What does tether_dun_required 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