Uncomplicated firewall for Ubuntu systems

The ufw is already installed on Ubuntu 16.04 and Ubuntu 18.04. It still uses the iptables service, but it offers a vastly simplified set of commands. Perform just one simple command to open the desired ports and another simple command to activate it, and you have a good, basic firewall. Whenever you perform a ufw command, it will automatically configure both the IPv4 and the IPv6 rules. This alone is a huge time-saver, and much of what we've had to configure by hand with iptables is already there by default.

There's a graphical frontend that you can use on desktop machines, but since we're learning about server security, we'll just cover the command-line utility here.