Re: Strange SMTP Server behaviour

From: Russ (Russ.Cooper_at_RC.ON.CA)
Date: 11/27/03

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    Date:         Thu, 27 Nov 2003 11:20:53 -0500

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    Many people responded to this indicating that it is a result of Cisco's PIX Firewall, or IOS IPFW, feature called "Mailguard".


    Paul Wakeford, Fabio Pietrosanti (naif), and Martin Blackstone all contributed to this message. Also, Brian Bergin provided the following detailed explanation;

    This is no worm.  I'm betting it's the result if improperly configured firewalls.  I would suggest that you're seeing a firewall that's doing SMTP traffic inspection like a Cisco PIX or a Cisco router with IP Firewall installed.  Others may do this too, but I can reproduce this in 20 seconds by enabling one of these commands in our PIX or our 3640 with IP FW:
    fixup protocol smtp 25  (in the PIX)
    inspect name FastEthernet_0_0 smtp (on IOS w/ IPFW)
    ip inspect name Serial_0_0 smtp (on IOS w/ IPFW)
    The proper commands for either PIX or IOS w/ IPFW when you run ESMTP are:
    no fixup protocol smtp 25  (in the PIX)
    no inspect name FastEthernet_0_0 smtp (on IOS w/ IPFW)
    no ip inspect name Serial_0_0 smtp (on IOS w/ IPFW)
    Cisco docs:
    (notice the last statement "Note: If you have an ESMTP server behind the PIX, you may need to turn off the Mailguard feature to allow mail to flow properly. Also, doing Telnet to port 25 may not work with the fixup protocol smtp command, especially with a Telnet client that does character mode.")
    IOS w/ IPFW:
    The problem is Exchange does ESMTP and those commands are not recognized by SMTP traffic inspectors and are therefore purged.  The ones from have properly configured firewalls that either don't do SMTP traffic inspection for their ESMTP servers OR they do ESMTP traffic inspection, the other ones are behind improperly configured firewalls for ESMTP traffic.  Cisco is supposedly due to release ESMTP traffic inspection into IOS and PIX OS software releases in 2004.  I'm sure there are other firewalls out there that don't do ESMTP traffic inspection so I'm guessing this problem isn't limited to Cisco, but IMHO, ESMTP has been out there for too many years for these enterprise-grade firewall vendors to still have no inspection schemes for ESMTP.
    NOTE: Please reply to the list so others may benefit from your thoughts.  If you're concerned it may not make it to the list, please cc: me on the reply.
    Terabyte Computers, Inc.
    Brian S. Bergin
    Russ - NTBugtraq Editor
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