At 04:55 08-05-2008, Tony Finch wrote:
RFC 2821 says:
If MX records are present, but none of them are usable, this situation
MUST be reported as an error.
This implies that a partially broken RRset is not grounds for rejecting a
message.
In Section 5.1 of draft-10, there is:
"If MX records are present, but none of them are usable, this situation
MUST be reported as an error."
which is exactly as the above.
2821bis says:
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.
Any other response, specifically including a value that will return a
CNAME record when queried, lies outside the scope of this standard.
This implies that it's OK to reject partially broken MX RRsets. Some
deployed software already does this.
I assume that you are referring to the last sentence. Quoting the
sentence following it from the draft:
"Any other response, specifically including a value that will return a
CNAME record when queried, lies outside the scope of this standard.
The prohibition on labels in the data that resolve to CNAMEs is
discussed in more detail in RFC 2181, Section 10.3 [31]."
This is more of a clarification about the CNAME issue that was
discussed previously. It also addresses the case where the MX lookup
returns an IP address instead of a domain name. I don't read that as
a change in MX lookup error handling as the grounds for rejection are
still when none of the RRs are usable.
Regards,
-sm