Add a local DNS zone to your k3s CoreDNS

In your private cluster, you may want to access resources located in your local network with a local DNS name, which is in some configurations not possible out of the box.

Althogh it is possible to change the CoreDNS configuration, that the cluster DNS server will resolve the declared zones using a specific DNS resolver.

To achieve this, it is required to add a line to the CoreDNS Corefile for each DNS zone with the corresponding DNS resolver ip address by setting forward myzone.local 10.0.0.255

To apply this configuration in your k3s, where CoreDNS is provided using a manifest file on each k3s master node, create a override file at /var/lib/rancher/k3s/server/manifests/coredns.override.yaml with the following content (which is basically a copy of the original ConfigMap with just the additional changes)

apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        health
        ready
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          fallthrough in-addr.arpa ip6.arpa
        }
        hosts /etc/coredns/NodeHosts {
          ttl 60
          reload 15s
          fallthrough
        }
        prometheus :9153
        forward myzone.local 10.0.0.255
        forward . /etc/resolv.conf
        log
        cache 60
        loop
        reload
        loadbalance
    }

Make sure to create this file on every master node in your k3s cluster, otherwise it will not be applied or only applied temporarly.