有些時候項目會碰到需要端口轉(zhuǎn)發(fā)的需求，比如：

1. 一個服務發(fā)布在公共端口上，客戶端進來的請求需要轉(zhuǎn)發(fā)到另一個本地端口斤寂，或者另一臺主機耿焊；或者
2. 一個公共服務需要轉(zhuǎn)到到多臺內(nèi)部服務上，輪詢遍搞，按優(yōu)先級等罗侯；或者
3. 一個服務請求進來之前需要做請求檢查，例如安全功能參數(shù)等檢查溪猿，類似網(wǎng)關功能钩杰，只有驗證通過的請求才能發(fā)送給實際服務器。
4. 等等等

下面是一個實現(xiàn)TCP端口轉(zhuǎn)發(fā)功能的代碼例子诊县。
這里例子將會起一個監(jiān)聽在端口fromport讲弄，然后把所有向此fromport發(fā)起的請求都轉(zhuǎn)發(fā)到端口toport，當然必須先有另一個服務已經(jīng)監(jiān)聽在toport接受請求服務翎冲。

package main

import (
    "os"
    "fmt"
    "net"
)

// Start a proxy server listen on fromport
// this proxy will then forward all request from fromport to toport
//
// Notice: a service must has been started on toport
func proxyStart(fromport, toport int) {
    proxyaddr := fmt.Sprintf(":%d", fromport)
    proxylistener, err := net.Listen("tcp", proxyaddr)
    if err != nil {
        fmt.Println("Unable to listen on: %s, error: %s\n", proxyaddr, err.Error())
        os.Exit(1)
    }
    defer proxylistener.Close()

    for {
        proxyconn, err := proxylistener.Accept()
        if err != nil {
            fmt.Printf("Unable to accept a request, error: %s\n", err.Error())
            continue
        }

        // Read a header firstly in case you could have opportunity to check request
        // whether to decline or proceed the request
        buffer := make([]byte, 1024)
        n, err := proxyconn.Read(buffer)
        if err != nil {
            fmt.Printf("Unable to read from input, error: %s\n", err.Error())
            continue
        }

        // TODO
        // Your choice to make decision based on request header

        targetaddr := fmt.Sprintf("localhost:%d", toport);
        targetconn, err := net.Dial("tcp", targetaddr)
        if err != nil {
            fmt.Println("Unable to connect to: %s, error: %s\n", targetaddr, err.Error())
            proxyconn.Close()
            continue
        }

        n, err = targetconn.Write(buffer[:n])
        if err != nil {
            fmt.Printf("Unable to write to output, error: %s\n", err.Error())
            proxyconn.Close()
            targetconn.Close()
            continue 
        }

        go proxyRequest(proxyconn, targetconn)
        go proxyRequest(targetconn, proxyconn)
    }
}

// Forward all requests from r to w
func proxyRequest(r net.Conn, w net.Conn) {
    defer r.Close()
    defer w.Close()

    var buffer = make([]byte, 4096000)
    for {
        n, err := r.Read(buffer)
        if err != nil {
            fmt.Printf("Unable to read from input, error: %s\n", err.Error())
            break
        }

        n, err = w.Write(buffer[:n])
        if err != nil {
            fmt.Printf("Unable to write to output, error: %s\n", err.Error())
            break
        }
    }
}