In addition to changing /etc/systemd/system/kubelet.service.d/10-kubeadm.conf, you should run kubeadm init with --service-dns-domain cluster.mydomain.local which will create correct manifest for kube-dns.

It's hard to tell why your mod didn't work without seeing what your current config is. Perhaps you can post the output of:

kubectl get pod -n kube-system -l k8s-app=kube-dns -o jsonpath={.items[0].spec.containers[0]}

so we can see what you got running.

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: v1
data:
  Corefile: |
    .:53 {
        reload 5s
        log
        errors
        health {
          lameduck 5s
        }
        ready
        template ANY A int {
          match "^([^.]+)\.([^.]+)\.int\.
quot;
          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\.
quot;
          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: ConfigMap
metadata:
  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

If you have deployed k8s with kubeadm, then you can change cluster.local in /var/lib/kubelet/config.yaml on every node. Also change it in kubeadm-config and kubelet-config-1.17 configmaps (kube-system namespace) if you are planing to add more nodes to cluster. And don't forget to restart nodes.

When you modify the /etc/kubernetes/manifests/ yaml files then you would need to restart kubelet again.

Additionally, if that doesn't work, double check the kubelet logs to see that the proper yaml files are being loaded.