Little Snitch for Linux https://obdev.at/products/littlesnitch-linux/index.html Every time an application on your computer opens a network connection, it does so quietly, without asking. Little Snitch for Linux makes that activity visible and gives you the option to do something about it. You can see exactly which applications are talking to which servers, block the ones you didn't invite, and keep an eye on traffic history and data volumes over time https://github.com/obdev/littlesnitch-linux
FYI, UI and client side is in Rust and open source but its core daemon on Linux is not. This app is from same authors who wrote macOS/OS X version. So use with caution as it is not 100% open source like many games you play on Linux.
@nixCraft OpenSnitch is open front to back.
Maybe not as nice but is doing the job for years.
opensnitch package versions - Repology

List of package versions for project opensnitch in all repositories

@regendans @squalouJenkins @nixCraft I have also been a happy OpenSnitch user, see no reason to recommend something with proprietary crap when there are good alternatives, if it is not full free software that is a huge disadvantage compared with OpenSnitch