UFW

Ubuntu 16.04 servers can use the UFW firewall to make sure only connections to certain services are allowed.application.

Different applications can register their profiles with UFW upon installation. These profiles allow UFW to manage these applications by name, e.g. OpenSSH, the service allowing us to connect to the server, has a profile registered with UFW.

sudo ufw app list

SSH
Allow SSH connections:

sudo ufw allow OpenSSH

Enabling the firewall

sudo ufw enable

Check firewall status

sudo ufw status