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17.2. BIND
This chapter covers
BIND
(Berkeley Internet Name Domain), the DNS server included in Red Hat Enterprise Linux. It focuses on the structure of its configuration files, and describes how to administer it both locally and remotely.
17.2.1. Configuring the named Service
When the
named
service is started, it reads the configuration from the files as described in Table 17.1, “The named service configuration files”.
Path | Description |
---|---|
/etc/named.conf | The main configuration file. |
/etc/named/ | An auxiliary directory for configuration files that are included in the main configuration file. |
The configuration file consists of a collection of statements with nested options surrounded by opening and closing curly brackets. Note that when editing the file, you have to be careful not to make any syntax error, otherwise the
named
service will not start. A typical /etc/named.conf
file is organized as follows:
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; };
Note
If you have installed the bind-chroot package, the BIND service will run in the
/var/named/chroot
environment. In that case, the initialization script will mount the above configuration files using the mount --bind
command, so that you can manage the configuration outside this environment. There is no need to copy anything into the /var/named/chroot
directory because it is mounted automatically. This simplifies maintenance since you do not need to take any special care of BIND
configuration files if it is run in a chroot
environment. You can organize everything as you would with BIND
not running in a chroot
environment.
The following directories are automatically mounted into
/var/named/chroot
if they are empty in the /var/named/chroot
directory. They must be kept empty if you want them to be mounted into /var/named/chroot
:
/var/named
/etc/pki/dnssec-keys
/etc/named
/usr/lib64/bind
or/usr/lib/bind
(architecture dependent).
The following files are also mounted if the target file does not exist in
/var/named/chroot
.
/etc/named.conf
/etc/rndc.conf
/etc/rndc.key
/etc/named.rfc1912.zones
/etc/named.dnssec.keys
/etc/named.iscdlv.key
/etc/named.root.key
17.2.1.1. Common Statement Types
The following types of statements are commonly used in
/etc/named.conf
:
-
acl
- The
acl
(Access Control List) statement allows you to define groups of hosts, so that they can be permitted or denied access to the nameserver. It takes the following form:acl acl-name { match-element; ... };
The acl-name statement name is the name of the access control list, and the match-element option is usually an individual IP address (such as10.0.1.1
) or a CIDR (Classless Inter-Domain Routing) network notation (for example,10.0.1.0/24
). For a list of already defined keywords, see Table 17.2, “Predefined access control lists”.Table 17.2. Predefined access control lists Keyword Description any
Matches every IP address. localhost
Matches any IP address that is in use by the local system. localnets
Matches any IP address on any network to which the local system is connected. none
Does not match any IP address. Theacl
statement can be especially useful in conjunction with other statements such asoptions
. Example 17.2, “Using acl in conjunction with options” defines two access control lists,black-hats
andred-hats
, and addsblack-hats
on the blacklist while grantingred-hats
a normal access.Example 17.2. Using acl in conjunction with options
acl black-hats { 10.0.2.0/24; 192.168.0.0/24; 1234:5678::9abc/24; }; acl red-hats { 10.0.1.0/24; }; options { blackhole { black-hats; }; allow-query { red-hats; }; allow-query-cache { red-hats; }; };
-
include
- The
include
statement allows you to include files in the/etc/named.conf
, so that potentially sensitive data can be placed in a separate file with restricted permissions. It takes the following form:include "file-name"
The file-name statement name is an absolute path to a file.Example 17.3. Including a file to /etc/named.conf
include "/etc/named.rfc1912.zones";
-
options
- The
options
statement allows you to define global server configuration options as well as to set defaults for other statements. It can be used to specify the location of thenamed
working directory, the types of queries allowed, and much more. It takes the following form:options { option; ... };
For a list of frequently used option directives, see Table 17.3, “Commonly used options” below.Table 17.3. Commonly used options Option Description allow-query
Specifies which hosts are allowed to query the nameserver for authoritative resource records. It accepts an access control list, a collection of IP addresses, or networks in the CIDR notation. All hosts are allowed by default. allow-query-cache
Specifies which hosts are allowed to query the nameserver for non-authoritative data such as recursive queries. Only localhost
andlocalnets
are allowed by default.blackhole
Specifies which hosts are not allowed to query the nameserver. This option should be used when particular host or network floods the server with requests. The default option is none
.directory
Specifies a working directory for the named
service. The default option is/var/named/
.dnssec-enable
Specifies whether to return DNSSEC related resource records. The default option is yes
.dnssec-validation
Specifies whether to prove that resource records are authentic via DNSSEC. The default option is yes
.forwarders
Specifies a list of valid IP addresses for nameservers to which the requests should be forwarded for resolution. forward
Specifies the behavior of theforwarders
directive. It accepts the following options:first
— The server will query the nameservers listed in theforwarders
directive before attempting to resolve the name on its own.only
— When unable to query the nameservers listed in theforwarders
directive, the server will not attempt to resolve the name on its own.
