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Using Statistics to Cause Spammers Pain
mlamb writes "Statistical mail classifiers like PopFile save time on the part of their users, but don't do anything to actively combat spam. I just published an article that suggests a way to use classifier output against a spammer while they're connected to your SMTP server, and I'm launching a project called TarProxy to implement it."
Most mail servers will only forward mail from users of their own domain. If the mailserver is sending spam for one of their legitimate users, I feel no pity for them if their server slows down.
If they forward mail from anyone who sends them mail, then they are an open relay, and again, they deserve what they get for leaving an open relay up.
Two things here. First, this article wasn't about preventing spammers from using your SMTP server as a relay, but in slowing down the reception of mail at the end-point SMTP server. This will ripple up the chain to hurt the spammers by slowing down the relays they use. Second, it doesn't matter whether I get 10 spam emails or 10,000. One of the goals of TarProxy is to be ubiquitous. I may only receive 10 spammy emails, but my running instance of TarProxy will determine that those are of sufficient spamminess to throttle bandwidth to each of those connections. At the same time, you're doing the same on your SMTP server, and Joe over there is, and so is Susie, and so on. If everybody (defined as "a large number of smtp servers", and not necessarily "everybody") is running such a service, the spammers will be hurt. You're right that a single individual using this won't make much difference, but that didn't seem to be the goal of the article.
In his article he actually does address this very question. He even gives, what I feel at least, is an interesting answer.
So, you don't run an open relay. You're not going to slow down the spammer directly, but you will slow down all the connections that come from that open relay to your mail server. For a particularly abused open relay, that could lead to such problems that the admin of that open relay will finally get a clue and look in to configuring their server properly.
Hence, a cascading effect that will eventually harm the spammers. Admins of open relays that get a clue will tighten their servers, thus depriving the spammers of one more relay they can abuse.
what if the spammer sends a message to a (good) SMTP server which haven't got the system, and the SMTP server in turn tries to deliver the "spammail" to the right SMTP server, won't that hurt the good SMTP server, who just tries to do it's job?
The situation you're describing is called relaying.
If you start with the assumption that spammers are evil, then the logical conclusion is that there is no such thing as a "good" SMTP server that would relay mail on a spammer's behalf. Servers that do are either in collusion with the spammer, or are mis-configured to allow anonymous relaying. A server that willingly acts in collusion with evil is, by definition, evil. The level of stupidity necessary to allow your sever to act as an open relay also, by definition, precludes being considered a "good" server.
So the short answer to your query is that it's a non-issue. A truly good server will, by definition, never relay spam!
If these tarpits were ubiquitous, they could completely change the economics of spam, creating a scarcity of bandwidth experienced only by spammers.
/dev/kqueue under FreeBSD, for example; and you can do the same, but with a bit more CPU wasted, using plain old select() on almost any Unix.
:(
Err, I don't think so. This just requires spammers to use more simultaneous connections to overcome the slowdown; it doesn't really increase their network requirements much, only their host CPU requirements. 20,000 simultaneous TCP connections from one process is quite possible with
I also don't understand the rationale behind processing the message incrementally. Why not just do your processing before sending back the final 2xx response to the DATA command? Most spam software does not hang up right after sending the final "\r\n.\r\n" from what I've heard from people who run tarpits.
How about this instead: when you are confident you are receiving spam, you stop reading from the socket entirely, and send perhaps 10MB of data back on the other side of the connection. (If the other endpoint isn't reading, and consequently you can only send one window worth of data, then do something to get your TCP stack to generate a lot of useless ACKs, or send your trash back one octet at a time and push between them, or something.) The intent being that sending spam to a large number of MTAs configured in this manner rapidly just becomes a way to DDOS *yourself*. Probably this is too disruptive for most sites to want to bother implementing, though
I don't know exactly what the profit margin for spammers is like, but I'm not convinced a small multiplier in network costs is going to matter. Anyway, a lot of these "countermeasures" are mostly going to hurt maintainers of open relays, but if that means they actually fix them, I suppose that is almost as good.
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