A simple HTTP proxy by Golang

HttpProxy.go

package main

import (
	"flag"
	"io"
	"log"
	"net"
	"net/http"
	"strings"
)

// Hop-by-hop headers. These are removed when sent to the backend.
// http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
var hopHeaders = []string{
	"Connection",
	"Keep-Alive",
	"Proxy-Authenticate",
	"Proxy-Authorization",
	"Te", // canonicalized version of "TE"
	"Trailers",
	"Transfer-Encoding",
	"Upgrade",
}

func copyHeader(dst, src http.Header) {
	for k, vv := range src {
		for _, v := range vv {
			dst.Add(k, v)
		}
	}
}

func delHopHeaders(header http.Header) {
	for _, h := range hopHeaders {
		header.Del(h)
	}
}

func appendHostToXForwardHeader(header http.Header, host string) {
	// If we aren't the first proxy retain prior
	// X-Forwarded-For information as a comma+space
	// separated list and fold multiple headers into one.
	if prior, ok := header["X-Forwarded-For"]; ok {
		host = strings.Join(prior, ", ") + ", " + host
	}
	header.Set("X-Forwarded-For", host)
}

type proxy struct {
}

func (p *proxy) ServeHTTP(wr http.ResponseWriter, req *http.Request) {
	log.Println(req.RemoteAddr, " ", req.Method, " ", req.URL)

	if req.URL.Scheme != "http" && req.URL.Scheme != "https" {
	  	msg := "unsupported protocal scheme "+req.URL.Scheme
		http.Error(wr, msg, http.StatusBadRequest)
		log.Println(msg)
		return
	}

	client := &http.Client{}

	//http: Request.RequestURI can't be set in client requests.
	//http://golang.org/src/pkg/net/http/client.go
	req.RequestURI = ""

	delHopHeaders(req.Header)

	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
		appendHostToXForwardHeader(req.Header, clientIP)
	}

	resp, err := client.Do(req)
	if err != nil {
		http.Error(wr, "Server Error", http.StatusInternalServerError)
		log.Fatal("ServeHTTP:", err)
	}
	defer resp.Body.Close()

	log.Println(req.RemoteAddr, " ", resp.Status)

	delHopHeaders(resp.Header)

	copyHeader(wr.Header(), resp.Header)
	wr.WriteHeader(resp.StatusCode)
	io.Copy(wr, resp.Body)
}

func main() {
	var addr = flag.String("addr", "127.0.0.1:8080", "The addr of the application.")
	flag.Parse()
	
	handler := &proxy{}
	
	log.Println("Starting proxy server on", *addr)
	if err := http.ListenAndServe(*addr, handler); err != nil {
		log.Fatal("ListenAndServe:", err)
	}
}

TL;DR: https traffic is encrypted, a proxy is just a dumb intermediary between the client and server in this case.

https traffic is encrypted with the session key at the client (and server) side. Therefore the traffic in the different
sessions is always different, even if they are transferring the same content. In order to serve something from
the cache a proxy should have decrypted the traffic. Being able to decrypt the intercepted https traffic means
a successful attack on https. Sometimes this is possible (google for OpenSSL heartbleed).
However in general it's not feasible. And in most cases it's considered illegal.

TL;DR: https traffic is encrypted, a proxy is just a dumb intermediary between the client and server in this case.

https traffic is encrypted with the session key at the client (and server) side. Therefore the traffic in the different
sessions is always different, even if they are transferring the same content. In order to serve something from
the cache a proxy should have decrypted the traffic. Being able to decrypt the intercepted https traffic means
a successful attack on https. Sometimes this is possible (google for OpenSSL heartbleed).
However in general it's not feasible. And in most cases it's considered illegal.

So just do the HTTPS Proxy without cache.

Mind... blown...

awesome guys!

Great gist! I have one question. Why do you create a new instance of http.Client for each request? As stated in Golang documentation for http Client, it is concurrent safe and can be used concurrently by multiple goroutines?

Is there some downside of having a global http Client?

if req.URL.Scheme != "http" && req.URL.Scheme != "https" {
		msg := "unsupported protocal scheme " + req.URL.Scheme
		http.Error(wr, msg, http.StatusBadRequest)
		log.Println(msg)
		return
	}

i think this part is redundant since you are using ListenAndServe

hopHeaders needs Content-Length i think?

The req.URL.Scheme is nonfunctional - req.URL will always have a blank Scheme, as discussed in golang/go#28940. So this will always error out.

VERSION = $(shell git describe --tags --always --dirty)
LDFLAGS=-ldflags "-X main.version=$(VERSION)"
OSARCH=$(shell go env GOHOSTOS)-$(shell go env GOHOSTARCH)

SIMPLEPROXYTOOL=\
	goproxytool-darwin-amd64 \
	goproxytool-linux-amd64 \
	goproxytool-windows-amd64.exe

proxytool: goproxytool-$(OSARCH)

$(SIMPLEPROXYTOOL):
	GOOS=$(word 2,$(subst -, ,$@)) GOARCH=$(word 3,$(subst -, ,$(subst .exe,,$@))) go build $(LDFLAGS) -o $@ ./$<

%-$(VERSION).zip: %.exe
	rm -f $@
	zip $@ $<

%-$(VERSION).zip: %
	rm -f $@
	zip $@ $<

clean:
	rm -f goproxytool-*


release:
	$(foreach bin,$(SIMPLEPROXYTOOL),$(subst .exe,,$(bin))-$(VERSION).zip)

rm:
	$(foreach bin,$(SIMPLEPROXYTOOL),$(subst .exe,,$(bin))-$(VERSION).zip)


build: proxytool $(SIMPLEPROXYTOOL) clean rm

.PHONY: build
all: build

A MakeFile following this link