Reverse Name Resolution Zone Files

A reverse name resolution zone file is used to translate an IP address in a particular namespace into an FQDN. It looks very similar to a standard zone file, except that PTR resource records are used to link the IP addresses to a fully qualified domain name.

The following illustrates the layout of a PTR record:

<last-IP-digit> IN PTR <FQDN-of-system> 

The <last-IP-digit> is the last number in an IP address which points to a particular system's FQDN.

In the following example, IP addresses 10.0.1.1 through 10.0.1.6 are pointed to corresponding FQDNs. It can be located in /var/named/example.com.rr.zone.

$ORIGIN 1.0.10.in-addr.arpa.
$TTL 86400
@ IN SOA dns1.example.com. hostmaster.example.com. (
   2001062501 ; serial
   21600      ; refresh after 6 hours
   3600       ; retry after 1 hour
   604800     ; expire after 1 week
   86400 )    ; minimum TTL of 1 day
;
@ IN NS dns1.example.com.
;
1 IN PTR dns1.example.com.
2 IN PTR dns2.example.com.
;
5 IN PTR server1.example.com.
6 IN PTR server2.example.com.
;
3 IN PTR ftp.example.com.
4 IN PTR ftp.example.com.

This zone file would be called into service with a zone statement in the named.conf file similar to the following:

zone "1.0.10.in-addr.arpa" IN {
 type master;
 file "example.com.rr.zone";
 allow-update { none; };
};

There is very little difference between this example and a standard zone statement, except for the zone name. Note that a reverse name resolution zone requires the first three blocks of the IP address reversed followed by .in-addr.arpa. This allows the single block of IP numbers used in the reverse name resolution zone file to be associated with the zone.