Common Statement Types

acl Statement

The acl statement (or access control statement) defines groups of hosts which can then be permitted or denied access to the nameserver.

An acl statement takes the following form:

acl <acl-name> {
 <match-element>;
 [<match-element>; ...]
};

In this statement, replace <acl-name> with the name of the access control list and replace <match-element> with a semi-colon separated list of IP addresses. Most of the time, an individual IP address or IP network notation (such as 10.0.1.0/24) is used to identify the IP addresses within the acl statement.

The following access control lists are already defined as keywords to simplify configuration:

• any - Matches every IP address

• localhost - Matches any IP address in use by the local system
• localnets - Matches any IP address on any network to which the local system is connected
• none - Matches no IP addresses

When used in conjunction with other statements (such as the options statement), acl statements can be very useful in preventing the misuse of a BIND nameserver.

The following example defines two access control lists and uses an options statement to define how they are treated by the nameserver:

acl black-hats {
 10.0.2.0/24;
 192.168.0.0/24;
};
acl red-hats {
 10.0.1.0/24;
};
options {
 blackhole { black-hats; };
 allow-query { red-hats; };
 allow-recursion { red-hats; };
};

This example contains two access control lists, black-hats and red-hats. Hosts in the black-hats list are denied access to the nameserver, while hosts in the red-hats list are given normal access.

include Statement

The include statement allows files to be included in a named.conf file. In this way, sensitive configuration data (such as keys) can be placed in a separate file with restrictive permissions.

An include statement takes the following form:

include "<file-name>"

In this statement, <file-name> is replaced with an absolute path to a file.

options Statement

The options statement defines global server configuration options and sets defaults for other statements. It can be used to specify the location of the named working directory, the types of queries allowed, and much more.

The options statement takes the following form:

options {
 <option>;
 [<option>; ...]
};

In this statement, the <option> directives are replaced with a valid option.

The following are commonly used options:

allow-recursion

Similar to allow-query, this option applies to recursive queries. By default, all hosts are allowed to perform recursive queries on the nameserver.

blackhole

Specifies which hosts are not allowed to query the server.

directory

Specifies the named working directory if different from the default value, /var/named/.

forwarders

Specifies a list of valid IP addresses for nameservers where requests should be forwarded for resolution.

forward

Specifies the forwarding behavior of a forwarders directive.

The following options are accepted:

• first - Specifies that the nameservers listed in the forwarders directive be queried before named attempts to resolve the name itself.

• only - Specifies that named does not attempt name resolution itself in the event that queries to nameservers specified in the forwarders directive fail.

listen-on

Specifies the network interface on which named listens for queries. By default, all interfaces are used.

Using this directive on a DNS server which also acts a gateway, BIND can be configured to only answer queries that originate from one of the networks.

The following is an example of a listen-on directive:

options {
 listen-on { 10.0.1.1; };
};

In this example, only requests that arrive from the network interface serving the private network (10.0.1.1) are accepted.

notify

Controls whether named notifies the slave servers when a zone is updated. It accepts the following options:

• yes - Notifies slave servers.

• no - Does not notify slave servers.
• explicit - Only notifies slave servers specified in an also-notify list within a zone statement.

pid-file

Specifies the location of the process ID file created by named.

root-delegation-only

Turns on the enforcement of delegation properties in top-level domains (TLDs) and root zones with an optional exclude list. Delegation is the process of dividing a single zone into multiple subzones. In order to create a delegated zone, items known as NS records are used. NameServer records (delegation records) announce the authoritative nameservers for a particular zone.

The following root-delegation-only example specifies an exclude list of TLDs from whom undelegated responses are expected and trusted:

options {
 root-delegation-only exclude { "ad"; "ar"; "biz"; "cr"; "cu"; "de"; "dm"; "id";
  "lu"; "lv"; "md"; "ms"; "museum"; "name"; "no"; "pa";
  "pf"; "se"; "sr"; "to"; "tw"; "us"; "uy"; };
};

statistics-file

Specifies an alternate location for statistics files. By default, named statistics are saved to the /var/named/named.stats file.

