Reverse

A normal DNS query would be of the form 'what is the IP of host=www in domain=mydomain.com'. There are times however when we want to be able to find out the name of the host whose IP address = x.x.x.x (or x:x:x:x:x for IPv6). Sometimes this is required for diagnostic purposes, more frequently these days it is used for security purposes to trace a hacker or spammer. Indeed, most modern mailing systems use reverse mapping to provide simple, first-cut, authentication using a dual look-up process - IP to name and name to IP. IPv4 reverse mapping is not mandatory though, as indicated by the mail example, it is essential for hosts that send mail, using either a Mail Transfer Agent (MTA) or a Mail User Agent (MUA). In the case of IPv6 reverse-mapping was originally mandatory but as part of the seemingly relentless move to relax the goals of the original specifications (doubtless for good operational reasons) it is no longer a mandatory requirement.