HOWTO Setup fail2ban

What fail2ban does

Fail2ban parses logfiles, and finds repeated-failures for various services. Once a specified number of failures within a given time is reached, the fail2ban makes an iptables-entry for you, banning (blocking) that IP-address. After a configurable length of time, the IP-address is unblocked.

Why we want fail2ban

One alternative is denyhosts, which requires tcpwrappers, and makes entries in your /etc/hosts.deny file. The problem is that regular-expressions are not supplied to filter more than SSH, and denyhosts can only scan a single log-file. One advantage of denyhosts is that it doesn't require iptables.
Our Gentoo systems use syslog-ng, with separate (from /var/log/messages) SSH auth.log file. Often, we supply vsftpd connectivity for users, which uses another separate log-file. We may also add other services.
So, the flexibility and integration with iptables is a major benefit.

Pre-Requisites

• iptables
• logrotate (watch out! Some stuff doesn't rotate; follow the link to fix it!)
• gamin (if you're following my suggestion, below)

Installing fail2ban

hostname ~ # emerge -v fail2ban
hostname ~ # emacs -nw /etc/fail2ban/jail.conf

Configuring fail2ban

In /etc/fail2ban/jail.conf, scroll to section "[ssh-iptables]" and enable it:

###enabled  = false
enabled  = true

In /etc/fail2ban/jail.conf, change the the SSH logfile we want to scan:

###logpath = /var/log/sshd.log
logpath = /var/log/auth.log

In /etc/fail2ban/jail.conf, change how we monitor failures, preferring a file-alteration monitor over polling:

###backend = auto
backend = gamin

In /etc/fail2ban/jail.conf, comment out the "mail-whois" actions.

Running fail2ban

/hostname ~ # etc/init.d/fail2ban start
* Starting fail2ban

hostname ~ # rc-update add fail2ban default
* fail2ban added to runlevel default

Monitoring and Verifying fail2ban

Check the log file:

hostname ~ # tail /var/log/fail2ban.log

and, for scrolling/real-time monitoring of additions to the log-file:

hostname ~ # tail -f /var/log/fail2ban.log

Check Iptables:

hostname ~ # iptables -L

Example output, for a web-server (port 80 open), with SSH and FTP services too. Note the banned SSH user, resulting from too many failed-login-attempts:

Chain INPUT (policy DROP)
target     prot opt source               destination         
fail2ban-VSFTPD  tcp  --  anywhere             anywhere            tcp dpt:ftp 
fail2ban-SSH  tcp  --  anywhere             anywhere            tcp dpt:ssh 
ACCEPT     all  --  anywhere             anywhere            
DROP       all  --  anywhere             anywhere            state INVALID 
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED 
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ftp-data 
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ftp 
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ssh 
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:http

Chain FORWARD (policy DROP)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED 

Chain fail2ban-SSH (1 references)
target     prot opt source               destination         
DROP       all  --  sr-01504.iat.sfu.ca  anywhere            
RETURN     all  --  anywhere             anywhere            

Chain fail2ban-VSFTPD (1 references)
target     prot opt source               destination         
RETURN     all  --  anywhere             anywhere