A couple of months back, our little server came under massive load; this had happened before, and normally in those situations it was a joe-job attack. Switching off all filtering and just collecting the targeted domain’s mail in a buffer for later processing would work to ameliorate the problem, by allowing the load to “drain”. Not this time, though.
Instead, when I turned off the filtering, the load was still too high — the massive volume of spam (and spam blowback / backscatter) was simply too much for the Postfix MTA. The MTA could not handle all the connections and SMTP traffic in time to simply collect all the data and store it in a file!
Looking into the “attack” afterwards, once the load was back under control, it looked likely that it wasn’t really an attack — it was just a volume spike. Massive SMTP load, caused by spammers increasing the volume of their output for no apparent reason. (Since then, spam volumes have been increasing still further on a nearly weekly basis.)
This is the effect of botnets — the amount of compromised hosts is now big enough to amplify spam attacks to server-swamping levels. Our server is not a big one, but it serves less than 50 users’ email I’d say; the user-to-CPU-power ratio is pretty good compared to most ISPs’ servers.
So here’s the thing. New SMTP-based methods of delivering nonspam email — whether based on DKIM, SPF, webs of trusted servers, or whatever — will not be able to operate if they have to compete for TCP connection slots with spammers, since spammers can now swamp the SMTP listener for port 25 with connections. In effect, spam will DDoS legitimate email, no matter what authentication system that legit mail uses to authenticate itself.
This, in my opinion, is a big problem.
What’s the fix? A “new SMTP” on a whole different port, where only authed email is permitted? How do you make that DoS-resistant? Ideas?
(Obviously, counting on spammers to notice or care is not a good approach.)