The revmap module offers a convenient way to automatically do reverse and forward mapping of domain names for large IP namespaces. It can algorithmically generate answers to in-addr.arpa reverse-mapping queries by giving back a domain name generated from the IP number. On the other hand, it also offers a forward-mapping zone companion to convert these domain names back to IP numbers. This is really useful when you maintain a large site: some programs require any connnecting host to have valid reverse-mapping information, which can sometimes be quite a pain.

The revmap and formap zone types defined in this module support standard queries only. They do not support zone transfers, so secondary nameservers for these reverse-mapping domains will either also need to use this module, or be set up using a master file or other static data. Also, this module is for IPv4 only.

An IP number is converted to a domain name by taking the combined hexadecimal representation of its labels. For example, a query for 1.1.168.192.in-addr.arpa might return a pointer to c0a80101.dyn.acdam.net., where a query for c0a80101.dyn.acdam.net. would return the address 192.168.1.1.

Maps back domain names in the in-addr.arpa namespace to domain names. Configuration options:

• prefix (domain name) - Domain name to append to the generated label. In our example, this was dyn.acdam.net.. This domain name should be a zone of the formap type to make forward queries work as well.

• nslist (list of addresses) - Nameservers for the zone. Note that these nameservers may not be in the reverse-mapping zone itself, since the zone cannot give addresses for the nameservers.

• ttl (number) - Time To Live value for the records sent back.

Companion to the “revmap” zone type. Configuration options:

• nslist (domain name) - As above.

• ttl (number) - As above.

The forward mapping zone will answer any A query as long as it is a subitem of the zone root with exactly 8 bytes, even if it is not a valid hexadecimal value. In such cases, the returned address will be garbage.

The formap and revmap zones have only one, non-customizable algorithm for generating domain names. This might cause interoperability problems with other DNS servers offering similar capabilities but with a different algorithm.