kubernetes中coredns组件的高级用法

通过coredns实现内外流量分离

场景

1. 旧业务固定了域名，无法通过内部service直接访问服务
2. 需要实现内部流量和外部流量自动拆分

实现

1. 通过coredns的rewrite功能实现以上能力,如以下内部访问tenant.msa.chinamcloud.com域名时，会将流量转发到tenantapi.yunjiao.svc.cluster.local域名，实现内外域名访问一致。
2. 部分版本nginx配置时候可能遇见无法访问的情况

[root@k8s-master1 ingress]# cat coredns.yaml
apiVersion: v1
data:
  Corefile: |
    .:53 {
        errors
        health
        rewrite name tenant.msa.chinamcloud.com tenantapi.yunjiao.svc.cluster.local
        rewrite name console.msa.chinamcloud.com console.yunjiao.svc.cluster.local
        rewrite name user.msa.chinamcloud.com userapi.yunjiao.svc.cluster.local
        rewrite name lims.msa.chinamcloud.com lims.yunjiao.svc.cluster.local
        rewrite name labapp.msa.chinamcloud.com limsapp.yunjiao.svc.cluster.local
        kubernetes cluster.local in-addr.arpa ip6.arpa {
           pods insecure
           upstream
           fallthrough in-addr.arpa ip6.arpa
        }
        prometheus :9153
        forward . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }
kind: ConfigMap
metadata:
  creationTimestamp: "2019-04-02T04:57:19Z"
  name: coredns
  namespace: kube-system
  resourceVersion: "197"
  selfLink: /api/v1/namespaces/kube-system/configmaps/coredns
  uid: cb686453-5503-11e9-8ea6-005056be93f5

检查

[root@k8s-master1 ingress]#  kubectl run -it --rm --restart=Never --image=infoblox/dnstools:latest dnstools
If you don't see a command prompt, try pressing enter.
dnstools# ping tenant.msa.chinamcloud.com
PING tenant.msa.chinamcloud.com (10.98.220.54): 56 data bytes
^C
--- tenant.msa.chinamcloud.com ping statistics ---
4 packets transmitted, 0 packets received, 100% packet loss

kubernetes内部实现hosts功能

coredns配置参考文档

场景

1. 通过kubernetes的coredns实现子域名解析
2. 实现kubernetes内部 hosts绑定功能

实现

创建pod时声明hosts(不推荐)

[root@k8s-master-1 coredns]# kubectl  explain  pods.spec.hostAliases
KIND:     Pod
VERSION:  v1

RESOURCE: hostAliases <[]Object>

DESCRIPTION:
     HostAliases is an optional list of hosts and IPs that will be injected into
     the pod's hosts file if specified. This is only valid for non-hostNetwork
     pods.

     HostAlias holds the mapping between IP and hostnames that will be injected
     as an entry in the pod's hosts file.

FIELDS:
   hostnames    <[]string>
     Hostnames for the above IP address.

   ip   
     IP address of the host file entry.

[root@k8s-master-1 coredns]#

coredns的hosts特性声明

hosts 字段部分指明了三个域名的解析地址

[root@k8s-master-1 coredns]# cat coredns-cm.yaml
apiVersion: v1
data:
  Corefile: |
    .:53 {
        errors
        health
        hosts {
            100.64.139.66 minio.chinamcloud.com
            100.64.139.66 registry.chinamcloud.com
            100.64.139.66 gitlab.chinamcloud.com
            fallthrough
        }
        kubernetes cluster.local in-addr.arpa ip6.arpa {
           pods insecure
           upstream
           fallthrough in-addr.arpa ip6.arpa
        }
        prometheus :9153
        forward . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system

根据域名指定上游dns服务器

sobeydemo.com 字段指明了解析该域名的dns服务器地址

[root@k8s-master-1 coredns]# cat coredns-cm.yaml
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
        forward . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }
    sobeydemo.com {
        forward . 100.64.134.250:53
    }
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system

检查

[root@k8s-master-1 coredns]#  kubectl run -it --rm --restart=Never --image=infoblox/dnstools:latest dnstools
If you don't see a command prompt, try pressing enter.
dnstools# host 0DJ01YUR.sobeydemo.com
0DJ01YUR.sobeydemo.com has address 100.64.148.116
0DJ01YUR.sobeydemo.com has IPv6 address 2002:6440:9474::6440:9474
dnstools# host minio.chinamcloud.com
minio.chinamcloud.com has address 100.64.139.66
Host minio.chinamcloud.com not found: 3(NXDOMAIN)
Host minio.chinamcloud.com not found: 3(NXDOMAIN)
dnstools#