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缘由：

用看代码、改代码的方式学习Go 语言。

正文：

参考解答：

放一个网上找到的，既容易理解，效果也不错的代码：

package main

import (
    "context"
    "fmt"
    "net"
    "os/exec"
    "strconv"
    "strings"
    "sync"
    "time"

    "golang.org/x/sync/semaphore"
)

/*
    https://medium.com/@KentGruber/building-a-high-performance-port-scanner-with-golang-9976181ec39d
*/

// 将对IP的端口扫描抽象成 PortScanner 结构体
type PortScanner struct {
    ip   string
    lock *semaphore.Weighted // 加权信号量
}

// 通过 os/exec 模块调用命令 `ulimit -n` 并将它输出作为返回
func Ulimit() int64 {
    out, err := exec.Command("ulimit", "-n").Output()
    if err != nil {
        panic(err)
    }

    s := strings.TrimSpace(string(out))

    i, err := strconv.ParseInt(s, 10, 64)
    if err != nil {
        panic(err)
    }

    return i
}

// 扫描原理的核心逻辑——使用 net.DialTimeout 进行连接尝试
func ScanPort(ip string, port int, timeout time.Duration) {
    target := fmt.Sprintf("%s:%d", ip, port)
    conn, err := net.DialTimeout("tcp", target, timeout)

    if err != nil {
        if strings.Contains(err.Error(), "too many open files") {
            time.Sleep(timeout)
            ScanPort(ip, port, timeout)
        } else {
            fmt.Println(port, "closed")
        }
        return
    }

    conn.Close()
    fmt.Println(port, "open")
}

func (ps *PortScanner) Start(f, l int, timeout time.Duration) {
    wg := sync.WaitGroup{}
    defer wg.Wait()

    for port := f; port <= l; port++ {
        ps.lock.Acquire(context.TODO(), 1) // 对每一个 ScanPort 调用都加锁处理
        wg.Add(1)
        go func(port int) {
            defer ps.lock.Release(1)
            defer wg.Done()
            ScanPort(ps.ip, port, timeout)
        }(port)
    }
}

func main() {
    start_time := time.Now()

    ps := &PortScanner{
        ip:   "127.0.0.1",
        lock: semaphore.NewWeighted(Ulimit()),
    }
    ps.Start(1, 65535, 500*time.Millisecond)

    fmt.Printf("Spend %v seconds.\n", time.Since(start_time).Seconds())
}

参考链接：

• 用Go 语言构建一个高性能的端口扫描器 #nice
  https://gist.github.com/picatz/9c0028efd7b3ced3329f7322f41b16e1.go
• https://github.com/w8rbt/netscan
  https://github.com/anvie/port-scanner
  https://github.com/gabbifish/golang-port-scanner
• PortScan 端口扫描器多种语言实现方案(Golang/Python)
  https://thief.one/2018/05/17/1/
• https://golang.org/pkg/sync/#example_WaitGroup
  https://golang.org/pkg/net/#JoinHostPort
• golang编写的网络端口扫描器
• Golang 写一个端口扫描器 #代码有问题，不能直接用
• 【golang】tcp syn scanner

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