Fail2Ban: Basic set up for an exposed system

Overview

This is a basic set up for Fail2Ban on an system that is directly exposed to the internet (i.e. not behind a separate firewall).

This set-up assumes you are using UFW as your firewall front-end and it's working correctly.

In addition to the standard SSHd jail, a separate jail that monitors UFW BLOCK reports (i.e. connection attempts to closed ports, etc.) is activated. This should aid in blocking 'scriptkiddies' and port-scanning attacks, reducing the resources your server has to allocate to processing bogus requests. F2B will automatically create UFW rules to drop connections from systems that try to make repeated invalid connection attempts and then remove the block automatically after the 'bantime' has expired.

Installing an up-to-date Fail2Ban version

The F2B version available via apt for Debian/Ubuntu is old and does not support IP6 (as at the time of this document being written). So let's grab a newer version from the source at github.

Note: Only versions 0.10+ offer IP6 support.

• Switch to your home directory or somewhere you can work with downloaded files.

• Get the latest version of Fail2Ban, switch to the created directory and install it using the python installer script.

  # get latest fail2ban version
  git clone https://github.com/fail2ban/fail2ban.git
  # change to the newly created directory containing f2b
  cd fail2ban
  # run the installer
  sudo python setup.py install
  

• Let's test the installation by running fail2ban-client. If it displays the help screen, then things are probably set up properly.

  fail2ban-client -h
  

• Now, let's configure systemd to load fail2ban automatically on system start-up.

  # copy the service file to the correct location
  sudo cp files/debian-initd /etc/init.d/fail2ban
  # tell systemd to refresh itself to recognize the new service
  sudo update-rc.d fail2ban defaults
  

• Let's go ahead and start the service to make sure it doesn't run into any problems. You shouldn't see any errors reported and should have a pleasant 'green dot' showing up.

  # start the service
  sudo systemctl start fail2ban.service
  # check it's status for any errors
  sudo systemctl status fail2ban.service
  

• One more test just to be sure everything is set up. You should see f2b report it's version without any errors being generated.

  fail2ban-client version
  

Customizing your set up

As with all Fail2Ban setups, you should do all your customization in the .local files and not the .conf files since those may be overwritten by updates.

/etc/fail2ban/fail2ban.conf

I recommend reviewing your the following settings at a minimum for any deployment:

loglevel

This sets the verbosity of the log output from F2B. The default setting of INFO is appropriate for most installs but, you should specify it anyways so you have any easy place to change it if you need to do so.

loglevel = INFO

logtarget

This controls the location of the F2B log file where it logs it's own actions. This is NOT the location of the log files it reads for banning! Again, the default is appropriate for most installs, but you should specify it in your custom configuration so you have an easy place to change it if needed.

logtarget = /var/log/fail2ban.log

dbpurgeage

This controls how long F2B keeps a record of systems it has banned for whatever reason. By default, this is set to one day. I prefer having a one week record so I can go back and review as necessary. You can set it to whatever you want, duration is expressed in seconds.

dbpurgeage = 604800

/etc/fail2ban/jail.local

This file overrides the defaults applied to all jail configurations used by F2B. This sets things like the default amount of time a system is banned, what actions should be used for banning systems and whether or not you get email notifications, etc.

ignoreip

This setting tells F2B which IP addresses/ranges/hostnames should never be banned. In general, this should be the localhost only. However, if you connect by remote using a particular machine, you might want to exempt it from any possible bans also. You can specify more than one entry by separating them with a space or comma. In this case, I've added the IP4 and IP6 defintions for localhost.

ignoreip = 127.0.0.1/8 ::1

Timeframes

You should customize the relevant timeframes to your requirements and this will likely take a little experimentation. F2B checks for a system making 'maxretry' failed attempts to connect or login within 'findtime' seconds and, if that happens, bans the system for 'bantime' seconds.

