k3s CoreDNS使用，实现内部服务互通

安装K3s后，会默认安装CoreDNS，实现集群内动态解析，在pod失效后会动态重新生成IP

## 查看CoreDNS安装运行情况
> kubectl get pod,svc -n kube-system

NAME                         READY   STATUS    RESTARTS   AGE
pod/coredns-d798c9dd-67mmc   1/1     Running   2          11d

NAME               TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)                  AGE
service/kube-dns   ClusterIP   10.43.0.10   <none>        53/UDP,53/TCP,9153/TCP   11d

验证CoreDNS

安装dig用来校验域名解析情况

apiVersion: v1
kind: Pod
metadata:
  name: dig
  namespace: default
spec:
  containers:
    - name: dig
      image:  docker.io/azukiapp/dig
      command:
        - sleep
        - "3600"
      imagePullPolicy: IfNotPresent
  restartPolicy: Always

测试

>kubectl exec -it dig -- nslookup kubernetes

Server:		10.43.0.10
Address:	10.43.0.10#53

Name:	kubernetes.default.svc.cluster.local
Address: 10.43.0.1

# 验证kubernetes正常，接下来验证外网

> kubectl exec -it dig -- nslookup www.baidu.com

Server:		10.43.0.10
Address:	10.43.0.10#53

Non-authoritative answer:
www.baidu.com	canonical name = www.a.shifen.com.
Name:	www.a.shifen.com
Address: 61.135.169.125
Name:	www.a.shifen.com
Address: 61.135.169.121

接下来验证内部Pod服务mysql

需要已安装mysql服务，本处需要说明Pod域名生成规则

pod域名生成规则：<svc-name>.<namespace>.svc.cluster.local

> kubectl exec -it dig -- nslookup mysql-1.dev-service.svc.cluster.local
Server:		10.43.0.10
Address:	10.43.0.10#53

Name:	mysql-1.dev-service.svc.cluster.local
Address: 10.43.34.109

使用nacos for mysql做测试

Tips : 测试使用nacos的镜像是1.2.0，测试时一直报错，无法连接，最后查看是因nacos的数据库链接版本较低导致的，mysql-connection 是5.1.34。如果数据库无法更改的情况下可以自己下载nacos源码打包修改驱动包版本。我自己因是测试环境，将数据库降为5.7版本后恢复正常。

apiVersion: v1
kind: ConfigMap
metadata:
  namespace: dev-app
  name: nacos-cm
data:
  mysql.host: "mysql-1.dev-service.svc.cluster.local"
  mysql.db.name: "nacos_devtest"
  mysql.port: "3306"
  mysql.user: "nacos"
  mysql.password: "nacos"
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  namespace: dev-app
  name: nacos-service
spec:
  serviceName: nacos-service
  replicas: 1
  template:
    metadata:
      labels:
        app: nacos-service
      annotations:
        pod.alpha.kubernetes.io/initialized: "true"
    spec:
      affinity:
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            - labelSelector:
                matchExpressions:
                  - key: "app"
                    operator: In
                    values:
                      - nacos-service
              topologyKey: "kubernetes.io/hostname"
      containers:
        - name: nacos-service
          imagePullPolicy: Always
          image: nacos/nacos-server:1.2.0
          resources:
            requests:
              memory: "2Gi"
              cpu: "500m"
          ports:
            - containerPort: 8848
              name: client
          env:
            - name: NACOS_REPLICAS
              value: "1"
            - name: MYSQL_SERVICE_HOST
              valueFrom:
                configMapKeyRef:
                  name: nacos-cm
                  key: mysql.host
            - name: MYSQL_SERVICE_DB_NAME
              valueFrom:
                configMapKeyRef:
                  name: nacos-cm
                  key: mysql.db.name
            - name: MYSQL_SERVICE_PORT
              valueFrom:
                configMapKeyRef:
                  name: nacos-cm
                  key: mysql.port
            - name: MYSQL_SERVICE_USER
              valueFrom:
                configMapKeyRef:
                  name: nacos-cm
                  key: mysql.user
            - name: MYSQL_SERVICE_PASSWORD
              valueFrom:
                configMapKeyRef:
                  name: nacos-cm
                  key: mysql.password
            - name: NACOS_SERVER_PORT
              value: "8848"
            - name: PREFER_HOST_MODE
              value: "hostname"
            - name: NACOS_SERVERS
              value: "nacos-service.dev-app.svc.cluster.local:8848"
  selector:
    matchLabels:
      app: nacos-service