CoreDNS Adjustment

1.Background

In general, CoreDNS uses deployment as workload. But in cloud-edge scenario, based on VPN tunnel that provided by Raven, domain name resolution cross NodePool will cause some latency, so we also recommend end users to use Daemonset or YurtAppDaemon to deploy CoreDNS. At the same time, we should also set the topologyKeys of kube-dns service as NodePool or HostName.

2 Configure CoreDNS Service

Add annotation to coredns service, which will make sure domain name resolution can be handled by CoreDNS instance in the same NodePool.

kubectl annotate svc kube-dns -n kube-system openyurt.io/topologyKeys='openyurt.io/nodepool'

The results of modifications:

apiVersion: v1
kind: Service
metadata:
  annotations:
    openyurt.io/topologyKeys: openyurt.io/nodepool
    prometheus.io/port: "9153"
    prometheus.io/scrape: "true"
  labels:
    k8s-app: kube-dns
    kubernetes.io/cluster-service: "true"
    kubernetes.io/name: KubeDNS
  name: kube-dns
  namespace: kube-system
spec:
  clusterIP: xx.xx.xx.xx
  ports:
  - name: dns
    port: 53
    protocol: UDP
    targetPort: 53
  - name: dns-tcp
    port: 53
    protocol: TCP
    targetPort: 53
  - name: metrics
    port: 9153
    protocol: TCP
    targetPort: 9153
  selector:
    k8s-app: kube-dns
  sessionAffinity: None
  type: ClusterIP

3 Use CoreDNS DaemonSet

The original CoreDNS is deployed by DaemonSet, please modify the settings manually (the CoreDNS image version can be adjusted to demand).

apiVersion: apps/v1
kind: DaemonSet
metadata:
  labels:
    k8s-app: kube-dns
  name: coredns
  namespace: kube-system
spec:
  selector:
    matchLabels:
      k8s-app: kube-dns
  template:
    metadata:
      labels:
        k8s-app: kube-dns
    spec:
      containers:
      - args:
        - -conf
        - /etc/coredns/Corefile
        image: registry.aliyuncs.com/google_containers/coredns:1.7.0
        livenessProbe:
          failureThreshold: 5
          httpGet:
            path: /health
            port: 8080
            scheme: HTTP
          initialDelaySeconds: 60
          periodSeconds: 10
          successThreshold: 1
          timeoutSeconds: 5
        name: coredns
        ports:
        - containerPort: 53
          name: dns
          protocol: UDP
        - containerPort: 53
          name: dns-tcp
          protocol: TCP
        - containerPort: 9153
          name: metrics
          protocol: TCP
        readinessProbe:
          failureThreshold: 3
          httpGet:
            path: /ready
            port: 8181
            scheme: HTTP
          periodSeconds: 10
          successThreshold: 1
          timeoutSeconds: 1
        resources:
          limits:
            memory: 170Mi
          requests:
            cpu: 100m
            memory: 70Mi
        securityContext:
          allowPrivilegeEscalation: false
          capabilities:
            add:
            - NET_BIND_SERVICE
            drop:
            - all
          readOnlyRootFilesystem: true
        volumeMounts:
        - mountPath: /etc/coredns
          name: config-volume
          readOnly: true
      dnsPolicy: Default
      nodeSelector:
        kubernetes.io/os: linux
      priorityClassName: system-cluster-critical
      serviceAccountName: coredns
      tolerations:
      - operator: Exists
      - key: CriticalAddonsOnly
        operator: Exists
      - effect: NoSchedule
        key: node-role.kubernetes.io/master
      volumes:
      - configMap:
          defaultMode: 420
          items:
          - key: Corefile
            path: Corefile
          name: coredns
        name: config-volume

4 Scale Down CoreDNS Deployment Replicas

Only support when CoreDNS is deployed by deployment workload.

kubectl scale --replicas=0 deployment/coredns -n kube-system