Linux install ufw

ufw ≡ Uncomplicated Firewall

sudo apt install -y ufw
sudo ufw default deny incoming
sudo ufw default allow outgoing
sudo ufw allow ssh
sudo sed -i s/IPV6=yes/IPV6=no/g /etc/default/ufw
sudo ufw enable
sudo ufw status
sudo ufw app list

/via: https://community.hetzner.com/tutorials/simple-firewall-management-with-ufw

sudo apt install -y fail2ban
sudo systemctl enable fail2ban
sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local
sudo vim /etc/fail2ban/jail.local # add enabled = true in [sshd] section

sudo groupadd su
sudo usermod -a -G su fidmag
sudo dpkg-statoverride --update --add root su 4750 /bin/su

/via: https://www.techrepublic.com/article/how-to-limit-access-to-the-su-command-in-linux/