XMODEM Gateway: File Transfers and SSH for Retro Hardware

Introduction to XMODEM Gateway

If you have a Commodore 64, an AltairDuino running CP/M, or any terminal that speaks telnet, XMODEM Gateway gives you file transfers, SSH access to remote servers, and AI chat — all from a single telnet connection. I’ve tested this with IMP8 on the Altairduino. IMP8 is on a a default floppy disk image that usually ships with your Altairduino. To see how to use this, visit the link for getting the Altairduino on the Internet.

XMODEM Gateway is a Rust server that listens on port 2323 (configurable) and serves three features through a simple menu-driven interface:

File Transfer — Upload and download files using the XMODEM protocol with CRC-16 error checking and automatic checksum fallback
SSH Gateway — Connect through the server to any SSH host, letting telnet-only hardware reach modern servers
AI Chat — Ask questions and get answers from a large language model, paginated to fit your screen.

The server auto-detects your terminal type when you connect. Press Backspace, and it figures out whether you are on a Commodore 64 (PETSCII), a modern terminal (ANSI), or a plain text connection (ASCII). Output is adapted accordingly — 40-column layouts for the C64, ANSI color codes for modern terminals, and clean plain text for everything else.

Setting up XMODEM Gateway

Install Rust if you do not have it:

sudo apt install build-essential cmake
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

Clone and build:

git clone https://github.com/rickybryce/xmodem-gateway.git
cd xmodem-gateway
cargo build --release
./target/release/xmodem-gateway

On first run, the server creates xmodem.conf with default settings and a transfer/ directory for files. Connect from any machine on your network:

telnet 192.168.1.100 2323

Transferring files using XMODEM Gateway

From the main menu, press F for File Transfer. Press U to upload or D to download. For uploads, enter a filename, then start your terminal program’s XMODEM send. For downloads, select a file from the list and start XMODEM receive on your end. The server supports CRC-16 and checksum modes, negotiated automatically with your client.

If you are transferring binary files that contain 0xFF bytes, toggle I to enable IAC escaping, which prevents the telnet protocol from misinterpreting data bytes as commands.

SSH Gateway

Press G then S from the main menu. Enter a hostname, port, username, and password. You get a full interactive shell on the remote machine. For PETSCII and ASCII terminals, the server strips ANSI escape sequences so the output is readable. Press Ctrl+] to disconnect.

AI Chat

Press A from the main menu. Type a question and the server sends it to the Groq API, displaying the response with pagination. To enable this feature, create a free account at https://console.groq.com, generate an API key, and add it to xmodem.conf:

“`
groq_api_key = gsk_your_key_here

Security

Telnet is inherently cleartext. This server is designed for local and private networks only. You can enable username/password authentication in xmodem.conf by setting security_enabled = true and configuring your credentials. Failed login attempts are locked out per IP after three tries.

Source

The project is open source at my github repository. Written by Ricky Bryce with Claude (Anthropic) as co-author.

— Ricky Bryce