A BIND 8 configuration file consists of two general features, statements and comments.

All statements end with a semicolon. Many statements allow substatements, which also terminate with a semicolon. BIND 8 supports the following statements:

logging

Specifies what the server logs, and where the log messages are sent.

options

Controls global server configuration options and sets defaults for other statements.

zone

Defines a zone.

acl

Defines a named IP address matching list, for access control and other uses.

key

Specifies key information for use in authentication and authorization.

trusted-keys

Defines DNSSEC keys that are preconfigured into the server and implicitly trusted.

server

Sets certain configuration options for individual remote servers.

controls

Declares control channels to be used by the ndc(1M) utility.

include

Includes another file.

The logging and options statements may only occur once per configuration, while the rest may appear numerous times. Further detail on each statement is provided in individual sections below.

Comments may appear anywhere that whitespace may appear in a BIND configuration file. To appeal to programmers of all kinds, they can be written in C, C++, shell or perl constructs.

C-style comments start with the two characters /* (slash, star) and end with */ (star, slash). Because comments are completely delimited by these characters, they can be used to comment either 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 comments start with the character # (hash or pound or number or octothorpe or whatever) and like C++ comments, continue to the end of the physical line. 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.

You can covert BIND 4.9.x configuration files to the new format by using named-bootconf(1M).

The elements described below are used throughout the BIND configuration file documentation. Elements which are only associated with one statement are described only in the section describing that statement.

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, as described in the ADDRESS MATCH LISTS section.

dotted-decimal

One or more integers valued 0 through 255 separated only by dots (``.''), such as 123, 45.67 or 89.123.45.67.

domain_name

A quoted string which will be used as a DNS name, for example "my.test.domain".

path_name

A quoted string which will be used as a pathname, such as "zones/master/my.test.domain".

ip_add

An IP address in with exactly four elements in dotted-decimal notation.

ip_port

An IP port number. number is limited to 0 through 65535, with values below 1024 typically restricted to root-owned processes. In some cases an asterisk ("*") character can be used as a placeholder to select a random high-numbered port.

ip_prefix

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. 1.2.3.0/28 is network 1.2.3.0 with netmask 255.255.255.240.

key_name

A string representing the name of a shared key, to be used for transaction security.

number

A non-negative 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 be further limited by the context in which it is used.

size_spec

number, the word unlimited, or the word default.

The maximum value of size_spec is that of unsigned long integers on the machine. unlimited requests unlimited use, or the maximum available amount. 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 really large number.

yes_or_no

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

Address match lists are primarily used to determine access control for various server operations. They are also used to define priorities for querying other name servers and to set the addresses on which in.named(1M) in.named will listen for queries. The elements which constitute an address match list can be any of the following:

• an ip-address (in dotted-decimal notation)
• an ip-prefix (in the '/'-notation)
• A key_id, as defined by the key statement
• the name of an address match list previously defined with the acl statement
• or, another address_match_list.

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

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

When a given IP address or prefix is compared to an address match list the list is traversed, in order, until an element matches. The interpretation of a match depends on whether the list is being used for access control, for defining listen-on ports, or as a topology, and whether the element is 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 at all in the list, access is denied. The clauses allow-query, allow-transfer, allow-update, allow-recursion and blackhole all use address match lists like this. Similarly, the listen-on option will cause the server to not accept queries on any of the machine's addresses that do not match the list.

When used with the topology option, a non-negated 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 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.

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

1.2.3/24; !1.2.3.13

the 1.2.3.13 element is completely useless, because the algorithm will match any lookup for 1.2.3.13 to the 1.2.3/24 element. Using

!1.2.3.13; 1.2.3/24

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

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 are multiple logging statements in a configuration, the first defined determines the logging, and warnings are issued for the others. If there is no logging statement, the logging configuration will be:

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 need not be consciously recalled if you ever do want to relocate the parser's messages.

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(3C) facility, 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 time stamp generated by in.named(1M), the category name, or severity level. The default is not to include any of those three.

