Jeremy Hylton : weblog : 2003-08-24

Blocking SOBIG.F with dynamic firewall updates

Sunday, August 24, 2003

Martijn Pieters came up with an excellent solution to get the mail flowing through (aka When a host tries to send too many copies of the SOBIG virus, block it from making any more connections with a Linux firewall rule. has been getting crushed by the SOBIG.F worm. There are so many machines attempting to deliver the virus to a address, that nothing is getting through. The bounces and virus notification messages are almost as bad. They just clog the server with useless mail.

We struggled coming up with a good solution to the problem. All sorts of second-order failures occurred; for example, the /var partition that holds the exim spools filled up. We eventually stopped new virus deliveries by filtering on the subject, but even then the number of attempted deliveries choked everything else out.

Martijn's solution is to scan the exim log and count the number of times an IP address attempts to deliver a virus. If it tries more than N times in M minutes, it is blocked from connecting to port 25 with an ipchains firewall rule. (M and N are 5 and 15 for now.)

Later, someone on one of the lists mentioned that this technique was described in a LISA paper last year:

Deeann M.M. Mikula, Chris Tracy, and Mike Holling. Spam Blocking with a Dynamically Updated Firewall Ruleset. Proceedings of LISA '02: 16th Systems Administration Conference, 13--20.

Abstract. In this paper, we detail our methods for controlling spam at a small ISP, reducing both resource usage and customer complaints. We will discuss our initial unsuccessful tactics, and the resulting development of our unique spam blocking system. Deny-Spammers classifies hosts as probable spammers and inserts those hosts into a dynamically updated firewall ruleset on our mail server, thereby effectively blocking the host from making an SMTP connection to our mail server. Our analysis demonstrates that this has been effective in reducing the amount of spam that our customers receive, and the burden on our limited resources.