本文将在前文的基础上介绍在kubernetes集群环境中配置dns服务，在k8s集群中，pod的生命周期是短暂的，pod重启后ip地址会产生变化，对于应用程序来说这是不可接受的，为解决这个问题，K8S集群巧妙的引入的dns服务来实现服务的发现，在k8s集群中dns总共需要使用4个组件，各组件分工如下：
etcd：DNS存储
kube2sky：将Kubernetes Master中的service（服务）注册到etcd。
skyDNS：提供DNS域名解析服务。
healthz：提供对skydns服务的健康检查。

一、下载相关镜像文件，并纳入本地仓库统一管理

# docker pull docker.io/elcolio/etcd 
# docker pull docker.io/port/kubernetes-kube2sky
# docker pull docker.io/skynetservices/skydns 
# docker pull docker.io/wu1boy/healthz

# docker tag docker.io/elcolio/etcd registry.fjhb.cn/etcd
# docker tag docker.io/port/kubernetes-kube2sky registry.fjhb.cn/kubernetes-kube2sky
# docker tag docker.io/skynetservices/skydns  registry.fjhb.cn/skydns
# docker tag docker.io/wu1boy/healthz registry.fjhb.cn/healthz

# docker push registry.fjhb.cn/etcd
# docker push registry.fjhb.cn/kubernetes-kube2sky
# docker push registry.fjhb.cn/skydns
# docker push registry.fjhb.cn/healthz
# docker images |grep fjhb

二、通过rc文件创建pod
这里面一个pod包含了4个组件，一个组件运行在一个docker容器中

# cat skydns-rc.yaml
apiVersion: v1
kind: ReplicationController
metadata:
  name: kube-dns
  namespace: default
  labels:
    k8s-app: kube-dns
    version: v12
    kubernetes.io/cluster-service: "true"
spec:
  replicas: 1
  selector:
    k8s-app: kube-dns
    version: v12
  template:
    metadata:
      labels:
        k8s-app: kube-dns
        version: v12
        kubernetes.io/cluster-service: "true"
    spec:
      containers:
      - name: etcd
        image: registry.fjhb.cn/etcd 
        resources:
          limits:
            cpu: 100m
            memory: 50Mi
          requests:
            cpu: 100m
            memory: 50Mi
        command:
        - /bin/etcd
        - --data-dir
        - /tmp/data
        - --listen-client-urls
        - http://127.0.0.1:2379,http://127.0.0.1:4001
        - --advertise-client-urls
        - http://127.0.0.1:2379,http://127.0.0.1:4001
        - --initial-cluster-token
        - skydns-etcd
        volumeMounts:
        - name: etcd-storage
          mountPath: /tmp/data
      - name: kube2sky
        image: registry.fjhb.cn/kubernetes-kube2sky
        resources:
          limits:
            cpu: 100m
            memory: 50Mi
          requests:
            cpu: 100m
            memory: 50Mi
        args:
        - -kube_master_url=http://192.168.115.5:8080
        - -domain=cluster.local
      - name: skydns
        image: registry.fjhb.cn/skydns 
        resources:
          limits:
            cpu: 100m
            memory: 50Mi
          requests:
            cpu: 100m
            memory: 50Mi
        args:
        - -machines=http://127.0.0.1:4001
        - -addr=0.0.0.0:53
        - -ns-rotate=false
        - -domain=cluster.local
        ports:
        - containerPort: 53
          name: dns
          protocol: UDP
        - containerPort: 53
          name: dns-tcp
          protocol: TCP
      - name: healthz
        image: registry.fjhb.cn/healthz
        resources:
          limits:
            cpu: 10m
            memory: 20Mi
          requests:
            cpu: 10m
            memory: 20Mi
        args:
        - -cmd=nslookup kubernetes.default.svc.cluster.local 127.0.0.1 >/dev/null
        - -port=8080
        ports:
        - containerPort: 8080
          protocol: TCP
      volumes:
      - name: etcd-storage
        emptyDir: {}
      dnsPolicy: Default

三、通过srv文件创建service

# cat skydns-svc.yaml   
apiVersion: v1
kind: Service
metadata:
  name: kube-dns
  namespace: default
  labels:
    k8s-app: kube-dns
    kubernetes.io/cluster-service: "true"
    kubernetes.io/name: "KubeDNS"
spec:
  selector:
    k8s-app: kube-dns
  clusterIP: 10.254.16.254
  ports:
  - name: dns
    port: 53
    protocol: UDP
  - name: dns-tcp
    port: 53
    protocol: TCP