The word null as the destination option for the channel will cause 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 will be 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 in.named will not write anything more to it until the file is reopened. That the size is exceeded does not automatically trigger a reopen. The default behavior does not limit the size of the file.

If you use the version logfile option, then in.named will retain the backup versions of the file by renaming them when it opens them. For example, if you choose to keep 3 old versions of the file lamers.log then just before it is opened lamers.log.1 is renamed to lamers.log.2, lamers.log.0 is renamed to lamers.log.1, and lamers.log is renamed to lamers.log.0. No rolled versions are kept by default. Any existing log file is simply appended. The unlimited keyword is synonymous with 99 in current BIND releases. Example usage of 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 in the syslog(3C) manual page. How syslogd(1M) will handle messages sent to this facility is described in the syslog.conf(4).

The severity clause works like the priority levels for syslog(), 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 warnings by means of syslog.conf will cause messages of severity info and notice to be dropped. If the situation were reversed, with in.named writing messages of only warning or higher, then syslogd will print all messages it receives from the channel.

The server can supply extensive debugging information when it is in debugging mode. If the server's global debug level is greater than zero, then the debugging mode will be active. The global debug level is set either by starting the in.named server with the -d flag followed by a positive integer, or by sending the running server the SIGUSR1 signal (for example, by using ndc trace). The global debug level can be set to zero and debugging mode turned off, by sending the server the SIGUSR2 signal (as with ndc notrace). 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;
};

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

If print-time has been turned on, then the date and time will be logged. print-time may be specified for a syslog() channel, but is usually unnecessary since syslog() also prints the date and time. If print-category is requested, then the category of the message will be logged as well. Finally, if print-severity is on, then the severity level of the message will be logged. The print- options may be used in any combination, and will always be printed in the following order: time, category, severity. Here is an example where all three print- options are on:

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

There are four predefined channels that are used for default logging in.named(1M). 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 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.

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

category default { default_syslog; default_debug; };

To log security events to a file but also keep the default logging behavior, 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; };

The following categories are available:

default

The catch-all. Many things still are not classified into categories, and they all end up here. Also, if you don not specify any channels for a category, the default category is used instead. If you do not define the default category, the following definition is used

category default { default_syslog; default_debug; };

config

High-level configuration file processing.

parser

Low-level configuration file processing.

queries

A short log message is generated for every query the server receives.

lame-servers

Messages like ``Lame server on ...''

statistics

Statistics.

panic

If the server has to shut itself down due to an internal problem, it will log the problem in this category as well as in the problem's native category. If you do not define the panic category, the following definition is used:

category panic { default_syslog; default_stderr; };

update

Dynamic updates.

ncache

Negative caching.

xfer-in

Zone transfers the server is receiving.

xfer-out

Zone transfers the server is sending

db

All database operations.

eventlib

Debugging information from the event system. Only one channel may be specified for this category, and it must be a file channel. If you do not define the eventlib category, the following definition is used:

category eventlib { default_debug; };

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 do not define the packet category, the following definition is used:

category packet { default_debug; };

notify

The Notify protocol.

cname

Messages like ``... points to a CNAME''.

security

Approved or unapproved requests.

os

Operating system problems.

insist

Internal consistency check failures.

maintenance

Periodic maintenance events.

load

Load.

response-checks

Messages arising from response checking, such as ``Malformed response ...'', ``wrong ans. name ...'', ``unrelated additional info ...'', ``invalid RR type ...'', and ``bad referral ...''.

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

version

The version the server should report by means of the ndc(1M) command or by means of a query of name version.bind in class chaos. The default is the real version number of the server.

