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Re: disabling SMTP

  • From: Richard Welty
  • Date: Mon Mar 29 07:56:06 2004

On Mon, 29 Mar 2004 07:20:47 -0500 Rob Nelson <> wrote:
> Richard Welty wrote:
> >when smtp fixup is on (default on many older pixes, i gather that there
> >may be some improvements on newer pixes), the smtp banner
> >is mostly obscured by * characters. the intent is a classic security
> >by obscurity play, to hide the type and verison of the MTA behind
> >the pix.

> Okay, so this is a problem when an SMTP server is hosted behind the PIX?


> I 
> thought the fixup statements were for outbound connections, and with it on 
> right now I get the full banner from SMTP servers. I don't host an SMTP 
> server myself, so can't check that.

nope, they mangle inbound connections too.

in addition to the banner obscuration, i (and others) have seen patterns of
intermittant, arbitrary disconnections of SMTP sessions when fixup is turned
on. this is harder to diagnose, though, because there is a TCP bug in some
variants of Outlook that causes similar behavior. those of us running exim
as an MTA a couple of revs back had to patch our installs to work around
the Outlook TCP bug. i believe that patch is now permanently part of exim,
as it is unlikely that the Outlook bug will ever entirely go away.

Richard Welty                               
Averill Park Networking                                         518-573-7592
    Java, PHP, PostgreSQL, Unix, Linux, IP Network Engineering, Security

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