There was some discussion a while back about tarpitting. If you don't know what
that is (I didn't when it first came up), it's the process of inserting a small
sleep in an SMTP session for each RCPT TO after some set number of RCPT TOs.
The idea is to thwart spammers who would hand your SMTP server a single message
with a long list of RCPT TOs.
The subject originally came up in a discussion of ways to run an open relay
safely (I didn't suggest it, and I don't do that kind of thing), but it could
also be useful in keeping your own dial-up customers from using you as a spam
relay.
I've made a simple patch to qmail-smtpd to allow it to do tarpitting. There are
two control files involved: control/tarpitcount and control/tarpitdelay.
tarpitcount is the number of RCPT TOs you accept before you start tarpitting,
and tarpitdelay is the number of seconds of delay to introduce after each
message. tarpitcount defaults to 0 (which means no tarpitting), and tarpitdelay
defaults to 5. If NOTARPIT is set in the environment (perhaps by tcpserver)
then no tarpitting is done. (I had considered doing this the other way
around--no tarpitting would be done unless TARPIT was set, irrespective of
control/tarpitcount. Any suggestions on this point?)
If anyone is interested, it's at http://www.palomine.net/qmail/tarpit.patch.
I'm not vouching for the effectiveness of doing tarpitting or whether it's a
good thing to do to your customers, but there was some interest in it, so there
it is.
Chris