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, for example, named.run, is this directory. If a directory is not specified, the working directory defaults to ".", the directory from which the server was started. The directory specified should be an absolute path.

named-xfer

The pathname to the named-xfer program that the server uses for inbound zone transfers. If not specified, the default is /usr/sbin/named-xfer.

dump-file

The pathname of the file to which the server dumps the database when it receives a SIGINT signal, for example, as sent by ndc dump. 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 deallocate-on-exit is yes. If not specified, the default is named.memstats.

pid-file

The pathname of the file in which the server writes its process ID. If not specified, the default is /var/run/named.pid.

statistics-file

The pathname of the file the server appends statistics to when it receives a SIGILL signal. If not specified, the default is named.stats.

auth-nxdomain

If the value is yes, then the AA bit is always set on NXDOMAIN responses, even if the server is not actually authoritative. The default is yes. Do not turn off auth-nxdomain unless you are sure you know what you are doing, as some older software will not like it.

deallocate-on-exit

If the value is yes, then when the server exits it will painstakingly deallocate every object it allocated, and then write a memory usage report to the memstatistics-file. The default is no because it is faster to let the operating system clean up. deallocate-on-exit is handy for detecting memory leaks.

dialup

If the value is yes, then the server treats all zones as if they are doing zone transfers across a dial on a demand dialup link, which can be brought up by traffic originating from this server. This has different effects according to the zone type. It 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 zone statement, in which case it overrides the options dialup statement.

If the zone is a master then the server will send out NOTIFY request to all the slaves. This will trigger the zone up to date checking in the slave, providing the slave supports NOTIFY, and allowing the slave to verify the zone while they call us up. If the zone is a slave or stub, then the server will suppress the regular zone up to date queries, and only perform them when the heartbeat-interval expires.

fake-iquery

If yes, the server will simulate the obsolete DNS query type IQUERY. The default is no.

fetch-glue

If yes (the default), the server will fetch ``glue'' resource records it does not have when it constructs the additional data section of a response. fetch-glue no can be used in conjunction with recursion no to prevent the server's cache from growing or becoming corrupted. However, it requires more work from the client.

has-old-clients

Setting the option to yes is equivalent to setting the following three options: auth-nxdomain yes, maintain-ixfr-base yes, and rfc2308-type1 no. has-old-clients with auth-nxdomain, maintain-ixfr-base, and rfc2308-type1 is order dependant.

host-statistics

If yes, then statistics are kept for every host with which the name server interacts. The default is no. Turning on host-statistics can consume huge amounts of memory.

host-statistics-max

The maximum number of host records that will be kept. When this limit is reached no new hosts will be added to the host statistics. If host-statistics-max is set to zero, then there is no limit set. The default value is zero.

maintain-ixfr-base

If yes, a IXFR database file is kept for all dynamically updated zones. This enables the server to answer IXFR queries, which speeds up zone transfers enormously. The default value is no.

multiple-cnames

If yes, then multiple CNAME resource records will be allowed for a domain name. The default is no. Allowing multiple CNAME records is against standards and is not 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

If yes (the default), DNS NOTIFY messages are sent when a zone for which the server is authoritative changes. The use of NOTIFY speeds convergence between the master and its slaves. Slave servers that receive a NOTIFY message and understand it will contact the master server for the zone and see if they need to do a zone transfer. If they do, they will initiate it immediately. The notify option may also be specified in the zone statement, in which case it overrides the options notify statement.

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 not on, the server will return a referral to the client if it does not know the answer. The default is yes. See also fetch-glue above.

rfc2308-type1

If yes, the server will send NS records along with the SOA record for negative answers. If you have an old BIND server using you as a forwarder, which does not understand negative answers that contain both SOA and NS records, or you have an old version of sendmail(1M), set this to no. The correct fix is to upgrade the broken server or sendmail. The default is no.

use-id-pool

If yes, the server will keep track of its own outstanding query ID's to avoid duplication and increase randomness. As a result, the server will consume 128KB more memory. The default is no.

treat-cr-as-space

If yes, the server will treat CR characters the same way it treats a space or tab. This may be necessary when loading zone files on a UNIX system that were generated on either an NT or a DOS machine. The default is no.

also-notify

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 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 get sent NOTIFY messages for that zone. The default is the empty list (no global notification list).

The forwarding facility can be used to create a large site-wide cache on a few servers. This reduces 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. If the forwarders do not 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-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, have different forward only or forward first behavior, or not forward at all. See THE ZONE STATEMENT section for more information.

