Hybrid AKS - CoreDNS customisation for internal DNS servers

[andytrickett]

AKS uses CoreDNS for all DNS functions within the cluster. As AKS is a managed cluster, making changes to the main configuration (CoreFile) is not supported.

Customised configuration is possible using ConfigMaps. The default set of properties can be seen using:

> kubectl -n kube-system get configmaps coredns -o yaml (snipped example below):

apiVersion: v1
data:
  Corefile: |
    .:53 {
        errors
        ready
        health
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          fallthrough in-addr.arpa ip6.arpa
        }
        prometheus :9153
        forward . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
        import custom/*.override
    }
    import custom/*.server
kind: ConfigMap

As you can see, name resolution is via /etc/resolv.conf (DNS settings on the underlying cluster nodes).

This works fine for many cases, but what happens when you need to use specific DNS servers for your internal hosts? In this case, you can add customisations via the coredns-custom configmap.

To add new configuration follow the steps below :

1. Create a new yaml file with the following contents:

internaldns.yaml

apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns-custom
  namespace: kube-system
data:
  myinternal.server: | # you may select any name here, but it must end with the .server file extension
    myinternal.com.au:53 {
        errors
        cache 30
        forward . 10.1.0.1 10.1.0.2 # Your internal DNS server(s)
    }

2. Apply the new configuration using kubectl :

> kubectl apply -f internaldns.yaml

Expected output :
configmap/coredns-custom configured

3. Kill existing coredns pods

To save waiting for the changes to propagate, kill the existing pods :

> kubectl delete pod --namespace kube-system --selector k8s-app=kube-dns

4. Complete!
You now have your internal hosts being resolved by your internal DNS servers.

To check:
- Ensure coredns pods are showing the mount for coredns-custom
- Ensure config from your yaml file is present in the running configmap