GlassWire - Visualize network activity in detail, get notified when new apps access the network, look out for...
ufw - Ufw stands for Uncomplicated Firewall, and is program for managing a netfilter firewall.
TinyWall - Lightweight and non-intrusive firewall
ferm - ferm is a tool to maintain complex firewalls, without having the trouble to rewrite the complex...
Little Snitch - Little Snitch is a firewall application that monitors and controls outbound internet traffic.
pfSense - pfSense is a free and open source firewall and router that also features unified threat management, load balancing, multi WAN, and more