CoreDNS介绍

本文介绍 CoreDNS 相关配置以及验证方法,实验环境为 Kubernetes 1.11,搭建方法参考kubeadm安装kubernetes V1.11.1 集群 。想要了解更多 Kubernetes 相关知识,可以阅读 Kubernetes 系列学习文章

busybox 的槽点

开始之前先吐槽一下busybox中的nslookup命令。这个命令应该是实现的不是很完全,导致我在测试DNS的成功,得到了错误的信息。先来看一下

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[root@devops-101 ~]# kubectl run busybox1 --rm -it --image=docker.io/busybox /bin/sh
If you don't see a command prompt, try pressing enter.
/ # nslookup kubernetes.default
Server: 172.17.0.10
Address: 172.17.0.10:53

** server can't find kubernetes.default: NXDOMAIN

*** Can't find kubernetes.default: No answer

看起来像是DNS没有响应,慢着,使用带nslookup的alphine试一下。

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[root@devops-101 ~]# kubectl run dig --rm -it --image=docker.io/azukiapp/dig /bin/sh
If you don't see a command prompt, try pressing enter.
/ # dig @172.17.0.10 kubernetes.default.svc.cluster.local +noall +answer

; <<>> DiG 9.10.3-P3 <<>> @172.17.0.10 kubernetes.default.svc.cluster.local +noall +answer
; (1 server found)
;; global options: +cmd
kubernetes.default.svc.cluster.local. 5 IN A 172.17.0.1
/ # nslookup kubernetes.default
Server: 172.17.0.10
Address: 172.17.0.10#53

Name: kubernetes.default.svc.cluster.local
Address: 172.17.0.1

/ # nslookup www.baidu.com
Server: 172.17.0.10
Address: 172.17.0.10#53

Non-authoritative answer:
www.baidu.com canonical name = www.a.shifen.com.
Name: www.a.shifen.com
Address: 220.181.112.244
Name: www.a.shifen.com
Address: 220.181.111.188

/ # nslookup kubernetes.default
Server: 172.17.0.10
Address: 172.17.0.10#53

Name: kubernetes.default.svc.cluster.local
Address: 172.17.0.1

好好的啊!就是这个原因,busybox坑了我好几天。

CoreDNS

CoreDNS在Kubernetes1.11版本已经做为GA功能释放,成为Kubernetes默认的DNS服务替代了Ku be-DNS,目前是kubeadm、kube-up、minikube和kops安装工具的默认选项。

kube-dns translates to the proxy in CoreDNS. The```federation```in kube-dns has an equivalent```federation```in CoreDNS.
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## 配置文件
使用kubeadm安装CoreDNS,会使用ConfigMap做为配置文件。这份配置文件,会默认使用宿主机的DNS服务器地址。
```bash
[root@devops-101 ~]# kubectl -n kube-system get configmap coredns -oyaml
apiVersion: v1
data:
Corefile: |
.:53 {
errors
health
kubernetes cluster.local in-addr.arpa ip6.arpa {
pods insecure
upstream
fallthrough in-addr.arpa ip6.arpa
}
prometheus :9153
proxy . /etc/resolv.conf
cache 30
reload
}
kind: ConfigMap
metadata:
creationTimestamp: 2018-08-20T07:01:55Z
name: coredns
namespace: kube-system
resourceVersion: "193"
selfLink: /api/v1/namespaces/kube-system/configmaps/coredns
uid: ec72baa4-a446-11e8-ac92-080027b7c4e9

配置文件各项目的含义

检查COreDNS运行状况

检查Pod状态

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[root@devops-101 ~]# kubectl -n kube-system get pods -o wide
NAME READY STATUS RESTARTS AGE IP NODE
coredns-78fcdf6894-52gp9 1/1 Running 4 4h 172.16.0.11 devops-101
coredns-78fcdf6894-mkvqn 1/1 Running 4 4h 172.16.0.10 devops-101
etcd-devops-101 1/1 Running 4 3h 192.168.0.101 devops-101

检查部署

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[root@devops-101 ~]# kubectl -n kube-system get deployments
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
coredns 2 2 2 2 4h

验证可以采用本文刚开始部分提到的方法。

参考资料

  1. nslookup not working with flannel as network: nameserver 10.96.0.10
  2. 使用 kubeadm 搭建 kubernetes1.10 集群
  3. CoreDNS for Kubernetes Service Discovery, Take 2
  4. Migration from kube-dns to CoreDNS
  5. Deploying Kubernetes with CoreDNS using kubeadm
  6. dns service discovery fails on 1.11 with coredns (new default)

cocowool

A FULL STACK DREAMER!