Slightly OT - MX RR Santity Check requested..

Barry Finkel b19141 at
Thu Mar 29 14:41:12 UTC 2007

"Kevin P. Knox" <bind-users at> 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 SMTP 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
This is a good case for split-views.  The external view has MX records
pointing to your border firewall mailer, and the internal view has
MX records pointing to your internal mailer.  We use split views for
this purpose.
Barry S. Finkel
Computing and Information Systems Division
Argonne National Laboratory          Phone:    +1 (630) 252-7277
9700 South Cass Avenue               Facsimile:+1 (630) 252-4601
Building 222, Room D209              Internet: BSFinkel at
Argonne, IL   60439-4828             IBMMAIL:  I1004994

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