net.LookupSRV()查询k8s无头服务
(金庆的专栏 2021.2)
如下创建 StatefulSet 和 Headless Service: test.yaml
```
apiVersion: v1
kind: Service
metadata:
name: headless-svc
labels:
app: headless-svc
spec:
ports:
- port: 80
name: aaaa
- port: 20080
name: bbbb
selector:
app: headless-pod
clusterIP: None
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: statefulset-test
spec:
serviceName: headless-svc
replicas: 3
selector:
matchLabels:
app: headless-pod
template:
metadata:
labels:
app: headless-pod
spec:
containers:
- name: myhttpd
image: httpd
ports:
- containerPort: 80
- containerPort: 20080
```
部署：
```
kubectl apply -f test.yaml
```
然后开一个 shell:
```
kubectl run mygolang -it --image golang -- bash
If you don't see a command prompt, try pressing enter.
root@mygolang:/go#
```
先装个 vim:
```
apt install vim
```
然后写个 golang 测试程序，向 DNS 查询 bbbb 服务的地址与端口：
```
root@mygolang:/jinqing# cat main.go
package main
import (
"fmt"
"github.com/davecgh/go-spew/spew"
"net"
)
func main() {
cname, addresses, err := net.LookupSRV("bbbb", "tcp", "headless-svc.default.svc.cluster.local")
if err != nil {
fmt.Printf("failed: %s\n", err)
}
fmt.Printf("cname: %s\n", cname)
spew.Dump(addresses)
}
root@mygolang:/jinqing#
```
运行结果为：
```
cname: _bbbb._tcp.headless-svc.default.svc.cluster.local.
([]*net.SRV) (len=3 cap=4) {
(*net.SRV)(0xc00000e220)({
Target: (string) (len=58) "statefulset-test-0.headless-svc.default.svc.cluster.local.",
Port: (uint16) 20080,
Priority: (uint16) 0,
Weight: (uint16) 33
}),
(*net.SRV)(0xc00000e1e0)({
Target: (string) (len=58) "statefulset-test-2.headless-svc.default.svc.cluster.local.",
Port: (uint16) 20080,
Priority: (uint16) 0,
Weight: (uint16) 33
}),
(*net.SRV)(0xc00000e200)({
Target: (string) (len=58) "statefulset-test-1.headless-svc.default.svc.cluster.local.",
Port: (uint16) 20080,
Priority: (uint16) 0,
Weight: (uint16) 33
})
}
```
多次运行发现结果项次序固定，并没有按权重随机。
Service yaml 定义中，必须为每个端口命名，不然没法查询。
```
cname, addresses, err := net.LookupSRV("bbbb", "tcp", "headless-svc.default.svc.cluster.local")
```
可利用默认域名后缀简写为
```
cname, addresses, err := net.LookupSRV("bbbb", "tcp", "headless-svc")
```
输出相同。