k8s中的服务发现插件-CoreDNS

k8s中默认的服务发现是通过dns，将service的名字自动的关联到clusterIP上

部署k8s内的资源配置清单HTTP服务：

在hdss7-200主机上，配置一个nginx的虚拟主机，用来提供k8s内网统一的资源配置清单访问入口：

[root@hdss7-200 conf.d]# cat k8s-yaml.od.com.conf
server {
    listen       80;
    server_name  k8s-yaml.od.com;
    location / {
        autoindex on;
        default_type text/plain;
        root /data/k8s-yaml;
    }
}
nginx -s reload
mkdir -p /data/k8s-yaml/coredns

内网dns中配置域名解析：

named]# vi od.com.zone
k8s-yaml             A    10.4.7.200
named]# systemctl restart named

解析验证：

named]# dig @10.4.7.11 k8s-yaml.od.com +short
10.4.7.200

安装部署CoreDNS，以容器的方式向k8s交付软件：

从docker-harbor拉取官方镜像：

docker pull coredns/coredns:1.6.1
k8s-yaml]# docker tag c0f6e815079e harbor.od.com/public/coredns:v1.6.1
[root@hdss7-200 k8s-yaml]# docker push harbor.od.com/public/coredns:v1.6.1

准备资源配置清单：

# pwd
/data/k8s-yaml/coredns
# cat rbac.yaml
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
---
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
# cat dp.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: coredns
  namespace: kube-system
  labels:
    k8s-app: coredns
    kubernetes.io/name: "CoreDNS"
spec:
  replicas: 1
  selector:
    matchLabels:
      k8s-app: coredns
  template:
    metadata:
      labels:
        k8s-app: coredns
    spec:
      priorityClassName: system-cluster-critical
      serviceAccountName: coredns
      containers:
      - name: coredns
        image: harbor.od.com/public/coredns:v1.6.1
        args:
        - -conf
        - /etc/coredns/Corefile
        volumeMounts:
        - name: config-volume
          mountPath: /etc/coredns
        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
      dnsPolicy: Default
      volumes:
        - name: config-volume
          configMap:
            name: coredns
            items:
            - key: Corefile
              path: Corefile
# cat cm.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        log
        health
        ready
        kubernetes cluster.local 192.168.0.0/16
        forward . 10.4.7.11
        cache 30
        loop
        reload
        loadbalance
       }
# cat svc.yaml
apiVersion: v1
kind: Service
metadata:
  name: coredns
  namespace: kube-system
  labels:
    k8s-app: coredns
    kubernetes.io/cluster-service: "true"
    kubernetes.io/name: "CoreDNS"
spec:
  selector:
    k8s-app: coredns
  clusterIP: 192.168.0.2
  ports:
  - name: dns
    port: 53
    protocol: UDP
  - name: dns-tcp
    port: 53
  - name: metrics
    port: 9153
    protocol: TCP

使用陈述式资源管理方法来应用我们的声明式资源配置清单：

[root@hdss7-21 ~]# kubectl apply -f http://k8s-yaml.od.com/coredns/rbac.yaml
serviceaccount/coredns created
clusterrole.rbac.authorization.k8s.io/system:coredns created
clusterrolebinding.rbac.authorization.k8s.io/system:coredns created
[root@hdss7-21 ~]# kubectl apply -f http://k8s-yaml.od.com/coredns/cm.yaml
configmap/coredns created
[root@hdss7-21 ~]# kubectl apply -f http://k8s-yaml.od.com/coredns/dp.yaml
deployment.apps/coredns created
[root@hdss7-21 ~]# kubectl apply -f http://k8s-yaml.od.com/coredns/svc.yaml
service/coredns created
[root@hdss7-21 ~]# kubectl get all -n kube-system
NAME                           READY   STATUS    RESTARTS   AGE
pod/coredns-6b6c4f9648-j7cv9   1/1     Running   0          34s
NAME              TYPE        CLUSTER-IP    EXTERNAL-IP   PORT(S)                  AGE
service/coredns   ClusterIP   192.168.0.2           53/UDP,53/TCP,9153/TCP   31s
NAME                      READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/coredns   1/1     1            1           34s
NAME                                 DESIRED   CURRENT   READY   AGE
replicaset.apps/coredns-6b6c4f9648   1         1         1       34s

解析验证coreDNS是否可用：

~]# dig -t A www.baidu.com +short @192.168.0.2
www.a.shifen.com.
14.215.177.39
14.215.177.38
~]# dig -t A harbor.od.com +short @192.168.0.2
10.4.7.200
[root@hdss7-21 ~]# kubectl get svc -n kube-public
NAME       TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)   AGE
nginx-dp   ClusterIP   192.168.47.82           80/TCP    10h

我们在集群外部必须使用全域名来解析service的ip

[root@hdss7-21 ~]# dig -t A nginx-dp.kube-public.svc.cluster.local. @192.168.0.2 +short
192.168.47.82

但是如果在集群内部，我们可以使用短域名，可以看解析的配置文件：

[root@hdss7-21 ~]# kubectl get pod -o wide
NAME             READY   STATUS    RESTARTS   AGE   IP           NODE                NOMINATED NODE   READINESS GATES
nginx-ds-rxfqd   1/1     Running   0          11h   172.7.22.2   hdss7-22.host.com              
nginx-ds-xm5l2   1/1     Running   0          11h   172.7.21.2   hdss7-21.host.com              
[root@hdss7-21 ~]# kubectl exec -it nginx-ds-xm5l2 -n default bash

默认k8s已经帮助我们配置来serch，即可以使用短域名来解析ip地址

root@nginx-ds-xm5l2:/# cat /etc/resolv.conf
nameserver 192.168.0.2
search default.svc.cluster.local svc.cluster.local cluster.local host.com
options ndots:5