This always start with a package installation:

$ aptitude install fail2ban

Then I simply create a local configuration file where I’ll put all my custom config:

$ touch /etc/fail2ban/jail.local

Here is the content of that file:

[DEFAULT]
# Do not filter connexion from my apartment and from the server itself
ignoreip  = 127.0.0.1 88.123.123.123 91.123.123.123
# Ban for a week
bantime   = 604800
maxretry  = 3
destemail = kevin@deldycke.com
banaction = iptables-allports
action    = %(action_mwl)s

[ssh]
enabled  = true
port     = 22
maxretry = 2

[ssh-ddos]
enabled = true
port     = 22

[apache]
# Apache basic auth
enabled   = true
maxretry  = 3
# Ban for 1 hour
bantime   = 3600

[apache-noscript]
enabled = true

[apache-overflows]
enabled = true

[apache-badbots]
enabled  = true
filter   = apache-badbots
port     = http,https
action   = iptables-allports
logpath  = /var/log/apache*/*access.log
maxretry = 1

[apache-nohome]
enabled  = true
filter   = apache-nohome
port     = http,https
action   = iptables-allports
logpath  = /var/log/apache*/*access.log
maxretry = 1

[exim]
enabled  = true
filter   = exim
port     = smtp,ssmtp
action   = iptables-allports
logpath  = /var/log/exim*/rejectlog
maxretry = 1

[exim-relay]
enabled  = true
filter   = exim-relay
port     = smtp,ssmtp
action   = iptables-allports
logpath  = /var/log/exim*/rejectlog
maxretry = 1

While adjusting Fail2Ban, I was surprised by how sensitive this software is. It can just refuse to start without any notice in the log or on the command line. Even if its log_level variable is set to 4 (= DEBUG) in /etc/fail2ban/fail2ban.conf.

In such a case, a sure way to find the culprit is to use a brute force debugging method: first set all the enabled variable of your jail.local‘s sections to false. Then activate one section after another until Fail2Ban refuse to restart.

For me, the problem was that I forgot to add my custom exim-relay filter to Fail2Ban. So I fixed my issue by creating an empty file at /etc/fail2ban/filter.d/exim-relay.conf in which I pasted the following content:

# Based on default exim.conf filter by Cyril Jaquier
# Real life exemaple:
# 2009-07-02 08:16:42 H=118-167-129-21.dynamic.hinet.net (91.121.198.84) [118.167.129.21] F=<titieueue@hotmail.com> rejected RCPT <s2288@mail2000.com.tw>: relay not permitted

[Definition]

# Option:  failregex
# Notes.:  regex to match use of my exim mail server as a relay it does not
#          allow.
# Values:  TEXT
#
failregex = \[<HOST>\] .*(?:relay not permitted)

# Option:  ignoreregex
# Notes.:  regex to ignore. If this regex matches, the line is ignored.
# Values:  TEXT
#
ignoreregex =

Speaking of custom filters, here is one to filter DFind scans (file located at /etc/fail2ban/filter.d/apache-w00tw00t.conf):

# Based on http://howflow.com/tricks/block_w00tw00t_scan_hosts_with_fail2ban
# Real life exemaple:
# [Sat Jun 27 16:43:08 2009] [error] [client 94.23.57.77] client sent HTTP/1.1 request without hostname (see RFC2616 section 14.23): /w00tw00t.at.ISC.SANS.DFind:)

[Definition]

# Option:  failregex
# Notes.:  regex to match the w00tw00t scan messages in the logfile.
# Values:  TEXT
failregex = ^.*\[client <HOST>\].*w00tw00t\.at\.ISC\.SANS\.DFind.*

# Option:  ignoreregex
# Notes.:  regex to ignore. If this regex matches, the line is ignored.
# Values:  TEXT
ignoreregex =

And here is the corresponding section from my jail.local file:

[apache-w00tw00t]
enabled  = true
filter   = apache-w00tw00t
action   = iptables-allports
logpath  = /var/log/apache*/*error.log
maxretry = 1