At 00:44 25-01-2009, Al Stu wrote:
"When a domain name associated with an MX RR is looked up and the
associated data field obtained, the data field of that response MUST
contain a domain name. That domain name, when queried, MUST
return at least one address record (e.g., A or AAAA RR) that gives
the IP address of the SMTP server to which the message should be directed."
Correct. And when a that domain name is a CNAME pointing to an A RR
the query returns not only the alias but also the real name and the
IP address from the A RR. Thus meeting the requirements to "return
at least one address record (e.t., A or AAAA RR)". But yet ISC
seems to find it necessary to throw a message that it is "illegal",
when it clearly is not.
That's a liberal interpretation of the specifications and it's the
opposite of the intent of the quoted paragraph. Implementors are
expected to query for an address record only. Any other behavior
such as the one described in your second paragraph is
undefined. Further reading of that section elaborates on what to do
if a CNAME is returned and there is a reference to RFC 2181 for a
discussion of the prohibition of CNAMEs on the right-end side. RFC
974 specifies the algorithm to build the list of RRs and discusses
about possible issues. It's the same algorithm in RFC 2821 and RFC 5321.
The confusion about CNAMEs in MX records stems from the
interpretation of the text about how CNAMEs on the left-hand side are
handled and that was clarified in the latest revision of the specifications.
Regards,
-sm
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