BIND 9 - Reverse lookups made by SMTP servers fail

Kevin Darcy kcd at
Tue Oct 24 20:51:50 UTC 2006

SZABO Stefan wrote:
> Hello,
> Our 2 Bind servers encounter problems to answer reverse requests made by
> SMTP gateways.
> The 2 dns servers are not in the same version (but both are 9.2.x).
> Reverse lookups fail on MX records, sometimes. 
What do you mean "reverse lookups fail on MX records"? MX records map 
mail domains to mail exchangers; there is no "reverse" mapping back from 
the mail exchangers to the mail domains that refer to them.
> In addition, the problem
> is random, and hard to reproduce on demand.
Then you might need to start a long-running network capture, and go back 
and analyze the results once you see a failure.
> SMTP gateways then consider that DNS did not anwser, as if the DNS
> did not send any answer to the reverse lookup request.
> Root Servers file is up to date.
> In Event Viewer , RCODE SERVFAIL and REFUSED messages can be found, but
> I'm not
> sure that those event are related to failed reverse lookups.
Correlate those error messages with the failures in the SMTP logs. 
Generally speaking, SERVFAIL is fairly common, but REFUSED is much less 
so, so maybe those would be worth following up, if you can associate one 
of the REFUSED messages with a failed lookup from the SMTP perspective.

                        - Kevin

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