Portmaster

Built by the German company Safing, Portmaster is a free application-layer firewall for Windows and Linux that gives desktop users granular, per-application control over outbound and inbound network traffic instead of leaving that decision entirely to the operating system. It intercepts DNS requests system-wide to block ad, tracker, and malware domains, supports DNS-over-TLS and DNS-over-HTTPS resolvers, and can restrict connections by country, network scope, or process. An optional add-on called SPN (Safing Privacy Network) layers onion-style multi-hop encryption over selected connections for a Tor-like privacy boost. Distributed under the GPLv3 license with an Electron-based notifier UI and a Go backend service, Portmaster targets privacy-conscious individuals who want visibility into background telemetry most consumer firewalls never surface.