Collectives™ on Stack Overflow

Find centralized, trusted content and collaborate around the technologies you use most.

Teams

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

I've implemented the most basic Reverse Proxy to pull a page and then add some content to the body. Unfortunately my attempt to add to the html isn't taking effect. The code below just shows the original page but without the "monkeys" I prepended to the response. What additional calls are needed to get this to work? I eventually want to use this to replace css for custom css.

package main
import (
    "log"
    "net/http"
    "net/http/httputil"
    "net/url"
    "fmt"
    "github.com/PuerkitoBio/goquery"
    "bytes"
type Director func(*http.Request)
func (f Director) Then(g Director) Director {
    return func(req *http.Request) {
        f(req)
        g(req)
func hostDirector(host string) Director {
    return func(req *http.Request) {
        req.Host = host
func UpdateResponse(r *http.Response) error {
    doc, err := goquery.NewDocumentFromReader(r.Body)
    if err != nil{
        //log.New("Research")
        log.Fatal("Bad doc %v", err)
        return err
    html, err := goquery.OuterHtml(doc.First())
    if err != nil{
        log.Fatal("Bad html %v", err)
        return err
    fmt.Printf("Body %v", html)
    r.Write(bytes.NewBufferString("monkeys"+html))
    return nil
func main() {
    url, _ := url.Parse("http://cnn.com/")
    proxy := httputil.NewSingleHostReverseProxy(url)
    d := proxy.Director
    // sequence the default director with our host director
    proxy.Director = Director(d).Then(hostDirector(url.Hostname()))
    proxy.ModifyResponse = UpdateResponse
    http.Handle("/", proxy)
    log.Fatal(http.ListenAndServe(":9090", nil))
                It doesn't show it in the browser, or in the raw response? Because if you're just looking in the browser, you likely won't see your added text because you're prepending it to an HTML file, before the doctype or root tag, which is syntactically invalid and may be discarded by the browser.
– Adrian
                Jul 16, 2018 at 15:00
Your ModifyResponse code is not modifying the response. In your code, you have:
r.Write(bytes.NewBufferString("monkeys"+html))
It seems that you are writing to the response, but nope. r is of *http.Response which has a Write method:
func (r *Response) Write(w io.Writer) error
The document says:
  
Write writes r to w in the HTTP/1.x server response format, including the status line, headers, body, and optional trailer. 
Since bytes.Buffer also implements a io.Writer, the code compiles, but instead of write to the response from the buffer, it write the response to the buffer, which is absolutely undesired.
The fact is, http.Response simply don't provide a way to modify its Body. It is clear that Bodey is an io.ReadCloser. What you can do is create a new io.ReadCloser with the body. Remember to change Content-Length field in the Header or it may cause problem.
func UpdateResponse(r *http.Response) error {
    b, _ := ioutil.ReadAll(r.Body)
    buf := bytes.NewBufferString("Monkey")
    buf.Write(b)
    r.Body = ioutil.NopCloser(buf)
    r.Header["Content-Length"] = []string{fmt.Sprint(buf.Len())}
    return nil
Playground: https://play.golang.org/p/_Tyvo6GVN3x
        
Thanks for contributing an answer to Stack Overflow!
Please be sure to answer the question. Provide details and share your research!
But avoid …
Asking for help, clarification, or responding to other answers.
Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.