/etc/named.conf

Configuration file for named

Name:

/etc/named.conf

Description:

BIND 8 is much more configurable than previous release of BIND. There are new areas of configuration, such as access control lists and categorized logging, and many options that previously applied to all zones can now be used selectively. These features, plus a consideration of future configuration needs, led to the creation of a new configuration file format.

This section: Contains:
Documentation definitions A description of the elements that are used throughout the named configuration file documentation
Statements Elements that are associated with only one statement
Comments Syntax rules for comments

Documentation definitions

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_address, ip_prefix or acl_name elements. It has a syntax like:
address_match_list    = 1*address_match_element

address_match_element = [ "!" ] (
    ip_address / 
    ip_prefix /
    acl_name /
    address_match_list)
      

An address_match_list is a list of elements. The elements can be any of the following:

The ACLs any, none, localhost and localnets are predefined. For more information, see the description of the acl statement.

You can use a leading "!" to negate elements.

When a given IP address or prefix is compared to an address match list, the list is traversed in order, and the first match (regardless of negation) is used. The interpretation of a match depends on whether the list is being used for access control or as a topology.

When used as an ACL, a nonnegated match allows access and a negated match denies access. If there's no match, access is denied. The clauses: allow-query, allow-transfer, and allow-update all use address match lists like this. Similarly, the listen-on clause can use negation to define local addresses that shouldn't be used to accept nameserver connections.

When used with the topology option, a nonnegated match returns a distance based on its position on the list (the closer the match is to the start of the list, the shorter the distance is between it and the server). A negated match is assigned the maximum distance from the server. If there's no match, the address gets a distance that's further than any nonnegated list element, and closer than any negated element.

Because of the first-match aspect of the algorithm, an element that defines a subset of another element in the list should come before the broader element, regardless of whether either is negated. For example, in:

1.2.3/24; ! 1.2.3.13
      

the 1.2.3.13 element is completely useless because the algorithm matches any lookup for 1.2.3.13 to the 1.2.3/24 element. Using:

! 1.2.3.13; 1.2.3/24
      

fixes that problem by having 1.2.3.13 blocked by the negation but all other 1.2.3.* hosts fall through.

domain_name
A quoted string that's used as a DNS name, for example "my.test.domain".
dotted-decimal
One or more integers valued 0 through 255 separated only by dots ("."), such as 123 or 45.67 or 89.123.45.67.
ip_addr
An IP address with exactly four elements in dotted-decimal notation.
ip_port
An IP port number. The port number is limited to 0 through 65535, with values below 1024 typically restricted to root-owned processes.
ip_prefix
An IP network specified in dotted-decimal form, followed by / and then the number of bits in the netmask. For example 127/8 is the network 127.0.0.0 with netmask 255.0.0.0, and 1.2.3.0/24 is network 1.2.3.0 with netmask 255.255.255.0.
number
A nonnegative integer with an entire range limited by the range of a C language signed integer (2,147,483,647 on a machine with 32-bit integers). Its acceptable value might further be limited by the context in which it's used.
path_name
A quoted string that's used as a pathname, such as "zones/master/my.test.domain".
size_spec
A number, the word unlimited, or the word default.

The maximum value of size_spec is that of unsigned long integers on the machine. The word unlimited requests unlimited use, or the maximum available amount. The word default uses the limit that was in force when the server was started.

A number can optionally be followed by a scaling factor: K or k for kilobytes, M or m for megabytes, and G or g for gigabytes, which scale by 1024, 1024*1024, and 1024*1024*1024, respectively.

Integer storage overflow is currently silently ignored during conversion of scaled values, resulting in values less than intended, possibly even negative. Using unlimited is the best way to safely set a very large number.

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

Statements

A named configuration consists of statements and comments. Statements end with a semicolon. Many statements contain a block of substatements, which are also terminated with a semicolon. The following statements are supported:

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

acl statement

acl name {
  address_match_list
};

The acl statement creates a named address match list. It gets its name from a primary use of address match lists: Access Control Lists (ACLs).


Note: You must use the acl statement to define the name of an address match list before you can use it elsewhere; no forward references are allowed.

The following ACLs are builtin:

any
Allow all hosts.
none
Deny all hosts.
localhost
Allow the IP addresses of all interfaces on the system.
localnets
Allow any host on a network for which the system has an interface.

controls statement

controls {
  [ inet ip_addr
    port ip_port
    allow { address_match_list; }; ]
};

The controls statement declares control channels to be used by system administrators to affect the operation of the local name server.

An inet control channel is a TCP/IP socket accessible to the Internet, created at the specified ip_port on the specified ip_addr. Modern telnet clients are capable of speaking directly to these sockets, and the control protocol is ARPAnet-style text. It's recommended that 127.0.0.1 be the only ip_addr used, and this only if you trust all nonprivileged users on the local host to manage your name server.

