In my use case, I run a K3D cluster in my home and wanted to resolved my local .home.arpa domain hosts (eg ponics.home.arpa) via my router DNS from the pods rather than setting static IPs in DHCP and manifests.
The solution was pretty simple: update in kubectl edit cm/coredns -n kube-system to replace in forward . 192.168.0.1 so domains not handled by CoreDNS used my router as the upstream.