一，基本概念
kube-dns主要实现K8S集群内部服务发现以及集群内部通过服务名进行访问。kubernetes集群中的服务都会有一个只用于内部访问的ClusterIP，如下：
[root@master ~]# kubectl get svc NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE kubernetes ClusterIP 10.254.0.1 <none> 443/TCP 54d mysql NodePort 10.254.194.156 <none> 3306:8606/TCP 1d myweb NodePort 10.254.244.155 <none> 8080:8500/TCP 20h
kube-dns可以实现集群内部的服务名与ClusterIP的解析，实现服务之间通过服务名的访问，方便部署管理
kubernetes提供的虚拟DNS服务1.6版本之后主要由kube-dns，dnsmasq，sidecar等3个容器组成。其中kube-dns监控kubernetes服务资源，并将记录缓存至内存中；dnsmasq从kube-dns中获取集群服务的DNS记录并向集群提供DNS服务，sidecar负责另外两个容器的健康监测。
二，安装部署
安装部署需要的yaml文件可以参考官网的github地址：https://github.com/kubernetes/kubernetes/tree/release-1.8/cluster/addons/dns；需要部署的文件为：kubedns-cm.yaml，kubedns-controller.yaml，kubedns-sa.yaml，kubedns-svc.yaml等
其中kubedns-cm.yaml，kubedns-sa.yaml两个文件不需要更改，可以直接部署
文件内容如下：
[root@master dns]# cat kubedns-cm.yaml apiVersion: v1 kind: ConfigMap metadata: name: kube-dns namespace: kube-system labels: addonmanager.kubernetes.io/mode: EnsureExists [root@master dns]# cat kubedns-sa.yaml apiVersion: v1 kind: ServiceAccount metadata: name: kube-dns namespace: kube-system labels: kubernetes.io/cluster-service: "true" addonmanager.kubernetes.io/mode: Reconcile
kubedns-controller.yaml文件修改后如下
# Copyright 2016 The Kubernetes Authors. # # # # Licensed under the Apache License, Version 2.0 (the "License"); # # you may not use this file except in compliance with the License. # # You may obtain a copy of the License at # # # # http://www.apache.org/licenses/LICENSE-2.0 # # # # Unless required by applicable law or agreed to in writing, software # # distributed under the License is distributed on an "AS IS" BASIS, # # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. # # See the License for the specific language governing permissions and # # limitations under the License. # # # Should keep target in cluster/addons/dns-horizontal-autoscaler/dns-horizontal-autoscaler.yaml # # in sync with this file. # # # __MACHINE_GENERATED_WARNING__ apiVersion: extensions/v1beta1 kind: Deployment metadata: name: kube-dns namespace: kube-system labels: k8s-app: kube-dns kubernetes.io/cluster-service: "true" addonmanager.kubernetes.io/mode: Reconcile 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. strategy: rollingUpdate: maxSurge: 10% maxUnavailable: 0 selector: matchLabels: k8s-app: kube-dns template: metadata: labels: k8s-app: kube-dns annotations: scheduler.alpha.kubernetes.io/critical-pod: '' spec: tolerations: - key: "CriticalAddonsOnly" operator: "Exists" volumes: - name: kube-dns-config configMap: name: kube-dns optional: true containers: - name: kubedns image: gcr.io/google_containers/k8s-dns-kube-dns-amd64:1.14.4 resources: # 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: httpGet: path: /healthcheck/kubedns port: 10054 scheme: HTTP initialDelaySeconds: 60 timeoutSeconds: 5 successThreshold: 1 failureThreshold: 5 readinessProbe: 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: - --domain=cluster.local. - --dns-port=10053 - --config-dir=/kube-dns-config - --v=2 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 volumeMounts: - name: kube-dns-config mountPath: /kube-dns-config - name: dnsmasq image: gcr.io/google_containers/k8s-dns-dnsmasq-nanny-amd64:1.14.4 livenessProbe: httpGet: path: /healthcheck/dnsmasq port: 10054 scheme: HTTP initialDelaySeconds: 60 timeoutSeconds: 5 successThreshold: 1 failureThreshold: 5 args: - -v=2 - -logtostderr - -configDir=/etc/k8s/dns/dnsmasq-nanny - -restartDnsmasq=true - -- - -k - --cache-size=1000 - --log-facility=- - --server=/cluster.local/127.0.0.1#10053 - --server=/in-addr.arpa/127.0.0.1#10053 - --server=/ip6.arpa/127.0.0.1#10053 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: 20Mi volumeMounts: - name: kube-dns-config mountPath: /etc/k8s/dns/dnsmasq-nanny - name: sidecar image: gcr.io/google_containers/k8s-dns-sidecar-amd64:1.14.4 livenessProbe: httpGet: path: /metrics port: 10054 scheme: HTTP initialDelaySeconds: 60 timeoutSeconds: 5 successThreshold: 1 failureThreshold: 5 args: - --v=2 - --logtostderr - --probe=kubedns,127.0.0.1:10053,kubernetes.default.svc.cluster.local,5,A - --probe=dnsmasq,127.0.0.1:53,kubernetes.default.svc.cluster.local,5,A ports: - containerPort: 10054 name: metrics protocol: TCP resources: requests: memory: 20Mi cpu: 10m dnsPolicy: Default # Don't use cluster DNS. serviceAccountName: kube-dns
需要修改的地方：
1）3个容器的镜像根据自身条件定义
2）原文中变量 '_PILLAR__DNS__DOMAIN__'更改为自己设定的域名，通常为cluster.local
kubedns-svc.yaml文件内容如下：
[root@master dns]# cat kubedns-svc.yaml # Copyright 2016 The Kubernetes Authors. # # # # Licensed under the Apache License, Version 2.0 (the "License"); # # you may not use this file except in compliance with the License. # # You may obtain a copy of the License at # # # # http://www.apache.org/licenses/LICENSE-2.0 # # # # Unless required by applicable law or agreed to in writing, software # # distributed under the License is distributed on an "AS IS" BASIS, # # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. # # See the License for the specific language governing permissions and # # limitations under the License. # # # __MACHINE_GENERATED_WARNING__ apiVersion: v1 kind: Service metadata: name: kube-dns namespace: kube-system labels: k8s-app: kube-dns kubernetes.io/cluster-service: "true" addonmanager.kubernetes.io/mode: Reconcile kubernetes.io/name: "KubeDNS" spec: selector: k8s-app: kube-dns clusterIP: 10.254.0.2 ports: - name: dns port: 53 protocol: UDP - name: dns-tcp port: 53 protocol: TCP
此文件只需要修改其中clusterIP，将kube-dns服务的clusterIP固定，可自行定义为kube-apiserver配置中--service-cluster-ip-range定义的IP段内的任意IP
4个文件都生成之后则可以启动了
kubectl create -f kubedns-cm.yaml -f kubedns-sa.yaml -f kubedns-controller.yaml -f kubedns-svc.yaml
启动上述文件之后，还需要在kubelet配置中添加参数，指定DNS服务器：
... --cluster-dns=10.254.0.2 \ --cluster-domain=cluster.local ...
之后就可以在集群之间使用服务名互相访问了
注意：
kube-dns按照上述文件进行配置只有1个副本，如果需要生成多个副本官网推荐以下两种方案：
1）参考https://github.com/kubernetes/kubernetes/tree/release-1.8/cluster/addons/dns-horizontal-autoscaler
2）使用命令：kubectl --namespace=kube-system scale deployment kube-dns --replicas=<NUM_YOU_WANT>