Create a DNS zone file

By Andres Jaimes

by FreeBSD.org

An example master zone file for example.org (existing within /etc/namedb/master/example.org) is as follows:

$TTL 3600        ; 1 hour default TTL
example.org.    IN      SOA      ns1.example.org. admin.example.org. (
                                2006051501      ; Serial
                                10800           ; Refresh
                                3600            ; Retry
                                604800          ; Expire
                                300             ; Negative Response TTL
                        )

; DNS Servers
                IN      NS      ns1.example.org.
                IN      NS      ns2.example.org.

; MX Records
                IN      MX 10   mx.example.org.
                IN      MX 20   mail.example.org.

                IN      A       192.168.1.1

; Machine Names
localhost       IN      A       127.0.0.1
ns1             IN      A       192.168.1.2
ns2             IN      A       192.168.1.3
mx              IN      A       192.168.1.4
mail            IN      A       192.168.1.5

; Aliases
www             IN      CNAME   example.org.

Note that every hostname ending in a “.” is an exact hostname, whereas everything without a trailing “.” is relative to the origin. For example, ns1 is translated into ns1.example.org.

The format of a zone file follows:

recordname      IN recordtype   value

The most commonly used DNS records:

  • SOA: start of zone authority
  • NS: an authoritative name server
  • A: a host address
  • CNAME: the canonical name for an alias
  • MX: mail exchanger
  • PTR: a domain name pointer (used in reverse DNS)
example.org. IN SOA ns1.example.org. admin.example.org. (
                        2006051501      ; Serial
                        10800           ; Refresh after 3 hours
                        3600            ; Retry after 1 hour
                        604800          ; Expire after 1 week
                        300 )           ; Negative Response TTL
  • example.org. : the domain name, also the origin for this zone file.
  • ns1.example.org. : the primary/authoritative name server for this zone.
  • admin.example.org. : the responsible person for this zone, email address with “@” replaced. (admin@example.org becomes admin.example.org)
  • 2006051501 : the serial number of the file. This must be incremented each time the zone file is modified. Nowadays, many admins prefer a yyyymmddrr format for the serial number. 2006051501 would mean last modified 05/15/2006, the latter 01 being the first time the zone file has been modified this day. The serial number is important as it alerts slave name servers for a zone when it is updated.
IN NS           ns1.example.org.

This is an NS entry. Every name server that is going to reply authoritatively for the zone must have one of these entries.

localhost       IN      A       127.0.0.1
ns1             IN      A       192.168.1.2
ns2             IN      A       192.168.1.3
mx              IN      A       192.168.1.4
mail            IN      A       192.168.1.5

The A record indicates machine names. As seen above, ns1.example.org would resolve to 192.168.1.2.

IN      A       192.168.1.1

This line assigns IP address 192.168.1.1 to the current origin, in this case example.org.

www             IN CNAME        @

The canonical name record is usually used for giving aliases to a machine. In the example, www is aliased to the “master” machine whose name happens to be the same as the domain name example.org (192.168.1.1). CNAMEs can never be used together with another kind of record for the same hostname.

IN MX   10      mail.example.org.

The MX record indicates which mail servers are responsible for handling incoming mail for the zone. mail.example.org is the hostname of a mail server, and 10 is the priority of that mail server.

One can have several mail servers, with priorities of 10, 20 and so on. A mail server attempting to deliver to example.org would first try the highest priority MX (the record with the lowest priority number), then the second highest, etc, until the mail can be properly delivered.

For in-addr.arpa zone files (reverse DNS), the same format is used, except with PTR entries instead of A or CNAME.

$TTL 3600

1.168.192.in-addr.arpa. IN SOA ns1.example.org. admin.example.org. (
                        2006051501      ; Serial
                        10800           ; Refresh
                        3600            ; Retry
                        604800          ; Expire
                        300 )           ; Negative Response TTL

        IN      NS      ns1.example.org.
        IN      NS      ns2.example.org.

1       IN      PTR     example.org.
2       IN      PTR     ns1.example.org.
3       IN      PTR     ns2.example.org.
4       IN      PTR     mx.example.org.
5       IN      PTR     mail.example.org.

This file gives the proper IP address to hostname mappings for the above fictitious domain.

It is worth noting that all names on the right side of a PTR record need to be fully qualified (i.e., end in a “.”).