一、Headless类型Service简介
在某些场景下，开发人员可能不想使用Service提供的负载均衡功能，而希望自己来控制负载均衡策略，针对这种情况，Kubernetes提供了Headless Service，这类Service不会分配ClusterIP，如果想要访问Service，只能通过service的域名进行查询
二、环境准备
编写deployment.yaml文件，内容如下：
apiVersion: v1
kind: Namespace
metadata:
name: dev
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: pc-deployment
namespace: dev
spec:
replicas: 3
selector:
matchLabels:
app: nginx-pod
template:
metadata:
labels:
app: nginx-pod
spec:
containers:
- name: nginx
image: nginx:1.17.1
ports:
- containerPort: 80
然后使用如下命令创建资源
[root@master service]# kubectl apply -f deployment.yaml
namespace/dev created
deployment.apps/pc-deployment created
[root@master service]#
查看创建的资源如下：
[root@master service]# kubectl get deploy,pod -n dev -o wide
NAME READY UP-TO-DATE AVAILABLE AGE CONTAINERS IMAGES SELECTOR
deployment.apps/pc-deployment 3/3 3 3 17m nginx nginx:1.17.1 app=nginx-pod
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
pod/pc-deployment-5ffc5bf56c-mqwst 1/1 Running 0 17m 10.244.2.191 node2 <none> <none>
pod/pc-deployment-5ffc5bf56c-qbznm 1/1 Running 0 17m 10.244.1.53 node1 <none> <none>
pod/pc-deployment-5ffc5bf56c-zxnsx 1/1 Running 0 17m 10.244.2.190 node2 <none> <none>
[root@master service]#
三、创建Headless类型的Service
编辑headless.yaml文件，内容如下：
apiVersion: v1
kind: Service
metadata:
name: service-headless
namespace: dev
spec:
selector:
app: nginx-pod
clusterIP: None
type: ClusterIP
ports:
- port: 80
targetPort: 80
使用如下命令创建服务
[root@master service]# kubectl apply -f headless.yaml
service/service-headless created
[root@master service]#
查看创建的资源
[root@master service]# kubectl get service,deployment,pod -n dev -o wide
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
service/service-headless ClusterIP None <none> 80/TCP 99s app=nginx-pod
NAME READY UP-TO-DATE AVAILABLE AGE CONTAINERS IMAGES SELECTOR
deployment.apps/pc-deployment 3/3 3 3 8m50s nginx nginx:1.17.1 app=nginx-pod
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
pod/pc-deployment-5ffc5bf56c-mqwst 1/1 Running 0 8m50s 10.244.2.191 node2 <none> <none>
pod/pc-deployment-5ffc5bf56c-qbznm 1/1 Running 0 8m50s 10.244.1.53 node1 <none> <none>
pod/pc-deployment-5ffc5bf56c-zxnsx 1/1 Running 0 8m50s 10.244.2.190 node2 <none> <none>
[root@master service]#
此时服务因为没有ClusterIP，所以此时不能使用ip去访问了，只能使用域名访问了，即 service-headless.dev.svc.cluster.local，这里面格式是
[service的名字].[命名空间].svc.cluster.local
首先登录pod查看域名解析
[root@master service]# kubectl exec -it pod/pc-deployment-5ffc5bf56c-mqwst -n dev /bin/bash
kubectl exec [POD] [COMMAND] is DEPRECATED and will be removed in a future version. Use kubectl exec [POD] -- [COMMAND] instead.
root@pc-deployment-5ffc5bf56c-9bg6w:/#
root@pc-deployment-5ffc5bf56c-9bg6w:/# cat /etc/resolv.conf
nameserver 10.96.0.10
search dev.svc.cluster.local svc.cluster.local cluster.local
options ndots:5
root@pc-deployment-5ffc5bf56c-9bg6w:/#
然后查看域名解析，具体查看域名的命令如下
[root@master service]# dig @10.96.0.10 service-headless.dev.svc.cluster.local
; <<>> DiG 9.11.4-P2-RedHat-9.11.4-26.P2.el7_9.9 <<>> @10.96.0.10 service-headless.dev.svc.cluster.local
; (1 server found)
;; global options: +cmd
;; Got answer:
;; WARNING: .local is reserved for Multicast DNS
;; You are currently testing what happens when an mDNS query is leaked to DNS
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 62573
;; flags: qr aa rd; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 1
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;service-headless.dev.svc.cluster.local. IN A
;; ANSWER SECTION:
service-headless.dev.svc.cluster.local. 30 IN A 10.244.2.190
service-headless.dev.svc.cluster.local. 30 IN A 10.244.1.53
service-headless.dev.svc.cluster.local. 30 IN A 10.244.2.191
;; Query time: 0 msec
;; SERVER: 10.96.0.10#53(10.96.0.10)
;; WHEN: Mon Apr 04 11:57:01 CST 2022
;; MSG SIZE rcvd: 229
[root@master service]#
