概述

Docker的流行激活了一直不温不火的PaaS，随着而来的是各类Micro-PaaS的出现，Kubernetes是其中最具代表性的一员，它是Google多年大规模容器管理技术的开源版本。本系列文章将逐一分析Kubernetes， 本文主要讲解如何开启DNS支持，以及其原理和使用方式。

1.开启DNS

Kubernetes支持2种服务发现方式，环境变量和DNS,其中环境变量是默认支持的，但是环境变量方式存在限制：Pod必须在Service之后创建，DNS则没有这个限制。
DNS是一种Cluster Add-on, 它随Kubernetes部署，但是需要配置启动：

- cluster turn-up

如果使用cluster turn-up，需要配置cluster/gce/config-default.sh

ENABLE_CLUSTER_DNS=true
DNS_SERVER_IP="10.0.0.10"
DNS_DOMAIN="cluster.local"
DNS_REPLICAS=1

- 手动方式

首先启动DNS server ReplicationController和Service,
配置文件模板：https://github.com/GoogleCloudPlatform/kubernetes/tree/v1.0.1/cluster/addons/dns

skydns-rc.yaml：

apiVersion: v1
kind: ReplicationController
metadata:
  name: kube-dns-v6
  namespace: kube-system
  labels:
    k8s-app: kube-dns
    version: v6
    kubernetes.io/cluster-service: "true"
spec:
  replicas: 1
  selector:
    k8s-app: kube-dns
    version: v6
  template:
    metadata:
      labels:
        k8s-app: kube-dns
        version: v6
        kubernetes.io/cluster-service: "true"
    spec:
      containers:
      - name: etcd
        image: gcr.io/google_containers/etcd:2.0.9
        resources:
          limits:
            cpu: 100m
            memory: 50Mi
        command:
        - /usr/local/bin/etcd
        - -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
      - name: kube2sky
        image: gcr.io/google_containers/kube2sky:1.11
        resources:
          limits:
            cpu: 100m
            memory: 50Mi
        args:
        # command = "/kube2sky"
        - --kube_master_url=http://192.168.3.146:8080 #change to your master url
        - -domain=cluster.local
      - name: skydns
        image: gcr.io/google_containers/skydns:2015-03-11-001
        resources:
          limits:
            cpu: 100m
            memory: 50Mi
        args:
        # command = "/skydns"
        - -machines=http://localhost:4001
        - -addr=0.0.0.0:53
        - -domain=cluster.local.
        ports:
        - containerPort: 53
          name: dns
          protocol: UDP
        - containerPort: 53
          name: dns-tcp
          protocol: TCP
      dnsPolicy: Default  # Don't use cluster DNS.

skydns-svc.yaml：

apiVersion: v1
kind: Service
metadata:
  name: kube-dns
  namespace: kube-system
  labels:
    k8s-app: kube-dns
    kubernetes.io/cluster-service: "true"
    kubernetes.io/name: "KubeDNS"
spec:
  selector:
    k8s-app: kube-dns
  clusterIP: 10.254.210.250
  ports:
  - name: dns
    port: 53
    protocol: UDP
  - name: dns-tcp
    port: 53
    protocol: TCP

创建后可以查看：

$ kubectl --namespace="kube-system" get pods
NAME                READY     STATUS    RESTARTS   AGE
kube-dns-v6-5y317   3/3       Running   0          2d

$ kubectl --namespace="kube-system" get services
NAME       LABELS               SELECTOR             IP(S)            PORT(S)
kube-dns   k8s-app=kube-dns...  k8s-app=kube-dns     10.254.210.250   53/UDP,53/TCP

DNS Server启动完成后，还需要配置kubelet：

--cluster_dns=10.254.210.250  #DNS service ip
--cluster_domain=cluster.local #default local domain

部署好的话可以验证下，现有以下services:

$ kubectl get services
NAME            LABELS                 SELECTOR            IP(S)            PORT(S)
frontend        name=frontend          name=frontend       10.254.159.131   80/TCP
redis-master    name=redis-master      name=redis-master   10.254.169.230   6379/TCP
redis-slave     name=redis-slave       name=redis-slave    10.254.70.184    6379/TCP

选择一个pod,进行DNS验证：

$ kubectl exec busybox -- nslookup frontend        
Server:    10.254.210.250
Address 1: 10.254.210.250

Name:      frontend
Address 1: 10.254.159.131

$ kubectl exec busybox -- nslookup redis-master
Server:    10.254.210.250
Address 1: 10.254.210.250

Name:      redis-master
Address 1: 10.254.169.230

$ kubectl exec busybox -- nslookup redis-slave
Server:    10.254.210.250
Address 1: 10.254.210.250

Name:      redis-slave
Address 1: 10.254.70.184

可以看到查询处理的ip和Service ip一致，说明DNS工作正常。

2.DNS说明

DNS Server包含3部分：
- skyDNS: 提供DNS解析服务
- etcd：用于skyDNS的存储
- kube2sky：连接Kubernetes和skyDNS

实际上kube2sky会监听Kubernetes，当有新的Service创建时，就生成相应记录到skyDNS，一个Service包括2条记录：

    <service_name>.<namespace_name>.<domain>
    <service_name>.<namespace_name>.svc.<domain>

然后kubelet会在容器启动的时候配置根据/etc/resolv.conf:

$ kubectl exec busybox cat /etc/resolv.conf 
nameserver 10.254.210.250
nameserver 218.85.157.99
search default.svc.cluster.local svc.cluster.local cluster.local
options ndots:5

可以看到配置了DNS Service IP作为域名服务器，然后设置了default.svc.cluster.local svc.cluster.local cluster.local作为默认域名。

以redis-master service为例:

$ kubectl exec busybox -- nslookup redis-master
Server:    10.254.210.250
Address 1: 10.254.210.250

Name:      redis-master
Address 1: 10.254.169.230

$ kubectl exec busybox -- nslookup redis-master.default.cluster.local
Server:    10.254.210.250
Address 1: 10.254.210.250

Name:      redis-master.default.cluster.local
Address 1: 10.254.169.230

$ kubectl exec busybox -- nslookup redis-master.default.svc.cluster.local
Server:    10.254.210.250
Address 1: 10.254.210.250

Name:      redis-master.default.svc.cluster.local
Address 1: 10.254.169.230

对于Headless services，域名则对于所有Endpoints：

$ kubectl describe service frontend-headless
Name:           frontend-headless
Namespace:      default
Labels:         name=frontend-headless
Selector:       name=frontend
Type:           ClusterIP
IP:             None
Port:           <unnamed>   80/TCP
Endpoints:      10.1.14.19:80,10.1.79.47:80,10.1.79.48:80
Session Affinity: None
No events.

$ kubectl exec busybox -- nslookup frontend-headless
Server:    10.254.210.250
Address 1: 10.254.210.250

Name:      frontend-headless
Address 1: 10.1.14.19
Address 2: 10.1.79.47
Address 3: 10.1.79.48

参考

• https://github.com/GoogleCloudPlatform/kubernetes/blob/v1.0.1/docs/services.md#discovering-services
• https://github.com/GoogleCloudPlatform/kubernetes/blob/v1.0.1/docs/dns.md

作者简介

吴龙辉，现任网宿科技高级运营工程师，致力于云计算PaaS的研究和实践，活跃于CloudFoundry,Docker,Kubernetes等开源社区，贡献代码和撰写技术文档。
邮箱：wulh@chinanetcenter.com/wlh6666@qq.com