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

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

[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。

[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。

Stubdomain and upstreamnameserver in kube-dns translates to the proxy in CoreDNS. The federation in kube-dns has an equivalent federation in CoreDNS.

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。

[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
名称|含义
---|---
errors|错误会被记录到标准输出
health|可以通过http://localhost:8080/health查看健康状况
kubernetes|根据服务的IP响应DNS查询请求,kubeadm的Cluster DomainService CIDR默认为cluster.local10.95.0.0/12,可以通过--service-dns-domain--service-cidr参数配置。
prometheus|可以通过http://localhost:9153/metrics获取prometheus格式的监控数据
proxy|本地无法解析后,向上级地址进行查询,默认使用宿主机的 /etc/resolv.conf 配置
cache|缓存时间

检查COreDNS运行状况

检查Pod状态

[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

[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。

39469-20180710163655709-89635310.png

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)

转载于:https://www.cnblogs.com/cocowool/p/kubernetes_coredns.html

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