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Re: rfc2821bis-07

2008-02-21 09:40:44

Alessandro Vesely wrote:
> Derek J. Balling wrote:
>> I know this may be a dead horse, but...
>> "The result of an MX lookup MUST NOT be a CNAME."
>> Can this *please* be slightly reworded? "The RR value of an MX
>> lookup..." perhaps?
It is the compound "MX lookup" that generates ambiguity, as it is
used to indicate the initial lookup of the domain name. Actually,
that's what the whole first paragraph in 5.1 is talking about. If it
were considered a minor change, I'd propose moving that sentence to
the end of the next paragraph, where its rationale can be grasped
more easily.

<techie hat on>

I'm thinking that the problem really lies in the paragraph being so long and covering multiple steps. If it were split apart like this, I think it would be more obvious where each statement applies in the flow:

   Once an SMTP client lexically identifies a domain to which mail will
   be delivered for processing (as described in sections Section 2.3.5
   and Section 3.6), a DNS lookup MUST be performed to resolve the
   domain name (RFC1035 [6]).  The names are expected to be fully-
   qualified domain names (FQDNs): mechanisms for inferring FQDNs from
   partial names or local aliases are outside of this specification.
   Due to a history of problems, SMTP servers used for initial
   submission of messages SHOULD NOT make such inferences (Message
   Submission Servers [41] have somewhat more flexibility) and
   intermediate (relay) SMTP servers MUST NOT make them.

   The lookup
   first attempts to locate an MX record associated with the name.  If a
   CNAME record is found instead, the resulting name is processed as if
   it were the initial name.  If no MX records are found, but an address
   RR (i.e., either an IPv4 A RR or an IPv6 AAAA RR, or their
   successors) is found, the address RR is treated as if it was
   associated with an implicit MX RR, with a preference of 0, pointing
   to that host.

   If one or more MX RRs are found for a given name, SMTP
   systems MUST NOT utilize any address RRs associated with that name
   unless they are located using the MX RRs; the "implicit MX" rule
   above applies only if there are no MX records present.  If MX records
   are present, but none of them are usable, this situation MUST be
   reported as an error.  The result of an MX lookup MUST NOT be a

</techie hat off>

        Tony Hansen

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