16、Service名称解析ClusterDNS

Cluster DNS（CoreDNS）是Kubernetes集群的必备附件，负责为Kubernetes提供名称解析和服务发现
  ⚫ 每个Service资源对象，在CoreDNS上都会自动生成一个遵循“..svc.”格式的名称
    ◆ ：当前Service对象的名称
    ◆ ：当前Service对象所属的名称空间
    ◆ ：当前Kubernetes集群使用的域名后缀，默认为“cluster.local”
  ⚫ 围绕该名称会生成一些DNS格式的资源记录

CoreDNS会持续监视API Server上的Service资源对象的变动，并实时反映到相关的DNS资源记录中

Pod中各容器默认会在/etc/resolv.conf中，将nameserver指向CoreDNS相关的Service的ClusterIP
  ⚫ 由kubelet创建Pod时根据指定的配置自动注入

kube-dns在kube-system名称空间下会自动被生成
[root@K8s-master01 chapter7]#kubectl get svc -n kube-system
NAME       TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)                  AGE
kube-dns   ClusterIP   10.96.0.10           53/UDP,53/TCP,9153/TCP   8d
kube-dns地址是固定的，10.96.0.10是可用地址范围内的第十个地址
 

这个地址段的第一个地址也被kubernetes固定使用
[root@K8s-master01 chapter7]#kubectl get svc -n default
NAME            TYPE        CLUSTER-IP       EXTERNAL-IP    PORT(S)        AGE
kubernetes    ClusterIP      10.96.0.1                443/TCP        8d

ClusterDNS服务的名称：kube-dns.kube-system.svc.cluster.local
固定的地址：Cluster Network地址范围内的第10个地址；

ClusterDNS服务会使用标签选择器选择合理的pod数量作为后端的端点，默认ClusterDNS工作为两个pod
[root@K8s-master01 chapter7]#kubectl get pods -n kube-system --show-labels
NAME                                   READY   STATUS    RESTARTS        AGE    LABELS
coredns-565d847f94-ql7cb               1/1     Running   0               78m    k8s-app=kube-dns,pod-template-hash=565d847f94
coredns-565d847f94-sr27n               1/1     Running   5 (11h ago)     8d     k8s-app=kube-dns,pod-template-hash=565d847f94
查看pod所在的地址和节点
[root@K8s-master01 chapter7]#kubectl get pods -n kube-system -l k8s-app=kube-dns -o wide
NAME                       READY   STATUS    RESTARTS      AGE   IP            NODE           NOMINATED NODE   READINESS GATES
coredns-565d847f94-ql7cb   1/1     Running   0             81m   10.244.2.2    k8s-master03              
coredns-565d847f94-sr27n   1/1     Running   5 (11h ago)   8d    10.244.0.12   k8s-master01              
查看service的详细信息
[root@K8s-master01 chapter7]#kubectl describe svc kube-dns -n kube-system
Name:              kube-dns
Namespace:         kube-system
Labels:            k8s-app=kube-dns
                   kubernetes.io/cluster-service=true
                   kubernetes.io/name=CoreDNS
Annotations:       prometheus.io/port: 9153
                   prometheus.io/scrape: true
Selector:          k8s-app=kube-dns
Type:              ClusterIP
IP Family Policy:  SingleStack
IP Families:       IPv4
IP:                10.96.0.10
IPs:               10.96.0.10
Port:              dns  53/UDP
TargetPort:        53/UDP
Endpoints:         10.244.0.12:53,10.244.2.2:53
Port:              dns-tcp  53/TCP
TargetPort:        53/TCP
Endpoints:         10.244.0.12:53,10.244.2.2:53
Port:              metrics  9153/TCP
TargetPort:        9153/TCP
Endpoints:         10.244.0.12:9153,10.244.2.2:9153  
Session Affinity:  None
Events:            
也可发现其所在的端点ip

[root@K8s-master01 chapter7]#kubectl exec demoapp-55c5f88dcb-6g9k4 -- cat /etc/resolv.conf 
nameserver 10.96.0.10
search default.svc.cluster.local svc.cluster.local cluster.local 
options ndots:5

