You can use OpenSnitch on Linux. These days it's quite stable, they even have binaries.
It works quite well but requires a GUI (obviously), it looks like it primarily supports GTK. If you're hoping to use the machine purely from the CLI (like, when sshing into your work machine) it won't work well.
It is significantly less powerful than LittleSnitch, some options don't exist (like, allowing access to a domain), but you get similar functionality in many cases.
Overall, it's definitely worth testing out to see if it works for you.
To make it no-less powerfull somebody has to invest time and expertise into extending the kernel for it - AFAIK LittleSnitch works this way on MacOS.
GUI is not GTK, but Qt.
> If you're hoping to use the machine purely from the CLI (like, when sshing into your work machine) it won't work well.
There's no cli tool published yet. There's a PoC though that works well.
> some options don't exist (like, allowing access to a domain)
Since version 1.0.0b you can filter by domain. And in latest version by domain, ip, network, uid, port, command line, command path, cmd environment variables, protocol, or any combination of them. You can't filter by interface, but if aomeone needs it open a new issue.
> It is significantly less powerful than LittleSnitch
What options do you miss?