Re: houston.rr.com MX fubar?

[Tony Finch <dot () dotat at>]

On Mon, 14 Jan 2008, Suresh Ramasubramanian wrote:
> On Jan 13, 2008 9:55 PM, Tony Finch <dot () dotat at> wrote:
> > On Sun, 13 Jan 2008, Suresh Ramasubramanian wrote:
> > > One operationally better way to go seems to be Mark Delany's mx0dot
> > > proposal, which started out as an internet draft, but seems to have
> > > lost momentum .. the concept is sound though.
> >
> > Exim implements this convention.
>
> Er, the concept is DNS related .. totally MTA independent.  Simply
> declaring that there is no MX record in a way that stops fallback to
> an A record.

It's slightly more subtle than that. MTAs have to interpret MX records, so
there is plenty of variation in semantics. If an MTA does not implement
the "." convention then it will look up the root's AAAA and A records,
which is stupid but should cause the message to bounce as desired. However
if it does implement the convention (just like the "usage rules" for a SRV
record target of "." in RFC 2782) then it can skip the address lookups and
save the root some work. (It can also produce a better error message.)
This really ought to be explained in draft-delany-nullmx.

Note that an MTA can't rely on its recursive DNS server to populate the
additional section of a DNS reply, because of the truncation rules in RFC
2181. So if the additional section is empty (as it would be for an MX
target of ".") it must explicitly look up the address records to find out
if they are really missing or were just truncated. So it's worth
implementing the "." convention explicitly. (See also
http://www1.ietf.org/mail-archive/web/ietf/current/msg49843.html
for the IPv6 implications of truncated MX records.)

Tony.
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