fowl 24.3.2

🤔 Why?

We sometimes pair-program but don’t like the idea of sending keystrokes over a third-party server. We also like avoiding the extra work of “set up a server on a public IP address”.

For more context, see my blog posts: Forwarding Streams over Magic Wormhole and Wizard Gardens vision.

To generalize this a little: there are many client/server programs the _can_ be self-hosted – fowl lets us use these sorts of programs in a peer-to-peer fashion. This means only depending on one general-purpose, public-IP-having server (the Magic Wormhole “mailbox server” used to set up connections).

🦃 What?

The command-line tool fowl allows you use any client/server program over Magic Wormhole. Magic Wormhole provides a persistent, strongly-encrypted session (end-to-end) with no need for pre-shared keys.

Conceptually, this is somewhat similar to combining ssh -R and ssh -L. fowl may be used to set up complex workflows directly between participants, integrating services that would “traditionally” demand a server on a public IP address.

Key features:

• no need to pre-exchange keys

• simple, one-time-use codes that are easy to transcribe

• secure (full-strength keys via SPAKE2)

• end-to-end encryption (and no possibility for unencrypted application data)

• integrate with any tools that can listen on or connect to localhost

This allows an author to write a “glue” program in any language that ties together unchanged networked progams. The communcation channel is:

• set up without pre-shared secrets;

• fully encrypted;

• and survives IP address changes or outages.

All this with no action required at the application level, it is just a normal localhost TCP (or UNIX) streaming socket.

✍ Motivational Example

When pair-programming using tty-share one handy option is to use the default, public server. However, I don’t like the idea of sending keystrokes over a third-party server that I don’t run. (Please note: I have no reason to believe this particular person is doing anything nefarious!)

I could fire up such a server myself and use it with my friends…

…but with fowl, one side can run a localhost tty-share server and the other side can run a tty-share client that connects to a localhost endpoint – data flows over the wormhole connection (only).

Key advantage: no need to expose keystrokes to a third-party server.

Additional advantage: no need to set up a server on a public IP address.

🐃 Why is This Particular Yak Being Shorn?

I wanted to write a pair-programming application in Haskell, but didn’t want to implement Dilation in the Magic Wormhole Haskell library (maybe one day!)

It also occurred to me that other people might like to experiment with Magic Wormhole (and advanced features like Dilation) in languages that lack a Magic Wormhole implementation – that is, most of them!

So, the first step in “write a Haskell pair-programming utility” became “write and release a Python program” :)

(p.s. the next-higher level Yak is now online at sr.ht but not “released”)

⌨ How Does It Work?

fowl uses the “Dilation” feature of the Magic Wormhole protocol.

This means that a Magic Wormhole Mailbox server is used to perform a SPAKE2 exchange via a short (but one-time only) pairing code. For details on the security arguments, please refer to the Magic Wormhole documentation. After this, an E2E-encrypted direct P2P connection (or, in some cases, via a “transit relay” service) is established between the two computers; that is, between the computer that created the wormhole code, and the one that consumed it.

The key encrypting messages on this connection is only known to the two computers; the Mailbox server cannot see any message contents. (It, like any attacker, could try a single guess at the wormhole code). See the Magic Wormhole documentation for more details on this.

The “Dilation” feature further extends the above protocol to provide subchannels and “durability” – this means the overall connection survives network changes, disconnections, etc. You can change WiFi networks or put one computer to sleep yet remain connected.

What fowl adds is a way to set up any number of localhost listeners on either end, forwarding data over subchannels. The always-present “control” subchannel is used to co-ordinate opening and closing such listeners.

With some higher-level co-ordination, fowl may be used to set up complex workflows between participants, integrating services that would “traditionally” demand a server on a public IP address.

Another way to view this: streaming network services can integrate the Magic Wormhole protocol without having to find, link, and use a magic-wormhole library (along with the implied code-changes) – all integration is via local streams. (There are implementations in a few languages so you could take that route if you prefer).

👤 Who Should Use This?

We handle and expect two main use-cases of this program: integrators and end-users.

Human CLI users can use fowl itself to set up and use connections, for any purpose.

For developers doing integration, fowld provides a simple protocol-based way for any runtime to use. That is, some “glue” code running fowld as a sub-process. This program will also co-ordinate the running of client-type or server-type networking applications that accomplish some goal useful to users. For example, “pair-programming” (for my case).