To use this interface with telnet, specify:

telnet ip_addr ip_port

When connected:

dumpdb
Dumps a copy of the name server's internal database to:
exec
Stops and then starts the name server. You can't specify command-line options for named; the name server just starts a new copy of itself with the same command-line arguments.
getpid
Prints the name server's current process ID.
notrace
Turns off debugging.
querylog or qrylog
Toggle logging all queries with syslog(). Logging takes place at priority LOG_INFO.
quit
Ends the control session.
reload [zone] ...
Reloads the name server. Send this command to a primary master name server after modifying its configuration file or one or more of its zone data files. Send this command to a 4.9 or later slave name server to have it update its slave zones if they aren't current. You can also specify one or more domain names of zones as arguments to reload; if you do, the name server reloads only these zones.
reconfig [-noexpired]
Tells the name server to check its configuration file for new or deleted zones. Send this command to a name server if you've added or deleted zones but haven't changed any existing zones' data.

Specifying -noexpired tells the name server not to bother you with error messages about zones that have expired. This comes in handy if your name server is authoritative for thousands of zones and you want to avoid seeing a flurry of expiration messages that you already know about.

stats [clear]
Appends the name server's statistics to:
status
Prints status information about the name server including its version, debug level, the number of zone transfers running, and whether query logging is on.
stop
Causes the name server to exit, writing dynamic zones to their zone data files.
trace [level]
Appends debugging information to:

Specifying higher debug levels increases the amount of detail in the debugging information.

include statement

include path_name;

The include statement inserts the specified file at the point that the include statement is encountered. It can't be used within another statement, so the following isn't allowed:

acl internal_hosts { include "internal_hosts.acl" }

Use include to break the configuration up into easily-managed chunks. For example the following could be used at the top of a BIND configuration file in order to include any ACL or key information:

include "/etc/security/keys.bind" 
include "/etc/acls.bind" 

Note: Be careful not to type "#include" as you would in a C program, because # is used to start a comment.

key statement

key key_id {
  algorithm algorithm_id;
  secret secret_string;
};

The key statement defines a key ID that can be used in a server statement to associate an authentication method with a particular name server.

A key ID must be created with the key statement before it can be used in a server definition or an address match list.

The arguments are as follows:

algorithm_id
A string that specifies a security/authentication algorithm. The only supported algorithm is "hmac-md5."
secret_string
The secret to be used by the algorithm, and is treated as a base-64 encoded string. This may be generated manually.

The key statement is intended for use in transaction security. Unless included in a server statement, it's not used to sign any requests. It's used to verify requests matching the key_id and algorithm_id, and sign replies to those requests.

logging statement

logging {
  [ channel channel_name {
    ( file path_name
       [ versions ( number | unlimited ) ]
       [ size size_spec ]
     | syslog ( kern | user | mail | daemon | auth | syslog |
                lpr | news | uucp | cron | authpriv | ftp | 
                local0 | local1 | local2 | local3 |
                local4 | local5 | local6 | local7 )
     | 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; ... ]
  }; ]
  ...
};

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

Sections in this description include:

Only one logging statement is used to define as many channels and categories as are wanted. If there are multiple logging statements in a configuration, the first defined determines the logging, and warnings are issued for the others. If there's no logging statement, the logging configuration is:

logging { category default { default_syslog; default_debug; };
          category panic { default_syslog; default_stderr; };
          category packet { default_debug; };
          category eventlib { default_debug; };
        };

The logging configuration is established as soon as the logging statement is parsed. If you want to redirect messages about processing of the entire configuration file, the logging statement must appear first. Even if you do not redirect configuration file parsing messages, we recommend always putting the logging statement first so that this rule needn't be consciously recalled if you ever do want the parser's messages relocated.

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 clause that says whether messages selected for the channel go to a file, to a particular syslog() facility, or are discarded. Also, it can optionally limit the message severity level that's 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 word NULL as the destination option for the channel causes all messages sent to it to be discarded; other options for the channel are meaningless.

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

The size option for files is simply a hard ceiling on log growth. If the file ever exceeds the size, then named just doesn't write anything more to it until the file is reopened; exceeding the size doesn't automatically trigger a reopen. The default behavior is to not limit the size of the file.

If you use the versions logfile option, then named retains 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's opened lamers.log.1 is renamed to lames.log.2, lamers.log.0 is renamed to lamers.log.1, and lamers.log is renamed to lamers.log.0. No rolled versions are kept by default. The unlimited keyword is synonymous with 99 in current releases.

Example usage of the size and versions options:

channel an_example_level {
  file "lamers.log" versions 3 size 20m;
  print-time yes;
  print-category yes;
};

The argument for the syslog clause is a syslog() facility as described for syslog(). In order to capture the log messages, you need to have syslogd running.

