PyTelnet: A Simple Telnet Implementation in Python

A lightweight and simple Telnet server and client implementation in Python. This project provides core Telnet protocol support (RFC 854), user authentication, and a basic command-line interface for interaction. It is designed for educational purposes to demonstrate the fundamentals of network socket programming and the Telnet protocol negotiation process.

Key Features

• Telnet Protocol Support: Implements standard Telnet protocol commands (IAC, DO, DONT, WILL, WONT).
• Option Negotiation: Supports key Telnet options like ECHO, SUPPRESS-GO-AHEAD, TERMINAL-TYPE, and NAWS (Negotiate About Window Size).
• Multi-threaded Server: The server uses threading to handle multiple client connections simultaneously.
• User Authentication: A basic authentication system with pre-defined usernames and passwords.
• Interactive Client: The client runs in raw terminal mode, allowing for character-by-character input and proper handling of server-side echo.
• Cross-Platform: Runs on any Unix-like system (Linux, macOS) with Python 3.6 or higher.

How It Works

The project consists of two main components:

1. server.py: This script creates a TCP socket server that listens for incoming connections on a specified port. When a client connects, the server spawns a new thread to handle it. The server and client then negotiate Telnet options to determine how communication will proceed. After successful authentication, the server processes commands sent by the client.
2. client.py: This script establishes a connection to the Telnet server. It sets the local terminal to "raw" mode to send individual keystrokes to the server immediately. It also responds to the server's Telnet option negotiations and displays data received from the server.

Requirements

• Python 3.6 or higher
• A Linux or other Unix-like operating system (for the termios and tty modules used by the client).

Getting Started

No special installation is required as the project only uses standard Python libraries.

1. Start the Server

Open a terminal and run the following command to start the server. It will bind to 0.0.0.0 on port 2323.

python3 server.py

You should see the output: Server listening on 0.0.0.0:2323

2. Connect with the Client

Open a second terminal and run the client script to connect to the server.

python3 client.py

By default, the client will attempt to connect to localhost:2323.

3. Log In

Once connected, the server will prompt you for a username and password. You can use one of the default credentials:

• Username: admin, Password: admin
• Username: guest, Password: guest
• Username: user, Password: 123456

Available Commands

Once you are authenticated, you can use the following commands:

Command	Description
help	Shows the list of available commands.
whoami	Displays your username and connection info.
users	Lists all currently connected users.
uptime	Shows the system uptime of the server.
date	Displays the current date and time on the server.
hostname	Shows the server's system hostname.
echo [msg]	Echoes back the message you provide.
exit / logout	Disconnects you from the server.