[release-1.24] Allow adding configuration to coredns .:53 block via *.override keys in coredns-custom ConfigMap

[brandond]

• Allow adding configuration to coredns .:53 block via *.override keys in coredns-custom ConfigMap #7639 backport

[ShylajaDevadiga]

Validated using v1.24.15-rc1+k3s1

Environment Details

Infrastructure
Cloud EC2 instance

Node(s) CPU architecture, OS, and Version:
Ubuntu 22.04

Applying some custom settings for the default server block as below and restarting coredns pod

cat custom.yaml 
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns-custom
  namespace: kube-system
data:
  log.override: |
    log
  custom.server: |
    why.local {
      forward . 127.0.0.1
    }

$ kubectl get configmaps --namespace=kube-system coredns-custom -o yaml
apiVersion: v1
data:
  custom.server: |
    why.local {
      forward . 127.0.0.1
    }
  log.override: |
    log
kind: ConfigMap
metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"v1","data":{"custom.server":"why.local {\n  forward . 127.0.0.1\n}\n","log.override":"log\n"},"kind":"ConfigMap","metadata":{"annotations":{},"name":"coredns-custom","namespace":"kube-system"}}
  creationTimestamp: "2023-06-15T22:43:06Z"
  name: coredns-custom
  namespace: kube-system
  resourceVersion: "2618"
  uid: 5991edbd-2e8f-4857-9adb-db69f166b8a8

$ kubectl delete  pod -n kube-system coredns-74448699cf-lt46z 
pod "coredns-74448699cf-lt46z" deleted

$ kubectl get pods -A
NAMESPACE     NAME                                          READY   STATUS      RESTARTS   AGE
kube-system   local-path-provisioner-597bc7dccd-f7n8s       1/1     Running     0          152m
kube-system   helm-install-traefik-crd-8x2x9                0/1     Completed   0          152m
kube-system   helm-install-traefik-bdvw8                    0/1     Completed   1          152m
kube-system   svclb-traefik-afb3f314-fs4bp                  2/2     Running     0          152m
kube-system   traefik-7467b667d9-m66ft                      1/1     Running     0          152m
kube-system   metrics-server-749b574fc-9p6bc                1/1     Running     0          152m
kube-system   svclb-nginx-loadbalancer-svc-453fd5ea-rnkb5   1/1     Running     0          148m
default       test-loadbalancer-5dc8984576-cx89t            1/1     Running     0          148m
default       test-loadbalancer-5dc8984576-khlq7            1/1     Running     0          148m
kube-system   coredns-74448699cf-h5cmf                      1/1     Running     0          9s

Validated custom config has taken effect as seen in the logs

$ kubectl logs -n kube-system pod/coredns-74448699cf-h5cmf   
.:53
why.local.:53
[INFO] plugin/reload: Running configuration SHA512 = 2e241110619e8c5ec252af702135deed1c90a4dab89bd410554347468d0c1d064fd0571098262fc611b97ef366f864627bb1c309b078c2dca755f55aae118364
CoreDNS-1.10.1
linux/amd64, go1.20, 055b2c3
[INFO] 127.0.0.1:38248 - 17065 "HINFO IN 3693267141280669377.4292237142184114153. udp 57 false 512" NXDOMAIN qr,rd,ra 132 0.013195785s