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

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 that define valid hostnames.

Three checking methods are available:

ignore

No checking is done.

warn

Names are checked against their expected client contexts. Invalid names are logged, but processing continues normally.

fail

Names are checked against their expected client contexts. Invalid names are logged, and the offending data is rejected.

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

The defaults are:

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

check-names 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 is not specified, as it can be deduced from the zone type.

Access to the server can be restricted based on the IP address of the requesting system or by means of shared secret keys. See ADDRESS MATCH LISTS for details on how to specify access criteria.

allow-query

Specifies which hosts are allowed to ask ordinary questions. allow-query may also be specified in the zone statement, in which case it overrides the options allow-query statement. If not specified, the default is:

allow-recursion

Specifies which hosts are allowed to ask recursive questions. allow-recursion may also be specified in the zone statement, in which case it overrides the options allow-recursion statement. If not specified, the default is to allow recursive queries from all hosts.

allow-transfer

Specifies which hosts are allowed to receive zone transfers from the server. allow-transfer may also be specified in the zone statement, in which case it overrides the options allow-transfer statement. If not specified, the default is to allow transfers from all hosts.

blackhole

Specifies a list of addresses that the server will not accept queries from or use to resolve a query. Queries from these addresses will not receive a response.

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

Multiple listen-on statements are allowed. For example,

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

will enable the name server on port 53 for the IP address 5.6.7.8, and on port 1234 of an address on the machine in net 1.2 that is not 1.2.3.4.

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

If the server does not know the answer to a question, it will query other name servers. query-source specifies the address and port used for such queries. If address is * or is omitted, a wildcard IP address (INADDR_ANY) will be used. If port is * or is omitted, a random unprivileged port will be used. The default is

query-source address * port *;

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

max-transfer-time-in

Inbound zone transfers ( named-xfer processes) running longer than max-transfer-time-in minutes will be terminated. The default value for max-transfer-time-in is 120 minutes (2 hours).

transfer-format

The server supports two zone transfer methods. one-answer uses one DNS message per resource record transferred. many-answers packs as many resource records as possible into a message. many-answers is more efficient, but is only known to be understood by BIND 8.1 and patched versions of BIND 4.9.5. The default is one-answer. 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

This option will be used in the future to limit the number of concurrent outbound zone transfers. It is checked for syntax, but is otherwise ignored.

transfers-per-ns

The maximum number of inbound zone transfers ( named-xfer processes) that can be concurrently transferred 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 the TCP connection used to fetch all zones transferred inbound by the server. 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 zones being transferred, if one is specified. This statement sets the transfer-source for all zones, but can be overriden on a per-zone basis by including a transfer-source statement within the zone block in the configuration file.

The server's usage of many system resources can be limited. Some operating systems do not support some of the limits. On such systems, a warning will be issued if the unsupported limit is used. Some operating systems do not support resource limits, and on these systems a

set resource limits on this system

will be 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. Other values include: unlimited requests, unlimited use, or the maximum available amount. The value default uses the limit that was in force when the server was started. See the definition of size_spec for more details.

coresize

The maximum size of a core dump. The default value is default.

datasize

The maximum amount of data memory the server may use. The default value is default.

files

The maximum number of files the server may have open concurrently. The default value is unlimited. Note that on some operating systems the server cannot set an unlimited value and cannot determine the maximum number of open files the kernel can support. On such systems, choosing unlimited will cause the server to use the larger of the rlim_max from getrlimit(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

The max-ixfr-log-size will be used in a future release of the server to limit the size of the transaction log kept for Incremental Zone Transfer.

stacksize

The maximum amount of stack memory the server may use. The default value is default.

cleaning-interval

The server will remove expired resource records from the cache every cleaning-interval minutes. The default is 60 minutes. If set to 0, no periodic cleaning will occur.

heartbeat-interval

The server will perform 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 will occur.

interface-interval

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

statistics-interval

Name server statistics will be logged every statistics-interval minutes. The default is 60. If set to 0, no statistics will be logged.

