The Ubuntu operating system must have an application firewall enabled.

Firewalls protect computers from network attacks by blocking or limiting access to open network ports. Application firewalls limit which applications are allowed to communicate over the network.

Verify the Uncomplicated Firewall is enabled on the system by running the following command:

$ systemctl status ufw.service | grep -i "active:" 

Active: active (exited) since Mon 2016-10-17 12:30:29 CDT; 1s ago

If the above command returns the status as "inactive", this is a finding.

If the Uncomplicated Firewall is not installed, ask the System Administrator if another application firewall is installed. If no application firewall is installed, this is a finding.

Enable the Uncomplicated Firewall by using the following command:

$ sudo systemctl enable ufw.service  

If the Uncomplicated Firewall is not currently running on the system, start it with the following command:

$ sudo systemctl start ufw.service