Have you ever noticed that the sshd on your publicly facing machines gets bombarded with dictionary attacks several times per day? This problem is mostly an annoyance, as it fills up the logs with lots of User authentication failed, wrong password for <username> messages. There are of course several ways to work around this problem, and the most common one is to run sshd on another port than 22. I find that approach cumbersome, because it means you'll always have to configure your client software to connect to a non-standard port, and in lots of cases a firewall at your location might be blocking the traffic as well. Isn't there a way to block these bothersome users instead?
I recently read an article in Linux Journal #210 that talked about a new feature in the Linux kernel called ipset. It allows you to create sets that store IP addresses which can dynamically be added to and removed from. This sets can be configured as selectors in iptables rules, so that you can perform actions on any IP address in the set, like dropping the packets. You can also create iptables rules that add or remove an IP from a set.
In Debian it is straight-forward to install ipset. Type in these commands to install ipset and the required kernel module. It should ensure it is automatically recompiled whenever you do a kernel upgrade.
$ apt-get install ipset xtables-addons-source $ module-assistant auto-install xtables-addons
I asked around on IRC if it was possible to make sshd execute a shell script once the amount of logins from an IP went above some configurable threshold, but apparently it is not possible. Someone pointed me at fail2ban which is a system that scans your syslog looking for failed logins and turn them into blocking iptables rules (and probably more, I didn't look very closely). I thought that this was too slow, as I wanted something that triggered as soon as multiple failed logins for a user reached a certain threshold, but I wanted the block to be temporary, not permanent.
That is when I remembered that syslog (rsyslog in my case) can be configured to run the log messages through a program. I put together the program below that reads incoming messages from rsyslog with auth.info facility/level (default for sshd on Debian) and does the necessary things to ensure the offending IP is added to my autoblock ipset if it triggers a certain amount of failed logins in a short period. The autoblock ipset uses the iptree storage module that has a --timeout parameter to automatically purge entries from the set after a given time. I set it to 3600 seconds (1 hour).
What is also quite cool, is that I have a rule that will immediately put the IP of anyone that tries to connect to my SMTP port (I don't run a mail server on my firewall) on the autoblock list. This particular bit happens completely inside netfilter, so its effect is immediate. Bye bye spammers! Try to connect to my (non-existent) email server and you're instantly blocked for an hour. And you didn't even know what hit you. :)
The reason I like the temporary block is that sometimes I port-scan my own server or do other strange things with it from remote computers, and the fact that I know the block will be lifted after an hour means I can continue without having to get physical access to server to remove the block. I can just smack myself in the forehead and wait an hour and continue whatever stupid thing I was doing.
PS: Remember that lots of IRC servers like to port-scan your IP when you connect to them, so you might need to put up some exceptions for those if you're an active IRC user and your particular IRC server likes to probe the SMTP port.