19.3.4. 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 through 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.