Fail2ban

We use the package fail2ban on all of our linux machines to help prevent ssh password brute-forcing. This package will block an IP address after a certain number (usually 10) of failed attempts.

Removing Fail2ban Block

If your address has been accidentally blocked on a server, and you are able to log in and control the server (usually via a different computer) you can remove your banned IP address from iptables.

To unban an IP address manually, it is necessary to know the chain name and the rule number. The following command can be used to acquire this information:

Code:

iptables -L -n --line-numbers

The relevant bits are at the end of the output. Here is an example chain with attendant rules:

Code:

Chain fail2ban-ssh (1 references)
num  target     prot opt source               destination
1    DROP       all  --  204.110.13.107       anywhere
2    DROP       all  --  1.234.20.21          anywhere
3    DROP       all  --  gw-tair-rp.rel.com.ua  anywhere
4    RETURN     all  --  anywhere             anywhere

In this example, three (3) IP addresses have been banned via the SSH jail (these are the DROP rules).

To unban an IP address, you would run the following command:

Code:

iptables -D [chain-name] [line-number]

To unban the IP address 1.234.20.21 (see above), the command would be:

Code:

iptables -D fail2ban-ssh 2

Reference: http://www.howtoforge.com/forums/showthread.php?t=51366&page=2

Installing Fail2ban

OS X

This is an advanced setup, and requires using the terminal to install and modify files.

1. Download the latest official source tarball: http://www.fail2ban.org/wiki/index.php/Downloads
2. Unpack by double clicking or command line
   • tar xvfj fail2ban-0.8.14.tar.bz2
3. Go into the folder and run the following command to install Fail2ban
   • cd fail2ban-0.8.14
   • sudo python setup.py install
4. Copy the init scipt to the launchdaemon directory
   • sudo cp files/macosx-initd /Library/LaunchDaemons/org.fail2ban.plist
5. Open the org.fail2ban.plist and delete the first two lines, such that the first line starts with <?xml ...
   • sudo nano /Library/LaunchDaemons/org.fail2ban.plist
6. Make a log file and give it proper permissions
   • sudo touch /var/log/fail2ban.log
   • sudo chgrp admin /var/log/fail2ban.log
7. Add these two lines to /etc/pf.conf
   • table <fail2ban> persist
   • block drop log quick from <fail2ban> to any

Optional Installs

1. Install IceFloor
2. Install Xcode and MacPorts: https://guide.macports.org/chunked/installing.html

add the below line to /opt/local/etc/fail2ban/filters.d/ssh.conf

^%(__prefix_line)s(?:error: PAM: )?unknown user for illegal user .* from <HOST>( via \S+)?\s*$