I like its simple UI, I will probably give it a try at some point.
Protected apps — Google Play Services
Whether it's a fature is arguable... Isn't a warning better?
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Btw. The most interesting feature I've seen in firewall app was: blocking all traffic, but showing a notification "X app tries to access internet" [Allow] [Block] [Allow once]
The Allow/Block options added it to white/black-list so the notification doesn't appear again.
But the "Allow once" option was the interesting part. It blocked by default, but when user choose to allow once it gained temporary permission to communicate over network. I don't remember if it allowed access for as long as the app stayed in foreground or just for some predefined time. Sadly after some app update it was removed and I never seen any other firewall implement that option since.
Is something like this outside of the scope of the project?