netcat-based chat

Did you know that you could run a chat service using only netcat/ncat?

BIG DISCLAIMER

This solution is by no mean production-safe. It's a toy.

• Anybody can usurp anybody's identity, there's no real control on the usernames.
• ncat is a hell of a tool, but even if it has a quite large --max-conns argument (set to 100 on *Nix, 60 on Windows), there's no guarantee that it can handle the network traffic or the CPU load.

Requirements

• ncat for servers & clients,
• awk,
• mawk (optional, only used in a "bare-bones" example below).
• rlwrap (optional, to use arrow keys in the client).

License

Since this client/server toolkit is basically stolen from here and there examples found on the Internet, that wouldn't be fair to assign a privative license to it.

So this piece of software is published under the terms of the WTFPL (v2).

Copyright © 2021 Bruno Bord bruno@jehaisleprintemps.net This work is free. You can redistribute it and/or modify it under the terms of the Do What The Fuck You Want To Public License, Version 2, as published by Sam Hocevar. See the COPYING file for more details.

Running the server

"Bare bones" servers

A "Bare bones" server would run using:

ncat --ssl --broker --listen -p <port-number>

You may assign any (available) port number for your server.

Another way is to use the --chat parameter of ncat. But it prefixes every message with an arbitrary identifier. It makes you sure that nobody is usurpating your identity, but it might not be readable to have <user1>, <user2> or <user3> talking to each other.

Our ncat server

For your convenience, we're providing a .sh script. The following will run a server on port 12345.

./ncat-server.sh

If you want to specify a dedicated port, you may use the following:

CHAT_PORT=<port-number> ./ncat-server.sh

You can use the Ctrl-C key shortcut to stop the server.

Running a client

"Bare bones" clients

Run a chat client using:

ncat --ssl <server-address-or-hostname> <port-number>

Tip: you can even test this on your own PC, in two separate shell sessions, where the server hostname would be: localhost.

If you want to make sure that your username/nickname is sent each time you're sending a message, you may try to use mawk along:

mawk -W interactive '$0="Alice: "$0' | ncat --ssl <server-address-or-hostname> <port-number>

About the SSL option

By default, the ncat-client.sh and ncat-server.sh scripts we're providing use SSL encryption.

It's a bit safer for your privacy, because it prevents intruders to sniff your messages over the network. Although, as long as someone knows both your hostname and port, they can access your messaging service, so… things are not as safe as they seem.

Please note that, if your server uses SSL, your client must use it also, otherwise you won't be able to send and receive messages through it.

And, if you intend to avoid using SSL, the clients connected to it shouldn't use the --ssl option either, and you'll have to use the barebones clients & servers for that.

Our client

For your convenience, there's even a .sh script. If you want to connect to the default server on your PC, run:

./ncat-client.sh

You'll then be connected to the server on localhost on port 12345. But the discussion here will be limited, since you'll be the only person speaking. And if you open another shell and run the same command, and try to write messages, you'll see that all messages are prefixed by your username@host (extracted from the result of the whoami & hostname commands).

Similarly to the server port, you can also use environment variables to change your connection parameters. See the following example:

CHAT_HOST=alice-laptop CHAT_PORT=9999 CHAT_USER=Bob ./ncat-client.sh

You can use the Ctrl-D key shortcut to disconnect.

Colors

You may chose the color with which your messages will be received by the other users. By default, no specific colour will be sent along. If you want to change that, use the CHAT_COLOR environment variable, such as:

CHAT_COLOR=cyan ./ncat-client.sh

Available colors:

• red
• green
• yellow
• blue
• magenta
• cyan
• white

If you're providing an unknown color, the "no-color" will be used.

User-friendly interface with rlwrap

You may notice that if you want to use arrows to move your cursor up/down or left/right, it inserts characters such as ^[[A^ or [[D. In order to enable arrow keys, you'd need to wrap launching the client script with the rlwrap tool, which wraps your program around GNU Readline.

Then you can edit your message before sending it using right/left keys, and browse your input history using up/down arrow keys.

Example usage:

rlwrap ./ncat-client.sh

Or, with environment variables:

CHAT_COLOR=blue CHAT_HOST=alice-laptop CHAT_PORT=9999 CHAT_USER=Bob rlwrap ./ncat-client.sh

Please note that the call to rlwrap should appear after the environment variable definitions, not before.