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Security

What do I have to consider when using Fail2ban?

Especially on systems which provide SSH/CGI/PHP services to unknown users, it is possible to block other users from ssh and probably other services. How would a user do so? The user could issue:

logger -p auth.warning -t 'sshd[123]' 'Illegal user user1 from 1.2.3.4'

Or the malicious user may write via PHP's openlog()/syslog() to syslog.

Solution #1: This security hazard can be handled via ownership/permissions of /dev/log, which allows logging to all the users by default. Just add a group log, add all daemons and root to that group and be happy.

What about log injection?

Fail2ban parses log files of other services and thus it can be vulnerable to log injection. Daniel B. Cid describes this kind of issues in Attacking Log analysis tools. I strongly suggest that you read this article. We will always try to provide safe configuration files. However, you can use fail2ban-regex to test your configuration files against forged log lines.

Troubleshooting

I have Postfix on my system but no "mail" command. How can I get e-mail notifications?

As of version 0.8.1, "mail" actions are deprecated. Please use the "sendmail" ones instead. E.g. sendmail-whois instead of mail-whois in your jail.[conf|local].

You probably have the sendmail command. Copy /etc/fail2ban/action.d/mail-whois.conf to /etc/fail2ban/action.d/mail-whois.local, edit this file and replace mail with sendmail. Here is an example:

actionban = echo -en "From:root <fail2ban>
            To: <dest>
            Subject: [Fail2Ban] <name>: banned <ip>
            Hi,\n
            The IP <ip> has just been banned by Fail2Ban after
            <failures> attempts against <name>.\n\n
            Here are more information about <ip>:\n
            `whois <ip>`\n
            Regards,\n
            Fail2Ban"|sendmail -t  

mail.conf can be modified too.

Why do my CVS users using SSH getting blocked?

If you are using the Eclipse CVS integration with SSH, then each access of the CVS results in a failed access before a valid one is done. As a consequence your CVS users get banned from time to time.

I get the error "Please check the format and your locale settings"

The error looks like this:

ERROR: time data did not match format: data=Mar 21 10:00:50 fmt=%b %d %H:%M:%S
ERROR: Please check the format and your locale settings.

This is a known bug. Since 0.6.1, Fail2ban uses your locale settings for date and time format. However, some daemons do not take care of locale and write their log messages using the POSIX standard. Please look at this bug for more details.

You can try to override the LANG variable:

# LANG=en_US /etc/init.d/fail2ban restart

You can get all the available locale with:

# locale -a

How do I increase verbosity?

In order to increase the verbosity of Fail2ban, use the command line option -vvv for fail2ban-client and fail2ban (only for 0.6.x). Set loglevel to 4 in /etc/fail2ban/fail2ban.conf (only for > 0.6.x).

Fail2ban is running but not banning SSH bruteforce

NB:This example is based on a Debian system, but can be easily done on any distro.

The package is well installed:

# dpkg -l |grep fail                                               
ii  fail2ban                      0.8.1-2                         bans IPs that 
cause multiple authentication

The service is running:

# /etc/init.d/fail2ban status                                      
Status of authentication failure monitor: fail2ban is running

SSH jail is set up and ready:

# fail2ban-client status                                           
Status                                                                          
|- Number of jail:      1                                                       
`- Jail list:           ssh

SSH bruteforce logs are identified by fail2ban:

# fail2ban-regex /var/log/auth.log /etc/fail2ban/filter.d/sshd.conf
....
Success, the total number of match is 30

So, check that all your logs are synchronized: all logs files (auth.log, syslog,..) must use the same time reference (if your server is not very busy, there will probably be an important difference between the output of [1]date command and the last event logged in syslog. You can force to generate a log in syslog using the logger command and check then with the output of date command)

# date                                                             
Wed Nov 28 13:49:02 CET 2007                                                    
# tail -2 /var/log/auth.log                                        
Nov 28 13:39:12 <SERVERNAME> sudo: pam_unix(sudo:session): session opened for user roo
t by <user>(uid=0)                                                              
Nov 28 13:39:12 <SERVERNAME> sudo: pam_unix(sudo:session): session closed for user roo
t

If time reference is not the same everywhere, then fail2ban won't ban any IP!