Protecting SSH With Fail2Ban on Ubuntu 23.04

Hello,

In this week’s feature highlight, How to Protect SSH With Fail2Ban on Ubuntu 23.04

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 the Fail2Ban package

Check for system updates and install them.

apt update -y

apt upgrade -y

Command to install the Fail2Ban

apt install fail2ban -y

Start the fail2ban using below command,

systemctl start fail2ban

Enable fail2ban, run the following command.

systemctl enable fail2ban

Check the status of the service, run the following command.

systemctl status fail2ban

Output:

root@ubuntu:~# systemctl status fail2ban
● fail2ban.service - Fail2Ban Service
     Loaded: loaded (/lib/systemd/system/fail2ban.service; enabled; preset: enabled)
     Active: active (running) since Mon 2023-04-24 19:54:31 UTC; 6s ago
       Docs: man:fail2ban(1)
   Main PID: 3064 (fail2ban-server)
      Tasks: 5 (limit: 3386)
     Memory: 34.4M
        CPU: 1.439s
     CGroup: /system.slice/fail2ban.service
             └─3064 /usr/bin/python3 /usr/bin/fail2ban-server -xf start

Apr 24 19:54:31 ubuntu systemd[1]: Started fail2ban.service - Fail2Ban Service.
Apr 24 19:54:31 ubuntu fail2ban-server[3064]: 2023-04-24 19:54:31,842 fail2ban.configreader   [3064]: WARNING 'allowipv6' no>
Apr 24 19:54:32 ubuntu fail2ban-server[3064]: Server ready

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 upgrades can overwrite this file.

Jail.local contains the 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 your IP address at the last of the command For ex.

ignoreip = 192.168.0.100 192.168.1.0/24

Multiple IP address can be whitelisted with the above configuration, they have to be separated with a single white space. You can also white list an IP block as shown above.

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 a negative value in the 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"]

Configure sending and receiving email addresses.

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

Example:

destemail = admin@xyz.com

sender = root@xyz.com

Fail2ban Client

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

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"
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