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 that aren't at least of the severity level given aren't selected for the channel; messages of higher severity levels are accepted.

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

The server can supply extensive debugging information when it's in debugging mode. If the server's global debug level is greater than zero, then debugging mode is active. The global debug level is set either by starting the server with the -d flag followed by a positive integer, or by sending the server the SIGUSR1 signal. The global debug level can be set to zero, and debugging mode turned off, by sending the server the SIGUSR2 signal. All debugging messages in the server have a debug level, and higher debug levels give more more detailed output. Channels that specify a specific debug severity, for example:

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

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

If print-time has been turned on, then the date and time are logged. It may also be specified for a syslog() channel, but is usually pointless since syslog() also prints the date and time. If print-category is requested, then the category of the message is logged as well. Finally, if print-severity is on, then the severity level of the message is logged.

The print- options may be used in any combination, and are always printed in the following order: time, category, severity.

Here's an example where all three print- options are on:

28-Apr-1997 15:05:32.863 default: notice: Ready to answer queries.

There are four predefined channels that are used for named's default logging as follows. How they are used is described in the next section, the category phrase.

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

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

channel default_stderr { # writes to stderr
    file "<stderr>"; # this is illustrative only; there's currently
                     # no way of specifying an internal file
                     # descriptor in the configuration language.
    severity info; # only send priority info and higher
};

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

Once a channel is defined, it can't be redefined. Thus you can't alter the builtin 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.

The following categories are available:

config
High-level configuration file processing.
cname
Messages such as "...points to a CNAME."
db
All database operations.
default
The catch-all. Many things still aren't classified into categories, and they all end up here. Also, if you don't specify any channels for a category, the default category is used instead. If you don't define the default category, the following definition is used:
category default { default_syslog; default_debug; }; 
      
eventlib
Debugging info from the event system. Only one channel may be specified for this category, and it must be a file channel. If you don't define the eventlib category, the following definition is used:
category eventlib { default_debug; }; 
      
insist
Internal consistency check failures.
lame-servers
Messages like "Lame server on ..."
load
Zone loading messages.
maintenance
Periodic maintenance events.
ncache
Negative caching.
notify
The NOTIFY protocol.
os
Operating system problems.
packet
Dumps of packets received and sent. Only one channel may be specified for this category, and it must be a file channel. If you don't define the packet category, the following definition is used:
category packet { default_debug; }; 
      
panic
If the server has to shut itself down due to an internal problem, it logs the problem in this category as well as in the problem's native category. If you don't define the panic category, the following definition is used:
category panic { default_syslog; default_stderr; }; 
      
parser
Low-level configuration file processing.
queries
A short log message is generated for every query the server receives.
response-checks
Messages arising from response checking, such as:
security
Approved/unapproved requests.
statistics
Statistics.
update
Dynamic updates.
xfer-in
Zone transfers the server is receiving.
xfer-out
Zone transfers the server is sending.

If you don't specify a list of channels for a category, then log messages in that category are 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 lame-servers { null; };
category cname { null; };

options statement

options {
      [ version version_string; ]
      [ directory path_name; ]
      [ named-xfer path_name; ]
      [ dump-file path_name; ]
      [ memstatistics-file path_name; ]
      [ pid-file path_name; ]
      [ statistics-file path_name; ]
      [ auth-nxdomain yes_or_no; ]
      [ deallocate-on-exit yes_or_no; ]
      [ dialup yes_or_no; ]
      [ fake-iquery yes_or_no; ]
      [ fetch-glue yes_or_no; ]
      [ has-old-clients yes_or_no; ]
      [ host-statistics yes_or_no; ]
      [ multiple-cnames yes_or_no; ]
      [ notify yes_or_no; ]
      [ recursion yes_or_no; ]
      [ rfc2308-type1 yes_or_no; ]
      [ use-id-pool yes_or_no; ]
      [ treat-cr-as-space yes_or_no; ]
      [ also-notify { ip_addr; [ ip_addr; ... ] }; ]
      [ forward ( only | first ); ]
      [ forwarders { [ ip_addr; [ip_addr; ... ] ] }; ]
      [ check-names (master|slave|response) (warn|fail|ignore); ]
      [ allow-query { address_match_list }; ]
      [ allow-transfer { address_match_list }; ]
      [ allow-recursion { address_match_list }; ]
      [ blackhole { address_match_list }; ]
      [ listen-on [ port ip_port ] { address_match_list }; ]
      [ query-source [ address (ip_addr|*) ] [ port (ip_port|*) ] ; ]
      [ lame-ttl number; ]
      [ max-transfer-time-in number; ]
      [ max-ncache-ttl number; ]
      [ min-roots number; ]
      [ serial-queries number; ]
      [ transfer-format ( one-answer | many-answers ); ]
      [ transfers-in  number; ]
      [ transfers-out number; ]
      [ transfers-per-ns number; ]
      [ transfer-source ip_addr; ]
      [ maintain-ixfr-base yes_or_no; ]
      [ max-ixfr-log-size number; ]
      [ coresize size_spec; ]
      [ datasize size_spec; ]
      [ files size_spec; ]
      [ stacksize size_spec; ]
      [ cleaning-interval number; ]
      [ heartbeat-interval number; ]
      [ interface-interval number; ]
      [ statistics-interval number; ]
      [ topology { address_match_list }; ]
      [ sortlist { address_match_list }; ]
      [ rrset-order { order_spec ; [ order_spec ; ... ] ] }; 
};

