Is Hotmail in the habit of ignoring MX records?

Mon Jul 30 01:46:44 UTC 2012

On Thu, Jul 26, 2012 at 10:45 PM, Mark Andrews <marka at isc.org> wrote:
> In message <B59A4092-CE2F-44E4-84F9-77C18493AD95 at kapu.net>, Michael J Wise writ
> es:
>> And maybe an endless loop for an MX lookup might be what is causing =
>> hotmail to panic and throw out the MX records.
>
> You don't lookup MX records for MX targets.  This is basic MTA
> processing.

Correct. An MX record points to a label containing one or more address
records. It does not chain. In principle the MX record could point to
a CNAME record which then chains until it reaches an address record
but I wouldn't depend on such a configuration working correctly. Ditto
the MX lookup fetching a CNAME which chains until it reaches a label
with an MX record.

> You don't depend on ALL (ANY) returning MX records as they may not
> be in the cache.  You need to make a explict MX query you get no
> MX records are returned in response to a ALL query.

Also correct.

> If the MX lookup fails, as apposed to returns nodata, you don't
> lookup the A/AAAA records and synthesis a MX record.  You treat it
> as a soft error and queue for retry later.  Again this is basic MTA
> processing.

Maybe. In principle this is correct but as you wander through various
bits of software in the name lookup process (which often consults more
than just the DNS -- even today DNS isn't the only game in town) it's
pretty easy to lose track of the difference between lookup failure and
success:no data.

Think about it... how is the MTA to respond if the primary lookup
reports success:no data (e.g. /etc/hosts) but a second tier lookup
(e.g. DNS) reports lookup failure? What if DNS is third tier and the
second tier is some kind of CIFS or NIS lookup which fails? Or reports
success:no data. Or the DNS gets translated through a middleman (like
NIS) which doesn't preserve the difference between fail and success no
data. Does the whole lookup fail because part did? Gets ambiguous.

Further, falling back to the address lookup in the absence of MX
records is correct behavior for an MTA.

What *should* happen here is that the guy's web server should reject
the port 25 connection (an SMTP soft fail condition) and on the next
retry hotmail should find the MX record and follow it.

Either way, I think I'd have to consider this -advanced- MTA
processing. You have to really know your stuff to get this one right.

Regards,
Bill Herrin

-- 
William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
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