Question about MX record

[Ashley Kitto]

In message <F0D08D901CD75840865A335B1D498235022418 at exchange01p.mec.i.midamerica
n.com>, "Klinkefus, David S" writes:
>Greetings!
>I had a request for any RFC, or reference material, that says 
>that a zone MX record must point at a host record instead of an IP address.

RFC1035, section 3.3.9:

3.3.9. MX RDATA format

    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                  PREFERENCE                   |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   EXCHANGE                    /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where:

PREFERENCE      A 16 bit integer which specifies the preference given to
                this RR among others at the same owner.  Lower values
                are preferred.

EXCHANGE        A <domain-name> which specifies a host willing to act as
                a mail exchange for the owner name.

(This is from the on-wire definition of the MX record, but logically, the
text representation has to correspond.)

>We discovered a zone that had just the IP address in the MX record,
>and we couldn't do a forward lookup on that IP address to trade e-mail.
>Our firewall sendmail process saw the IP address as a sub-domain name
>because
>of the dots in the address.

It's a common misconfiguration.

Ashley Kitto
Nominum Support & Training
http://www.nominum.com