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

[brandond]

From @andrewroffey

Allow CoreDNS override extensions in default Corefile

Other K8s implementations such as AKS[1] and DigitalOcean[2] have similar configuration:
1: https://learn.microsoft.com/en-us/azure/aks/coredns-custom
2: https://docs.digitalocean.com/products/kubernetes/how-to/customize-coredns/

This would enable ConfigMaps to adjust the CoreDNS settings without creating a whole copy of coredns.yaml.

[aganesh-suse]

k3s commit: b66a118

kubectl get configmaps --namespace=kube-system coredns -o yaml

shows .:53 having:

import /etc/coredns/custom/*.override

k apply -f 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
    }

k delete pod coredns-77ccd57875-l7fnq -n kube-system
k get pods -A 
NAME                                     READY   STATUS             RESTARTS   AGE
coredns-77ccd57875-l7fnq                 1/1     Running            0          4s
helm-install-apache-q6hgj                0/1     Completed          0          141m
helm-install-traefik-6jcqb               0/1     Completed          1          121m
helm-install-traefik-crd-jglnf           0/1     Completed          0          152m
local-path-provisioner-957fdf8bc-2c94g   1/1     Running            0          152m
metrics-server-648b5df564-qfb24          1/1     Running            0          152m
svclb-traefik-429dbe72-4424p             2/2     Running            0          120m
svclb-traefik-429dbe72-6kbzs             2/2     Running            0          120m
svclb-traefik-429dbe72-8nhs8             2/2     Running            0          120m
svclb-traefik-429dbe72-pc764             2/2     Running            0          120m

kubectl describe pod coredns-77ccd57875-l7fnq -n kube-system

shows:

Volumes:
  config-volume:
    Type:      ConfigMap (a volume populated by a ConfigMap)
    Name:      coredns
    Optional:  false
  custom-config-volume:
    Type:      ConfigMap (a volume populated by a ConfigMap)
    Name:      coredns-custom
    Optional:  true

kubectl get configmaps --namespace=kube-system coredns-custom -o yaml

displays the updated config from above ^^

k -n kube-system logs pod/coredns-77ccd57875-l7fnq
.:53
why.local.:53
[INFO] plugin/reload: Running configuration SHA512 = 2e241110619e8c5ec252af702135deed1c90a4dab89bd410554347468d0c1d064fd0571098262fc611b97ef366f864627bb1c309b078c2dca755f55aae118364
CoreDNS-1.10.1
linux/amd64, go1.20, 055b2c3
[INFO] 127.0.0.1:50386 - 25704 "HINFO IN 735618883049447441.6322315480693977090. udp 56 false 512" NXDOMAIN qr,rd,ra 131 0.019542746s

[gfrankliu]

I hope the logs won't be filled with warnings when people aren't using this feature and the .override doesn't exist. Currently we have this issue with .server in the configmap:

kubectl -n kube-system logs coredns-59b4f5bbd5-4shsp
...
[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.server
[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.server
[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.server
[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.server
[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.server
[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.server
[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.server
[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.server
[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.server

[gfrankliu]

I just installed the latest k3s and confirmed this feature adds more warning in the log. Every 30 seconds, below two lines are added in coredns log:

[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.override
[WARNING] No files matching import glob pattern: /etc/coredns/custom/*.server

@brandond Can this be fixed?

[brandond]

@gfrankliu no. See #4919 (comment) and coredns/coredns#3600 (comment)