DNS / Reverse Resolution

Configurations - IPv4 Blocks

In order to configure the inverse resolution for an IPv4 address block it is necessary first to configure a DNS server to answer for a specific domain name.

For instance an considering the IPv4 address block 192.0.2.0/24, the domain name to be configured would be the 2.0.192.in-addr.arpa.

Considering also a DNS server running the BIND software (Berkeley Internet Name Domain), developed by Internet Systems Consortium (ISC), in an Unix environment, one would have to follow the steps:

To create a general configuration file containing the following lines (not to mention other configuration lines needed for correct server operation):

zone "2.0.192.in-addr.arpa"{

type master;

file "2.0.192.in-addr.arpa.db";

};

The lines above indicates the zone for which this servers should answer as authority, the type of server (master or slave), and the file name where specific informations about the zone will reside. The file name could be any; in this example we used one that is based on the name of the zone.

Then, next step would be to create the file which will contain information about the zone itself. The file must have the same name as indicated in the general configuration (as seen on previous step). In this example the file name will be 2.0.192.in-addr.arpa.db.

Basically, this file will contain the following information set:

2.0.192.in-addr.arpa. IN SOA ns.example.org root.example.org (

1 ; serial

3600 ; refresh

3600 ; retry

3600 ; expire

900) ; minimum

2.0.192.in-addr.arpa. IN NS ns.example.org.

1 IN PTR host1.example.org.

2 IN PTR host2.example.org.

The lines above indicates the zone name and its RR (Resource Records), as for instance, the SOA (Start of Authority), which indicates the server with authority for this zone. The information that follows are used for synchronization purposes among master and slaves servers. And then, there is information about DNS servers for the zone, which can be more than one. And finally the information about the inverse resolution, which are the ones indicating names assigned to each IP address in this block. The Resource Record PTR indicates a pointer between an IP address and its name. For instance, the IP address 192.0.2.1 has the name host1.example.org