The options statement sets up global options to be used by named. This statement may appear only once in a configuration file; if more than one occurrence is found, the first occurrence determines the actual options used, and a warning is generated. If there's no options statement, an options block with each option set to its default is used.

Sections in this description include:

Pathnames

The following options are available:

directory path_name;
The working directory of the server. Any nonabsolute pathnames in the configuration file are taken as relative to this directory. The default location for most server output files (e.g. named.run) is this directory. If a directory isn't specified, the working directory defaults to ".", the directory from which the server was started. The directory specified should be an absolute path.
dump-file path_name;
The pathname of the file the server dumps the database to when it receives SIGINT signal. If not specified, the default is named_dump.db.
memstatistics-file path_name;
The pathname of the file the server writes memory usage statistics to on exit, if deallocate-on-exit is yes. If not specified, the default is named.memstats.
named-xfer path_name;
The pathname to the named-xfer program that the server uses for inbound zone transfers. If not specified, the default is system dependent (e.g. /usr/sbin/named-xfer).
pid-file path_name;
The pathname of the file the server writes its process ID in. If not specified, the default is operating system dependent, but is usually /var/run/named.pid or /etc/named.pid. The pid-file is used by programs (such as ndc) that want to send signals to the running nameserver.
statistics-file path_name;
The pathname of the file the server appends statistics to when it receives SIGILL signal. If not specified, the default is named.stats.
version version_string;
The version the server should report via a query of name version.bind in class chaos. The default is the real version number of the server, but some server operators prefer the string "surely you must be joking."

Boolean options

The following options are available:

auth-nxdomain yes_or_no;
If yes, then the AA bit is always set on NXDOMAIN responses, even if the server isn't actually authoritative. The default is yes. Don't turn off auth-nxdomain unless you are sure you know what you are doing, as some older software won't like it.
deallocate-on-exit yes_or_no;
If yes, then when the server exits, it painstakingly deallocates every object it allocated, and then writes a memory-usage report to the memstatistics-file. The default is no, because it's faster to let the operating system clean up. This option is handy for detecting memory leaks.
dialup yes_or_no;
If yes, the server treats all zones as if they are doing zone transfers across a dial on demand dialup link, which can be brought up by traffic originating from this server. This has different effects according to zone type and concentrates the zone maintenance so that it all happens in a short interval, once every heartbeat-interval and hopefully during the one call. It also suppresses some of the normal zone maintainance traffic. The default is no. This option may also be specified in the zone statement, in which case it overrides the options dialup statement.

If the zone is a master zone, the server sends out a NOTIFY request to all the slaves. This triggers the "zone up to date checking" in the slave (providing it supports NOTIFY), allowing the slave to verify the zone while the call us up.

If the zone is a slave or stub zone, the server suppresses the regular "zone up to date" queries and only perform them when the heartbeat-interval expires.

fake-iquery yes_or_no;
If yes, the server simulates the obsolete DNS query type IQUERY. The default is no.
fetch-glue yes_or_no;
If yes (the default), the server fetches "glue" resource records it doesn't have when constructing the additional data section of a response. This option can be used in conjunction with recursion no to prevent the server's cache from growing or becoming corrupted (at the cost of requiring more work from the client).
has-old-clients yes_or_no;
Setting the option to yes is equivalent to setting the following three options:

The use of has-old-clients with auth-nxdomain, maintain-ixfr-base and rfc2308-type1 is order-dependant.

