What is the RFC requirement for Reverse DNS? That it exist? Or that it match the official server name?
RFC1912 2.1 says that "Every Internet-reachable host should have a name", and "For every IP address, there should be a matching PTR record in the in-addr.arpa domain." The host name in the PTR record should have an A record pointing back to the original IP address.
I thought that it only had to exist.
That is correct. A few militant anti-spam people will do things such as check to see if the reverse DNS entry matches something else, but that isn't necessary.
In our case (and in many cases as I talk with more admins) the reverse DNS for our static Ips is not delegated by our ISP, but returns their name in the reverse DNS lookup. I have been told repeatedly that this is OK, as long as something is returned.
It's OK for your reverse DNS entry to have the ISP's domain name in it (that's the case in our situation, too).
-Scott
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