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, 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 the least preferred.

The default topology is:

topology { localhost; localnets; };

When returning multiple resource records ("RRs"), the name server will normally return them in round robin, that is, after each request, the first RR is put to the end of the list. As the order of RRs is not defined, this should not cause any problems.

The client resolver code should rearrange the RRs as appropriate, for example, using any addresses on the local network before other addresses. However, not all resolvers can do this, or are not correctly configured to do so.

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

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

Each top level statement in the sortlist must itself be an explicit address match list with one or two elements. The first element of each top level list, which may be an IP address, an IP prefix, an acl name or nested address match 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 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 that prefer addresses on any of the locally connected networks. Next most preferred are addresses on the 192.168.1/24 network, and after that either the 192.168.2/24 or 192.168.3/24 network with no preference shown between these two networks. Queries received from a host on the 192.168.1/24 network will prefer other addresses on that network to the 192.168.2/24 and 192.168.3/24 networks. Queries received from a host on the 192.168.4/24 or the 192.168.5/24 network will only prefer other addresses on their directly connected networks.

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

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

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

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. Perhaps you want a random shuffle of the records as they are returned. The rrset-order statement permits you to configure the order 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 no class is specified, the default is ANY. If no type is specified, the default is ANY. If no name is specified, the default is "*".

The legal values for ordering are:

fixed

Records are returned in the order they are defined in the zone file.

random

Records are returned in some random order.

cyclic

Records are returned in a round-robin order.

For example:

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

will cause any responses for type A records in class IN that have "rc.vix.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.

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

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

lame-ttl

Sets the number of seconds to cache a lame server indication. 0 disables caching. The default is 600 (10 minutes). The maximum value is 1800 (30 minutes).

max-ncache-ttl

To reduce network traffic and increase performance, the server store 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 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 than 7 days.

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.

The zone statement is used to define how information about particular DNS zones is managed by the server. There are five different 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 that the slave contacts to update its copy of the zone. If a port is specified, it then checks to see if the zone is current and makes zone transfers to the port given. If a file is specified, then the replica will be written to the named file. Use of the file clause is highly recommended, since it often speeds server startup and eliminates a needless waste of bandwidth.

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, as described in THE OPTIONS STATEMENT section. The specification of options in such a zone will override any global options declared in the options statement.

If no forwarders clause is present in the zone or an empty list for forwarders is given, then no forwarding will be 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 only the behavior of the global forward option, and not the servers used, then you also need to respecify 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.

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

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 for an information service from MIT's Project Athena. It is used to share information about various systems databases, such as users, groups, and printers. More information can be found at ftp://athena-dist.mit.edu/pub/ATHENA/usenix/athena_changes.PS. The keyword hs is a synonym for hesiod.

Another MIT development was CHAOSnet, a LAN protocol created in the mid-1970s. It is still sometimes seen on LISP stations and other hardware in the AI community, and zone data for it can be specified with the chaos class.

check-names

See the subsection on Name Checking in THE OPTIONS STATEMENT.

allow-query

See the description of allow-query in the Access Control subsection of THE OPTIONS STATEMENT.

allow-update

Specifies which hosts are allowed to submit dynamic DNS updates to the server. The default is to deny updates from all hosts.

allow-transfer

See the description of allow-transfer in the Access Control subsection of THE OPTIONS STATEMENT.

transfer-source

transfer-source determines which local address will be bound to the TCP connection used to fetch this zone. 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 this zone if one is specified.

max-transfer-time-in

See the description of max-transfer-time-in in the Zone Transfers subsection of THE OPTIONS STATEMENT.

dialup

See the description of dialup in the Boolean Options subsection of THE OPTIONS STATEMENT.

notify

See the description of notify in the Boolean Options subsection of the THE OPTIONS STATEMENT.

also-notify

also-notify is 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 for the zone (other than the primary master), plus any IP addresses specified with also-notify. also-notify is not meaningful for stub zones. The default is the empty list.

forward

forward 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

The forwarders option in a zone is 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.

pubkey

The DNSSEC flags, protocol, and algorithm are specified, as well as a base-64 encoded string representing the key.

