Wait how is this easy on macOS? Do you mean with Little Snitch? Or is there something built into the OS?

I don't suppose you have any pointers on how to make that a reality in Ubuntu/Debian? I too would be very interested in being able to configure whitelist only network connections.

The linux desktop security model is severely broken [1]. Just use Qubes if you want to control access to resources without losing your mind.

[1] https://forums.whonix.org/t/fixing-the-desktop-linux-securit...

The concept is called a full system policy https://gitlab.com/apparmor/apparmor/-/wikis/FullSystemPolic...