DNS服务

服务自发现，是在服务启动后，有自发现服务自己发现启动服务的 IP 和 Port 并保存下来提供使用。
在 K8S 中，服务自发现通过两种方式：环境变量或 DNS。在没有 DNS 的时候， K8S 就用环境变量的方式，但一旦有很多 Service，环境变量就会变得极其复杂（每个服务都会有对应的环境变量），我们就需要用 DNS 的方式。

安装 DNS 服务

下载资料包： https://www.qstack.com.cn/skydns.zip

资料中有三个文件：

• skydns-rc.yaml，是启动 dns 服务的（是个 Deployment）
• skydns-svc.yaml，启动服务，其中配置了端口
• test_dns_pod.yaml，用于测试

apiVersion: extensions/v1beta1
kind: Deployment # 资源类型
metadata:
  name: kube-dns
  namespace: kube-system # 指定了 namespace
  labels:
    k8s-app: kube-dns
    kubernetes.io/cluster-service: "true"
spec:
  replicas: 1
  # 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.
  strategy: 
    rollingUpdate: # 升级属性
      maxSurge: 10%
      maxUnavailable: 0
  selector:
    matchLabels:
      k8s-app: kube-dns
  template:
    metadata:
      labels:
        k8s-app: kube-dns
      annotations:
        scheduler.alpha.kubernetes.io/critical-pod: ''
        scheduler.alpha.kubernetes.io/tolerations: '[{"key":"CriticalAddonsOnly", "operator":"Exists"}]'
    spec:
      containers:
      - name: kubedns
        image: myhub.fdccloud.com/library/kubedns-amd64:1.9 # 第一个镜像
        resources: # 资源使用的请求和限制
          # TODO: Set memory limits when we've profiled the container for large
          # clusters, then set request = limit to keep this container in
          # guaranteed class. Currently, this container falls into the
          # "burstable" category so the kubelet doesn't backoff from restarting it.
          limits: 
            memory: 170Mi
          requests:
            cpu: 100m
            memory: 70Mi
        livenessProbe: # 容器的探针服务，检查有没有运行起来，4xx/5xx 为不正常
          httpGet:
            path: /healthz-kubedns
            port: 8080
            scheme: HTTP
          initialDelaySeconds: 60
          timeoutSeconds: 5
          successThreshold: 1
          failureThreshold: 5
        readinessProbe: # 检查服务能否可用
          httpGet:
            path: /readiness
            port: 8081
            scheme: HTTP
          # we poll on pod startup for the Kubernetes master service and
          # only setup the /readiness HTTP server once that's available.
          initialDelaySeconds: 3
          timeoutSeconds: 5
        args: # 参数，指定了域名、端口、配置
        - --domain=cluster.local.
        - --dns-port=10053
        - --config-map=kube-dns
        - --kube-master-url=http://172.16.156.128:8080 # 这里要改成自己 api-server 的地址
        # This should be set to v=2 only after the new image (cut from 1.5) has
        # been released, otherwise we will flood the logs.
        - --v=0
        #__PILLAR__FEDERATIONS__DOMAIN__MAP__
        env:
        - name: PROMETHEUS_PORT
          value: "10055"
        ports:
        - containerPort: 10053
          name: dns-local
          protocol: UDP
        - containerPort: 10053
          name: dns-tcp-local
          protocol: TCP
        - containerPort: 10055
          name: metrics
          protocol: TCP
      - name: dnsmasq
        image: myhub.fdccloud.com/library/kube-dnsmasq-amd64:1.4 # 第二个容器
        livenessProbe:
          httpGet:
            path: /healthz-dnsmasq
            port: 8080
            scheme: HTTP
          initialDelaySeconds: 60
          timeoutSeconds: 5
          successThreshold: 1
          failureThreshold: 5
        args:
        - --cache-size=1000
        - --no-resolv
        - --server=127.0.0.1#10053
        #- --log-facility=-
        ports:
        - containerPort: 53
          name: dns
          protocol: UDP
        - containerPort: 53
          name: dns-tcp
          protocol: TCP
        # see: https://github.com/kubernetes/kubernetes/issues/29055 for details
        resources:
          requests:
            cpu: 150m
            memory: 10Mi
      - name: dnsmasq-metrics # 指标，配合做监控使用
        image: myhub.fdccloud.com/library/dnsmasq-metrics-amd64:1.0
        livenessProbe:
          httpGet:
            path: /metrics
            port: 10054
            scheme: HTTP
          initialDelaySeconds: 60
          timeoutSeconds: 5
          successThreshold: 1
          failureThreshold: 5
        args:
        - --v=2
        - --logtostderr
        ports:
        - containerPort: 10054
          name: metrics
          protocol: TCP
        resources:
          requests:
            memory: 10Mi
      - name: healthz
        image: myhub.fdccloud.com/library/exechealthz-amd64:1.2 # 健康检查的镜像
        resources:
          limits:
            memory: 50Mi
          requests:
            cpu: 10m
            # Note that this container shouldn't really need 50Mi of memory. The
            # limits are set higher than expected pending investigation on #29688.
            # The extra memory was stolen from the kubedns container to keep the
            # net memory requested by the pod constant.
            memory: 50Mi
        args:
        - --cmd=nslookup kubernetes.default.svc.cluster.local 127.0.0.1 >/dev/null
        - --url=/healthz-dnsmasq
        - --cmd=nslookup kubernetes.default.svc.cluster.local 127.0.0.1:10053 >/dev/null
        - --url=/healthz-kubedns
        - --port=8080
        - --quiet
        ports:
        - containerPort: 8080
          protocol: TCP
      dnsPolicy: Default  # Don't use cluster DNS.

# Warning: This is a file generated from the base underscore template file: skydns-svc.yaml.base
apiVersion: v1
kind: Service
metadata:
  name: kube-dns
  namespace: kube-system # 指定了 namespace
  labels:
    k8s-app: kube-dns
    kubernetes.io/cluster-service: "true"
    kubernetes.io/name: "KubeDNS"
spec:
  selector:
    k8s-app: kube-dns
  clusterIP: 10.254.230.254 # v IP 地址
  ports:
  - name: dns
    port: 53
    protocol: UDP
  - name: dns-tcp
    port: 53
    protocol: TCP

注意，这两个文件都指定了 namespace。以下是查看所有跑起来服务的截图。

image.png

要想使用此 DNS，我们需要改一下所有 kubelet 的配置文件。

# /etc/kubernetes/kubelet
KUBELET_ARGS="--cluster_dns=10.254.230.254 --cluster_domain=cluster.local"
# 更改完后重启 kubelet： systemctl restart kubelet

测试 DNS 生效

资料包中 test_dns_pod.yaml 文件是通过 busybox 来测试 DNS 的

apiVersion: v1
kind: Pod
metadata:
  labels:
    name: busybox
    role: master
  name: busybox2
spec:
  containers:
  - name: busybox
    image: docker.io/busybox:latest
    imagePullPolicy: IfNotPresent
    command:
    - sleep
    - "3600"

通过创建它，并进入容器

kubectl create -f test_dns_pod.yaml
kubectl exec -it busybox2 sh # 此容器通过 sh 进入，没有 bash

在容器中查询其他服务的名称，结果可以查询到服务地址：

image.png

注意，只有新启动的服务才能使用 DNS 服务，如上面的 busybox，其他服务需要重启才能查询。

上文中 tomcat 连 mysql 服务的 ip 地址可以改为服务名进行连接。

image.png

最后重启 tomcat 服务。（连接数据库的项目没做，所以部分内容忽略，先做笔记做参考）