The best open source alternative to Little Snitch is LuLu by Objective-See, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Little Snitch and nine of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Little Snitch are OpenSnitch (Free, Open Source), AFWall+ (Free, Open Source), Portmaster (Free, Open Source) and NetGuard (Freemium, Open Source).
All-Around System Privacy Since the Portmaster sits on your kernel, you can finally protect your whole computer, not just your browser. Do you want to block Ads and Analytics for your whole operating system? The Portmaster has you covered.
NetGuard provides simple and advanced ways to block access to the internet - no root required. Applications and addresses can individually be allowed or denied access to your Wi-Fi and/or mobile connection.
Douane is a personal firewall that protects a user's privacy by allowing a user to control which applications can connect to the internet from their GNU/Linux computer. You can allow network traffic for some applications and deny network traffic for others.