Hi all!
The recent MS viruses has been bothersome for quite a few of us, I
presume, because of the noise it creates. I have configured my Exim4
install to reject MS executables at SMTP-time, so I don't see a lot of
the actual virus.
But I suppose I get a few bounces, like everyone, because MyDoom tries
to send itself to "common name"@some.domain, with a forged return path
it found somewhere. This is rather annoying, so it is important to
configure the servers to avoid it, I figure. If you agree, please read
on! :-)
If I've understood the configuration I have tried to make correctly, if
you reject the virus in the SMTP-dialog, either due to a unknown
username (in the RCPT TO) or because it has a MS executable (in DATA),
that bounce should not go to the address in the return-path or MAIL
FROM: Which is good, because it is trivially forged, and so, a bounce
that goes to the addresses there will often end up at an innocent
third-party.
If, OTOH, you first accept the message, _then_ bounce, the bounce will
go to that innocent third party. So, one shouldn't do that. If the
message is accepted, it is too late to bounce.
I've seen quite a few of these bounces, and since I'm not very
experienced myself, whenever I've seen bounces from Exim4 installs,
I've dropped the postmaster of that domain a note, telling them what
happen, and that this must be an error, and I'd like to discuss it, in
case my install behaves similarly. I haven't had a response from
anyone, though.
But now, I got a bounce from Debian's servers, and I thought "et tu,
Brute":
[EMAIL PROTECTED]
unknown local-part "linda" in domain "debian.org"
------ This is a copy of the message, including all the headers. ------
Return-path: <[EMAIL PROTECTED]>
Received: from gluck.debian.org [192.25.206.10]
by master.debian.org with esmtp (Exim 3.35 1 (Debian))
id 1ApPpz-0005Vz-00; Sat, 07 Feb 2004 04:37:23 -0600
Received: from catv-d5de9094.bp04catv.broadband.hu
(learn-orienteering.org) [213.222.144.148]
by gluck.debian.org with esmtp (Exim 3.35 1 (Debian))
id 1ApPps-00050t-00; Sat, 07 Feb 2004 03:37:18 -0700
So, it is pretty clear that it didn't come from me, eh... :-) But I got
the bounce. Why is this happening?
Well, for one thing, it seems like gluck.debian.org has accepted the
message and sent it on to master.debian.org. Is that the reason? Since
gluck accepted the message, there's nothing master can do about
rejecting it and the bounce wrongly ends up in my mailbox.
gluck is the 2nd MX, as far as I can see, but would the same thing have
happened if master had been handling the message?
My main worry is of course that my own setup does the same thing, so
that my bounces end up in random people's mail boxes. I haven't got a
2nd MX (but I hope to get one soon), and the rejection at RCTP TO I
haven't tweaked, it just rejects unrouteable addresses.
My rule to reject MS executables looks like this:
deny message = $found_extension files not accepted (may contain MS
virus)
demime = com:exe:vbs:bat:pif:scr
and can be found in my ACL config. I believe this should only reject at
SMTP time.
So, under what circumstances could this reject a message after it has
been accepted and result in a bounce to an innocent third-party?
If I get around to get a 2nd MX, would I have to set up this 2nd MX to
reject viruses and spam at SMTP-time, or is this something that could
be left to the primary MX? From the above story, I think it looks like
the 2nd MX would have to handle the rejection, that it can't be handed
over to the primary MX.
Your thoughts on this subject?
Best,
Kjetil
--
Kjetil Kjernsmo
Astrophysicist/IT Consultant/Skeptic/Ski-orienteer/Orienteer/Mountaineer
[EMAIL PROTECTED] [EMAIL PROTECTED] [EMAIL PROTECTED]
Homepage: http://www.kjetil.kjernsmo.net/ OpenPGP KeyID: 6A6A0BBC
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