News Group Posts

Sendmail DNS lookups

From: richard@ starjump.org (Richard Thomas)
Newsgroups: comp.unix.sco.misc
Subject: Re: Default Route being invoked by unknown process
Date: Tue, 08 Jun 1999 11:23:55 GMT
Message-ID: <375cfa34.13362752@nflintra> 
References: <375c484c.2518134@news.dial.pipex.com> 

On Mon, 07 Jun 1999 22:32:13 GMT, BobD@dial.pipex.com (Bob Dawson)
wrote:

>We have a 5.0.4 system with supplement loaded as a server in a small network of
>approx 20 PC's. We have installed a 3com ISDN Lan Modem, and set this up as a
>default route, so that request outside the network go via this to the Internet.
>
>We have enabled DNS on our server, and declared the IP addresses of our PC's,
>and other peripheral devices (one or two test servers, and a Specialix MTS).
>
>For some unknown reason, the amount of times the ISDN line comes up is
>excessive, and it appears that some proces on the server cannot resolve an IP
>address, so it looks to the default gateway to try to resolve it, hence the line
>comes up. Tracking down this errant proces is proving something of a headache,
>and I wonder if others have experienced a similar problem, and have a solution
>in identifying the culprit.

<snip>

Are you running sendmail? If so, the domain parts of any messages sent

checked in DNS as soon as they are sent by default. If you have the 
external mailer prog (the one used for internet addresses) flagged as
an expensive mailer, and sendmail set to queue "expensive" mail
for later delivery, you get the following "interesting" situation:

1. User sends email to an address that isn't in the local DNS
2. Sendmail tries to lookup address in DNS
3. DNS refers query to external server
4. Internet connection opened (modem, ISDN or whatever)
5. Address verified
6. Message is sent to sendmail queue
7. When sendmail processes its queue (once an hour by default),
   internet connection is opened again and message is sent

To stop spurious connections while retaining periodic connection
to send email, find the line in sendmail.cf that tells it to do the
lookup, comment it out, and restart sendmail. (Clue: it's in
Ruleset 3, and it has a set of square brackets in the rewriting
rule!!).

The downside of this fix is that domain errors in addresses
won't be spotted and returned to the user until the queue
is run (up to 59 minutes from when the message was sent).

Of course, if you aren't running sendmail, this won't help you,
but hopefully it's a usefull addition to the dejanews archive.

Richard.

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