Slightly OT - MX RR Santity Check requested...

Merton Campbell Crockett m.c.crockett at
Thu Mar 29 03:14:31 UTC 2007

Why is "" even defined?  Remove it.  It's  

Configure mailertable, smarthost, etc. on ""  
to relay mail for "" to "".

Why waste other people's time and resources attempting to send mail  
to "firewalled" that is not reachable?

Merton Campbell Crockett

On 28 Mar 2007, at 18:50:24, Mosemann, Russell wrote:

> Lowest preference value, not lowest preference. You are blocking  
> the MX
> server that all mail servers are required to contact.
> --
> Russell Mosemann, Ph.D.
> Associate Professor of Computer Science
> From: Kevin P. Knox
> Sent: Wednesday, March 28, 2007 8:35 PM
> It's the high pref that's not.
> IN      MX      5
> IN      MX      10
> Pref 10 is reachable.  Pref 5 is only reachable via TCP/25 by
>, which is configured to relay SMTP for the
> domain.
> A very, VERY few sending SMTP servers never even try to query the  
> for
> ... Kev
> On Wednesday 28 March 2007 09:23 pm, you wrote:
>>> You mean "their" configuration is broken?  The sending mail server
> is NOT
>>> ours.  We're on the receiving end.
>> 	No.  Your configuration is broken.  The lowest preference
>> 	MXs MUST always be reachable.  You cannot depend upon
>> 	fallback to higher preference MXs.  The sending side is not
>> 	required to try them.  It is required to try all the lowest
>> 	preference MXs.
>>> Our MX RRs are not empty.  We've got two MX RRs for the domain in
>>> question. Pref 5 is an internally firewalled server.  Pref 10 is a
>>> (world reachable) SMTP server.  99% of the SMTP servers sending us
> mail,
>>> query the MX RRs, select the most preferred, but the connection
> times out
>>> because THAT MX host isn't world reachable.  So they fall back to
> our
>>> pref 10 mailserver, which spools and delivers to the firewalled
>>> mailserver.
>>> ...except for a few, which I'm trying to narrow down to some common
>>> factor.
>>> RFC 2821 section 5 certainly strongly suggests that mail transport
> agents
>>> support multiple MX records.
>>> ... Kev
>> 	A minimal SMTP client only has to try all the lowest preference
> MXs.
>> 	Your lowest preference MXs are unreachable.  That make you
> wrong.
>> 	[ I don't know why anyone would write a minimal SMTP client
>> 	  as a minimal SMTP client still has to be prepared to try
>> 	  multiple hosts and it really is not much more work to add
>> 	  all the hosts in the RRset to the list of servers to try.  ]
>> 	Mark
>>> On Wednesday 28 March 2007 08:04 pm, Mark Andrews wrote:
>>>>> I've encountered a specific problem FOUR times in the past six
> months
>>>>> now and
>>>>> am kindly asking Bind-Users for some insight.
>>>>> The problem is sending SMTP servers that don't ever query past
> the
>>>>> first (hi pref) MX RRs.  The first time we encountered this
> problem,
>>>>> it was with an e-mail list server appliance (don't know the
> exact
>>>>> type/make/model) at a local university in our area.
>>>>> The second and third times were with new MS Exchange servers.
>>>>> Now today, I'm working on the same problem with a domain who's
>>>>> services are hosted by Network Solutions Inc. (NSI).
>>>>> We use a strategy whereby our lowest numbered (high pref) MX RR
> is a
>>>>> firewalled host.  The higher numbered (lower pref) MX RR
> designates
>>>>> our DMZ SMTP server, which handles e-mail on behalf of the
> server in
>>>>> the other MX RR.
>>>>> The DMZ SMTP server is world reachable on TCP/25.  It's straight
> out
>>>>> of the ORA Nutshell book, "Building Internet Firewalls".  We
> process
>>>>> 4 million messages per month, so I'm pretty sure that other
>>>>> organizations are still using MX and firewalls to force mail
> through
>>>>> the DMZ SMTP server, and then deliver back to a better protected
> mail
>>>>> server.
>>>>> I've verified that the sending SMTP server only ever queries the
>>>>> first (low numbered - high pref) MX RR.  After that...NOTHING.
> It
>>>>> never tries the second.
>>>>> The net result is that the sender (in this case) will queue SMTP
>>>>> traffic for our domain indefinitely....because they never look
> up MX
>>>>> RRs any lower than the highest pref MX RR.
>>>>> Has anybody else run into this lately?
>>>>> For the curious....   YES!  We plan on configuring transports in
>>>>> place of the
>>>>> old Firewall/MX strategy on our Postfix servers ASAP.
>>>>> Thanks in advance. :-)
>>>>> ... Kev
>>>> 	Your configuration is broken.
>>>> RFC 974
>>>>    If the list of MX RRs is not empty, the mailer should try to
> deliver
>>>>    the message to the MXs in order (lowest preference value tried
>>>>    first).  The mailer is required to attempt delivery to the
> lowest
>>>>    valued MX.  Implementors are encouraged to write mailers so
> that
>>>> they try the MXs in order until one of the MXs accepts the
> message, or
>>>> all the MXs have been tried.  A somewhat less demanding system, in
>>>> which a fixed number of MXs is tried, is also reasonable.  Note
> that
>>>> multiple MXs may have the same preference value.  In this case,
> all MXs
>>>> at with a given value must be tried before any of a higher value
> are
>>>> tried.  In addition, in the special case in which there are
> several MXs
>>>> with the lowest preference value,  all of them should be tried
> before a
>>>> message is deemed undeliverable.

Merton Campbell Crockett
m.c.crockett at

More information about the bind-users mailing list