Fiddle in the middle with HTTP/HTTPS traffic using Node.js
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MiddleFiddle is an outbound local proxy which lets to modify your outbound request and responses via Connect middleware. It support HTTP and HTTPS, the latter through a hijacking of the request with locally generated SSL certs.


npm install middlefiddle


Change your user agent

Changes your outbound user-agent depending on the URL

var Mf = require('middlefiddle');
var iPhoneUA = "Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A543a Safari/419.3"
Mf.createProxy(Mf.logger(), Mf.user_agent(iPhoneUA, /google\.com/)).listen(8088).listenHTTPS(8089);

Add headers to a response

var Mf = require('middlefiddle');

var addCSP = function(urlRegex) {
  return function(req, res, next){
    if (req.fullUrl.match(urlRegex)) {
      var writeHead = res.writeHead;
      res.writeHead = function(){
        var headers = arguments[arguments.length-1];
        var statusCode = arguments[0];
        headers['x-content-security-policy'] = "allow 'self'";, statusCode, headers);


Streaming MP3 recorder

Grab any mp3 downloaded or streamed to your browser:

var Mf = require('middlefiddle');
var url = require('url');
var fs = require('fs');

var mp3Grab = function() {
  return function(req, res, next){
    var path = url.parse(req.url).pathname;
    var filename;
    if (path) {filename = path.split('/').pop()}
    if (filename && filename.match(/\.mp3$/)){
      console.log("Beginning capture of " + filename);
      var file = fs.createWriteStream(filename);
      res.addListener('data', function (chunk) {
      res.addListener("end", function(chunk) {
        if (chunk) {
        file = undefined;
        console.log("Downloaded - " + filename);


HTTPS Hijacking

When an HTTPS request is first seen, MiddleFiddle generates a certificate for that domain, signs it with it's own generated root cert, and stores the cert for future use in ~/.middlefiddle/certs

In order to make this look legit to your browser, you'll need to add the generated root cert in ~/.middlefiddle/ca.crt to your keychain. This cert is auto generated just for your machine, so you won't be compromising your browser security.

Things to note

Connect typically doesn't have a simple way to hijack downstream responses, so middlefiddle emits events on the response along with writing to the stream.

You've also got a couple helper properties:

  • req.fullUrl #=> The full requested URL, including the schema
  • req.isSecure #=> Did it come via SSL?


  • Clean up cert generation
  • Expand logging
  • Add more middleware

Want to contribute

This is my first node project, criticism is gladly accepted as long as it's in the form of a pull request.


MiddleFiddle is written in CoffeeScript. It's set up with a Cakefile for building files in src/ to lib/ and running tests with nodeunit. There's also a docs task that generates Docco documentation from the source in src/.

Released under the MIT license.

Mark Percival