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).

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

Allows all hosts.

none

Denies all hosts.

localhosts

Allows the IP addresses of all interfaces on the system.

localnets

Allows any host on a network for which the system has an interface.

The key statement defines a key ID which can be used in a server statement to associate with a particular name server a method of authentication that is more rigorous than simple IP address matching. 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 algorithm_id is a string that specifies a security/authentication algorithm. secret_string is the secret to be used by the algorithm, and is treated as a base-64 encoded string. If you have a secret_string in your named.conf file, make sure that it is not be readable by anyone beside superuser.

The trusted-keys statement is for use with DNSSEC-style security, originally specified in RFC 2065. DNSSEC is meant to provide three distinct services: key distribution, data origin authentication, and transaction and request authentication.

The contributed section of the ISC BIND distribution includes a dns_signer utility to sign zone data according to the DNSSEC specifications. The utility is provided as-is, without any expressed or implied warranties. The contributed source could be retrieved from the /isc/bind/src/cur/bind-8 directory at ISC's FTP site, ftp.isc.org.

Each trusted key is associated with a domain name. Its attributes are the non-negative integral flags, protocol, and algorithm, as well as a base-64 encoded string representing the key.

Any number of trusted keys can be specified.

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

If you mark a server as bogus, all other addresses for that server will be marked as bogus when a match is made when looking up a server's address by name.

The server supports two zone transfer methods. The first, one-answer, uses one DNS message per resource record transferred. The second method, many-answers packs as many resource records as possible into a message. many-answers is more efficient, but is only understood by BIND 8.1 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.

The transfers 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 is checked for syntax but is otherwise ignored.

The key 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.

The key statement is intended for future use by the server. It is checked for syntax but is otherwise ignored.

The controls statement declares control channels to be used by system administrators to affect the operation of the local name server. These control channels are used by the ndc(1M) utility to send commands to and retrieve non-DNS results from a name server.

A UNIX control channel is a FIFO in the file system, and access to it is controlled by normal file system permissions. It is created by in.named(1M) with the specified file mode bits, user and group owner. See chmod(1). Note that, unlike chmod, the mode bits specified for perm will normally have a leading 0 so the number is interpreted as octal. Also note that the user and group ownership specified as owner and group must be given as numbers, not names. It is recommended that the permissions be restricted to administrative personnel only, or else any user on the system may be able to manage 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 is recommended that 127.0.0.1 be the only ip_addr used, and this only if you trust all non-privileged users on the local host to manage your name server.

The include statement inserts the specified file at the point that the include statement is encountered. It cannot be used within another statement, though, so a line such as

acl internal_hosts { include internal_hosts.acl; };

is not allowed.

Use include to break the configuration up into easily-managed chunks. For example:

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

could be used at the top of a BIND configuration file in order to include any acl or key information.

Be careful not to use ``#include," like you would in a C program, because ``#'' is used to start a comment.

The simplest configuration file that is still realistically useful is one which simply defines a hint zone that has a full path to the root servers file, for example:

zone "." in {
        type hint;
        file "/var/named/root.cache";
};

     

Here is a more typical real-world example.

/*
* A simple BIND 8 configuration
*/

logging {
        category lame-servers { null; };
        category cname { null; };
};

options {
        directory "/var/named";
};

controls {
         inet * port 52 allow { any; };                  // a bad idea
         unix "/var/run/ndc" perm 0600 owner 0 group 0;  // the default
};

zone "isc.org" in {
         type master;
         file "master/isc.org";
};

zone "vix.com" in {
         type slave;
         file "slave/vix.com";
         masters { 10.0.0.53; };
};

zone "0.0.127.in-addr.arpa" in {
         type master;
         file "master/127.0.0";
};

zone "." in {
         type hint;
         file "root.cache";
};