sudo apt update
sudo apt install fail2ban
sudo systemctl enable fail2ban
sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local

Debian 11

sudo systemctl start fail2ban.service
sudo systemctl status fail2ban.service

Debian 12

sudo nano /etc/fail2ban/jail.conf

backend = systemd

sudo systemctl start fail2ban.service
sudo systemctl status fail2ban.service

sudo systemctl status fail2ban.service
sudo systemctl restart fail2ban.service
sudo fail2ban-client status sshd
sudo fail2ban-client set <jail-name> unbanip <ip>
sudo fail2ban-client set <jail-name> banip <ip>