Is Hotmail in the habit of ignoring MX records?

David Miller dmiller at tiggee.com
Fri Jul 27 20:40:18 CDT 2012


On 7/27/2012 9:00 PM, Jimmy Hess wrote:
> On 7/27/12, Tony Finch <dot at dotat.at> wrote:
>> That would be a seriously broken violation of the SMTP specification.
> I would definitely agree it would be quite broken behavior,  but you
> know,  I never said Hotmail's processing wasn't broken --  only that
> they seem to honor MX records in the common case.    If you are doing
> something unusual like    "mail MX    bla bla"
>
> I would say you can't rule that out as a possible cause,  just because
> some RFC suggests it should be OK.
>
> The spec does say that you're not allowed to chain MX records.     But
> i'm not so sure that the specification actually prohibits a SMTP
> server from doing that, if someone
> does try to chain MX records.
>
>  it   may also be out of spec to have a  "MX"  record   point to a
> dns  label  that a MX record exists for  in the first place.

MX records don't "chain".

If they did, then

example.com. 1800 IN MX 10 example.com.

would be an infinite loop.  This isn't an infinite loop and is instead a
perfectly valid configuration.

If you made a DNS query for the MX records for example.com you would get
back an answer that might include:

;; ANSWER SECTION:
example.com.            1800    IN      MX      10 example.com.

;; ADDITIONAL SECTION:
example.com.            1800    IN      A       10.10.10.10

>From RFC 1035:

3.3.9. MX RDATA format

    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                  PREFERENCE                   |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   EXCHANGE                    /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where:

PREFERENCE      A 16 bit integer which specifies the preference given to
                this RR among others at the same owner.  Lower values
                are preferred.

EXCHANGE        A <domain-name> which specifies a host willing to act as
                a mail exchange for the owner name.

MX records cause type A additional section processing for the host
specified by EXCHANGE.  The use of MX RRs is explained in detail in
[RFC-974].

-DMM





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