I’ve recently setup fail2ban  on a bunch of my servers. It’s purpose is to ban IP addresses associated with password guessing – or whatever other criteria for badness you configure. It supports Linux, OpenBSD  and probably most Unix type OSs too. I run Debian so I’ve been using the Debian packages of fail2ban.
The first thing to note is that it is very easy to install and configure (for the common cases at least). For a long time installing it had been on my todo list but I didn’t make the time to do it, after installing it I realised that I should have done it years ago, it was so easy.
Generally to configure it you just create a file under /etc/fail2ban/jail.d with the settings you want, any settings that are different from the defaults will override them. For example if you have a system running dovecot on the default ports and sshd on port 999 then you could put the following in /etc/fail2ban/jail.d/local.conf:
[dovecot] enabled = true [sshd] port = 999
By default the Debian package of fail2ban only protects sshd.
When fail2ban is running on Linux the command “iptables -L -n -v|grep f2b” will show the rules that match inbound traffic and the names of the chains they direct traffic to. To see if fail2ban has acted to protect a service you can run a command like “iptables -L f2b-sshd -n” to see the iptables rules.
The fail2ban entries in the INPUT table go before other rules, so it should work with any custom iptables rules you have configured as long as either fail2ban is the last thing to be started or your custom rules don’t flush old entries.
There are hooks for sending email notifications etc, that seems excessive to me but it’s always good to have options to extend a program.
In the past I’ve tried using kernel rate limiting to minimise hostile activity. That didn’t work well as there are legitimate end users who do strange things (like a user who setup their web-cam to email them every time it took a photo).
Fail2ban has some good features. I don’t think it will do much good at stopping account compromise as anything that is easily guessed could be guessed using many IP addresses and anything that has a good password can’t be guessed without taking many years of brute-force attacks while also causing enough noise in the logs to be noticed. What it does do is get rid of some of the noise in log files which makes it easier to find and fix problems. To me the main benefit is to improve the signal to noise ratio of my log files.