package main

import (
	"bytes"
	"os/exec"
	"strings"
	"time"
)

const (
	pingInterval = 60 * time.Second // 每10秒执行一次
	ipToPing     = "203.34.49.130"  // 这里填写要ping的IP地址
)

// checkErr 输出错误信息并退出程序
func checkErr(err error) {
	if err != nil {
		//log.Fatalf("Error: %s", err)
	}
}

// ping 检查是否可以到达指定的IP地址
func ping(ip string) bool {
	cmd := exec.Command("ping", "-c", "1", ip)
	var out bytes.Buffer
	cmd.Stdout = &out
	err := cmd.Run()
	output := out.String()

	if strings.Contains(output, "1 packets transmitted, 1 received") ||
		strings.Contains(output, "1 packets transmitted, 1 received, 0% packet loss") {
		return true
	}
	return err == nil
}

// checkNet 解析ip route命令的输出，返回接口与默认网关的映射以及当前默认网卡名
func checkNet(net string) (map[string]string, string) {
	var defaultNet string
	m := make(map[string]string)
	split := strings.Split(net, "\n")

	for i := 0; i < len(split)-1; i++ {
		line := strings.TrimSpace(split[i])
		if line == "" {
			continue
		}

		defaultNets := strings.Fields(line)
		if len(defaultNets) > 1 {
			for k := 0; k < len(defaultNets)-1; k++ {
				if defaultNets[k] == "dev" && defaultNets[0] == "default" {
					defaultNet = defaultNets[k+1]
				}
			}
		}

		i2 := strings.Fields(line)
		if len(i2) > 1 {
			for j := 0; j < len(i2)-1; j++ {
				if i2[j] == "dev" {
					netName := i2[j+1]
					i3 := strings.Split(i2[j-1], ".")
					if len(i3) > 1 {
						ip := i3[0] + "." + i3[1] + "." + i3[2] + ".1"
						m[netName] = ip
					}
				}
			}
		}
	}

	//fmt.Println("Routes:", m)
	//fmt.Println("Default Net:", defaultNet)
	return m, defaultNet
}

// updateRoute 更新默认路由
func updateRoute(routes map[string]string, currentDefault, newDefault string) {
	if currentDefault != newDefault {
		cmdDel := exec.Command("ip", "route", "del", "default", "via", routes[currentDefault], "dev", currentDefault)
		cmdAdd := exec.Command("ip", "route", "add", "default", "via", routes[newDefault], "dev", newDefault)

		checkErr(cmdDel.Run())
		checkErr(cmdAdd.Run())

		//fmt.Printf("切换默认网卡  从 %s 到 %s\n", currentDefault, newDefault)
	} else {
		//fmt.Println("不需要切换默认网卡")
	}
}

func main() {
	for {
		b := ping(ipToPing)
		if b {
			//fmt.Println("网络连接正常")

			// 获取所有可用的网络接口及其默认网关
			cmd := exec.Command("ip", "route")
			var out bytes.Buffer
			cmd.Stdout = &out
			checkErr(cmd.Run())
			output := out.String()
			routes, currentDefault := checkNet(output)

			// 判断当前默认网卡是否为 eth 网卡
			if !strings.Contains(currentDefault, "eth") {
				//fmt.Println("当前为无线网卡，正在切换到有线网卡")
				for k := range routes {
					if strings.Contains(k, "eth") {
						updateRoute(routes, currentDefault, k)
						break
					}
				}
			} else {
				//fmt.Println("当前为有线网卡，无需切换")
			}
		} else {
			//fmt.Println("网络未接通正在重启....")

			// 获取所有可用的网络接口及其默认网关
			cmd := exec.Command("ip", "route")
			var out bytes.Buffer
			cmd.Stdout = &out
			checkErr(cmd.Run())
			output := out.String()
			routes, currentDefault := checkNet(output)

			// 找到一个新的默认网卡
			newDefault := ""
			for k := range routes {
				if k != currentDefault {
					newDefault = k
					break
				}
			}

			if newDefault != "" {
				updateRoute(routes, currentDefault, newDefault)

				// 再次检查是否可以 ping 通
				b := ping(ipToPing)
				if b {
					//fmt.Println("切换网卡后网络连接正常")
				} else {
					//fmt.Println("切换网卡后仍然无法 ping 通")
				}
			} else {
				//fmt.Println("没有找到其他可用的网卡")
			}
		}
		time.Sleep(pingInterval)
	}
}