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Mitm.js

![NPM version][npm-badge] [npm-badge]: https://badge.fury.io/js/mitm.png

Mitm.js is a library for Node.js to intercept and mock network TCP and HTTP connections. Mitm.js intercepts all remote connections and gives you an instance of Net.Socket to communicate as the remote server. For HTTP requests it even gives you instances of Http.IncomingMessage and Http.ServerResponse — just like you're used to when writing Node.js servers. Except there's no actual server running, it's all just In-Process Interception™.

Mitm.js should work both on the stable Node v0.10.24 and up and v0.11.11 and up and has automated tests to ensure it will stay that way.

Tour

  • Intercept both TCP socket connections (Net.connect) and HTTP requests (Http.request and Https.request).

  • Hooks to Node.js's network functions at a very low level with the goal of not having to patch existing classes and have everything behave as if bytes were arriving from the network.

  • Does not have any kitchen sink features or yet another API to assert on intercepted connections.
    That's a different responsibility handled better by assertion libraries (you'll do no better than to pick Must.js for that ;-).

  • Use an API you already to know to assert or respond to requests — Mitm.js gives you access to a vanilla Net.Socket to respond with:

    mitm.on("connection", function(socket) { socket.write("Hello back!") })

var socket = Net.connect(22, "example.org")
socket.write("Hello!")
socket.setEncoding("utf8")
socket.read() // => "Hello back!"

  • When you do HTTP or HTTPS requests, Mitm.js gives you both a Http.IncomingMessage and Http.ServerResponse to play the server with. That means you'll be using an API you're already familiar with rather than yet another idiosyncratic domain specific language.

    Mitm.js comes very handy to ensure your code makes requests with the appropriate parameters:

    mitm.on("request", function(req, res) {
  req.headers.authorization.must.equal("OAuth DEADBEEF")
})

Http.get("http://example.org")

    It's also useful to see if your code behaves as you'd expect if everything is not 200 OK:

    mitm.on("request", function(req, res) {
  res.statusCode = 402
  res.end("Pay up, sugar!")
})

Http.get("http://example.org", function(res) {
  res.setEncoding("utf8")
  res.statusCode // => 402
  res.on("data", console.log) // => "Pay up, sugar!"
})

    Http.IncomingMessage and Http.ServerResponse are the same objects you get when you write Node.js HTTP servers with Net.Server or use a library like Express.js.

  • Developed with automated tests. Yeah, I know, why should one list this a feature when writing tests is just a sign of professionalism and respect towards other developers? But in a world where so many libraries and "production" software are released without any tests, I like to point out that I even write tests for testing libraries. ;-)

Note: This is a fairly early release of Mitm.js, so it might not cover all use cases you may come across. I've developed this on a need-to basis for testing Monday Calendar's syncing, so if you find a use-case I haven't come across, please fling me an email, a tweet or create an issue on GitHub.

Installing

npm install mitm

From v1.0.0 Mitm.js will follow semantic versioning, but until then, breaking changes may appear between minor versions (the middle number).

Using

Require Mitm.js and invoke it as a function to both create an instance of Mitm and enable intercepting:

var Mitm = require("mitm")
var mitm = Mitm()

Mitm.js will then intercept all requests until you disable it:

mitm.disable()

Intercepting in tests

In tests, it's best to use the before and after hooks to enable and disable intercepting for each test case:

beforeEach(function() { this.mitm = Mitm() })
afterEach(function() { this.mitm.disable() })

Intercepting TCP connections

After you've called Mitm(), Mitm.js will intercept and emit connection on itself for each new connection.
The connection event will be given a server side Net.Socket for you to reply with:

mitm.on("connection", function(socket) { socket.write("Hello back!") })

var socket = Net.connect(22, "example.org")
socket.write("Hello!")
socket.setEncoding("utf8")
socket.read() // => "Hello back!"

Intercepting HTTP/HTTPS requests

After you've called Mitm(), Mitm.js will intercept and emit request on itself for each new HTTP or HTTPS request.
The request event will be given a server side Http.IncomingMessage and Http.ServerResponse.

For example, asserting on HTTP requests would look something like this:

mitm.on("request", function(req, res) {
  req.headers.authorization.must.equal("OAuth DEADBEEF")
})

Http.get("http://example.org")

Responding to requests is just as easy and exactly like you're used to from using Node.js HTTP servers (or from libraries like Express.js):

mitm.on("request", function(req, res) {
  res.statusCode = 402
  res.end("Pay up, sugar!")
})

Http.get("http://example.org", function(res) {
  res.statusCode // => 402
  res.setEncoding("utf8")
  res.on("data", console.log) // => "Pay up, sugar!"
})

Please note that HTTPS requests are currently "morphed" into HTTP requests. That's to save us from having to set up certificates and disable their verification. But if you do need to test this, please ping me and we'll see if we can get Mitm.js to support that.

Events

All events that Mitm will emit on an instance of itself (see Using Mitm.js for examples):

Event Description
connect Emitted when a TCP connection is made.
Given the client side Net.Socket.
connection Emitted when a TCP connection is made.
Given the server side Net.Socket.
request Emitted when a HTTP/HTTPS request is made.
Given the server side Http.IncomingMessage and Http.ServerResponse.

License

Mitm.js is released under a Lesser GNU Affero General Public License, which in summary means:

  • You can use this program for no cost.
  • You can use this program for both personal and commercial reasons.
  • You do not have to share your own program's code which uses this program.
  • You have to share modifications (e.g. bug-fixes) you've made to this program.

For more convoluted language, see the LICENSE file.

About

Andri Möll typed this and the code.
Monday Calendar supported the engineering work.

If you find Mitm.js needs improving, please don't hesitate to type to me now at [email protected] or create an issue online.