How to Protect SSH With Fail2Ban on Debian 11

Fail2Ban is an intrusion prevention framework written in the Python programming language. It works by reading SSH, ProFTP, Apache logs, etc. And uses iptables profiles to block brute-force attempts.

Installing Fail2ban package

Check for system updates and install it.

apt update -y

apt upgrade -y

Command to install the fail2ban.

apt install fail2ban -y

To enable fail2ban, run the following command.

systemctl enable fail2ban

To check the status of the service, run the following command.

systemctl status fail2ban

Output:

[email protected]:~#
● fail2ban.service - Fail2Ban Service
     Loaded: loaded (/lib/systemd/system/fail2ban.service; enabled; vendor preset: enabled)
     Active: active (running) since Wed 2021-08-04 14:06:32 GMT; 2min 26s ago
       Docs: man:fail2ban(1)
   Main PID: 73604 (fail2ban-server)
      Tasks: 5 (limit: 4677)
     Memory: 15.5M
        CPU: 743ms
     CGroup: /system.slice/fail2ban.service
             └─73604 /usr/bin/python3 /usr/bin/fail2ban-server -xf start

Aug 04 14:06:32 server systemd[1]: Starting Fail2Ban Service...

Configuring Fail2ban

Jail.conf contains a section in which Configuration settings can be done for the fail2ban, we are not going to edit this file because package upgrade can overwrite this file.

Jail.local contains same sections where "jail.conf" file contains and it can override these values.

Command to create a jail.local configuration file by copying the default jail.conf file.

cp /etc/fail2ban/jail.{conf,local}

Open file to configure.

nano /etc/fail2ban/jail.local

Whitelisting IP addresses

Find the following line in the config file /etc/fail2ban/jail.local and uncomment it to whitelist the IP address.

ignoreip = 127.0.0.1/8 ::1 

Once you uncomment it, add the your IP address at the last of command For ex.

ignoreip = 127.0.0.1/8 ::1 <Your IP address here>

IPs are separated by a single white space, for example, ignoreip = 127.0.0.1/8 ::1 198.100.0.10 198.100.0.20 10.10.0.20

Ban settings

3 main options in these settings,

  • bantime: is the number of sec/hours/day that an IP address is banned.
  • findtime: is the window that fail2ban will pay attention to when looking for repeated failed authentication attempts.
  • maxretry: is the maximum try which will be given before blocking.

Find these lines in the config file /etc/fail2ban/jail.local and change as you required.

Default values of the option are,

bantime  = 10m

findtime  = 10m

maxretry = 5

Note: If you want to block IP address permanently use negative value in bantime option.

Get e-mail notifications

Note: To receive email alerts, you need to have an SMTP installed on your server.

To receive email alerts with relevant logs, find the following line in the config file /etc/fail2ban/jail.local and make sure that the following line is present.

# ban & send an e-mail with whois report and relevant log lines
# to the destemail.
  action_mwl = %(action_)s
               %(mta)s-whois-lines[sender="%(sender)s", dest="%(destemail)s", logpath="%(logpath)s", chain="%(chain)s"]

To configure sending and receiving email addresses.

Find the following line in the config file /etc/fail2ban/jail.local and update the details.

Example:

destemail = [email protected]

sender = [email protected]

Fail2ban Client

To interact with the Fail2ban service there is a command-line tool called fail2ban-client.

To check its available options enter the following command.

fail2ban-client -h

Here is a few examples that can be performed by using this tool,

Check the jail status.

fail2ban-client status sshd

To unban an IP.

fail2ban-client set sshd unbanip "IP address here"

To Ban an IP.

fail2ban-client set sshd banip "IP address here"