listen-on
Specifies the IPv4 network interface on which to listen for queries. On a DNS server that also acts as a gateway, you can use this option to answer queries originating from a single network only. All IPv4 interfaces are used by default. listen-on-v6
Specifies the IPv6 network interface on which to listen for queries. On a DNS server that also acts as a gateway, you can use this option to answer queries originating from a single network only. All IPv6 interfaces are used by default. max-cache-size
Specifies the maximum amount of memory to be used for server caches. When the limit is reached, the server causes records to expire prematurely so that the limit is not exceeded. In a server with multiple views, the limit applies separately to the cache of each view. The default option is 32M
.notify
Specifies whether to notify the secondary nameservers when a zone is updated. It accepts the following options:yes
— The server will notify all secondary nameservers.no
— The server will not notify any secondary nameserver.master-only
— The server will notify primary server for the zone only.explicit
— The server will notify only the secondary servers that are specified in thealso-notify
list within a zone statement.
pid-file
Specifies the location of the process ID file created by the named
service.recursion
Specifies whether to act as a recursive server. The default option is yes
.statistics-file
Specifies an alternate location for statistics files. The /var/named/named.stats
file is used by default.Important
To prevent distributed denial of service (DDoS) attacks, it is recommended that you use theallow-query-cache
option to restrict recursive DNS services for a particular subset of clients only.See the BIND 9 Administrator Reference Manual referenced in Section 17.2.7.1, “Installed Documentation”, and thenamed.conf
manual page for a complete list of available options.Example 17.4. Using the options statement
options { allow-query { localhost; }; listen-on port 53 { 127.0.0.1; }; listen-on-v6 port 53 { ::1; }; max-cache-size 256M; directory "/var/named"; statistics-file "/var/named/data/named_stats.txt"; recursion yes; dnssec-enable yes; dnssec-validation yes; };
-
zone
- The
zone
statement allows you to define the characteristics of a zone, such as the location of its configuration file and zone-specific options, and can be used to override the globaloptions
statements. It takes the following form:zone zone-name [zone-class] { option; ... };
The zone-name attribute is the name of the zone, zone-class is the optional class of the zone, and option is azone
statement option as described in Table 17.4, “Commonly used options”.The zone-name attribute is particularly important, as it is the default value assigned for the$ORIGIN
directive used within the corresponding zone file located in the/var/named/
directory. Thenamed
daemon appends the name of the zone to any non-fully qualified domain name listed in the zone file. For example, if azone
statement defines the namespace forexample.com
, useexample.com
as the zone-name so that it is placed at the end of host names within theexample.com
zone file.For more information about zone files, see Section 17.2.2, “Editing Zone Files”.Table 17.4. Commonly used options Option Description allow-query
Specifies which clients are allowed to request information about this zone. This option overrides global allow-query
option. All query requests are allowed by default.allow-transfer
Specifies which secondary servers are allowed to request a transfer of the zone's information. All transfer requests are allowed by default. allow-update
Specifies which hosts are allowed to dynamically update information in their zone. The default option is to deny all dynamic update requests.Note that you should be careful when allowing hosts to update information about their zone. Do not set IP addresses in this option unless the server is in the trusted network. Instead, use TSIG key as described in Section 17.2.5.3, “Transaction SIGnatures (TSIG)”.file
Specifies the name of the file in the named
working directory that contains the zone's configuration data.masters
Specifies from which IP addresses to request authoritative zone information. This option is used only if the zone is defined as type
slave
.notify
Specifies whether to notify the secondary nameservers when a zone is updated. It accepts the following options:yes
— The server will notify all secondary nameservers.no
— The server will not notify any secondary nameserver.master-only
— The server will notify primary server for the zone only.explicit
— The server will notify only the secondary servers that are specified in thealso-notify
list within a zone statement.
type
Specifies the zone type. It accepts the following 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 (Top-Level Domain) 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 ahint
zone.master
— Designates the nameserver as authoritative for this zone. A zone should be set as themaster
if the zone's configuration files reside on the system.slave
— Designates the nameserver as a slave server for this zone. Master server is specified inmasters
directive.
