goproxy.go

package main

import (
	"github.com/elazarl/goproxy"
	"log"
	"net/http"
)

func main() {
	proxy := goproxy.NewProxyHttpServer()
	proxy.OnRequest().DoFunc(func(req *http.Request, ctx *goproxy.ProxyCtx) (*http.Request, *http.Response) {
		req.Header.Set("user-agent", "GO!")
		return req, nil
	})
	proxy.Verbose = true
	log.Fatal(http.ListenAndServe(":8888", proxy))
}

http://httpbin.org/user-agent
for test