[AusNOG] PTR Records

[Aaron Wigley]

David Beveridge <dave at bevhost.com> asked:
> Can anyone point me to any RFC which says that there are restrictions on
>the name a PTR can point to.

RFC 1912, under "2.1 Inconsistent, Missing, or Bad Data: Make sure your
PTR and A records match.
For every IP address, there should be a matching PTR record in the
in-addr.arpa domain."


So, if there is an A RR for mail.mycompany.com, there should be a PTR RR
pointing back to it.  This is commonly used for email spam detection
(https://en.wikipedia.org/wiki/Anti-spam_techniques#PTR.2Freverse_DNS_check
s)

> I have been told by a certain ISP that I cannot have a PTR pointing to
>mycompany.com.au <http://mycompany.com.au> as it is NON-standard and
>which cannot and should not be done.  They insist on setting the PTR to
>mail.mycompany.com.au <http://mail.mycompany.com.au> and refuse to do as
>I ask.

As others have noted, that ISP is likely trying to keep their allocated
blocks of IP addresses out of email blacklists by ensuring that PTR
records are correct for any MX servers hosted on their IP addresses.

Regards,

-- 
Aaron Wigley