https://www.flysnow.org/2016/12/24/golang-http-proxy.html

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package main

import (
    "bytes"
    "fmt"
    "io"
    "log"
    "net"
    "net/url"
    "strings"
)

func main() {
    log.SetFlags(log.LstdFlags | log.Lshortfile)
    l, err := net.Listen("tcp", ":8080")
    if err != nil {
        log.Panic(err)
    }

    for {
        client, err := l.Accept()
        if err != nil {
            log.Panic(err)
        }

        go handleClientRequest(client)
    }
}

func handleClientRequest(client net.Conn) {
    if client == nil {
        return
    }
    defer client.Close()

    log.Println(client.RemoteAddr())
    var b [1024]byte
    n, err := client.Read(b[:])
    if err != nil {
        log.Println(err)
        return
    }
    var method, host, address string
    fmt.Sscanf(string(b[:bytes.IndexByte(b[:], '\n')]), "%s%s", &method, &host)
    hostPortURL, err := url.Parse(host)
    if err != nil {
        log.Println(err)
        return
    }

    if hostPortURL.Opaque == "443" { //https访问
        address = hostPortURL.Scheme + ":443"
    } else {                                            //http访问
        if strings.Index(hostPortURL.Host, ":") == -1 { //host不带端口, 默认80
            address = hostPortURL.Host + ":80"
        } else {
            address = hostPortURL.Host
        }
    }

    //获得了请求的host和port,就开始拨号吧
    server, err := net.Dial("tcp", address)
    if err != nil {
        log.Println(err)
        return
    }
    if method == "CONNECT" {
        fmt.Fprint(client, "HTTP/1.1 200 Connection established\r\n\r\n")
    } else {
        server.Write(b[:n])
    }
    //进行转发
    go io.Copy(server, client)
    io.Copy(client, server)
}