What is reverse DNS lookup?

- Reverse DNS lookup is the fact of checking an IP against a domain name server and being able to retrieve
the corresponding domain name.
This can only work if a "ptr" record exists for the concerned IP adress.

It is used by some ISPs in a silly attempt at forcing mail sending servers to "authenticate":
if your server's name can't be retrieved from your ip, you (supposedly) are a spammer...

Which is technically very wrong: many servers do not allow reverse dns lookup for good reasons,
and many spam servers do allow it Isps do this, very stupidly, to show they're doing something.

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(Created by Emperor on 05-17-2004 17:40)