"I researched a bit, found OpenSnitch, several command line tools, and various security systems built for servers. None of these gave me what I wanted: see which process is making which connections, and in the best case deny with a single click." https://obdev.at/blog/little-snitch-for-linux/
I've used OpenSnitch for years, and while LittleSnitch definitely has a better UI for showing which process is making which connections over time, OpenSnitch does a pretty good job here. I get a modal popup when a program that hasn't made a connection tries to make a connection, and I can either allow/deny in one click, or further customize the rule e.g. allowing ntpd to connect, but only to pool.ntp.org on port 123.
Where LittleSnitch is definitely ahead is showing process connections over time after said process has been allowed.
When I looked at OpenSnitch (years ago), it didn't support running headless on a server. Am I mistaken about this, or has it changed?
You can run daemons on several nodes (different machines) and view them all through a central ui, it is pretty cool.
The UI is a separate package. Though you might just configure the firewall yourself at that point.