deploying the dns cluster addon

we will have dns based service discovery for inside running applications in place by using CoreDNS - the k8s builtin dns application

deploy CoreDNS cluster addon on master

# apply coredns.yaml
> kubectl apply -f \
  https://raw.githubusercontent.com/mmumshad/kubernetes-the-hard-way/master/deployments/coredns.yaml

# output
  ---
  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


apiVersion: v1
kind: ServiceAccount
metadata:
  name: coredns
  namespace: kube-system
---
apiVersion: rbac.authorization.k8s.io/v1beta1
kind: ClusterRole
metadata:
  labels:
    kubernetes.io/bootstrapping: rbac-defaults
  name: system:coredns
rules:
- apiGroups:
  - ""
  resources:
  - endpoints
  - services
  - pods
  - namespaces
  verbs:
  - list
  - watch
---
apiVersion: rbac.authorization.k8s.io/v1beta1
kind: ClusterRoleBinding
metadata:
  annotations:
    rbac.authorization.kubernetes.io/autoupdate: "true"
  labels:
    kubernetes.io/bootstrapping: rbac-defaults
  name: system:coredns
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: system:coredns
subjects:
- kind: ServiceAccount
  name: coredns
  namespace: kube-system
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        health
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          upstream
          fallthrough in-addr.arpa ip6.arpa
        }
        prometheus :9153
        proxy . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: coredns
  namespace: kube-system
  labels:
    k8s-app: kube-dns
    kubernetes.io/name: "CoreDNS"
spec:
  replicas: 2
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxUnavailable: 1
  selector:
    matchLabels:
      k8s-app: kube-dns
  template:
    metadata:
      labels:
        k8s-app: kube-dns
    spec:
      serviceAccountName: coredns
      tolerations:
        - key: node-role.kubernetes.io/master
          effect: NoSchedule
        - key: "CriticalAddonsOnly"
          operator: "Exists"
      containers:
      - name: coredns
        image: coredns/coredns:1.2.2
        imagePullPolicy: IfNotPresent
        resources:
          limits:
            memory: 170Mi
          requests:
            cpu: 100m
            memory: 70Mi
        args: [ "-conf", "/etc/coredns/Corefile" ]
        volumeMounts:
        - name: config-volume
          mountPath: /etc/coredns
          readOnly: true
        ports:
        - containerPort: 53
          name: dns
          protocol: UDP
        - containerPort: 53
          name: dns-tcp
          protocol: TCP
        - containerPort: 9153
          name: metrics
          protocol: TCP
        securityContext:
          allowPrivilegeEscalation: false
          capabilities:
            add:
            - NET_BIND_SERVICE
            drop:
            - all
          readOnlyRootFilesystem: true
        livenessProbe:
          httpGet:
            path: /health
            port: 8080
            scheme: HTTP
          initialDelaySeconds: 60
          timeoutSeconds: 5
          successThreshold: 1
          failureThreshold: 5
      dnsPolicy: Default
      volumes:
        - name: config-volume
          configMap:
            name: coredns
            items:
            - key: Corefile
              path: Corefile
---
apiVersion: v1
kind: Service
metadata:
  name: kube-dns
  namespace: kube-system
  annotations:
    prometheus.io/port: "9153"
    prometheus.io/scrape: "true"
  labels:
    k8s-app: kube-dns
    kubernetes.io/cluster-service: "true"
    kubernetes.io/name: "CoreDNS"
spec:
  selector:
    k8s-app: kube-dns
  clusterIP: 10.96.0.10
  ports:
  - name: dns
    port: 53
    protocol: UDP
  - name: dns-tcp
    port: 53
    protocol: TCP

list kube-dns deployment

> kubectl get pods -l k8s-app=kube-dns -n kube-system                                                     
  --- 
  NAME                       READY   STATUS    RESTARTS   AGE
  coredns-69cbb76ff8-c522h   1/1     Running   0          48s
  coredns-69cbb76ff8-x9mcd   1/1     Running   0          48s

verfify

# create a busybox deployment
> kubectl run --generator=run-pod/v1  busybox \
  --image=busybox:1.28 --command -- sleep 3600

# list busyboy deployment
> kubectl get pods -l run=busybox
  ---
  NAME      READY   STATUS    RESTARTS   AGE
  busybox   1/1     Running   0          29s

execute a dns lookup for the kubernetes service inside the busybox pod

> kubectl exec -ti busybox -- nslookup kubernetes
  ---
  Server:    10.96.0.10
  Address 1: 10.96.0.10 kube-dns.kube-system.svc.cluster.local

  Name:      kubernetes
  Address 1: 10.96.0.1 kubernetes.default.svc.cluster.local

— copyleft —

all commands shown on this page are from mmumshad’s fork of “kubernetes-the-hard-way” by kelseyhightower on github