查看service
[root@K8s-master01 ~]#kubectl get svc
NAME             TYPE      CLUSTER-IP       EXTERNAL-IP    PORT(S)        AGE
demoapp-svc  ClusterIP      10.97.72.1               80/TCP         5h57m

创建客户端对service进行域名解析
[root@K8s-master01 chapter7]#kubectl run client-$RANDOM --image=ikubernetes/admin-box:v1.2 --restart=Never -it --command -- /bin/bash
If you don't see a command prompt, try pressing enter.
root@client-22289 /# host -t A demoapp-svc

demoapp-svc.default.svc.cluster.local has address 10.97.72.1

对service进行IP解析
root@client-22289 /# host -t PTR 10.97.72.1

1.72.97.10.in-addr.arpa domain name pointer demoapp-svc.default.svc.cluster.local.

对service进行端口解析
root@client-22289 /# host -t SRV _http._tcp.demoapp-svc.default.svc.cluster.local.

_http._tcp.demoapp-svc.default.svc.cluster.local has SRV record 0 100 80 demoapp-svc.default.svc.cluster.local.

DNS解析策略
1、Default：从运行在的节点继承DNS名称解析相关的配置
如：
[root@K8s-master01 chapter7]#host -t A www.baidu.com
www.baidu.com is an alias for www.a.shifen.com.
www.a.shifen.com has address 220.181.38.150
www.a.shifen.com has address 220.181.38.149
2、ClusterFirst：于集群DNS服务上解析集群域内的名称，其他域名的解析则交由从节点继承而来的上游名称服务器
3、ClusterFirstWithHostNet：专用于在设置了hostNetwork的Pod对象上使用的ClusterFirst策略
4、None：用于忽略Kubernetes集群的默认设定，而仅使用由dnsConfig自定义的配置


DNS名称解析机制
⚫ nameservers ：DNS名称服务器列表，附加于由dnsPolicy生成的DNS名称服务器之后
⚫ searches ：DNS名称解析时的搜索域，附加由于dnsPolicy生成的搜索域之后
⚫ options ：DNS解析选项列表，同dnsPolicy生成的解析选项合并成最终生效的定义

[root@K8s-master01 chapter7]#cat pod-with-dnspolicy.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: pod-with-dnspolicy
  namespace: default
spec:
  containers:
  - name: demo
    image: ikubernetes/demoapp:v1.0
    imagePullPolicy: IfNotPresent
  dnsPolicy: None
  dnsConfig:
    nameservers:
    - 10.96.0.10
    - 223.5.5.5
    - 223.6.6.6
    searches: 
    - svc.cluster.local
    - cluster.local
    - ilinux.io
    options:
    - name: ndots
      value: "5"
创建pod
[root@K8s-master01 chapter7]#kubectl apply -f pod-with-dnspolicy.yaml 
pod/pod-with-dnspolicy created

查看域名解析
[root@K8s-master01 chapter7]#kubectl exec pod-with-dnspolicy -- cat /etc/resolv.conf 
nameserver 10.96.0.10
nameserver 223.5.5.5
nameserver 223.6.6.6
search svc.cluster.local cluster.local ilinux.io
options ndots:5

[root@K8s-master01 chapter7]#kubectl get cm -n kube-system
NAME                                 DATA   AGE
coredns                              1      8d
extension-apiserver-authentication   6      8d
kube-proxy                           2      8d
kube-root-ca.crt                     1      8d
kubeadm-config                       1      8d
kubelet-config                       1      8d

把DNS配置文件导出来修改，在用kubectl apply -f 
[root@K8s-master01 chapter7]#kubectl get cm coredns -o yaml -n kube-system > DNS.yaml
apiVersion: v1
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
    }
kind: ConfigMap
metadata:
  creationTimestamp: "2022-11-07T02:23:24Z"
  name: coredns
  namespace: kube-system
  resourceVersion: "241"
  uid: cde90501-030a-465a-979f-d3253c71583f
具体配置方法可互联网搜索