8、kubernetes组件——CoreDns

kubernetes组件——CoreDns

在Kubernetes集群推荐使用Service Name作为服务的访问地址，因此需要一个Kubernetes集群范围的DNS服务实现从Service Name到Cluster Ip的解析，这就是Kubernetes基于DNS的服务发现功能。

从Kubernetes 1.11开始，可使用CoreDNS作为Kubernetes的DNS插件进入GA状态，这意味着CoreDNS将通过安装工具包（例如kubeadm，kube-up，minikube和kops）作为Kubernetes中的标准功能提供。Kubernetes推荐使用CoreDNS作为集群内的DNS服务。

网站：

https://github.com/coredns/coredns
https://coredns.io/
https://github.com/coredns/deployment/tree/master/kubernetes

安装

准备工作

由于coredns安装时默认使用的时国外源，这里我们将镜像下载到本地harbor

root@k8s-deploy-01:~# docker pull coredns/coredns:1.9.3
root@k8s-deploy-01:~# docker tag coredns/coredns:1.9.3  xmtx.harbor.com/baseimages/coredns-coredns:1.9.3
root@k8s-deploy-01:~# docker push xmtx.harbor.com/baseimages/coredns-coredns:1.9.3

corefile配置

#corefile默认配置
data:
  Corefile: |
    .:53 {
        errors
        health {
            lameduck 5s
        }
        ready
        kubernetes __DNS__DOMAIN__ in-addr.arpa ip6.arpa {
            pods insecure
            fallthrough in-addr.arpa ip6.arpa
            ttl 30
        }
        prometheus :9153
        forward . /etc/resolv.conf {
            max_concurrent 1000
        }
        cache 30
        loop
        reload
        loadbalance
    }

• errors：错误记录到标准输出。
• health：在 处提http://localhost:8080/health供 CoreDNS 的健康报告。
• ready：在端口 8181 上提供的一个 HTTP 末端，当所有能够表达自身就绪的插件都已就绪时，在此末端返回 200 OK。
• kubernetes：CoreDNS 将基于 Kubernetes service name 进行DNS 查询。 你可以使用 ttl 来定制响应的 TTL。默认值是 5 秒钟。TTL 的最小值可以是 0 秒，最大值为 3600 秒。将 TTL 设置为 0 可以禁止对 DNS 记录进行缓存。
• pods insecure 选项是为了与 kube-dns 向后兼容。你可以使用 pods verified 选项，该选项使得 仅在相同名称空间中存在具有匹配 IP 的 Pod 时才返回 A 记录。如果你不使用 Pod 记录，则可以使用 pods disabled选项。
• prometheus：CoreDNS 的度量指标值以 Prometheus的 key-value 格式在http://localhost:9153/metrics 上提供。
• forward: 不在 Kubernetes 集群域内的任何查询都将转发到 预定义的解析器 (/etc/resolv.conf).
• cache：启用service解析缓存，单位为秒。
• loop：检测转发环，如果发现是死循环，则强制中止 CoreDNS 进程(kubernetes会重建)。
• reload：允许自动重新加载已更改的 Corefile。 编辑 ConfigMap 配置后，默认两分钟后会自动加载，以使更改生效。
• loadbalance：轮询DNS域名解析，如果一个域名存在多个记录则轮训解析。