host-statistics yes_or_no;
If yes, then statistics are kept for every host that the nameserver interacts with. The default is no. Turning this option on can consume huge amounts of memory.
maintain-ixfr-base yes_or_no;
If yes, a transaction log is kept for Incremental Zone Transfer. The default is no.
multiple-cnames yes_or_no;
If yes, then multiple CNAME resource records are allowed for a domain name. The default is no. Allowing multiple CNAME records is against standards and isn't recommended. Multiple CNAME support is available because previous versions of BIND allowed multiple CNAME records, and these records have been used for load balancing by a number of sites.
notify yes_or_no;
If yes (the default), DNS NOTIFY messages are sent when a zone the server is authoritative for changes. The use of NOTIFY speeds convergence between the master and its slaves. Slave servers that receive a NOTIFY message and understand it contact the master server for the zone and see if they need to do a zone transfer, and if they do, initiate it immediately. This option may also be specified in the zone statement, in which case it overrides the options notify statement.
recursion yes_or_no;
If yes (the default), and a DNS query requests recursion, then the server attempts to do all the work required to answer the query. If recursion isn't on, the server returns a referral to the client if it doesn't know the answer. See also fetch-glue above.
rfc2308-type1 yes_or_no;
If yes, the server sends NS records along with the SOA record for negative answers. You need to set this to no if you have an old BIND server using you as a forwarder that doesn't understand negative answers which contain both SOA and NS records or you have an old version of sendmail. The correct fix is to upgrade the broken server or sendmail. The default is no.
treat-cr-as-space yes_or_no;
If yes, the server treats "\r" characters the same way it treats a " " or "\t". This may be necessary when loading zone files on a UNIX system that were generated on an NT or DOS machine. The default is no.
use-id-pool yes_or_no;
If yes, the server keeps track of its own outstanding query ID's to avoid duplication and increase randomness. This results in 128KB more memory being consumed by the server. The default is no.

Also-Notify

also-notify { ip_addr; [ ip_addr; ... ] };
Defines a global list of IP addresses that also get sent NOTIFY messages whenever a fresh copy of the zone is loaded. This helps to ensure that copies of the zones will quickly converge on "stealth" servers. If an also-notify list is given in a zone statement, it overrides the options also-notify statement. When a zone notify statement is set to no, the IP addresses in the global also-notify list won't get sent NOTIFY messages for that zone. The default is the empty list (no global notification list).

Forwarding

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

Forwarding options include:

forward ( only | first );
Meaningful only if the forwarders list isn't 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 then looks for the answer itself. If only is specified, the server queries only the forwarders.
forwarders { [ ip_addr; [ip_addr; ... ] ] };
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-zone basis, allowing for the global forwarding options to be overridden in a variety of ways. You can set particular zones to use different forwarders, or have different forward only/first behavior, or to not forward at all. See the zone statement for more information.

Future versions of BIND 8 will provide a more powerful forwarding system. The syntax described above should continue to be supported.

Name checking

The server can check domain names based upon their expected client contexts. For example, a domain name used as a hostname can be checked for compliance with the RFCs defining valid hostnames.

Three checking methods are available:

fail
Names are checked against their expected client contexts. Invalid names are logged, and the offending data is rejected.
ignore
No checking is done.
warn
Names are checked against their expected client contexts. Invalid names are logged, but processing continues normally.

The server can check names in three areas: master zone files, slave zone files, and in responses to queries the server has initiated. If check-names response fail has been specified, and answering the client's question would require sending an invalid name to the client, the server sends a REFUSED response code to the client.

The defaults are:

    check-names master fail;
    check-names slave warn;
    check-names response ignore;

The check-names option may also be specified in the zone statement, in which case it overrides the options check-names statement. When used in a zone statement, the area isn't specified (because it can be deduced from the zone type).

Access control

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

The access control options are:

blackhole { address_match_list };
Specifies a list of addresses that the server won't accept queries from or use to resolve a query. Queries from these addresses won't be responded to.
allow-query { address_match_list };
Specifies which hosts are allowed to ask ordinary questions. The allow-query option may also be specified in the zone statement, in which case it overrides the options allow-query statement. If not specified, the default is to allow queries from all hosts.
allow-recursion { address_match_list };
Specifies which hosts are allowed to make recursive queries through this server. If not specified, the default is to allow recursive queries from all hosts.
allow-transfer { address_match_list };
Specifies which hosts are allowed to receive zone transfers from the server. The allow-transfer option may also be specified in the zone statement, in which case it overrides the options allow-transfer statement. If not specified, the default is to allow transfers from all hosts.

Interfaces

You can specify the interfaces and ports that the server answer queries from by using the listen-on option, which takes an optional port and an address_match_list. The server listens on all interfaces allowed by the address match list. If a port isn't specified, port 53 is used.

Multiple listen-on clauses are allowed. For example:

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

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

Query address

If the server doesn't know the answer to a question, it queries other nameservers. The query-source option specifies the address and port used for such queries. If address is * or is omitted, a wildcard IP address (INADDR_ANY) is used. If port is * or is omitted, a random unprivileged port is used. The default is:

query-source address * port *;

Note: The query-source option currently applies only to UDP queries; TCP queries always use a wildcard IP address and a random unprivileged port.

