This CoreDNS based resolver can be used to improve name resolutin inside Azure Networks when using Private Link DNS Zones.
When using Private Links with Private DNS Zones in Azure the name resolution for resources that also use Private Links but aren't in the linked Private DNS Zone can not be resolved.
This resolver improves that by first querying the internal Azure resolver 168.63.129.16. If this resolver returns a NXDOMAIN the request is forwarded to an external resolver.