//client.go
package main

import (
    "fmt"
    "log"
    "net"
    "os"
    "strconv"
    "strings"
    "time"
)

var tag string

const HAND_SHAKE_MSG = "我是打洞消息"

func main() {
    if len(os.Args) < 2 {
        fmt.Println("请输入一个客户端标志")
        os.Exit(0)
    }
    // 当前进程标记字符串,便于显示
    tag = os.Args[1]
    srcAddr := &net.UDPAddr{IP: net.IPv4zero, Port: 9901} // 注意端口必须固定
    dstAddr := &net.UDPAddr{IP: net.ParseIP("192.168.1.102"), Port: 9527}
    conn, err := net.DialUDP("udp", srcAddr, dstAddr)
    if err != nil {
        fmt.Println(err)
    }
    if _, err = conn.Write([]byte("hello, I'm new peer:" + tag)); err != nil {
        log.Panic(err)
    }
    data := make([]byte, 1024)
    n, remoteAddr, err := conn.ReadFromUDP(data)
    if err != nil {
        fmt.Printf("error during read: %s", err)
    }
    conn.Close()
    anotherPeer := parseAddr(string(data[:n]))
    fmt.Printf("local:%s server:%s another:%s\n", srcAddr, remoteAddr, anotherPeer.String())
    // 开始打洞
    bidirectionHole(srcAddr, &anotherPeer)
}
func parseAddr(addr string) net.UDPAddr {
    t := strings.Split(addr, ":")
    port, _ := strconv.Atoi(t[1])
    return net.UDPAddr{
        IP:   net.ParseIP(t[0]),
        Port: port,
    }
}
func bidirectionHole(srcAddr *net.UDPAddr, anotherAddr *net.UDPAddr) {
    conn, err := net.DialUDP("udp", srcAddr, anotherAddr)
    if err != nil {
        fmt.Println(err)
    }
    defer conn.Close()
    // 向另一个peer发送一条udp消息(对方peer的nat设备会丢弃该消息,非法来源),用意是在自身的nat设备打开一条可进入的通道,这样对方peer就可以发过来udp消息
    if _, err = conn.Write([]byte(HAND_SHAKE_MSG)); err != nil {
        log.Println("send handshake:", err)
    }
    go func() {
        for {
            time.Sleep(10 * time.Second)
            if _, err = conn.Write([]byte("from [" + tag + "]")); err != nil {
                log.Println("send msg fail", err)
            }
        }
    }()
    for {
        data := make([]byte, 1024)
        n, _, err := conn.ReadFromUDP(data)
        if err != nil {
            log.Printf("error during read: %s\n", err)
        } else {
            log.Printf("收到数据:%s\n", data[:n])
        }
    }
}

客户端1显示如下：

ykdeMac-mini:study yekai$ ./client yekai1
local:0.0.0.0:9901 server:192.168.1.102:9527 another:192.168.1.126:9902
2019/04/03 14:52:57 收到数据:我是打洞消息
2019/04/03 14:52:57 error during read: read udp 192.168.1.102:9901->192.168.1.126:9902: recvfrom: connection refused
2019/04/03 14:53:07 收到数据:from [yekai2]
2019/04/03 14:53:17 收到数据:from [yekai2]

客户端2显示如下：

localhost:zhuhai yk$ ./client yekai2
local:0.0.0.0:9902 server:192.168.1.102:9527 another:192.168.1.102:9901
2019/04/03 14:53:07 收到数据:from [yekai1]
2019/04/03 14:53:17 收到数据:from [yekai1]

备注：本文中的公网服务器使用的是192.168.1.102进行替代，测试时并没有实际走NAT映射，不过其他童鞋可以用代码在公网服务器进行验证！