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Charlie Martin
Asked: 2011-02-05 14:43:09 +0800 CST

Configuring sendmail to use one outbound MTA exclusively

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I have a sendmail problem, and I'm anything but a sendmail guru -- I could use some help.

My problem is that I have a system intended to be more or less an "appliance" -- it's not intended to have an admin. Because of this, it needs to be able to "call home" by sending email. As we have configured it, this works fine -- using sendmail, it finds the appropriate relay by looking up an MX record and everything works fine.

Now, however, because of security concerns, we want to limit it to using exactly one relay, so for example relay.corp.example.com. Should the user configure it to use, say, fubar.example.com, the mail sending should fail or be deferred.

I thought that by configuring sendmail with a /etc/mail/server.switch file containing

hosts   files

without dns, I'd get that effect. This doesn't work -- instead, if it gets mail addressed to [email protected], it tries to talk directly to example.com, and ignores the configured server.

Any ideas?

Update

Okay, I tied both schemes experimentally, and they both seem to work. Wish I could mark them both correct.

sendmail

2 Answers

  1. Best Answer

    Are you using m4 to build your sendmail.cf? If so, you should add

    define(`SMART_HOST',`relay.corp.example.com')

    to your sendmail.mc and regenerate your sendmail.cf. It looks like adding

    DSrelay.corp.example.com

    right to the sendmail.cf will do the trick too.

    • 6

  2. Here is an M4 file for a SuSE system for simply forwarding ALL email to the relay host.
    (Its included in the SuSE distribution as well). You simply do m4 x.mc >x.cf where x.mc is the file below and x.cf would be the file to replace your sendmail.cf file (usually in either /etc/sendmail.cf or /etc/mail/sendmail.cf).

     divert(-1)
 #
 # Copyright (c) 1999 SuSE GmbH Nuernberg, Germany.
 # Copyright (c) 2006 SuSE LINUX Products GmbH, Germany.
 # Author: Werner Fink
 # Please send feedback to http://www.suse.de/feedback/
 #
 # This is a special case -- it creates a stripped down configuration
 # file containing nothing but support for forwarding all mail to a
 # central hub via a local SMTP-based network.  The argument is the
 # name of that hub.
 #

 include(`/usr/share/sendmail/m4/cf.m4')
 divert(0)dnl
 VERSIONID(`@(#)Setup for SuSE Linux     0.2 (SuSE Linux) 2002/01/14')
 dnl
 dnl  This is the default configuration for SuSE Linux.
 dnl  See /usr/share/sendmail/ostype/suse-linux.m4
 dnl  and /usr/share/sendmail/README for more information.
 dnl
 OSTYPE(`suse-linux')dnl
 dnl
 dnl  A stripped down configuration. Replace the mailhub.domain.notused
 dnl  with your real mailhub.
 dnl
 FEATURE(`nocanonify')dnl
 FEATURE(`nullclient', `relay.corp.example.com')dnl
 dnl
 dnl  This line is required for formating the /etc/sendmail.cf
 dnl
 LOCAL_CONFIG
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