portmaster

Portmaster is a free and open-source application firewall for Windows and Linux desktops. A firewall controls which programs on your computer are allowed to connect to the internet and which connections get blocked. Portmaster watches every network connection at a low level, identifies which application made it, and lets you allow or block traffic on a per-app basis. It also automatically blocks connections to known tracker and malware domains using built-in filter lists, so your privacy improves from the moment you install it without requiring manual setup.

The tool provides a graphical interface where you can see all current and recent network activity, apply global rules that affect all applications, and override settings for individual apps. For example, you can allow a web browser to access the internet while blocking a different program from making any outbound connections at all. DNS queries, which are the requests your computer makes to look up domain names, are intercepted and sent through encrypted resolvers to prevent your internet provider from seeing which websites you visit.

Some features require a paid subscription. Recording and searching your network history, monitoring per-app bandwidth usage, and access to a privacy network called SPN are paid tiers. The SPN routes traffic through multiple servers using layered encryption, similar in concept to Tor but run by Safing (the company behind Portmaster) and community-hosted nodes. The README notes speeds above 100 Mbit/s.

Portmaster is developed in Austria and runs entirely locally on your device. It is built in Go and integrates into the operating system at the network stack level, intercepting packets before they leave the machine. Documentation, compatibility notes, and installation guides are available on the project's wiki.