This common DNS trick is a part of every DNS geek's knowledge base. It works by using a CNAME record on an in-addr.arpa hostname instead of a PTR record. The idea being that the owner of the hostname of the CNAME record will be able to manipulate it to his will. This is commonly used for when an ISP wants to relinquish only partial control over an in-addr.arpa zone (i.e. for colocate purposes). Confused? Here's an example:

Normally, an entry in a reverse resolve zone would look like this:

1.20.168.192.in-addr.arpa. IN PTR   targethost.foo.com.

However, an entry in the reverse resolve zone using the CNAME trick looks like this:

1.20.168.192.in-addr.arpa. IN CNAME 192.168.20.1.foo.com.

Moving over to foo.com:

192.168.20.1.foo.com.      IN PTR   targethost.foo.com.

That way, the owner of foo.com has ultimate control of the reverse resolve by using his own zone. This is opposed to having to get ahold of the owner of the in-addr.arpa zone every time the hostname associated with the IP address (targethost.foo.com and 192.168.20.1, respectively) changes.

Cool, huh?

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