coredns_coredns配置域名

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网上的coredns.yaml文档都是粘贴复制的，不知所以然，授人以鱼不如授人以渔，官方coredns yaml文件下载地址：https://github.com/kubernetes/kubernetes/blob/master/cluster/addons/dns/coredns/coredns.yaml.base

1.下载coredns yaml

wget不下来的手动去复制吧

root@master01:mkdir /data/work/yaml/coredns/
root@master01:~# cd /data/work/yaml/coredns/
root@master01:/data/work/yaml/coredns# wget https://github.com/kubernetes/kubernetes/blob/master/cluster/addons/dns/coredns/coredns.yaml.base 
root@master01:/data/work/yaml/coredns# cp coredns.yaml.base coredns.yaml

Jetbrains全家桶1年46，售后保障稳定

2.修改coredns.yaml

拿到yaml文件需要修改几处配置

#修改k8s集群后缀名称__DNS__DOMAIN__，一般为cluster.local
#77         kubernetes __DNS__DOMAIN__ in-addr.arpa ip6.arpa {
kubernetes cluster.local in-addr.arpa ip6.arpa {

#修改coredns谷歌地址为dockerhub地址，容易下载
#142         image: k8s.gcr.io/coredns/coredns:v1.8.6   
image: coredns/coredns:1.8.6
#修改pod启动内存限制大小，300Mi即可
#146             memory: __DNS__MEMORY__LIMIT__
memory: 300Mi
#修改coredns的svcIP地址，一般为svc网段的第二位，10.100.0.2，第一位为apiserver的svc
#212   clusterIP: __DNS__SERVER__
clusterIP: 10.100.0.2

#修改coredns副本数，默认是1，且没有replicas字段
replicas: 3

coredns.yaml修改前后对比

完整coredns.yaml配置文件如下

cat coredns.yaml

# __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 cluster.local 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
    }
---
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: 3
  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: coredns/coredns:v1.8.6
        imagePullPolicy: IfNotPresent
        resources:
          limits:
            memory: 500Mi
          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
        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
  ports:
  - name: dns
    port: 53
    protocol: UDP
  - name: dns-tcp
    port: 53
    protocol: TCP
  - name: metrics
    port: 9153
    protocol: TCP

3.创建coredns

root@master01:/data/work/yaml/coredns# 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

启动成功

4.测试coredns域名解析功能

部署nginx，tomcat容器及svc进行测试

cat nginx.yaml
kind: Deployment
#apiVersion: extensions/v1beta1
apiVersion: apps/v1
metadata:
  labels:
    app: nginx-deployment-label
  name: nginx-deployment
  namespace: test
spec:
# replicas: 1
  selector:
    matchLabels:
      app: nginx-selector
  template:
    metadata:
      labels:
        app: nginx-selector
    spec:
      containers:
      - name: linux66-nginx-container
        image: nginx
        #command: ["/apps/tomcat/bin/run_tomcat.sh"]
        #imagePullPolicy: IfNotPresent
        imagePullPolicy: Always
        ports:
        - containerPort: 80
          protocol: TCP
          name: http
        - containerPort: 443
          protocol: TCP
          name: https
        env:
        - name: "password"
          value: "123456"
        - name: "age"
          value: "18"
# resources:
# limits:
# cpu: 2
# memory: 2Gi
# requests:
# cpu: 500m
# memory: 1Gi


---
kind: Service
apiVersion: v1
metadata:
  labels:
    app: nginx-service-label
  name: nginx-service
  namespace: test
spec:
  type: NodePort
  ports:
  - name: http
    port: 80
    protocol: TCP
    targetPort: 80
    nodePort: 30006
  - name: https
    port: 443
    protocol: TCP
    targetPort: 443
    nodePort: 30443
  selector:
    app: nginx-selector

tomcat.yaml

kind: Deployment
#apiVersion: extensions/v1beta1
apiVersion: apps/v1
metadata:
  labels:
    app: tomcat-app1-deployment-label
  name: tomcat-app1-deployment
spec:
  replicas: 1
  selector:
    matchLabels:
      app: tomcat-app1-selector
  template:
    metadata:
      labels:
        app: tomcat-app1-selector
    spec:
      containers:
      - name: tomcat-app1-container
        image: tomcat:7.0.94-alpine 
        #command: ["/apps/tomcat/bin/run_tomcat.sh"]
        #imagePullPolicy: IfNotPresent
        imagePullPolicy: Always
        ports:
        - containerPort: 8080
          protocol: TCP
          name: http
        env:
        - name: "password"
          value: "123456"
        - name: "age"
          value: "18"
        resources:
          limits:
            cpu: 1
            memory: "512Mi"
          requests:
            cpu: 500m
            memory: "512Mi"
---
kind: Service
apiVersion: v1
metadata:
  labels:
    app: tomcat-app1-service-label
  name: tomcat-app1-service
spec:
  #type: NodePort
  ports:
  - name: http
    port: 80
    protocol: TCP
    targetPort: 8080
    #nodePort: 40003
  selector:
    app: tomcat-app1-selector

进入容器进行网络测试,网络正常

root@master01:~# kubectl  exec  -it tomcat-app1-deployment-6c86988859-5rj9g  sh 
kubectl exec [POD] [COMMAND] is DEPRECATED and will be removed in a future version. Use kubectl exec [POD] -- [COMMAND] instead.
/usr/local/tomcat # ping nginx-service.test.svc.cluster.local
PING nginx-service.test.svc.cluster.local (10.100.82.138): 56 data bytes
64 bytes from 10.100.82.138: seq=0 ttl=64 time=0.161 ms
64 bytes from 10.100.82.138: seq=1 ttl=64 time=0.134 ms
64 bytes from 10.100.82.138: seq=2 ttl=64 time=0.148 ms
64 bytes from 10.100.82.138: seq=3 ttl=64 time=0.146 ms

/usr/local/tomcat # nslookup nginx-service.test.svc.cluster.local 10.100.0.2
Server:    10.100.0.2
Address 1: 10.100.0.2 kube-dns.kube-system.svc.cluster.local

Name:      nginx-service.test.svc.cluster.local
Address 1: 10.100.82.138 nginx-service.test.svc.cluster.local

5.pod无法正常域名解析排查思路

1.测试pod网络是否通讯，即使dns服务正常，因为pod内ping 域名，无法通过coredns pod返回解析结果也无法解析，这里要做的是排除calico，fannel等网络插件引起的pod之间网络不通的故障

2.查看pod中的dns配置是否是coredns服务的IP，pod内的容器本身就是一个完整的操作系统，dns地址配置错误也会引起无法正常接卸，这里的pod的中显示是ameserver 10.100.0.2是kubelet服务中的config.yaml 的clusterDNS字段决定

root@master01:~# kubectl  exec  -it tomcat-app1-deployment-6c86988859-5rj9g  sh
/usr/local/tomcat # cat /etc/resolv.conf 
nameserver 10.100.0.2         #kubelet服务中的config.yaml 的clusterDNS字段决定
search default.svc.cluster.local svc.cluster.local cluster.local local
options ndots:5
/usr/local/tomcat #

3.第二条中pod配置的nameserver 为svc的clusterIP，svc背后是真正的coredns服务，有一层代理转发，这里知道了流量走向，
要排查两个方向，一.转发动作是否有完成,排查kube-proxy是否还正常工作，svc的转发是依靠ipvs中的规则转发（或者iptables 转发），kube-proxy不能正常工作，svc将没有ipvs规则支撑进行转发。二.clusterIP 10.100.0.2 svc是否匹配到coredns pod，执行kubectl get ep 查看后端是否匹配到pod。