Dave Crocker wrote:
I keep trying to understand why the SMTP use of AAAA records should be any
different than its use of A records. Haven't heard a solid explanation,
nevermind seen consensus forming around it.
It seems there are two ways of looking at this:
(1) AAAA records in the IPv6 world should do exactly same things as A
records in the IPv4 world, so SMTP should look for an AAAA record in the
absence of an MX record, just as A records are used in the absence of MX
(2) Although some SMTP servers will continue to be found through A
records for legacy reasons, there is no longer a good reason for any new
server not to have a published MX record. SMTP clients (senders) will,
of course, need to continue to look up A records, but since there is
currently no significant use of AAAA records for email routing, we
should not perpetuate this legacy in IPv6 as it is in IPv4.
These are both reasonable positions, but I'm in camp (2). The
additional use of AAAA records for email address resolution would add
complexity to at least some implementations and test cases, and it
something that should never be needed: v6 mail handlers will just
publish MX records. There is probably a small DNS efficiency argument
as well, especially if the MX, A, and AAAA requests are not made together.
I wish that 2821bis made a stronger statement deprecating the use of A
records for email address resolution, but it seems not to have been the
consensus to do so. Writing a separate BCP on this point is not a good
use of anyone's time.
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