Zone transfers

The following options are available:

max-transfer-time-in number;
Inbound zone transfers (named-xfer processes) running longer than this many minutes are terminated. The default is 120 minutes (2 hours).
serial-queries number;
Slave servers periodically query master servers to find out if zone serial numbers have changed. Each such query uses a minute amount of the slave server's network bandwidth, but more importantly each query uses a small amount of memory in the slave server while waiting for the master server to respond. This option sets the maximum number of concurrent serial-number queries allowed to be outstanding at any given time. The default is four (4).

If a server loads a large (tens or hundreds of thousands) number of slave zones, this limit should be raised to the high hundreds or low thousands -- otherwise the slave server may never actually become aware of zone changes in the master servers. Beware, though, that setting this limit arbitrarily high can spend a considerable amount of your slave server's network, CPU, and memory resources. As with all tunable limits, this one should be changed gently and monitored for its effects.

transfer-format ( one-answer | many-answers );
The server supports two zone transfer methods:

The transfer-format option may be overridden on a per-server basis by using the server statement.

transfers-in number;
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 number;
This option will be used in the future to limit the number of concurrent outbound zone transfers. It's checked for syntax, but is otherwise ignored.
transfers-per-ns number;
The maximum number of inbound zone transfers (named-xfer processes) that can be concurrently transferring from a given remote nameserver (default value is 2). Increasing transfers-per-ns may speed up the convergence of slave zones, but it also may increase the load on the remote nameserver. This option may be overridden on a per-server basis by using the transfers phrase of the server statement.
transfer-source ip_addr;
Determines which local address is bound to the TCP connection and fetches all zones transferred inbound by the server. If not set, it defaults to a system controlled value which is usually the address of the interface "closest to" the remote end. This address must appear in the remote end's allow-transfer option for the zone being transferred, if one is specified. This statement sets the transfer-source for all zones, but can be overridden on a per-zone basis by including a transfer-source clause within the zone block in the configuration file.

Resource Limits

The server's usage of many system resources can be limited. Some operating systems don't support some of the limits. On such systems, a warning will be issued if the unsupported limit is used. Some operating systems don't support limiting resources, and on these systems a "cannot set resource limits on this system" message is logged.

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 size_spec for more details.

coresize size_spec;
The maximum size of a core dump. The default is default.
datasize size_spec;
The maximum amount of data memory the server may use. The default is default.
files size_spec;
The maximum number of files the server may have open concurrently. The default is unlimited.

On some operating systems the server can't set an unlimited value and can't determine the maximum number of open files the kernel can support. On such systems, choosing unlimited causes the server to use the larger of the rlim_max for RLIMIT_NOFILE and the value returned by sysconf(_SC_OPEN_MAX). If the actual kernel limit is larger than this value, use limit files to specify the limit explicitly.

max-ixfr-log-size number;
This option will be used in a future release of the server to limit the size of the transaction log kept for Incremental Zone Transfer.
stacksize size_spec;
The maximum amount of stack memory the server may use. The default is default.

Periodic task intervals

The following options are available:

cleaning-interval number;
The server removes expired resource records from the cache every number minutes. The default is 60 minutes. If set to 0, no periodic cleaning occurs.
heartbeat-interval number;
The server performs zone maintenance tasks for all zones marked dialup yes whenever this interval expires. The default is 60 minutes. Reasonable values are up to 1 day (1440 minutes). If set to 0, no zone maintenance for these zones occur.
interface-interval number;
The server scans the network interface list every number minutes. The default is 60 minutes. If set to 0, interface scanning occurs only when the configuration file is loaded. After the scan, listeners are started on any new interfaces (provided they are allowed by the listen-on configuration). Listeners on interfaces that have gone away are cleaned up.
statistics-interval number;
Nameserver statistics are logged every number minutes. The default is 60. If set to 0, no statistics are logged.

Topology

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

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

prefers servers on network 10 the most, followed by hosts on network 1.2.0.0 (netmask 255.255.0.0) and network 3, 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; };

Resource Record sorting

When returning multiple RRs, the nameserver normally returns them in Round Robin -- after each request, the first RR is put to the end of the list. As the order of RRs isn't defined, this shouldn't cause any problems.

The client resolver code should rearrange the RRs as appropriate -- using any addresses on the local net in preference to other addresses. However, not all resolvers can do this, or aren't correctly configured.

When a client is using a local server, the sorting can be performed in the server, based on the client's address. This only requires configuring the nameservers, not all the clients.

The sortlist option takes an address match list and interprets it even more specially than the topology option does.

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 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, the second element is treated like the address match list in a topology option. Each top level element is assigned a distance and the address in the response with the minimum distance is moved to the beginning of the response.

