On Thu 22/Jan/2015 02:27:02 +0100 Justin Vallon wrote:
> I am having a "discussion" in a support ticket about an MX record
> pointing to a CNAME.
>
> Searching for references for MX and CNAME leads to RFC 2181:
>
> {{{
> 10.3. MX and NS records
>
> The domain name used as the value of a NS resource record, or part of
> the value of a MX resource record must not be an alias. Not only is
> the specification clear on this point, but using an alias in either
> of these positions neither works as well as might be hoped, nor well
> fulfills the ambition that may have led to this approach. This
> }}}
>
> What is confusing here is the "Not only is the specification clear" - is
> this a reference to another RFC?
The current SMTP specification refers to the above for "more detail" from
Section 5.1 http://tools.ietf.org/html/rfc5321#section-5.1
Note that the domain part of an address can be a CNAME, as explicitly allowed
in the second paragraph of Section 5.1.
My understanding of why CNAMEs are prohibited for MX hosts is that they can
introduce loops. The last paragraph of Section 5.1 explains how a sender
should attempt to locate itself in the list of MXes, ordered by preference.
You may want to compare that paragraph with the historic discussion in RFC 974,
which, under "Minor Special Issues", says:
Note that the algorithm to delete irrelevant RRs breaks if LOCAL has
a alias and the alias is listed in the MX records for REMOTE. (E.g.
REMOTE has an MX of ALIAS, where ALIAS has a CNAME of LOCAL). This
can be avoided if aliases are never used in the data section of MX
RRs.
hth
Ale
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