Corefile

# __MACHINE_GENERATED_WARNING__

apiVersion: v1
kind: ServiceAccount
metadata:
  name: coredns
  namespace: kube-system
  labels:
      kubernetes.io/cluster-service: "true"
      addonmanager.kubernetes.io/mode: Reconcile
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  labels:
    kubernetes.io/bootstrapping: rbac-defaults
    addonmanager.kubernetes.io/mode: Reconcile
  name: system:coredns
rules:
- apiGroups:
  - ""
  resources:
  - endpoints
  - services
  - pods
  - namespaces
  verbs:
  - list
  - watch
- apiGroups:
  - ""
  resources:
  - nodes
  verbs:
  - get
- apiGroups:
  - discovery.k8s.io
  resources:
  - endpointslices
  verbs:
  - list
  - watch
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  annotations:
    rbac.authorization.kubernetes.io/autoupdate: "true"
  labels:
    kubernetes.io/bootstrapping: rbac-defaults
    addonmanager.kubernetes.io/mode: EnsureExists
  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
  labels:
      addonmanager.kubernetes.io/mode: EnsureExists
data:
  Corefile: |
    .:53 {
        errors
        health {
            lameduck 5s
        }
        ready
        kubernetes xmtx.local  in-addr.arpa ip6.arpa {    #将xmtx.local替换为自己的
            pods insecure
            fallthrough in-addr.arpa ip6.arpa
            ttl 30
        }
        prometheus :9153
        forward . /etc/resolv.conf {
            max_concurrent 1000
        }
        cache 30
        loop
        reload
        loadbalance
    }
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: coredns
  namespace: kube-system
  labels:
    k8s-app: kube-dns
    kubernetes.io/cluster-service: "true"
    addonmanager.kubernetes.io/mode: Reconcile
    kubernetes.io/name: "CoreDNS"
spec:
  # replicas: not specified here:
  # 1. In order to make Addon Manager do not reconcile this replicas parameter.
  # 2. Default is 1.
  # 3. Will be tuned in real time if DNS horizontal auto-scaling is turned on.
  replicas: 2             #副本数量
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxUnavailable: 1
  selector:
    matchLabels:
      k8s-app: kube-dns
  template:
    metadata:
      labels:
        k8s-app: kube-dns
    spec:
      securityContext:
        seccompProfile:
          type: RuntimeDefault
      priorityClassName: system-cluster-critical
      serviceAccountName: coredns
      affinity:
        podAntiAffinity:
          preferredDuringSchedulingIgnoredDuringExecution:
          - weight: 100
            podAffinityTerm:
              labelSelector:
                matchExpressions:
                  - key: k8s-app
                    operator: In
                    values: ["kube-dns"]
              topologyKey: kubernetes.io/hostname
      tolerations:
        - key: "CriticalAddonsOnly"
          operator: "Exists"
      nodeSelector:
        kubernetes.io/os: linux
      containers:
      - name: coredns
        image: xmtx.harbor.com/baseimages/coredns-coredns:1.9.3      #使用本地harbor下载镜像
        imagePullPolicy: IfNotPresent
        resources:
          limits:  #资源限制
            memory: 512Mi  
            cpu: 200m
          requests:
            cpu: 200m    
            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
        livenessProbe:
          httpGet:
            path: /health
            port: 8080
            scheme: HTTP
          initialDelaySeconds: 60
          timeoutSeconds: 5
          successThreshold: 1
          failureThreshold: 5
        readinessProbe:
          httpGet:
            path: /ready
            port: 8181
            scheme: HTTP
        securityContext:
          allowPrivilegeEscalation: false
          capabilities:
            add:
            - NET_BIND_SERVICE
            drop:
            - all
          readOnlyRootFilesystem: true
      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"
    addonmanager.kubernetes.io/mode: Reconcile
    kubernetes.io/name: "CoreDNS"
spec:
  selector:
    k8s-app: kube-dns
  clusterIP: 10.100.0.2   #自己的clusterIP
  ports:
  - name: dns
    port: 53
    protocol: UDP
  - name: dns-tcp
    port: 53
    protocol: TCP
  - name: metrics
    port: 9153
    protocol: TCP

应用coredns

root@k8s-deploy-01:/opt/soft# kubectl apply -f coredns.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

验证

root@k8s-master-01:~# kubectl get pods -A
kube-system   coredns-8fc4ff95b-bv5ct                    1/1     Running   0             5m17s
kube-system   coredns-8fc4ff95b-fd8mh                    1/1     Running   0             13m
#进入容器ping外网
root@k8s-master-01:~# kubectl exec -it net-test1 bash -n myserver
[root@net-test1 /]# ping www.baidu.com
PING www.wshifen.com (103.235.46.40) 56(84) bytes of data.
64 bytes from 103.235.46.40 (103.235.46.40): icmp_seq=1 ttl=127 time=202 ms
64 bytes from 103.235.46.40 (103.235.46.40): icmp_seq=2 ttl=127 time=204 ms