OpenSnitch

OpenSnitch is a GNU/Linux port of the Little Snitch firewall. You can launch the GUI from the icon or from the system menu. The daemon will start intercepting connections, prompting you to allow or deny them. If you don't apply an action, after 15 seconds (configurable) it'll apply the default action configured. When you open the GUI, you'll see all the connections and processes that the daemon has intercepted. Double click on a row to view the details of a process, rule, host or user. Once you know which are the common processes, IPs and hosts that your machine is connecting to, you can start creating permanent rules (Duration: always) to deny or allow them. You can also convert temporary rules to permanent by right-clicking on a temporary rule or by double-clicking on it, and then edit it. By default OpenSnitch UI listens on a local Unix socket in /tmp/osui.sock.