Custom Wildcard Domain for CoreDNS

Image by KTC Services from Pixabay

I normally forward the CoreDNS to an external DNS server for some custom wildcard domain requirement. This is achieved by adding the following forward plugin into the coredns config map,

apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    apps.k3s.io.cpak:53 {
      errors
      cache 30
      forward . 192.168.10.79
      reload
    }
   .: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 . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }
    import /etc/coredns/custom/*.server

But the precondition is that the UDP traffic is allowed from the cluster nodes to the custom DNS server.

I have a K3S cluster where the UDP traffic between the nodes are not allowed. To overcome this, let’s explore the built-in custom “file” plugin support of CoreDNS.

RFC1035 Style File

Create a following text file,

$ORIGIN {{ .domain }}.
$TTL 60
@ IN SOA ns1.{{ .domain }}. hostmaster.{{ .domain }}. (…