Modifying the response body of an `httputil.ReverseProxy` response

June 25, 2024, 8:26 p.m.

I'm currently working with the maintainer of the beautiful sql-studio to make it possible to serve the application from a subpath, which would allow me to add support for running sql-studio as dependency-management-data's web interface.

As part of some investigations around this, I wanted to make a somewhat cursed attempt to perform some server-side rewriting so I could make changes like:

- <script type="module" crossorigin src="https://www.jvt.me/assets/index-Dc4xAj-D.js"></script>
- <link rel="stylesheet" crossorigin href="https://www.jvt.me/assets/index-DpDMcPjm.css">
+ <script type="module" crossorigin src="https://www.jvt.me/sql-studio/assets/index-Dc4xAj-D.js"></script>
+ <link rel="stylesheet" crossorigin href="https://www.jvt.me/sql-studio/assets/index-DpDMcPjm.css">
 </head>

To do this, I've been using httputil.ReverseProxy, with an initial setup like:

package main

import (
 "log"
 "net/http"
 "net/http/httputil"
 "net/url"
)

func main() {
 u, err := url.Parse("http://localhost:3030")
 if err != nil {
 log.Fatal(err)
 }

 proxy := httputil.NewSingleHostReverseProxy(u)
 mux := http.NewServeMux()
 mux.Handle("/sql-studio", proxy)
 log.Fatal(http.ListenAndServe(":8080", mux))
}

To perform the rewriting inside the proxy, I found this StackOverflow which indicated that I should use the ModifyResponse function, which allows us to do something like:

package main

import (
 "bytes"
 "io"
 "log"
 "net/http"
 "net/http/httputil"
 "net/url"
 "strconv"
 "strings"
)

func main() {
 u, err := url.Parse("http://localhost:3030")
 if err != nil {
 log.Fatal(err)
 }

 proxy := httputil.NewSingleHostReverseProxy(u)
 proxy.ModifyResponse = func(r *http.Response) error {
 b, err := io.ReadAll(r.Body)
 if err != nil {
 return err
 }
 defer r.Body.Close()
 // this is pretty hacky, but as a step towards https://github.com/frectonz/sql-studio/issues/16 in the meantime of upstream support
 b = bytes.Replace(b,
 []byte("href=\"/"),
 []byte("href=\"/sql-studio/"),
 -1)
 // ...

 // make sure we set the body, and the relevant headers for well-formed clients to respect
 r.Body = io.NopCloser(bytes.NewReader(b))
 r.ContentLength = int64(len(b))
 r.Header.Set("Content-Length", strconv.Itoa(len(b)))
 return nil
 }

 mux := http.NewServeMux()
 mux.Handle("/sql-studio/", proxy)
 log.Fatal(http.ListenAndServe(":8080", mux))
}

Et voila, we can now rewrite the response body from the proxied service, but before it goes back to the caller.