How to change the cluster.local default domain on kubernetes 1.9 deployed with kubeadm?

I had a similar problem where I have been porting a microservices based application to Kubernetes. Changing the internal DNS zone to cluster.local was going to be a fairly complex task that we didn't really want to deal with.

In our case, we switched from KubeDNS to CoreDNS, and simply enabled the coreDNS rewrite plugin to translate our our.internal.domain to ourNamespace.svc.cluster.local.

After doing this, the corefile part of our CoreDNS configmap looks something like this:

data:  Corefile: |    .:53 {        errors        health        kubernetes cluster.local in-addr.arpa ip6.arpa {          pods insecure          upstream          fallthrough in-addr.arpa ip6.arpa        }        prometheus :9153        rewrite name substring our.internal.domain ourNamespace.svc.cluster.local        proxy . /etc/resolv.conf        cache 30    }

This enables our kubernetes services to respond on both the default DNS zone and our own zone.

I deployed internal instance of ingress controller, and added CNAME to coreDNS config. to deploy internal nginx-ingress

helm install int -f ./values.yml stable/nginx-ingress --namespace ingress-nginx

values.yaml:

controller:  ingressClass: 'nginx-internal'  reportNodeInternalIp: true  service:    enabled: true    type: ClusterIP

to edit coreDNS config: KUBE_EDITOR=nano kubectl edit configmap coredns -n kube-system

My coredns file:

apiVersion: v1data:  Corefile: |    .:53 {        reload 5s        log        errors        health {          lameduck 5s        }        ready        template ANY A int {          match "^([^.]+)\.([^.]+)\.int\.$"          answer "{{ .Name }} 60 IN CNAME int-nginx-ingress-controller.ingress-nginx.svc.cluster.local"          upstream 127.0.0.1:53        }        template ANY CNAME int {          match "^([^.]+)\.([^.]+)\.int\.$"          answer "{{ .Name }} 60 IN CNAME int-nginx-ingress-controller.ingress-nginx.svc.cluster.local"          upstream 127.0.0.1:53        }        kubernetes cluster.local in-addr.arpa ip6.arpa {          pods insecure          upstream          fallthrough in-addr.arpa ip6.arpa        }        prometheus :9153        forward . "/etc/resolv.conf"        cache 30        loop        reload        loadbalance    }kind: ConfigMapmetadata:  annotations:    kubectl.kubernetes.io/last-applied-configuration: |      {"apiVersion":"v1","data":{"Corefile":".:53 {\n    errors\n    health {\n      lameduck 5s\n    }\n    ready\n    kubernetes >  creationTimestamp: "2020-02-27T16:02:20Z"  name: coredns  namespace: kube-system  resourceVersion: "16293672"  selfLink: /api/v1/namespaces/kube-system/configmaps/coredns  uid: 8f0ebf84-6451-4f9b-a6e1-c386d44f2d43

If you now add to ingress resource ..int domain, and add proper annotation to use nginx-internal ingress, you can have shorter domain, for example you can configure it like this in jenkins helm chart:

master:  ingress:    annotations:      kubernetes.io/ingress.class: nginx-internal    enabled: true    hostName: jenkins.devtools.int

I assume you are using CoreDNS.

You can change the cluster base DNS by editing the kubelet config file on ALL Nodes, located here /var/lib/kubelet/config.yaml or set the clusterDomain during kubeadm init.

Change

clusterDomain: cluster.local

to:

clusterDomain: my.new.domain

Now you also need to change the CoreDNS configuration. CoreDNS uses a ConfigMap for this. You can get your current CoreDNS ConfigMap by running

kubectl get -n kube-system cm/coredns -o yaml

Then change

kubernetes cluster.local in-addr.arpa ip6.arpa {    ...}

to match your new domain like this:

kubernetes my.new.domain in-addr.arpa ip6.arpa {    ...}

Now apply the changes to the CoreDNS ConfigMap.If you restart kubelet and your CoreDNS pods then your cluster should use the new domain.

If you have for example a service called grafana-service, this can now be accessed with this address:grafana-service.default.svc.my.new.domain

# kubectl get serviceNAME              TYPE         CLUSTER-IP      EXTERNAL-IP   PORT(S)    AGEgrafana-service   ClusterIP    <Internal-IP>   <none>        3000/TCP   100m# nslookup grafana-service.default.svc.my.new.domainServer:    <Internal-IP>Address 1: <Internal-IP> kube-dns.kube-system.svc.my.new.domainName:      grafana-service.default.svc.my.new.domainAddress 1: <Internal-IP> grafana-service.default.svc.my.new.domain