Fallback MX host and custom mailer interaction

David F. Skoll

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Feb 10, 2013, 3:29:23 PM2/10/13

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Hi,

I have hit a weird edge-case in Sendmail and wonder if anyone has
some advice.

I have this in my sendmail.cf (generated from sendmail.mc):

O FallbackMXhost=[colo3vs1.roaringpenguin.com]

which usually works very well. However, for one domain, I have this
in /etc/mail/mailertable:

example.com esmtp8889:[email.example.com]

where esmtp8889 is a custom SMTP mailer that delivers on port 8889:

Mesmtp8889, P=[IPC], F=mDFMuXa, S=EnvFromSMTP/HdrFromSMTP, R=EnvToSMTP, E=\r\n, L=990,
T=DNS/RFC822/SMTP,
A=TCP $h 8889

So here's the problem: If email.example.com happens not to be reachable
on port 8889, Sendmail falls back to colo3vs1.roaringpenguin.com as it
should. But it tries to connect on port 8889 rather than port 25. It seems
that the esmtp8889 mailer is "sticky".

Is there any way to specify which mailer to use for the FallbackMXhost
regardless of what is in the mailertable?

Regards,

David.

Message has been deleted

Andrzej Adam Filip

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Feb 12, 2013, 5:41:32 PM2/12/13

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You may add F=0 flag to esmtp8889 mailer flags.

The flag turns off MX lookups (>=8.8) and additionally FallbackMXhost
use (>=8.12.2) according to RELEASE_NOTES. You seem to turn off MX
lookups anyway in the mailertable entry.

David F. Skoll

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Feb 13, 2013, 2:24:29 PM2/13/13

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Andrzej Adam Filip wrote:

> You may add F=0 flag to esmtp8889 mailer flags.

> The flag turns off MX lookups (>=8.8) and additionally FallbackMXhost
> use (>=8.12.2) according to RELEASE_NOTES. You seem to turn off MX
> lookups anyway in the mailertable entry.

But I *want* it to use the FallbackMXHost, only on port 25 instead of
port 8889. I want to be able to specify the FallbackMXHost as:

esmtp:[colo3vs1.roaringpenguin.com]

but it seems that's not possible. :(

Regards,

David.

Carl Byington

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Feb 13, 2013, 2:53:30 PM2/13/13

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-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On Wed, 13 Feb 2013 14:24:29 -0500, David F. Skoll wrote:

> But I *want* it to use the FallbackMXHost, only on port 25 instead of
> port 8889. I want to be able to specify the FallbackMXHost as:

> esmtp:[colo3vs1.roaringpenguin.com]

O FallbackMXhost=[127.0.0.25]

Then run a localhost port redirector (xinetd will work) to forward
127.0.0.25:25 to colo3vs1.roaringpenguin.com:25, and also forward
127.0.0.25:8889 to colo3vs1.roaringpenguin.com:25

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David F. Skoll

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Feb 13, 2013, 5:29:27 PM2/13/13

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Carl Byington wrote:

> O FallbackMXhost=[127.0.0.25]

> Then run a localhost port redirector (xinetd will work) to forward
> 127.0.0.25:25 to colo3vs1.roaringpenguin.com:25, and also forward
> 127.0.0.25:8889 to colo3vs1.roaringpenguin.com:25

Well, yeah. Or I could just get colo3vs1 to listen on port 8889 and
port 25. That's be easier because the real Sendmail listens on *:25
which makes it annoying to have something else listen on 127.0.0.25:25.
(I think... haven't actually tried...)

In case you're wondering what the heck I'm trying to achieve :), we
run a hosted spam filtering service and we let our customers deliver
mail to non-standard ports. Under the hood, we generate a new
sendmail.cf file whenever a new non-standard port appears. [I realize
we'll hit trouble when more than about 12 different non-standard ports
are needed, but so far that's not an issue.]

We have a fallback host to handle the not-uncommon situation of a customer
back-end server being down... we don't want to clog the queues on our
main scanners.

So I think my strategy will be to make Sendmail on the fallback machine
listen on all the non-standard ports that our customers use.

Regards,

David.