netguard

NetGuard is the first free and open source no-root firewall for Android.

It’s not perfect, either.

don’t use an f-droid build

F-Droid builds are not supported because I have no control over if and when the F-Droid version of NetGuard will be updated, so I cannot guarantee timely updates, for example if there is a critical or security issue.

don’t use a google play store build

You need 3 packages (applications) enabled (use search in NetGuard to find them quickly):
com.android.vending (Play store)
com.google.android.gms (Play services)
com.android.providers.downloads (Download manager)

DO use a github build

https://github.com/M66B/NetGuard

ad blocking with hosts file

Blackhole DNS lookups for a lot of known ads/malware:

https://github.com/M66B/NetGuard/blob/master/ADBLOCKING.md

Test it here.

netguard keeps dying

A userland app is subject to being killed by the Android LMK, or by aggressive battery saving automation. These mechanisms are uniformly not very smart.

On most devices, NetGuard will keep running in the background with its foreground service. On some devices (in particular some Samsung models), where there are lots of applications competing for memory, Android may still stop NetGuard as a last resort.

…

You can workaround this problem by enabling the watchdog in the NetGuard advanced options to check every 10-15 minutes.

Or more frequently. On my Titan, netguard dies regularly, and without warning.

documentation

Netguard FAQ