- https://github.com/IngoZenz/personaldnsfilter/blob/master/app/src/main/java/dnsfilter/android/DNSFilterService.java
- https://android.googlesource.com/platform/development/+/master/samples/ToyVpn/src/com/example/android/toyvpn/ToyVpnService.java
- https://developer.android.com/guide/components/services#StartingAService
- https://developer.android.com/guide/components/services#Foreground
- https://stackoverflow.com/questions/600207/how-to-check-if-a-service-is-running-on-android
- https://www.vogella.com/tutorials/AndroidServices/article.html#communication-with-services
- https://github.com/googlecodelabs/bazel-android-intro/blob/master/sample/mediarecorder/java/com/example/android/mediarecorder/MainActivity.java