ローカルフォワードプロキシでHTTP要求時に機能横断な処理を差し込み

package main

import (
	"crypto/tls"
	"fmt"
	"github.com/elazarl/goproxy"
	"io"
	"log"
	"net/http"
	"os"
)

func main() {

	proxy := goproxy.NewProxyHttpServer()
	proxy.Tr = &http.Transport{TLSClientConfig: &tls.Config{InsecureSkipVerify: true}, Proxy: nil}
	proxy.ConnectDial = nil
	proxy.Verbose = true
	go http.ListenAndServe(":8000", proxy) // ローカルにフォワードプロキシを起動

	resp, err := http.Get(os.Args[1]) // 引数で渡されたURLにアクセス
	if err != nil {
		log.Fatalln(err)
	}
	defer resp.Body.Close()

	all, err := io.ReadAll(resp.Body)
	if err != nil {
		log.Fatalln(err)
	}
	fmt.Println(string(all))

}

proxy.OnRequest(goproxy.DstHostIs("www.reddit.com")).DoFunc(
    func(r *http.Request,ctx *goproxy.ProxyCtx)(*http.Request,*http.Response) {
        if h,_,_ := time.Now().Clock(); h >= 8 && h <= 17 {
            return r,goproxy.NewResponse(r,
                    goproxy.ContentTypeText,http.StatusForbidden,
                    "Don't waste your time!")
        }
        return r,nil

proxy.OnRequest().DoFunc(
    func(r *http.Request,ctx *goproxy.ProxyCtx)(*http.Request,*http.Response) {
        r.Header.Set("X-GoProxy","yxorPoG-X")
        return r,nil
    })

// Add handlers to httpsHandlers
proxy.OnRequest(Some ReqConditions).HandleConnect(YourHandlerFunc())

// Add handlers to reqHandlers
proxy.OnRequest(Some ReqConditions).Do(YourReqHandlerFunc())

// Add handlers to respHandlers
proxy.OnResponse(Some RespConditions).Do(YourRespHandlerFunc())

func exampleMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// 何かしらの処理のフック
		next.ServeHTTP(recorder, r)
	})
}

go http.ListenAndServe(":8000", exampleMiddleware(proxy))

type StatusRecorder struct {
	http.ResponseWriter
	Status int
}

func (r *StatusRecorder) WriteHeader(status int) {
	r.Status = status
	r.ResponseWriter.WriteHeader(status)
}

func loggingMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		recorder := &StatusRecorder{ResponseWriter: w}
		next.ServeHTTP(recorder, r)
		fmt.Println("★★★StatusCode", recorder.Status)
	})
}

go http.ListenAndServe(":8000", loggingMiddleware(proxy))

func (r *StatusRecorder) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	h, ok := r.ResponseWriter.(http.Hijacker)
	if !ok {
		return nil, nil, errors.New("hijack not supported")
	}
	return h.Hijack()
}

> go run main.go  http://httpbin.org/ip
2021/08/26 13:16:36 [001] INFO: Got request /ip httpbin.org GET http://httpbin.org/ip
2021/08/26 13:16:36 [001] INFO: Sending request GET http://httpbin.org/ip
2021/08/26 13:16:36 [001] INFO: Received response 200 OK
2021/08/26 13:16:36 [001] INFO: Copying response to client 200 OK [200]
2021/08/26 13:16:36 [001] INFO: Copied 30 bytes to client error=<nil>
★★★StatusCode 200
{
  "origin": "111.98.xx.x"
}

>go run main.go  https://httpbin.org/ip
2021/08/26 13:17:39 [001] INFO: Running 0 CONNECT handlers
2021/08/26 13:17:39 [001] INFO: Accepting CONNECT to httpbin.org:443
★★★ 0
{
  "origin": "111.98.xx.x"
}