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Re: SMTP store and forward requires DSN for integrity (was Re:Cluelessanti-virus )

  • From: Jon Lewis
  • Date: Sat Dec 10 11:50:08 2005

On Sat, 10 Dec 2005, Douglas Otis wrote:

With the high prevalence of viruses having a forged return-path, the
concern is largely about _false_ detections.  These are not actual
numbers, but perhaps more realistic than figures suggested previously.
Imagine the false positive error rate for an email AV filter runs about
1 in 1000 malwares.  While indeed this may not be a tragedy having a few
valid emails lost without notice in an AV effort, this loss is not
required when "valid" DSN recognition is deployed.
The loss is also not required when virus/malware scanning is done during the SMTP conversation. Google for QHPSI. Messages don't have to disappear and bogus DSNs don't have to be sent. People just need to modernize their MTAs.

The AV filter then bounce technique has been used for many years, where
DSNs must be filtered at the DSN recipient.  Rather than seemingly
Like many other things on the internet, just because it's been in place for many years doesn't mean its a good idea or still a viable system.

will also recover the valid 1 in 1000 DSNs.  This BATV automation would
also ensure no DSNs with forged return-paths, created at any point where
acceptance criteria differs between MTAs, will be accepted before the
data phase.  BATV should be almost as effective as a DNS-BL.  You can
even use automate BATV refusals by others to add to your own temp BL.
That still leaves "our" (the people not sending bogus DSNs) systems having to do lots of work (validating signitures) to decide how to handle DSNs that should never have been sent.

Interesting that you should mention DNSBLs. I've seen people asking for DNSBLs of bogus DSN senders for years. I hope the integration of AV filtering and MTAs will improve before we see widespread use of bogus DSN sender DNSBLs. Unfortunately, for some people, experiencing pain is the only way they can be convinced to clean up their problems.

----------------------------------------------------------------------
Jon Lewis | I route
Senior Network Engineer | therefore you are
Atlantic Net |
_________ http://www.lewis.org/~jlewis/pgp for PGP public key_________




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