I like using settings as below which state, "ban any system for 30 minutes that makes 5 invalid connection attempts within a 5 minute period".

bantime = 1800
maxretry = 5
findtime = 300

Some people find this too aggressive and prefer settings such as 10 attempts in 20 minutes, for example, which would look like:

bantime = 1800
maxretry = 10
findtime = 1200

Again, this will be up to you to determine what is appropriate for your environment and users. Remember that invididual jails can override these defaults.

Actions

Notication options

If you choose actions that involve sending email notifications, you need to let F2B know where to send those emails and who should send them. It's pretty straightforward, so this is the general setup:

destemail = account@domain.tld
sender = thismachine@domain.tld
mta = sendmail

The 'mta' field is very likely correct for your system, but if you are using a different MTA, you'll want to specify that here.

Shortcuts

This is where you tell F2B what exactly to do when it finds a reason to ban a system based on the jail configuration. Again, individual jails can override these settings. The settings are defined backwards in this file, so I'll take a second to explain.

'action' is performed each time a system should be banned. There are several predefined actions listed in the /etc/fail2ban/jail.conf file which you can use and are often sufficient for most setups. Read the comments in that file to understand what each predefined action does. In my case, I like getting an email along with a few lines from the log telling me what they did to get banned.

Within 'action' is 'banaction' which is a link over to a specific configuration file telling F2B what to do on the system to enforce the ban. In this setup, we direct F2B to look at the ufw.conf file to see how to modify UFW's rules so it drops packets from the offending system. Details on that file are found later in this document.

The general setup as described above is as follows:

banaction = ufw
action = %(action_mwl)s

Jails

F2B uses 'jail configurations' specified either in /etc/fail2ban/jail.conf, /etc/fail2ban/jail.local or in /etc/fail2ban/jail.d/. The latter is my preference since it allows for each jail to be contained in it's own configuration file which makes debugging and maintaining them much easier.

sshd (/etc/fail2ban/jail.d/ssh.conf)

I usually just define a jail for sshd which is the SSH server. You can add additional SSH jails as you wish to this file, but I keep it pretty simple. One note, I run my SSH server on a non-standard port, so be sure you fill in the correct port for your environment such as my example below of port 222:

[sshd]
port    = 222
...

If you are running on the standard port 22, then you can actually omit this line entirely since it's already defined in the default .conf files. Also note that if you have customized your SSHd configuration to use non-standard logging, you'll want to specify a logfile location in the jail also, like this:

[sshd]
...
logpath     = /path/to/your/log.file
...

UFW port probing

This is probably the what you are really looking for in this entire set-up. We will create a custom jail that monitors UFW's logs for any mention of [UFW BLOCK] and then proceeds to ban those systems attempting to connect to blocked ports as per your timeframe settings. I've commented the ufw-probe file but I'll run though it here also for convenience.

Name of the jail

You can call this anything that has meaning to you, I've chosen 'ufw-probe'. Just change what it says in the [square brackets]

[ufw-probe]
...

Ports and IPs

Since this is searching for port probing, we will tell F2B to look for attempts made to connect to any and all ports. The 'ignoreip' parameter is only necessary IF it's different from what you've already set in 'jail.local'.

port        = all
ignoreip    = 127.0.0.1/8 ::1

Timeframes

This section is also optional and is only needed if it's different from what you have in your 'jail.local'. I like keeping it in this configuration file though since the settings for this jail are often different from others.

maxretry    = 5
findtime    = 300

Jail-specific settings

In order for this jail to function, you need to give F2B a little information. First, we need to specify what log file it should be parsing. In this case, it's the UFW log file which is, by default, located at /var/log/ufw.log. If you've changed this, then update the 'logpath' parameter. We also need to tell it what filter to use when parsing the file, in this case, it's a filter I've called 'ufw-probe' (change this if you change the filename) which is located at /etc/fail2ban/filter.d/ufw-probe.conf [details here]. Finally, we tell F2B to enable this jail.

logpath     = /var/log/ufw.log
filter      = ufw-probe
enabled     = true