clients-per-query and max-clients-per-query options (see the description about clients-per-query.) This corresponds to the dropped counter of previous versions of BIND 9.
QryFailure |  | Other query failures. This corresponds to the failure counter of previous versions of BIND 9. Note: this counter is provided mainly for backward compatibility with the previous versions. Normally a more fine-grained counters such as AuthQryRej and RecQryRej that would also fall into this counter are provided, and so this counter would not be of much interest in practice.
XfrReqDone |  | Requested zone transfers completed.
UpdateReqFwd |  | Update requests forwarded.
UpdateRespFwd |  | Update responses forwarded.
UpdateFwdFail |  | Dynamic update forward failed.
UpdateDone |  | Dynamic updates completed.
UpdateFail |  | Dynamic updates failed.
UpdateBadPrereq |  | Dynamic updates rejected due to prerequisite failure.
RPZRewrites |  | Response policy zone rewrites.
RateDropped |  | Responses dropped by rate limits.
RateSlipped |  | Responses truncated by rate limits.

#### Zone Maintenance Statistics Counters

Symbol | Description
NotifyOutv4 | IPv4 notifies sent.
NotifyOutv6 | IPv6 notifies sent.
NotifyInv4 | IPv4 notifies received.
NotifyInv6 | IPv6 notifies received.
NotifyRej | Incoming notifies rejected.
SOAOutv4 | IPv4 SOA queries sent.
SOAOutv6 | IPv6 SOA queries sent.
AXFRReqv4 | IPv4 AXFR requested.
AXFRReqv6 | IPv6 AXFR requested.
IXFRReqv4 | IPv4 IXFR requested.
IXFRReqv6 | IPv6 IXFR requested.
XfrSuccess | Zone transfer requests succeeded.
XfrFail | Zone transfer requests failed.

#### Resolver Statistics Counters

Symbol | BIND8 Symbol | Description
Queryv4 | SFwdQ | IPv4 queries sent.
Queryv6 | SFwdQ | IPv6 queries sent.
Responsev4 | RR | IPv4 responses received.
Responsev6 | RR | IPv6 responses received.
NXDOMAIN | RNXD | NXDOMAIN received.
SERVFAIL | RFail | SERVFAIL received.
FORMERR | RFErr | FORMERR received.
OtherError | RErr | Other errors received.
EDNS0Fail |  | EDNS(0) query failures.
Mismatch | RDupR | Mismatch responses received. The DNS ID, response's source address, and/or the response's source port does not match what was expected. (The port must be 53 or as defined by the port option.) This may be an indication of a cache poisoning attempt.
Truncated |  | Truncated responses received.
Lame | RLame | Lame delegations received.
Retry | SDupQ | Query retries performed.
QueryAbort |  | Queries aborted due to quota control.
QuerySockFail |  | Failures in opening query sockets. One common reason for such failures is a failure of opening a new socket due to a limitation on file descriptors.
QueryTimeout |  | Query timeouts.
GlueFetchv4 | SSysQ | IPv4 NS address fetches invoked.
GlueFetchv6 | SSysQ | IPv6 NS address fetches invoked.
GlueFetchv4Fail |  | IPv4 NS address fetch failed.
GlueFetchv6Fail |  | IPv6 NS address fetch failed.
ValAttempt |  | DNSSEC validation attempted.
ValOk |  | DNSSEC validation succeeded.
ValNegOk |  | DNSSEC validation on negative information succeeded.
ValFail |  | DNSSEC validation failed.
QryRTTnn |  | Frequency table on round trip times (RTTs) of queries. Each nn specifies the corresponding frequency. In the sequence of nn_1, nn_2, ..., nn_m, the value of nn_i is the number of queries whose RTTs are between nn_(i-1) (inclusive) and nn_i (exclusive) milliseconds. For the sake of convenience we define nn_0 to be 0. The last entry should be represented as nn_m+, which means the number of queries whose RTTs are equal to or over nn_m milliseconds.

#### Socket I/O Statistics Counters

Socket I/O statistics counters are defined per socket types, which are UDP4 (UDP/IPv4), UDP6 (UDP/IPv6), TCP4 (TCP/IPv4), TCP6 (TCP/IPv6), Unix (Unix Domain), and FDwatch (sockets opened outside the socket module). In the following table <TYPE> represents a socket type. Not all counters are available for all socket types; exceptions are noted in the description field.

Symbol | Description
<TYPE>Open | Sockets opened successfully. This counter is not applicable to the FDwatch type.
<TYPE>OpenFail | Failures of opening sockets. This counter is not applicable to the FDwatch type.
<TYPE>Close | Sockets closed.
<TYPE>BindFail | Failures of binding sockets.
<TYPE>ConnFail | Failures of connecting sockets.
<TYPE>Conn | Connections established successfully.
<TYPE>AcceptFail | Failures of accepting incoming connection requests. This counter is not applicable to the UDP and FDwatch types.
<TYPE>Accept | Incoming connections successfully accepted. This counter is not applicable to the UDP and FDwatch types.
<TYPE>SendErr | Errors in socket send operations. This counter corresponds to SErr counter of BIND 8.
<TYPE>RecvErr | Errors in socket receive operations. This includes errors of send operations on a connected UDP socket notified by an ICMP error message.

#### Compatibility with BIND 8 Counters

Most statistics counters that were available in BIND 8 are also supported in BIND 9 as shown in the above tables. Here are notes about other counters that do not appear in these tables.

RFwdR,SFwdR

  These counters are not supported because BIND 9 does not adopt the notion of forwarding as BIND 8 did.

RAXFR

  This counter is accessible in the Incoming Queries section.

RIQ

  This counter is accessible in the Incoming Requests section.

ROpts

  This counter is not supported because BIND 9 does not care about IP options in the first place.

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