# kubectl create -f skydns-rc.yaml 
# kubectl create -f skydns-svc.yaml 
# kubectl get rc
# kubectl get pod
# kubectl get svc

# kubectl describe svc kube-dns

# kubectl describe rc kube-dns

# kubectl describe pod kube-dns-9fllp
Name:           kube-dns-9fllp
Namespace:      default
Node:           192.168.115.6/192.168.115.6
Start Time:     Tue, 23 Jan 2018 10:55:19 -0500
Labels:         k8s-app=kube-dns
                kubernetes.io/cluster-service=true
                version=v12
Status:         Running
IP:             172.16.37.5
Controllers:    ReplicationController/kube-dns
Containers:
  etcd:
    Container ID:       docker://62ad76bfaca1797c5f43b0e9eebc04074169fce4cc15ef3ffc4cd19ffa9c8c19
    Image:              registry.fjhb.cn/etcd
    Image ID:           docker-pullable://docker.io/elcolio/etcd@sha256:3b4dcd35a7eefea9ce2970c81dcdf0d0801a778d117735ee1d883222de8bbd9f
    Port:
    Command:
      /bin/etcd
      --data-dir
      /tmp/data
      --listen-client-urls
      http://127.0.0.1:2379,http://127.0.0.1:4001
      --advertise-client-urls
      http://127.0.0.1:2379,http://127.0.0.1:4001
      --initial-cluster-token
      skydns-etcd
    Limits:
      cpu:      100m
      memory:   50Mi
    Requests:
      cpu:              100m
      memory:           50Mi
    State:              Running
      Started:          Tue, 23 Jan 2018 10:55:23 -0500
    Ready:              True
    Restart Count:      0
    Volume Mounts:
      /tmp/data from etcd-storage (rw)
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-6pddn (ro)
    Environment Variables:      <none>
  kube2sky:
    Container ID:       docker://6b0bc6e8dce83e3eee5c7e654fbaca693730623fb7936a1fd9d73de1a1dd8152
    Image:              registry.fjhb.cn/kubernetes-kube2sky
    Image ID:           docker-pullable://docker.io/port/kubernetes-kube2sky@sha256:0230d3fbb0aeb4ddcf903811441cf2911769dbe317a55187f58ca84c95107ff5
    Port:
    Args:
      -kube_master_url=http://192.168.115.5:8080
      -domain=cluster.local
    Limits:
      cpu:      100m
      memory:   50Mi
    Requests:
      cpu:              100m
      memory:           50Mi
    State:              Running
      Started:          Tue, 23 Jan 2018 10:55:25 -0500
    Ready:              True
    Restart Count:      0
    Volume Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-6pddn (ro)
    Environment Variables:      <none>
  skydns:
    Container ID:       docker://ebc2aaaa54e2f922e370e454ec537665d813c69d37a21e3afd908e6dad056627
    Image:              registry.fjhb.cn/skydns
    Image ID:           docker-pullable://docker.io/skynetservices/skydns@sha256:6f8a9cff0b946574bb59804016d3aacebc637581bace452db6a7515fa2df79ee
    Ports:              53/UDP, 53/TCP
    Args:
      -machines=http://127.0.0.1:4001
      -addr=0.0.0.0:53
      -ns-rotate=false
      -domain=cluster.local
    Limits:
      cpu:      100m
      memory:   50Mi
    Requests:
      cpu:              100m
      memory:           50Mi
    State:              Running
      Started:          Tue, 23 Jan 2018 10:55:27 -0500
    Ready:              True
    Restart Count:      0
    Volume Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-6pddn (ro)
    Environment Variables:      <none>
  healthz:
    Container ID:       docker://f1de1189fa6b51281d414d7a739b86494b04c8271dc6bb5f20c51fac15ec9601
    Image:              registry.fjhb.cn/healthz
    Image ID:           docker-pullable://docker.io/wu1boy/healthz@sha256:d6690c0a8cc4f810a5e691b6a9b8b035192cb967cb10e91c74824bb4c8eea796
    Port:               8080/TCP
    Args:
      -cmd=nslookup kubernetes.default.svc.cluster.local 127.0.0.1 >/dev/null
      -port=8080
    Limits:
      cpu:      10m
      memory:   20Mi
    Requests:
      cpu:              10m
      memory:           20Mi
    State:              Running
      Started:          Tue, 23 Jan 2018 10:55:29 -0500
    Ready:              True
    Restart Count:      0
    Volume Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-6pddn (ro)
    Environment Variables:      <none>
Conditions:
  Type          Status
  Initialized   True 
  Ready         True 
  PodScheduled  True 
Volumes:
  etcd-storage:
    Type:       EmptyDir (a temporary directory that shares a pod's lifetime)
    Medium:
  default-token-6pddn:
    Type:       Secret (a volume populated by a Secret)
    SecretName: default-token-6pddn
QoS Class:      Guaranteed
Tolerations:    <none>
Events:
  FirstSeen     LastSeen        Count   From                    SubObjectPath                   Type            Reason        Message
  ---------     --------        -----   ----                    -------------                   --------        ------        -------
  7m            7m              1       {default-scheduler }                                    Normal          Scheduled     Successfully assigned kube-dns-9fllp to 192.168.115.6
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{etcd}           Normal          Pulling       pulling image "registry.fjhb.cn/etcd"
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{etcd}           Normal          Pulled        Successfully pulled image "registry.fjhb.cn/etcd"
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{etcd}           Normal          Created       Created container with docker id 62ad76bfaca1; Security:[seccomp=unconfined]
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{kube2sky}       Normal          Pulled        Successfully pulled image "registry.fjhb.cn/kubernetes-kube2sky"
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{etcd}           Normal          Started       Started container with docker id 62ad76bfaca1
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{kube2sky}       Normal          Pulling       pulling image "registry.fjhb.cn/kubernetes-kube2sky"
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{kube2sky}       Normal          Created       Created container with docker id 6b0bc6e8dce8; Security:[seccomp=unconfined]
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{skydns}         Normal          Pulled        Successfully pulled image "registry.fjhb.cn/skydns"
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{skydns}         Normal          Pulling       pulling image "registry.fjhb.cn/skydns"
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{kube2sky}       Normal          Started       Started container with docker id 6b0bc6e8dce8
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{skydns}         Normal          Created       Created container with docker id ebc2aaaa54e2; Security:[seccomp=unconfined]
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{skydns}         Normal          Started       Started container with docker id ebc2aaaa54e2
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{healthz}        Normal          Pulling       pulling image "registry.fjhb.cn/healthz"
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{healthz}        Normal          Pulled        Successfully pulled image "registry.fjhb.cn/healthz"
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{healthz}        Normal          Created       Created container with docker id f1de1189fa6b; Security:[seccomp=unconfined]
  7m            7m              1       {kubelet 192.168.115.6} spec.containers{healthz}        Normal          Started       Started container with docker id f1de1189fa6b