Some other ideas to get you started:

• “private” / invite-only streaming (one side runs video source, invited sides see it)

• on-demand tech support or server access (e.g. set up limited-time SSH, VNC, etc)

• …

💼 Installation and Basic Usage

fowl and fowld are Python programs using the Twisted asynchronous networking library.

You may install them with pip:

pip install fowl

Once this is done, fowl and fowld will appear on your PATH. Run either for instructions on use.

In accordance with best practices, we recommend using a virtualenv to install all Python programs. Never use ``sudo pip``. To create a virtualenv in your checkout of fowl, for example:

python -m venv venv
./venv/bin/pip install --upgrade pip
./venv/bin/pip install fowl
# or: ./venv/bin/pip install --editable .
./venv/bin/fowl

💬 Hello World: Chat!

The actual “hello world” of networked applications these days is chat, amirite? 😉

We will use two venerable network utilities (nc and telnet) to implement a simple, secure, and e2e-encrypted chat.

Yes, that’s correct: we will make secure chat over telnet. The first insight here is that we can make nc listen on a localhost-only port, and we can make telnet connect to a localhost TCP port.

At first we can prove the concept locally, from one terminal to another. Open two terminals. In the first, run: nc -l localhost 8888 This tells nc (aka “net cat”) to listen on the localhost TCP port “8888” (it will echo anything that comes in, and send anything you type).

In the second terminal: telnet localhost 8888 This instructs telnet to connect to localhost TCP port 8888 – that is, the very netcat instance running in the first terminal. Type “hello world” into either of the terminals, and you should see it appear on the other side.

Goal achieved!, partially. We have “chat” over nc and telnet. It’s not pretty, but it works fine.

However, we want to talk to other machines. This means we need: * encryption; * and a way to arrange network connectivity

These additional features are exactly what ``fowl`` gives us.

So, we still run the exact same nc and telnet commands, but first do some fowl magic on each machine.

On the second machine (the one running telnet) we’ll need to add in something that listens on port 8888. This thing is: fowl --listen 8888 invite When connected to the public Mailbox Server, this will print out a <secret code> like 1-foo-bar

Next we want all the information this listener gets to be magically forwarded to the first machine (the one running nc). So, on it we run: fowl --allow-connect 8888 accept <secret code>. The <secret code> comes from the “invite” above, and is communicated – usually via a human or two – to the second machine.

Note that we could swap “invite” and “accept” around if it’s more convenient for one or the other human to go first.

What happens under the hood is that the two fowl programs establish a secure connection, via the public Mailbox Server. They then use this connection to maintain a persistent (possibly changing) TCP connection between each other (worst case, using the public Transit Relay) to send end-to-end encrypted messages.

fowl uses this connection to communicate via a simple protocol that can establish listeners on either end or ask for fresh connections. These result in “subchannels” (in the Magic Wormhole Dilation protocol) that can send bytes back or forth.

Any bytes received at either end of the connection are simply forwarded over the subchannel.

Full example, computer one:

$ nc -l localhost 8888
$ fowl --allow-connect 8888 invite
Invite code: 1-foo-bar

Computer two:

$ fowl --listen 8888 accept 1-foo-bar
$ telnet localhost 8888

Now we have encrypted chat.

These two programs can run anywhere on the Internet. Like TCP promises, all bytes are delivered in-order. In addition, they are encrypted. Also the stream will survive changing networks (disconnects, new IP addresses, etc); that is, the actual inter-computer TCP connection is re-stablished, but to the applications (nc, telnet) it looks uninterupted.

📦 Other Platforms

We welcome contributions from people experienced with packaging for other installation methods; please get in touch!

🚚 Stability and Releases

This is an early release of, essentially, a proof-of-concept. While we intend to make it a stable base to put co-ordination software on top, it is not yet there. APIs may change, options may change. If you are developing on top of fowl, please get in touch so we know what you need 😊

All releases are on PyPI with versioning following a CalVer variant: year.month.number, like 23.4.0 (for the first release in April, 2023).

See NEWS.rst for specific release information.

🧙 Contributors

• meejah: main development

• balejk: early feedback, proof-reading, review and testing