Tony Finch wrote:
>> Perhaps, it is a misunderstanding of a suggestion of rfc974:
>>
>> 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.
>
> No, that's about MX pointing at CNAME,
> CNAME pointing at MX is a different problem,
There is an interesting history of confusion. See below.
> The requirement in RFC 1123 is a restatement of
> RFC 821 section 3.7 (last paragraph) and page 30 (penultimte paragraph).
At that time, there was no MX-like mechanism in domain names
and rfc819 discusess domain->address mapping only. A mail
to a domain is delivered to domain's address.
As such, rfc974 specifies:
It is possible that the list of MXs in the response to the query will
be empty. This is a special case. If the list is empty, mailers
should treat it as if it contained one RR, an MX RR with a preference
value of 0, and a host name of REMOTE. (I.e., REMOTE is its only
MX).
Thus, what rfc821 prohibits is alias->address, CNAME pointing to
A, with an obvious reason to prevent mail loops, if mail software
is not robust enough not to be able to recognize its identity
involving aliases.
With introduction of MX, what should have been prohibited is
still CNAME pointing to A, that is, MX pointing to CNAME then
to A (as rfc974 mentions), not CNAME pointing to MX (rfc1123).
Masataka Ohta
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