How to check the default CoreDNS configmap of a Rancher Kubernetes Engine (RKE) Kubernetes version

• Rancher 2.6.x/ 2.7/2.8.x
• CoreDNS default configuration

Task

You might have modified the default configmap for CoreDNS using Rancher Kubernetes Engine's (RKE) cluster configuration YAML (cluster.yml). In this case, you may want to know the default configmap before upgrading Kubernetes. This verification step will help you to add all of the default/mandatory parameters to the modified configmap in RKE's cluster.yml, upon upgrade.

Pre-requisites

• Python installed.

Resolution

Download the kontainer-metadata according to the Rancher version you are running.

Rancher 2.6.x:

curl -O https://raw.githubusercontent.com/rancher/kontainer-driver-metadata/release-v2.6/data/data.json

Rancher v2.7.x:

curl -O https://raw.githubusercontent.com/rancher/kontainer-driver-metadata/dev-v2.7/data/data.json

Rancher v2.8.x:

curl -O https://raw.githubusercontent.com/rancher/kontainer-driver-metadata/dev-v2.8/data/data.json

Get the available template list:

python -c "import sys, json; d=json.load(sys.stdin)['K8sVersionedTemplates']['coreDNS']; print (json.dumps(d,indent=4))" <data.json

Output:

{
    ">=1.16.0-alpha <1.17.0-alpha": "coredns-v1.16",
    ">=1.17.0-alpha <1.20.15-rancher1-2": "coredns-v1.17",
    ">=1.20.15-rancher1-2 <1.21.0-rancher1-1": "coredns-v1.8.3-rancher2",
    ">=1.21.0-rancher1-1 <1.21.9-rancher1-2": "coredns-v1.8.3",
    ">=1.21.9-rancher1-2": "coredns-v1.8.3-rancher2",
    ">=1.8.0-rancher0 <1.16.0-alpha": "coredns-v1.8"
}

Translation of one of the entries from the list is as follows:

">=1.21.9-rancher1-2": "coredns-v1.8.3-rancher2",

If the Kubernetes version is greater than or equal to 1.21.9-rancher1-2 , then the CoreDNS key we have to use in the next step is coredns-v1.8.3-rancher2.

python -c "import sys, json; print (json.load(sys.stdin)['K8sVersionedTemplates']['templateKeys']['coredns-v1.8.3-rancher2'])" <data.json

The configmap will be printed on the screen along with other YAML template specs associated with CoreDNS.

Sample output:

---
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        health {
          lameduck 5s
        }
        ready
        kubernetes {{.ClusterDomain}} {{ if .ReverseCIDRs }}{{ .ReverseCIDRs }}{{ else }}{{ "in-addr.arpa ip6.arpa" }}{{ end }} {
          pods insecure
          fallthrough in-addr.arpa ip6.arpa
        }
        prometheus :9153
	{{- if .UpstreamNameservers }}
        forward . {{range $i, $v := .UpstreamNameservers}}{{if $i}} {{end}}{{.}}{{end}}
	{{- else }}
        forward . "/etc/resolv.conf"
	{{- end }}
        cache 30
        loop
        reload
        loadbalance
    } # STUBDOMAINS - Rancher specific change
---