(Warning to DNSOP and IESG -- this response is going to descend
quickly into SMTP esoterica and is probably safe to ignore from
a DNS perspective)
--On Friday, July 18, 2014 22:39 +0100 Tony Finch <d...@dotat.at>
wrote:
> John C Klensin <john-i...@jck.com> wrote:
>
>> > MX target names should obey the LDH host name rules.
>>
>> I completely agree, but SMTP imposes no such requirement.
>
> RFC 974 specifies that the target of an MX record is a host
> name.
And so? RFC 2821 obsoleted 974. As far as I can tell, it says
nothing at all about the DATA field in an MX record, leaving
pulling that information out and looking it up in the DNS to be
inferred from context and a parenthetical comment "(perhaps
taken from the preferred MX record)". It does use the term
"host addresses" and "destination host", but in a too-informal
context to assume that people would infer "host name" (aside:
whatever that means) in the DNS sense. After 14+ years, I no
longer remember and cannot reconstruct whether that was a DRUMS
screwup or mine although the WG is ultimately responsible for
not reviewing carefully enough.
When RFC 5321 was done, we obviously knew there was a problem
with the 2821 text because its text improves on the situation.
The pre-5321 change log doesn't say much that sheds light on the
change, other than saying that the description of MX handing
was changed in -10 (one of several post-Last-Call versions).
The relevant section is more clear and it does explicitly say
"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)". But that is it: "domain name".
One can read that as an invocation of the discussion in Section
2.3.5 but that isn't the only possible reading. The section
invokes 1035 and "a sequence of letters, digits, and hyphens
drawn from the ASCII character set." The following sentence
reads "Domain names are used as names of hosts and of other
entities in the domain name hierarchy." so, while 2.3.5 (if one
believes that the text in 5.1 invokes it) requires LDH, it does
not require a "host name". If one doesn't believe that 2.3,5
is invoked, then a domain name is presumably just an FQDN
conforming to 1034/1035 as interpreted by 2181). There is,
fwiw, some additional test in 2.3.5 that further muddies the
waters.
That is definitely not a good example of clarity or precision.
Mea culpa, with or without comments about quality of review. I
have tightened things up in the 5321bis editing copy (yes, there
is such a thing).
>...
>> Does that imply that this spec should contain a warning that
>> loading one of these "null MX" records may require a
>> configuration change in one's nameserver?
>
> No, because "." doesn't violate LDH syntax. (What admin GUIs
> do is another matter...)
Those admin GUIs or text input converters were what I was
concerned about (should have said "nameserver configuration
interface" or words to that effect). Your call (and John L.'s).
john
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