What does tether_dun_required do? I see lots of mentions of this option on Android sites, including Android.StackExchange.com, but I don't see any technical exploitation for what it does. What does this option actually do?
Tells telephony manager dial up networking emulation not required, not sure if thats actually required if you use net.tethering.noprovisioning=true in build.prop you can see the telephony and tethering code in the links below

https://android.googlesource.com/platform/frameworks/base/+/master/telephony/java/android/telephony/TelephonyManager.java

https://github.com/android/platform_frameworks_base/blob/355dbae680994002c48d7a66cb276a65393ecbbb/services/core/java/com/android/server/connectivity/Tethering.java