Cosmos Telnet Server 🚀

Cosmos.Network.Telnet is a Telnet server made in C# for the Cosmos operating system construction kit.

It is for Cosmos Gen3 kernels (NativeAOT). Every client that connects gets a console session of its own, with a thread that runs the shell you give the server, so the same Console.ReadLine() and Console.WriteLine() code runs for a remote terminal as on the display. It is built on the console sessions of Cosmos.Kernel.System and on System.Net.Sockets, which a Gen3 kernel runs on its own network stack.

Usage

Add the package to your kernel .csproj:

<ItemGroup>
    <PackageReference Include="Cosmos.Network.Telnet" Version="1.0.0" />
</ItemGroup>

The kernel needs networking and the scheduler (CosmosEnableNetwork, CosmosEnableScheduler), the kernel console, since every session's screen is a grid on the display's canvas, and an IP configuration (DHCP or static). Start() returns at once: the server accepts connections on a thread of its own, and runs the shell of each on another.

using System;
using Cosmos.Network.Telnet;

TelnetServer server = new(session =>
{
    Console.WriteLine("Welcome, " + session.Name);

    while (Console.ReadLine() is { } line && line != "exit")
    {
        Console.WriteLine("You typed: " + line);
    }
})
{
    // Optional: one line per connection, disconnection and failure.
    Log = message => Cosmos.Kernel.System.Diagnostics.Log.WriteString(message + "\n"),
};

server.Start(); // port 23 by default: new TelnetServer(shell, 2323) picks another

// Later, to stop accepting connections; the sessions already open stay connected:
server.Stop();

The connection closes when the shell returns. The sessions are listed and numbered with the kernel's others by SessionManager, and Alt with a function key shows one on the display, where the local keyboard can type into it.

Reaching a kernel running in QEMU

With QEMU user-mode networking, forward a host port to the guest's port 23. With the Cosmos CLI:

cosmos run --nic e1000e --hostfwd tcp::2323-:23
telnet localhost 2323

With plain QEMU, the forward is an option of the user-mode NIC: -nic user,model=e1000e,hostfwd=tcp::2323-:23.

Authors

👤 @valentinbreiz

🤝 Contributing

Contributions, issues and feature requests are welcome!

Feel free to check issues page.

📝 License

Copyright © 2026 CosmosOS.

This project is BSD Clause 3 licensed.