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

sortlist {
         { localhost;         // IF   the local host
           { localnets;       // THEN first fit on the
             192.168.1/24;    //      following nets
             { 192,168.2/24; 192.168.3/24; }; }; };
         { 192.168.1/24;      // IF   on class C 192.168.1
           { 192.168.1/24;    // THEN use .1, or .2 or .3
             { 192.168.2/24; 192.168.3/24; }; }; };
         { 192.168.2/24;      // IF   on class C 192.168.2
           { 192.168.2/24;    // THEN use .2, or .1 or .3
             { 192.168.1/24; 192.168.3/24; }; }; };
         { 192.168.3/24;      // IF   on class C 192.168.3
           { 192.168.3/24;    // THEN use .3, or .1 or .2
             { 192.168.1/24; 192.168.2/24; }; }; };
         { { 192.168.4/24; 192.168.5/24; }; // if .4 or .5, prefer that net
         };
};

The following example gives reasonable behaviour for the local host and hosts on directly connected networks. It's similar to the behavior of the address sort in BIND 4.9.x. Responses sent to:

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

RRset Ordering

When multiple records are returned in an answer it may be useful to configure the order the records are placed into the response. For example the records for a zone might be configured to always be returned in the order they are defined in the zone file. Or perhaps a random shuffle of the records as they are returned is wanted. The rrset-order statement permits configuration of the ordering made of the records in a multiple record response. The default, if no ordering is defined, is a cyclic ordering (round robin).

An order_spec is defined as follows:

[ class class_name ][ type type_name ][ name "FQDN" ] order ordering
If this option isn't specified: The default is:
class ANY
type ANY
name "*"

The legal values for ordering are:

cyclic
Records are returned in a round-robin order.
fixed
Records are returned in the order they are defined in the zone file.
random
Records are returned in some random order.

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

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

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

If no rrset-order statement is specified, the following is used as the default:

rrset-order { class ANY type ANY name "*" order cyclic ; };

Tuning

The following options are available:

lame-ttl number;
Sets the number of seconds to cache a lame server indication; 0 disables caching. Default is 600 (10 minutes). Maximum value is 1800 (30 minutes).
max-ncache-ttl number;
To reduce network traffic and increase performance the server stores negative answers. This option sets a maximum retention time for these answers in the server in seconds. The default is 10800 seconds (3 hours). This value can't exceed the maximum retention time for ordinary (positive) answers (7 days) and will be silently truncated to 7 days if set to a value which is greater that 7 days.
min-roots number;
Minimum number of root servers that's required for a request for the root servers to be accepted. Default is 2.

server statement

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

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

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

The server supports two zone transfer methods:

You specify the method to use for a server with the transfer-format clause. If transfer-format isn't specified, then transfer-format (specified by the options statement) is used.

The transfers clause will be used in a future release of the server to limit the number of concurrent in-bound zone transfers from the specified server. It's checked for syntax but is otherwise ignored.

The keys clause is used to identify a key_id defined by the key statement, to be used for transaction security when talking to the remote server. The key statement must come before the server statement that references it. When a request is sent to the remote server, a request signature is generated using the key specified here and appended to the message. A request originating from the remote server isn't required to be signed by this key.

zone statement

zone domain_name [ ( in | hs | hesiod | chaos ) ] {
  type master;
  file path_name;
  [ forward ( only | first ); ]
  [ forwarders { [ ip_addr ; [ ip_addr ; ... ] ] }; ]
  [ check-names ( warn | fail | ignore ); ]
  [ allow-update { address_match_list }; ]
  [ allow-query { address_match_list }; ]
  [ allow-transfer { address_match_list }; ]
  [ dialup yes_or_no; ]
  [ notify yes_or_no; ]
  [ also-notify { ip_addr; [ ip_addr; ... ] }; ]
  [ ixfr-base  path_name; ]
};

zone domain_name [ ( in | hs | hesiod | chaos ) ] { 
  type ( slave | stub );
  [ file path_name; ]
  [ ixfr-base  path_name; ]
  masters [ port ip_port ] { ip_addr; [ ip_addr; ... ] };
  [ forward ( only | first ); ]
  [ forwarders { [ ip_addr ; [ ip_addr ; ... ] ] }; ]
  [ check-names ( warn | fail | ignore ); ]
  [ allow-update { address_match_list }; ]
  [ allow-query { address_match_list }; ]
  [ allow-transfer { address_match_list }; ]
  [ transfer-source ip_addr; ]
  [ dialup yes_or_no; ]  
  [ max-transfer-time-in number; ]
  [ notify yes_or_no; ]
  [ also-notify { ip_addr; [ ip_addr; ... ] };
};


zone domain_name [ ( in | hs | hesiod | chaos ) ] { 
  type forward;
  [ forward ( only | first ); ]
  [ forwarders { [ ip_addr ; [ ip_addr ; ... ] ] }; ]
  [ check-names ( warn | fail | ignore ); ]
};


zone "." [ ( in | hs | hesiod | chaos ) ] { 
  type hint;
  file path_name;
  [ check-names ( warn | fail | ignore ); ]
};

Sections in this description include:

Zone types

The following types are available:

master
The master copy of the data in a zone.
slave
A slave zone is a replica of a master zone. The masters list specifies one or more IP addresses that the slave contacts to update its copy of the zone.

