19.2. /etc/named.conf
The
named.conf file is a collection of statements using nested options surrounded by opening and closing ellipse characters, { }. Administrators must be careful when editing named.conf to avoid syntax errors as many seemingly minor errors prevent the named service from starting.
A typical
named.conf file is organized similar to the following example:
<statement-1> ["<statement-1-name>"] [<statement-1-class>] {
<option-1>;
<option-2>;
<option-N>;
};
<statement-2> ["<statement-2-name>"] [<statement-2-class>] {
<option-1>;
<option-2>;
<option-N>;
};
<statement-N> ["<statement-N-name>"] [<statement-N-class>] {
<option-1>;
<option-2>;
<option-N>;
};19.2.1. Common Statement Types
The following types of statements are commonly used in
/etc/named.conf:
19.2.1.1. 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 addresslocalhost— Matches any IP address in use by the local systemlocalnets— Matches any IP address on any network to which the local system is connectednone— 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.
19.2.1.2. 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.
19.2.1.3. 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-query - Specifies which hosts are allowed to query this nameserver. By default, all hosts are allowed to query. An access control list, or collection of IP addresses or networks, may be used here to allow only particular hosts to query the nameserver.
-
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
namedworking 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
forwardersdirective.The following options are accepted:first— Specifies that the nameservers listed in theforwardersdirective be queried beforenamedattempts to resolve the name itself.only— Specifies thatnameddoes not attempt name resolution itself in the event that queries to nameservers specified in theforwardersdirective fail.
-
listen-on - Specifies the network interface on which
namedlistens 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 alisten-ondirective: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
namednotifies 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 analso-notifylist 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-onlyexample 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,
namedstatistics are saved to the/var/named/named.statsfile.
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 Section 19.7.1, “Installed Documentation” and the
bind.conf man page for more details.
19.2.1.4. 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.
Note
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 Section 19.3, “Zone Files”.
The most common
zone statement options include the following:
-
allow-query - Specifies the clients that are allowed to request information about this zone. The default is to allow all query requests.
-
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
namedservice. -
file - Specifies the name of the file in the
namedworking 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
typeslave. -
notify - Specifies whether or not
namednotifies 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 analso-notifylist 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 asNXDOMAIN. 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 ahintzone.master— Designates the nameserver as authoritative for this zone. A zone should be set as themasterif 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
namedto keep statistics concerning this zone, writing them to either the default location (/var/named/named.stats) or the file listed in thestatistics-fileoption in theserverstatement. Refer to Section 19.2.2, “Other Statement Types” for more information about theserverstatement.
19.2.1.5. 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.
