Basically, I want to specify certain apps to always run in “offline mode,” even while my internet connection remains active, so that the application is blocked from sending any outbound data. Switching Steam into offline mode and running all apps through the Steam client usually doesn’t work.
In Windows 10, I used the built-in firewall for this, but SteamOS has no built-in firewall and I can’t find any way to install ufw or firewalld in SteamOS, assuming either of those apps would have the same ability to block internet traffic on a per-application basis.
The primary reason I want to do this is for privacy reasons (e.g. preventing Unity from tracking my game installations), but there are other benefits as well. For example, Doom Eternal usually requires a Bethesda account to play, but if the executable is blocked in the firewall you can play without one.
So, is there any app or other method that can do this in SteamOS?
Open the system settings in desktop mode. There, you’ll see some firewall settings. It might ask you to install a package before using them, but once you do, it works fine for this purpose.
That was my first attempt, but for whatever reason SteamOS does not come with a firewall pre-installed. The firewall settings window suggests installing ufw or firewalld, but neither is available for SteamOS.