mitm

Mitm.js
=======
[![NPM version][npm-badge]](https://www.npmjs.com/package/mitm)
[![Build status][build-badge]](https://github.com/moll/node-mitm/actions/workflows/node.yaml)

Mitm.js is a library for Node.js (and Io.js) to intercept and mock outgoing
network TCP and HTTP connections. Mitm.js intercepts and gives you
a Net.Socket to communicate as if you were the remote server. For **HTTP
requests** it even gives you 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™_.

Intercepting connections and requests is **extremely useful to test and ensure
your code does what you expect**. Assert on request parameters and send back
various responses to your code without ever having to hit the real network.
Fast as hell and **a lot easier to develop with than external test
servers**.

Mitm.js works on all Node versions: ancient v0.10, v0.11 and v0.12 versions, previous and current LTS versions like v4 to v12 and the newest v22 and beyond. For all it has automated tests to ensure it will stay that way.

I've developed Mitm.js on a need-to basis for testing [Monday
Calendar][monday]'s syncing, so if you find a use-case I haven't come across,
please fling me an [email][email], a [tweet][twitter] or [create an
issue][issues] on GitHub.

[npm-badge]: https://img.shields.io/npm/v/mitm.svg
[build-badge]: https://github.com/moll/js-j6pack/actions/workflows/node.yaml/badge.svg

$3

- 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][must] for that ;-).

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

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

var socket = Net.connect(22, "example.org")
socket.write("Hello!")
socket.setEncoding("utf8")
socket.on("data", console.log) // => "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:
`javascript
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:
`javascript
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][express].

- Bypass interception selectively for some connections (such as your SQL
server) and let them connect as usual.
`javascript
mitm.on("connect", function(socket, opts) {
if (opts.host == "sql.example.org" && opts.port == 5432) socket.bypass()
})
`

- 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. ;-)

[must]: https://github.com/moll/js-must
[express]: http://expressjs.com


Installing
----------
`
npm install mitm
`

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

[semver]: http://semver.org/


Using
-----
Require Mitm.js and invoke it as a function to both create an instance of Mitm
and enable intercepting:
`javascript
var Mitm = require("mitm")
var mitm = Mitm()
`

Mitm.js will then intercept all requests until you disable it:
`javascript
mitm.disable()
`

$3


In tests, it's best to use the _before_ and _after_ hooks to enable and disable
intercepting for each test case:
`javascript
beforeEach(function() { this.mitm = Mitm() })
afterEach(function() { this.mitm.disable() })
`

$3

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:

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

var socket = Net.connect(22, "example.org")
socket.write("Hello!")
socket.setEncoding("utf8")
socket.on("data", console.log) // => "Hello back!"
`

$3


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:
`javascript
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][express]):
`javascript
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.

#### Custom HTTP Methods
Unfortunately because Node.js's web server doesn't seem to support custom HTTP methods (that is, ones beyond require("http").METHODS), Mitm.js doesn't support them out of the box either. The Node.js HTTP parser throws an error given a request with an unsupported method. However, as Mitm.js also supports intercepting at the TCP level, you could hook in your own HTTP parser. I've briefly alluded to it in issue #63.

$3


You can bypass connections listening to the connect event on the Mitm instance
and then calling bypass on the given socket. To help you do
so selectively, connect is given the options object that was given to
Net.connect:

`javascript
mitm.on("connect", function(socket, opts) {
if (opts.host == "sql.example.org" && opts.port == 5432) socket.bypass()
})
`

Bypassed connections do not emit connection or request events. They're
ignored by Mitm.js.

In most cases you don't need to bypass because by the time you call Mitm in
your tests to start intercepting, all of the long-running connections, such as
database or cache connections, are already made.

You might need to bypass connections you make to localhost when you're running
integration tests against the HTTP server you started in the test process, but
still want to intercept some other connections that this request might invoke.
The following should suffice:

`javascript
mitm.on("connect", function(socket, opts) {
if (opts.host == "localhost") socket.bypass()
})
`


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 and options from Net.connect.
connection | Emitted when a TCP connection is made.
Given the server side Net.Socket and options from Net.connect.
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][moll] typed this and the code.
[Monday Calendar][monday] supported the engineering work.

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

[email]: mailto:andri@dot.ee
[issues]: https://github.com/moll/node-mitm/issues
[moll]: http://themoll.com
[monday]: https://mondayapp.com
[twitter]: https://twitter.com/theml