Most changes to the/etc/named.conf
file of a primary or secondary nameserver involve adding, modifying, or deletingzone
statements, and only a small subset ofzone
statement options is usually needed for a nameserver to work efficiently.In Example 17.5, “A zone statement for a primary nameserver”, the zone is identified asexample.com
, the type is set tomaster
, and thenamed
service is instructed to read the/var/named/example.com.zone
file. It also allows only a secondary nameserver (192.168.0.2
) to transfer the zone.Example 17.5. A zone statement for a primary nameserver
zone "example.com" IN { type master; file "example.com.zone"; allow-transfer { 192.168.0.2; }; };
A secondary server'szone
statement is slightly different. The type is set toslave
, and themasters
directive is tellingnamed
the IP address of the master server.In Example 17.6, “A zone statement for a secondary nameserver”, thenamed
service is configured to query the primary server at the192.168.0.1
IP address for information about theexample.com
zone. The received information is then saved to the/var/named/slaves/example.com.zone
file. Note that you have to put all slave zones to/var/named/slaves
directory, otherwise the service will fail to transfer the zone.Example 17.6. A zone statement for a secondary nameserver
zone "example.com" { type slave; file "slaves/example.com.zone"; masters { 192.168.0.1; }; };
17.2.1.2. Other Statement Types
The following types of statements are less commonly used in
/etc/named.conf
:
-
controls
- The
controls
statement allows you to configure various security requirements necessary to use therndc
command to administer thenamed
service.See Section 17.2.3, “Using the rndc Utility” for more information on therndc
utility and its usage. -
key
- The
key
statement allows you to define a particular key by name. Keys are used to authenticate various actions, such as secure updates or the use of therndc
command. Two options are used withkey
:algorithm algorithm-name
— The type of algorithm to be used (for example,hmac-md5
).secret "key-value"
— The encrypted key.
See Section 17.2.3, “Using the rndc Utility” for more information on therndc
utility and its usage. -
logging
- The
logging
statement allows you to use multiple types of logs, so called channels. By using thechannel
option within the statement, you can construct a customized type of log with its own file name (file
), size limit (size
), versioning (version
), and level of importance (severity
). Once a customized channel is defined, acategory
option is used to categorize the channel and begin logging when thenamed
service is restarted.By default,named
sends standard messages to thersyslog
daemon, which places them in/var/log/messages
. Several standard channels are built into BIND with various severity levels, such asdefault_syslog
(which handles informational logging messages) anddefault_debug
(which specifically handles debugging messages). A default category, calleddefault
, uses the built-in channels to do normal logging without any special configuration.Customizing the logging process can be a very detailed process and is beyond the scope of this chapter. For information on creating custom BIND logs, see the BIND 9 Administrator Reference Manual referenced in Section 17.2.7.1, “Installed Documentation”. -
server
- The
server
statement allows you to specify options that affect how thenamed
service should respond to remote nameservers, especially with regard to notifications and zone transfers.Thetransfer-format
option controls the number of resource records that are sent with each message. It can be eitherone-answer
(only one resource record), ormany-answers
(multiple resource records). Note that while themany-answers
option is more efficient, it is not supported by older versions of BIND. -
trusted-keys
- The
trusted-keys
statement allows you to specify assorted public keys used for secure DNS (DNSSEC). See Section 17.2.5.4, “DNS Security Extensions (DNSSEC)” for more information on this topic. -
view
- The
view
statement allows you to create special views depending upon which network the host querying the nameserver is on. This allows some hosts to receive one answer regarding a zone while other hosts receive totally different information. Alternatively, certain zones may only be made available to particular trusted hosts while non-trusted hosts can only make queries for other zones.Multiple views can be used as long as their names are unique. Thematch-clients
option allows you to specify the IP addresses that apply to a particular view. If theoptions
statement is used within a view, it overrides the already configured global options. Finally, mostview
statements contain multiplezone
statements that apply to thematch-clients
list.Note that the order in which theview
statements are listed is important, as the first statement that matches a particular client's IP address is used. For more information on this topic, see Section 17.2.5.1, “Multiple Views”.
17.2.1.3. Comment Tags
Additionally to statements, the
/etc/named.conf
file can also contain comments. Comments are ignored by the named
service, but can prove useful when providing additional information to a user. The following are valid comment tags:
-
//
- Any text after the
//
characters to the end of the line is considered a comment. For example:notify yes; // notify all secondary nameservers
-
#
- Any text after the
#
character to the end of the line is considered a comment. For example:notify yes; # notify all secondary nameservers
/*
and*/
- Any block of text enclosed in
/*
and*/
is considered a comment. For example:notify yes; /* notify all secondary nameservers */