There are several other options also available, many of which rely upon one another to work properly. Refer to the BIND 9 Administrator Reference Manual referenced in "Installed Documentation" and the bind.conf man page for more details.

zone Statement

A zone statement defines the characteristics of a zone, such as the location of its configuration file and zone-specific options. This statement can be used to override the global options statements.

A zone statement takes the following form:

zone <zone-name> <zone-class> {
 <zone-options>;
 [<zone-options>; ...]
};

In this statement, <zone-name> is the name of the zone, <zone-class> is the optional class of the zone, and <zone-options> is a list of options characterizing the zone.

The <zone-name> attribute for the zone statement is particularly important. It is the default value assigned for the $ORIGIN directive used within the corresponding zone file located in the /var/named/ directory. The named daemon appends the name of the zone to any non-fully qualified domain name listed in the zone file.

If you have installed the caching-nameserver package, the default configuration file will be in /etc/named.rfc1912.zones.

For example, if a zone statement defines the namespace for example.com, use example.com as the <zone-name> so it is placed at the end of hostnames within the example.com zone file.

For more information about zone files, refer to "Zone Files".

The most common zone statement options include the following:

allow-transfer

Specifies the slave servers that are allowed to request a transfer of the zone's information. The default is to allow all transfer requests.

allow-update

Specifies the hosts that are allowed to dynamically update information in their zone. The default is to deny all dynamic update requests.

Be careful when allowing hosts to update information about their zone. Do not enable this option unless the host specified is completely trusted. In general, it is better to have an administrator manually update the records for a zone and reload the named service.

file

Specifies the name of the file in the named working directory that contains the zone's configuration data.

masters

Specifies the IP addresses from which to request authoritative zone information and is used only if the zone is defined as type slave.

notify

Specifies whether or not named notifies the slave servers when a zone is updated. This directive accepts the following options:

• yes - Notifies slave servers.

• no - Does not notify slave servers.
• explicit - Only notifies slave servers specified in an also-notify list within a zone statement.

type

Defines the type of zone.

Below is a list of valid options:

• delegation-only - Enforces the delegation status of infrastructure zones such as COM, NET, or ORG. Any answer that is received without an explicit or implicit delegation is treated as NXDOMAIN. This option is only applicable in TLDs or root zone files used in recursive or caching implementations.

• forward - Forwards all requests for information about this zone to other nameservers.
• hint - A special type of zone used to point to the root nameservers which resolve queries when a zone is not otherwise known. No configuration beyond the default is necessary with a hint zone.
• master - Designates the nameserver as authoritative for this zone. A zone should be set as the master if the zone's configuration files reside on the system.
• slave - Designates the nameserver as a slave server for this zone. Also specifies the IP address of the master nameserver for the zone.

zone-statistics

Configures named to keep statistics concerning this zone, writing them to either the default location (/var/named/named.stats) or the file listed in the statistics-file option in the server statement. Refer to "Other Statement Types" for more information about the server statement.

Sample zone Statements

Most changes to the /etc/named.conf file of a master or slave nameserver involves adding, modifying, or deleting zone statements. While these zone statements can contain many options, most nameservers require only a small subset to function efficiently. The following zone statements are very basic examples illustrating a master-slave nameserver relationship.

The following is an example of a zone statement for the primary nameserver hosting example.com (192.168.0.1):

zone "example.com" IN {
 type master;
 file "example.com.zone";
 allow-update { none; };
};

In the statement, the zone is identified as example.com, the type is set to master, and the named service is instructed to read the /var/named/example.com.zone file. It also tells named not to allow any other hosts to update.

A slave server's zone statement for example.com is slightly different from the previous example. For a slave server, the type is set to slave and in place of the allow-update line is a directive telling named the IP address of the master server.

The following is an example slave server zone statement for example.com zone:

zone "example.com" {
 type slave;
 file "example.com.zone";
 masters { 192.168.0.1; };
};

This zone statement configures named on the slave server to query the master server at the 192.168.0.1 IP address for information about the example.com zone. The information that the slave server receives from the master server is saved to the /var/named/example.com.zone file.