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Middleware as a proxy for HTTP/HTTPS traffic


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 -g middlefiddle
# Depends on Node 0.6.x
$ git clone git://
$ cd middlefiddle
$ npm install
$ npm link #If you want to use it globally

By default middlefiddle will start logging traffic and modifying requests based on site specific middleware found in '.middlefiddle/sites'

You can find an example in .middlefiddle/sites

# Start middlefiddle with default options
$ middlefiddle
# Proxy will be at port 8080
$ middlefiddle logger
# Now open http://localhost:8411

# Only log for a certain URL
$ middlefiddle logger --url

# Only log certain statuses
$ middlefiddle logger --status 404

# Only log responses containing text
$ middlefiddle logger --grep "setTimeout"
# Also work with regex
$ middlefiddle logger -r --grep "Mark(Percival)?"
# And case insensitive
$ middlefiddle logger -ri --grep "m@mdp\.im"

MiddleFiddle can alter requests based on the host name. You'll find some examples in .middlefiddle/sites

Simple add the middleware to your ~/.middlefiddle/sites directory, with the appropriate hostname. For example, ~/.middlefiddle/sites/' would get injected on any request to

MiddleFiddle middleware is connect compatible. Anything you can do with Connect, you can do with middlefiddle middleware.

For example, lets say you want to save all the streamed mp3's from

Found in

fs = require 'fs'
module.exports = (Mf) ->
  (req, res, next) ->
    res.on 'body', ->
      if res.headers["content-type"] == 'audio/mpeg'
        fileName = req.path.replace(/^[a-zA-Z]/, '').split('.')[0]
        path = "#{process.env["HOME"]}/Downloads/soundcloud#{fileName}.mp3"
        fs.writeFile path, res.body, ->
          console.log "Saved #{res.body.length} bytes to #{path}"

In this case we used the MiddleFiddle helper 'replace'

Found in

module.exports = (Mf) ->
  replacement = (string, req, res) ->
    contentType = res.headers['content-type'] || ''
    if >= 0
      string.replace(/repositories/ig, "suppositories")
  return  Mf.replace(replacement)

Here we are going to change the user agent to GoogleBot

Found in

module.exports = (Mf) ->
  # I'm the Google, let me in!
  (req, res, next) ->
    req.headers['user-agent'] = "GoogleBot"

By default MiddleFiddle logs all outbound traffic to a web based logger on localhost:8411

MiddleFiddle looks for a .middlefiddle directory in the current working directory, or at ~/.middlefiddle.

Inside you'll find a file, https certs, and a sites directory.

You'll need to add the https certs to you keychain if you want to avoid the browser warning. The certs are generated on the first launch of middlefiddle and are therefor unique to each machine.

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.

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

res.on 'data', (chunk) ->
  console.log chunk.toString()

res.on 'end', (chunk) ->
  console.log chunk.toString()

res.on 'close', (chunk) ->
  console.log "Closed response"

You've also got a couple helper properties:

  • req.href #=> String: The full requested URL, including the scheme, host, path, and query params
  • req.ssl #=> Boolean: Did it come via SSL?
  • req.startTime #=> Datetime: When the request was started
  • res.endTime #=> Datetime: I'll let you guess

Response headers can be modified before they are sent to the browser. Just wait till they're available:

Example in

Modifying the a response body means buffering the stream, waiting for it to finish, then making the replacement and sending it back downstream. The 'replace' middleware provides this.

Tests can be run from within the repo

npm install
npm test
  • Clean up cert generation
  • Expand logging
  • Add more middleware

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