similar to greylisting, but not quite the same: with greylisting you would
always reject the first delivery attempt with a temporary failure. with the
hope that spammers won't retry their deliveries. the spams i've been receiving
seem to be quite consistent though. and we would keep rejecting them anyway.
we slow down the spammy connections to waste some of the resources of a
spammer. this may slow their campaigns down a bit, leaving a bit more time to
take measures.
we do the same with connections that have their 3rd authentication failure,
typically password guess attempts.
when we accept a message by a first-time sender, we sleep for 15 seconds before
actually delivering them. known-good senders don't have to wait. if the message
turns out to be a spammer, at least we've consumed one of their connections,
and they cannot deliver at too high a rate to us because of the max open
connection limit.
limiting is done based on remote ip's, with 3 ip mask variants to limit networks
of machines. often with two windows, enabling short bursts of activity, but not
sustained high activity. currently only for imap and smtp, not yet http.
limits are currently based on:
- number of open connections
- connection rate
- limits after authentication failures. too many failures, and new connections will be dropped.
- rate of delivery in total number of messages
- rate of delivery in total size of messages
the limits on connections and authentication failures are in-memory. the limits
on delivery of messages are based on stored messages.
the limits themselves are not yet configurable, let's use this first.
in the future, we may also want to have stricter limits for senders without any
reputation.
and change thunderbird autoconfiguration to use it.
unfortunately, for microsoft autodiscover, there appears to be no way to
request secure password negotiation. so it will default to plain text auth.
cram-md5 is less secure than scram-sha-*, but thunderbird does not yet support
scram auth. it currently chooses "plain", sending the literal password over the
connection (which is TLS-protected, but we don't want to receive clear text
passwords). in short, cram-md5 is better than nothing...
for cram-md5 to work, a new set of derived credentials need to be stored in the
database. so you need to save your password again to make it work. this was
also the case with the scram-sha-1 addition, but i forgot to mention it then.
the idea is that clients may not support SCRAM-SHA-256, but may support
SCRAM-SHA-1. if they do support the 256 variant, they'll use it.
unfortunately, thunderbird does not support scram-sha-1 either.
named "traceauth" and "tracedata".
with this, you can (almost) enable trace logging without fear of logging
sensitive data or ddos'ing your log server.
the caveat is that the imap login command has already printed the line as
regular trace before we can decide it should not be. can be fixed soon.
