proxy that can handle/modify html responses

proxy.go

package main

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

func main() {
	p := &proxy{}
	http.Handle("/proxy", p.Handler())
	http.ListenAndServe(":8080", nil)
}

// proxy provides an http handler for proxying pages.
type proxy struct{}

// Handler is an http handler for net/http.
func (p *proxy) Handler() http.Handler {
	f := func(w http.ResponseWriter, r *http.Request) {
		if r.Method != "GET" {
			http.Error(w, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
			return
		}
		url := r.FormValue("url")
		if url == "" {
			http.Error(w, "Missing 'url' param", http.StatusBadRequest)
			return
		}
		p.do(w, url)
	}
	return http.HandlerFunc(f)
}

// 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",
}

// do makes the upstream request and proxy the response back to client w.
func (p *proxy) do(w http.ResponseWriter, url string) {
	resp, err := http.Get(url)
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
		return
	}
	defer resp.Body.Close()
	// remove certain headers from the response
	for _, h := range hopHeaders {
		resp.Header.Del(h)
	}
	// copy headers
	for k, vv := range resp.Header {
		for _, v := range vv {
			w.Header().Set(k, v)
		}
	}
	// direct proxy non http 200 text/html
	ok := strings.HasPrefix(resp.Header.Get("Content-Type"), "text/html")
	if !ok || resp.StatusCode != http.StatusOK {
		w.WriteHeader(resp.StatusCode)
		io.Copy(w, resp.Body)
		return
	}
	p.handleBody(w, resp.Body)
}

// handleBody handles the response body and writes to client w.
func (p *proxy) handleBody(w http.ResponseWriter, body io.Reader) {
	io.Copy(w, body)
}