KTHW - DNS inside a Pod Network

August 16, 2020 - Reading time: 3 minutes

DNS inside a Pod Network

The DNS service is used by pods to find other pods. The service will also set the DNS settings inside the containers, this is useful to reach other pods inside the cluster.

The original guide I was following to deploy the K8S cluster uses kube-dns, but a newer version of the guide uses coreDNS. Here are the main differences between the two services:

• CoreDNS is a single container per instance, vs kube-dns which uses three.
• Kube-dns uses dnsmasq for caching, which is single threaded C. CoreDNS is multi-threaded Go.
• CoreDNS enables negative caching in the default deployment. Kube-dns does not.

Source: https://coredns.io/2018/11/27/cluster-dns-coredns-vs-kube-dns

Due to the fact that I have low-resource workers, I decided to go with CoreDNS.

cloud_user@client:~$ curl -sLO https://storage.googleapis.com/kubernetes-the-hard-way/coredns-1.7.0.yaml
cloud_user@client:~$ grep kind coredns-1.7.0.yaml
kind: ServiceAccount
kind: ClusterRole
kind: ClusterRoleBinding
  kind: ClusterRole
- kind: ServiceAccount
kind: ConfigMap
kind: Deployment
kind: Service

The yaml file contains a ServiceAccount (used for processes inside a container to contact the apiserver) Then creates a cluster Role/Binding. A ConfigMap is used to pass the coreDNS configuration to the container. Then a deployment is created with two pods and a new service with a clusterIP of 10.32.0.10

cloud_user@client:~$ kubectl create -f coredns-1.7.0.yaml
serviceaccount/coredns created
clusterrole.rbac.authorization.k8s.io/system:coredns created
clusterrolebinding.rbac.authorization.k8s.io/system:coredns created
configmap/coredns created
deployment.apps/coredns created
service/kube-dns created

Once the service is deployed:

cloud_user@client:~$ kubectl get deployment -n kube-system
NAME      READY   UP-TO-DATE   AVAILABLE   AGE
coredns   2/2     2            2           40s
cloud_user@client:~$ kubectl get svc -n kube-system
NAME       TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)                  AGE
kube-dns   ClusterIP   10.32.0.10   <none>        53/UDP,53/TCP,9153/TCP   100s
cloud_user@client:~$ kubectl get pods -l k8s-app=kube-dns -n kube-system
NAME                       READY   STATUS    RESTARTS   AGE
coredns-5677dc4cdb-6ssp5   1/1     Running   0          12m
coredns-5677dc4cdb-m5xtm   1/1     Running   0          12m

Now to test the new service, we launch a busybox pod:

cloud_user@client:~$ kubectl run busybox --image=busybox:1.28 --command -- sleep 3600
pod/busybox created
cloud_user@client:~$ kubectl exec -ti  busybox -- cat /etc/resolv.conf
search default.svc.cluster.local svc.cluster.local cluster.local mylabserver.com
nameserver 10.32.0.10
options ndots:5
cloud_user@client:~$ kubectl exec -ti  busybox -- nslookup kubernetes
Server:    10.32.0.10
Address 1: 10.32.0.10 kube-dns.kube-system.svc.cluster.local

Name:      kubernetes
Address 1: 10.32.0.1 kubernetes.default.svc.cluster.local