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All the Perl that's Practical to Extract and Report

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  • Disclaimer: I do AV/Anti-spam for ~ 250,000 folk. Many of my e-mail addresses are also plastered all over the 'net, so I get plenty of these bounces too.

    Still, from my perspective, mail must *not* get lost. Failure to deliver a message to it's recipient must *always* generate a bounce message (i.e., an SMTP 5xx error).

    Why? Because I don't trust anti-virus software to always do the right thing. I don't trust anti-spam software to always do the right thing. I don't trust *BLs and local block lists to

    • I fail to see how an automated message saying "A message you didn't send to someone you don't know couldn't be delivered" is useful.

      • I fail to see how an automated message saying "A message you didn't send to someone you don't know couldn't be delivered" is useful.

        It's not. And if you've got an algorithm that can determine when to silently drop mail on the floor with no false positives, I'm all ears. But RFC 2821, s4.2.5 is quite clear on an MTA's responsibilities after it accepts a message. I don't think picking and choosing which bits of an RFC to implement is a good idea.

        Yes, 2821 is in need of an update to deal with today's In

        • If you can detect that the message contains a virus, don't send the virus back. If you can detect which virus the message contains, you can tell whether the virus spoofs e-mail addresses. If it does, don't even send a bounce.

          I gather from the fact that so many of these bounce messages say "Your message tested positive for Sobig" that both points are actually possible — and pratical.

          • If you can detect that the message contains a virus, don't send the virus back.

            Doesn't work if you're trying to save cycles for wanted mail, and rejecting messages based on attachment types, or other content (e.g., the presence of web bugs).

            To be specific, consider three sites, A, B, and C. B has the virus, and is sending mail to C, with forged headers that look like it came from A.

            If C refuses to accept the message (SMTP 5xx), it's B that generates the bounce message to A. The mail logs at B shoul

            • If C accepted the message and silently refused to deilver it then why would B retry? That makes no sense.

              As Schwern pointed out the anti-virus vendors do know which virus is which and they do know which ones spoof sender addresses so of course they shouldn't bounce those ones back to the 'sender'. They should simply say '200 Hmm Yummy' and do nothing more.

              But I have an even simpler rule ... Never generate a bounce response when a virus is detected. Any virus. Ever. By all means have your virus scan

              • Never generate a bounce response when a virus is detected. Any virus.

                As I say -- doesn't work if you're bouncing because of something else in the message (e.g., an attachment type that you don't want to see -- .exe, .pif, etc). This is much simpler to check for than doing a full virus scan, so it runs faster, so it's a better use of resources.

                By all means have your virus scanning software alert the recipient - as a human being, they can eyeball the sender address and decide whether they want to do anyt

                • As I say -- doesn't work if you're bouncing because of something else in the message (e.g., ... .exe, .pif, etc).

                  Nonsense. You are treating messages with those file types as if they contained a virus. So the rule still stands don't bounce it, drop it.

                  If your SMTP server makes a quantitative decision that it can't handle the message (eg: unknown user or out of disk space) then by all means bounce it. On the other hand if your server examines the contents of the message and makes a qualitative decisi

                  • Nonsense. You are treating messages with those file types as if they contained a virus. So the rule still stands don't bounce it, drop it.

                    Not true. It's being treated as 'content not wanted'. There's a large amount of content that's not wanted, and 'has a virus' is just a subset of it. And on a typical day, virus infected content is a tiny fraction of the stuff that gets 5xx'd.

                    If your SMTP server makes a quantitative decision that it can't handle the message (eg: unknown user or out of disk space) then by all means bounce it. On the other hand if your server examines the contents of the message and makes a qualitative decision that it doesn't want to accept it due to your security policy then generating a reply which details those policies helps no-one but a potential attacker.

                    Again, not true. Differentiated response messages make troubleshooting much easier, and assist the technical support at the sender's site in determining whether the problem is at their end or ours.

                    And there's really not that much information you can leak in 70 or so characters that's going to be of any use to an attacked. And, to be clear, I'm not talking about a system that composes a multi page reply and fires it off to the envelope FROM -- I'm talking about a SMTP 5xx response code.

                    If your system doesn't want to handle bounce messages and other adminstrivia, send mail from <> to /dev/null (or just refuse to accept it). That'll generate a double-bounce, and the recipient site is free to choose how it wants to handle double bounces.

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