If a port is specified, it checks to see if the zone is current, and if so, zone transfers are done to the port given.

If file is specified, then the replica is written to the file. We recommend using file 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's best to use a two level naming scheme for zone file names. For example, a slave server for the zone vix.com might place the zone contents into a file called vi/vix.com where vi/ is just the first two letters of the zone name. (Most operating systems behave very slowly if you put 100K files into a single directory.)

stub
A stub zone is like a slave zone, except that it replicates only the NS records of a master zone instead of the entire zone.
forward
A forward zone is used to direct all queries in it to other servers. The specification of options in such a zone will override any global options declared in the options statement.

If either no forwarders clause is present in the zone or an empty list for forwarders is given, no forwarding is done for the zone, cancelling 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, and not the servers used, you also need to respecify the global forwarders.

hint
The initial set of root nameservers is specified using a hint zone. When the server starts up, it uses the root hints to find a root nameserver and get the most recent list of root nameservers.

Note: Previous releases of named used the term primary for a master zone, secondary for a slave zone, and cache for a hint zone.

Class

The zone's name may optionally be followed by one of the following classes:

in
Short for "Internet." The default when a class isn't specified.
hesiod
An information service from MIT's Project Athena. It's used to share information about various systems databases, such as users, groups, printers, etc.
hs
A synonym for hesiod.
chaos
CHAOSnet -- a LAN protocol created in the mid-1970s by MIT. It's still sometimes seen on LISP stations and other hardware in the AI community, and zone data for it can be specified using this class.

Zone option descriptions

The following options are available for zone:

allow-query { address_match_list };
See the description of allow-query in the "Access control" section in options statement. In general, this should be more restrictive than the similar global option of the same name; otherwise, confusing and nonworthwhile delegations are returned.
allow-transfer { address_match_list };
See the description of allow-transfer in the "Access control" section in options statement.
allow-update { address_match_list };
Specifies which hosts are allowed to submit Dynamic DNS updates to the server. The default is to deny updates from all hosts.
also-notify { ip_addr; [ ip_addr; ... ] };
This option is only meaningful if notify is active for this zone. The set of machines that receive a DNS NOTIFY message for this zone is made up of all the listed nameservers for the zone (other than the primary master) plus any IP addresses specified with also-notify. Note that also-notify isn't meaningful for stub zones. The default is the empty list.
check-names ( warn | fail | ignore );
See the "Name checking" section in options statement.
dialup yes_or_no;
See the description of dialup in the "Boolean options" section in options statement.
forward ( only | first );
This option is only meaningful if the zone has a forwarders list. The only value causes the lookup to fail after trying the forwarders and getting no answer, while first would allow a normal lookup to be tried.
forwarders { [ ip_addr ; [ ip_addr ; ... ] ] };
Used to override the list of global forwarders. If it's not specified in a zone of type forward, no forwarding is done for the zone; the global options aren't used.
ixfr-base path_name;
Specifies the file name used for the IXFR transaction log file.
max-transfer-time-in number;
See the description of max-transfer-time-in in the "Zone transfers" section in options statement.
notify yes_or_no;
See the description of notify in the "Boolean options" section in options statement.
transfer-source ip_addr;
Determines the local address that's bound to the TCP connection used to fetch this zone. If not set, it defaults to a system controlled value which is usually the address of the interface "closest to" remote end. This address must appear in the remote end's allow-transfer option for this zone if one is specified.

Comments

/* This is a comment as in C */

// This is a comment as in C++

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

Comments may appear anywhere that whitespace may appear in a named 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 a portion of a line or to span multiple lines.

C-style comments can't be nested. For example, the following isn't 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 can't 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's logically
// part of the previous comment.

Shell-style (or perl-style, if you prefer) comments start with the pound sign (#) and continue to the end of the physical line, like C++ comments. For example:

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

Note: You can't use a semicolon (;) to start a comment such as you would in a zone file. The semicolon indicates the end of a configuration statement, so whatever follows it is interpreted as the start of the next statement.

See also:

named, named-xfer, syslogd

syslogd() in the Library Reference

TCP/IP Network Administration

DNS and BIND by Paul Albitz and Cricket Liu, O'Reilly & Associates (ISBN 1-56592-010-4)