k8s集群搭建DNS服务

一、k8s集群中DNS服务介绍 

作为服务发现机制的基本功能，在集群内需要能够通过服务名对服务进行访问，这就需要一个集群范围的DNS服务来完成服务名到ClusterIP的解析。

 

DNS在k8s集群中发展的三个阶段

1. k8s版本 < 1.4 ，DNS由SkyDNS提供

  SkyDNS有4个容器组成

   kube2sky: 监控k8s集群中Service资源的变化

   skydns: 从etcd中读取DNS记录，并为客户端容器应用提供DNS查询服务

   healthz: 提供对skydns服务的健康检查功能

   etcd:  用于保存DNS记录

 

skydns架构图如下：

2. k8s版本 1.4~1.11

  从k8s1.4版本以后SkyDNS组件便被KubeDNS替换,KubeDNS去掉了etcd直接保存在内存中，以提高查询性能。

  KubeDNS由3个容器组成

   kubedns: 监控k8s集群中Service资源的变化，根据Service的名称和IP地址生成DNS记录

   dnsmasq：从kubedns中获取DNS记录，提供DNS缓存，为客户端容器应用提供DNS查询服务

   sidecar: 提供对kubedns和dnsmasq服务的健康检查功能。

 

KubeDNS架构图

 

3. k8s版本大于1.11以后(至今1.19)

   从k8s1.11版本开始，DNS服务由CoreDNS提供。CoreDNS是CNCF基金会的一个项目，是用Go语言实现的高性能、插件式、易扩展的DNS服务器。

CoreDNS只使用了一个容器coredns

架构图如下：

 

 

二、CoreDNS服务搭建

1.在创建DNS服务之前修改每个Node上kubelet的启动参数

修改每个kubelet的启动参数，加上以下两个参数：

--cluster-dns=clusterIP  #clusterIP是具体配置的k8s可用的ip地址

--cluster-domain=cluster.local #配置DNS服务中设置的域名

修改启动参数可在：10-kubeadm.conf 或在kubelet其他加载的配置文件中添加配置(比如1.18的, /etc/sysconfig/kubelet)

配置完之后重启kubelet

 

2.搭建DNS服务

项目源码：https://github.com/coredns/coredns

官网中文介绍：https://kubernetes.io/zh/docs/tasks/administer-cluster/dns-custom-nameservers/

对应的yaml文件：https://github.com/kubernetes/kubernetes/blob/master/cluster/addons/dns/coredns/coredns.yaml.base

 

将coredns对应的yaml文件下载，需要将配置文件中的大写属性进行修改

如： __PILLAR__DNS__DOMAIN__ 改成刚才配置的--cluster-domain的值

__PILLAR__DNS__MEMORY__LIMIT__、__PILLAR__DNS__SERVER__

如果因为防火墙的问题拉取不到image: k8s.gcr.io/coredns:1.7.0，建议将coredns的镜像配置修改，

coredns的参考yaml文件(coredns.yaml)：

# __MACHINE_GENERATED_WARNING__

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
- apiGroups:
  - ""
  resources:
  - nodes
  verbs:
  - get
---
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
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
  labels:
      addonmanager.kubernetes.io/mode: EnsureExists
data:
  Corefile: |
    .:53 {
        errors
        health {
            lameduck 5s
        }
        ready
        kubernetes cluster.local in-addr.arpa ip6.arpa {
            pods insecure
            fallthrough in-addr.arpa ip6.arpa
            ttl 30
        }
        prometheus :9153
        forward . /etc/resolv.conf {
            max_concurrent 1000
        }
        cache 30
        loop
        reload
        loadbalance
    }
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: coredns
  namespace: kube-system
  labels:
    k8s-app: kube-dns
    kubernetes.io/cluster-service: "true"
    addonmanager.kubernetes.io/mode: Reconcile
    kubernetes.io/name: "CoreDNS"
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:
    type: RollingUpdate
    rollingUpdate:
      maxUnavailable: 1
  selector:
    matchLabels:
      k8s-app: kube-dns
  template:
    metadata:
      labels:
        k8s-app: kube-dns
      annotations:
        seccomp.security.alpha.kubernetes.io/pod: 'runtime/default'
    spec:
      priorityClassName: system-cluster-critical
      serviceAccountName: coredns
      affinity:
        podAntiAffinity:
          preferredDuringSchedulingIgnoredDuringExecution:
          - weight: 100
            podAffinityTerm:
              labelSelector:
                matchExpressions:
                  - key: k8s-app
                    operator: In
                    values: ["kube-dns"]
              topologyKey: kubernetes.io/hostname
      tolerations:
        - key: "CriticalAddonsOnly"
          operator: "Exists"
      nodeSelector:
        kubernetes.io/os: linux
      containers:
      - name: coredns
        image: coredns/coredns:1.7.0
        imagePullPolicy: IfNotPresent
        resources:
          limits:
            memory: 170Mi
          requests:
            cpu: 100m
            memory: 70Mi
        args: [ "-conf", "/etc/coredns/Corefile" ]
        volumeMounts:
        - name: config-volume
          mountPath: /etc/coredns
          readOnly: true
        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
        readinessProbe:
          httpGet:
            path: /ready
            port: 8181
            scheme: HTTP
        securityContext:
          allowPrivilegeEscalation: false
          capabilities:
            add:
            - NET_BIND_SERVICE
            drop:
            - all
          readOnlyRootFilesystem: true
      dnsPolicy: Default
      volumes:
        - name: config-volume
          configMap:
            name: coredns
            items:
            - key: Corefile
              path: Corefile
---
apiVersion: v1
kind: Service
metadata:
  name: kube-dns
  namespace: kube-system
  annotations:
    prometheus.io/port: "9153"
    prometheus.io/scrape: "true"
  labels:
    k8s-app: kube-dns
    kubernetes.io/cluster-service: "true"
    addonmanager.kubernetes.io/mode: Reconcile
    kubernetes.io/name: "CoreDNS"
spec:
  selector:
    k8s-app: kube-dns
  clusterIP: 10.96.0.100
  ports:
  - name: dns
    port: 53
    protocol: UDP
  - name: dns-tcp
    port: 53
    protocol: TCP
  - name: metrics
    port: 9153
    protocol: TCP

 

$  kubectl create -f coredns.yaml

     创建CoreDNS服务

 

$ kubectl get deployment --namespace=kube-system

  查看对应的coredns Deployment是否正常启动

 

$ kubectl get pods --namespace=kube-system

查看对应的coredns Pod是否正常启动

 

$  kubectl get services --namespace=kube-system

 查看对应的core-dns service是否正常启动

 

创建busybox的pod来验证dns功能

文件名：busybox.yaml

apiVersion: v1
kind: Pod
metadata:
 name: busybox
 namespace: default
spec:
 containers:
 - name: busybox
   image: busybox:1.28.4
   imagePullPolicy: IfNotPresent
   command:
   - sleep
   - "3600"

 

创建该busybox的Pod

$  kubectl create busybox.yaml

 

验证dns服务是否正常

$ kubectl exec busybox -- nslookup redis-master

看到系统可以根据服务名进行解析

 

问题与解决：

如果执行以上指令出现了dns can't resolve kubernetes.default and/or cluster.local错误

https://github.com/kubernetes/kubernetes/issues/66924

则很可能是因为busybox的镜像版本导致的，busybox不能使用最新版本，要使用1.28.4及以下版本。

 

best wish!