Forward vs. Reverse DNS

Forward DNS is what you typically think of when you think of DNS: it resolves a domain name to an IP address.  Reverse DNS does just the opposite; it resolves an IP address to it's domain name.

When you register a domain, you specify the forward DNS servers with your registrar.  To define reverse DNS, you must contact your ISP.  Reverse DNS uses a series of delegations starting at the root of the in-addr.arpa domain name and works it's way down through to your ISPs DNS server.  Each server along the way must either answer the request, or once again delegate the request to the next server.

Reverse DNS has been used for quite some time as a way to combat spam.  While not particularly effective, it has helped weed of some of the less dedicated spammers.

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