On 2008-04-05 11:32:10 -0700, Bill McQuillan wrote:
On Sat, 2008-04-05, John C Klensin wrote:
If I correctly understand what you are suggesting, it is that,
if I have
foo.example.com. IN A #.#....
IN AAAA #:#:...
My SMTP client should somehow generate, not the implicit MX
called for by 2821bis (and 2821), which is
foo.example.com. IN MX 0 foo.example.com.
but some sort of MX arrangement that points to the A RR only and
not to the AAAA RR. There is just no way to do that, at least
without putting completely new language in 2821bis that
distinguishes between the interpretation rules for implicit and
explicit MX records (and getting hopelessly entangled with and
overriding RFC 3484).
It seems to me that the above concept of "implicit MX" requires an extra
step that is not dictated by RFC 2821.
No, it's RFC 2821 which requires an extra step which wasn't there in RFC
974 (and isn't there in RFC 3974, either, and isn't performed by at
least one widely deployed MTA (postfix)). This step is
"look up an A record to decide whether an MX record should be
In this concept the implicit MX is automatically synthesized when no
explicit MX was found. Then "address records" would be looked up under
that name (foo.example.com. above).
Extracting the important sentence from 2821, section 5:
If no MX records are found, but an A RR is found, the A RR is treated as
if it was associated with an implicit MX RR, with a preference of 0,
pointing to that host.
My reading of this is: when an MX RR is not found then look for a an A RR
with the same name. If the A RR is found, *treat it as* though it was the
result of previously having found an MX pointing to that host name. Thus
there is no need for a subsequent lookup of both A and AAAA RRs.
More importantly there is no need to do an A lookup at that time. Just
synthesize the MX and be done with it. The A lookup may fail later, but
that can also happen for any hostname contained in an MX record. For an
IPv6-only host there is no reason to do an A lookup at all.
_ | Peter J. Holzer | It took a genius to create [TeX],
|_|_) | Sysadmin WSR | and it takes a genius to maintain it.
| | | hjp(_at_)hjp(_dot_)at | That's not engineering, that's art.
__/ | http://www.hjp.at/ | -- David Kastrup in comp.text.tex
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