Create CoreDNS K8s charm for better configurability (custom Corefile)

[Drew Freiberger]

We have a customer who would like to have the DNS configuration map updated in their cluster, but have found that the charm reverts this setting on each update-status when dns-provider is set to auto or core-dns on kubernetes-master.

$ kubectl describe cm/coredns -n kube-system
Name: coredns
Namespace: kube-system
Labels: <none>
Annotations: kubectl.kubernetes.io/last-applied-configuration:
                {"apiVersion":"v1","data":{"Corefile":".:53 {\n errors\n health\n ready\n kubernetes cluster.local in-addr.arpa ip6.arpa {\n ...

Data
====
Corefile:
----
.:53 {
    errors
    health
    ready
    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
}

To workaround, we have set dns-provider=none, deployed a manual core-dns deployment, and set kubelet-extra-specs to point to ClusterDNS set to the ClusterIP of the manually deployed kube-dns service. Customer would like to not have to own and manage the core-dns deployment, but just charm config the ConfigMap.

[George Kraft (cynerva)]

We don't have plans to add this to kubernetes-master/cdk-addons, but we do plan to move coredns out and create a new k8s charm for it that will be more configurable.

[Cory Johns (johnsca)]

https://github.com/charmed-kubernetes/charm-coredns/pull/3
https://github.com/charmed-kubernetes/charm-kubernetes-master/pull/126
https://github.com/charmed-kubernetes/kubernetes-docs/pull/478