apt-get update
apt-get upgrade

apt-get install fail2ban

service fail2ban status

service fail2ban stop
service fail2ban start
service fail2ban restart

root@raspberrypi3:/home/pi# service fail2ban status
● fail2ban.service - Fail2Ban Service
     Loaded: loaded (/lib/systemd/system/fail2ban.service; enabled; vendor preset: enabled)
     Active: active (running) since Wed 2024-01-31 13:19:50 CET; 1min 3s ago
       Docs: man:fail2ban(1)
    Process: 20877 ExecStartPre=/bin/mkdir -p /run/fail2ban (code=exited, status=0/SUCCESS)
   Main PID: 20878 (fail2ban-server)
      Tasks: 5 (limit: 1595)
        CPU: 1.857s
     CGroup: /system.slice/fail2ban.service
             └─20878 /usr/bin/python3 /usr/bin/fail2ban-server -xf start

Jan 31 13:19:50 raspberrypi3 systemd[1]: Starting Fail2Ban Service...
Jan 31 13:19:50 raspberrypi3 systemd[1]: Started Fail2Ban Service.
Jan 31 13:19:52 raspberrypi3 fail2ban-server[20878]: Server ready

ls -l /etc/fail2ban/
total 64
drwxr-xr-x 2 root root  4096 Jan 31 13:19 action.d
-rw-r--r-- 1 root root  2816 Nov 23  2020 fail2ban.conf
drwxr-xr-x 2 root root  4096 Jul 12  2021 fail2ban.d
drwxr-xr-x 3 root root  4096 Jan 31 13:19 filter.d
-rw-r--r-- 1 root root 24996 Nov 23  2020 jail.conf
drwxr-xr-x 2 root root  4096 Jan 31 13:19 jail.d
-rw-r--r-- 1 root root   645 Nov 23  2020 paths-arch.conf
-rw-r--r-- 1 root root  2827 Nov 23  2020 paths-common.conf
-rw-r--r-- 1 root root   573 Nov 23  2020 paths-debian.conf
-rw-r--r-- 1 root root   738 Nov 23  2020 paths-opensuse.conf

https://wiki.ubuntuusers.de/fail2ban/
https://www.digitalocean.com/community/tutorials/how-to-protect-ssh-with-fail2ban-on-ubuntu-20-04