四、修改kubelet配置文件并重启服务
注意:
--cluster-dns参数要和前面svc文件中的clusterIP参数一致
--cluster-domain参数要和前面rc文件中的-domain参数一致
集群内所有的kubelet节点都需要修改

# grep 'KUBELET_ADDRESS' /etc/kubernetes/kubelet     
KUBELET_ADDRESS="--address=192.168.115.5 --cluster-dns=10.254.16.254 --cluster-domain=cluster.local"
# systemctl restart kubelet

五、运行一个busybox和curl进行测试

# cat busybox.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: busybox
spec:
  containers:
  - name: busybox
    image: docker.io/busybox
    command:
    - sleep
- "3600"

# cat curl.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: curl
spec:
  containers:
  - name: curl
    image: docker.io/webwurst/curl-utils
    command:
    - sleep
- "3600"

# kubectl create -f busybox.yaml 
# kubectl create -f curl.yaml

通过busybox容器对kubernetes的service进行解析，发现service被自动解析成了对应的集群ip地址，而并不是172.16网段的docker地址

# kubectl get svc
# kubectl exec busybox -- nslookup frontend
# kubectl exec busybox -- nslookup redis-master
# kubectl exec busybox -- nslookup redis-slave

通过curl容器访问前面创建的php留言板

# kubectl exec curl -- curl frontend