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Preventing Relays with Sendmail

Posted on 1998-12-10
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Last Modified: 2010-03-18
I have a small LAN, where the computers on the inside use the firewall to send messages.  While this works exactly as configured, I wish to minimizer my chances of someone discovering that the port is opened and using the server to SPAM others.  

Anyone can offer insite on the .M4 entries to place in the sendmail.m4 file?  I have been given direct modifications to do with sendmail.cf, but I would much rather have the appropriate M4 entries.

I would of posted my M4 file.  But since it contains information on the mail server location and addess I decided that it might be a security risk.  It can be given at a later time if the need for it becomes evident.
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Question by:mfoster
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by:wqclatre
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If you upgrade to sendmail version 8.9 forwarding of SMTP messages is not permitted.

http://www.sendmail.org/tips/relaying.html
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by:mfoster
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That's exactly what I did.  Allowed me to add the necessary changes VIA M4 instead of actually having to modify the actualy .cf file.   Mind you I could of done it.  But I was in no mood to completely botch up sendmail by trying to decipher what it did.  
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davidmwilliams earned 100 total points
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 Make the upgrade to sendmail 8.9.1 and then just rebuild your cf file from your existing m4 file.  You won't need to touch your current m4 file at all - just build a new cf file from it.  By default, sendmail 8.9 does not permit relaying - this option must be switched off explicitly to permit it.
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