OpenSnitch, an application firewall for Linux, is no longer under active development. This didn't mean it was the end for this project though, because Gustavo forked it about 8 months ago, and has been constantly improving it since.
OpenSnitch (fork) is a free an open source application-level firewall for Linux that's made of a daemon (written in Go) and a GUI (PyQt5). It monitors outbound connections that your applications are trying to make, preventing or permitting their connection based on a set of rules (the user is prompted to allow or deny access when no existing rules are found). The application is inspired by Little Snitch, a commercial host-based application firewall for macOS.