Resource Records in Zone Files

A resource record in a zone file contains the following fields, some of which are optional, depending on the record type:

Name

Domain name or IP address.

TTL (time to live)

The maximum time that a name server caches a record before it checks whether a newer one is available.

Class

Always IN for the Internet.

Type

Type of record, for example:

A (address): IPv4 address corresponding to a host.
AAAA (address): IPv6 address corresponding to a host.
CNAME (canonical name): Alias name corresponding to a host name.
MX (mail exchange): Destination for email addressed to the domain.
NS (name server): Fully qualified domain name of an authoritative name server for a domain.
PTR (pointer): Host name that corresponds to an IP address for address-to-name lookups (reverse-name resolution).
SOA (start of authority): Authoritative information about a zone, such as the primary name server, the email address of the domain's administrator, and the domain's serial number. All records following a SOA record relate to the zone that it defines up to the next SOA record.

Data

Information that the record stores, such as an IP address in an A record, or a host name in a CNAME or PTR record.

The following example shows the contents of a typical zone file such as /var/named/master-data:

$TTL 86400        ; 1 day
@ IN SOA dns.us.example.com. root.us.example.com. (
            57 ; serial
            28800 ; refresh (8 hours)
            7200 ; retry (2 hours)
            2419200 ; expire (4 weeks)
            86400 ; minimum (1 day)
            )
              IN  NS      dns.us.example.com.

dns           IN  A       192.168.2.1
us.example.com  IN  A       192.168.2.1
svr01         IN  A       192.168.2.2
www           IN  CNAME   svr01
host01        IN  A       192.168.2.101
host02        IN  A       192.168.2.102
host03        IN  A       192.168.2.103
...

A comment on a line is preceded by a semicolon (;).

The $TTL directive defines the default time-to-live value for all resource records in the zone. Each resource record can define its own time-to-live value, which overrides the global setting.

The SOA record is mandatory and includes the following information:

us.example.com: The name of the domain.
dns.us.example.com.: The fully qualified domain name of the name server, including a trailing period (.) for the root domain.
root.us.example.com.: The email address of the domain administrator.
serial: A counter that, if incremented, tells named to reload the zone file.
refresh: The time after which a primary name server notifies backup name servers that they should refresh their database.
retry: If a refresh fails, the time that a backup name server should wait before attempting another refresh.
expire: The maximum elapsed time that a backup name server has to complete a refresh before its zone records are no longer considered authoritative and it will stop answering queries.
minimum: The minimum time for which other servers should cache information obtained from this zone.

An NS record declares an authoritative name server for the domain.

Each A record specifies the IP address that corresponds to a host name in the domain.

The CNAME record creates the alias www for svr01.

For more information, see the BIND 9 Administrator Reference Manual.