CobolCraft

A Minecraft server written in COBOL. It supports Minecraft 1.21.4 (the latest version at time of writing).

Features

The following features are already working:

• infinite terrain generation and dynamic chunk loading
• persisting world and player data to disk
• support for Minecraft's file formats (import existing worlds)
• multiplayer (up to 10 concurrent players)
• ping/server status (i.e., show as online in the server list)
• breaking and placing blocks
• block interaction (right-clicking, e.g., to open doors)
• player inventory (limited to creative mode)
• chat
• commands (in-game and via an interactive console)
• whitelist (single player, edit main.cob to enable/set username)

Note that blocks with multiple states, orientations, or interactive blocks require large amounts of specialized code to make them behave properly, which is way beyond the scope of this project. Some are supported, however:

• torches (all variants)
• slabs (all variants)
• stairs (non-connecting)
• rotated pillars, such as logs or basalt
• buttons (non-interactive)
• doors (including interaction)
• trapdoors (including interaction)
• beds

How-to

CobolCraft was developed using GnuCOBOL and is meant to be run on Linux. Support for other operating systems such as Windows has not been tested. However, it is possible to use Docker for a platform-independent deployment.

To deploy on Linux, make sure all prerequisites are installed:

• cobc (e.g., from the gnucobol APT package on Debian)
• make
• g++
• zlib (e.g. zlib1g-dev on Debian)
• curl (needed to download the official server .jar)
• a recent version of Java (needed to extract data from the server .jar)

Then execute make to build, followed by make run to start a server on port 25565.

Or, using Docker:

docker build -t cobolcraft .
docker run --rm -p 25565:25565 -it cobolcraft

To configure the server, edit the variables in main.cob (limited options available).

Note: By default, the server is only accessible via localhost (i.e., only on your own system via localhost:25565). To make it accessible from the outside (your local network, via VPN, port forwarding, on a rented server, ...), you can start the Docker container like this:

docker run --rm -p 0.0.0.0:25565:25565 -it cobolcraft

Why?

Well, there are quite a lot of rumors and stigma surrounding COBOL. This intrigued me to find out more about this language, which is best done with some sort of project, in my opinion. You heard right - I had no prior COBOL experience going into this.

Writing a Minecraft server was perhaps not the best idea for a first COBOL project, since COBOL is intended for business applications, not low-level data manipulation (bits and bytes) which the Minecraft protocol needs lots of. However, quitting before having a working prototype was not on the table! A lot of this functionality had to be implemented completely from scratch, but with some clever programming, data encoding and decoding is not just fully working, but also quite performant.

If you too have never written COBOL before but are interested in CobolCraft, I recommend reading the GnuCOBOL Programmer's Guide: https://gnucobol.sourceforge.io/HTML/gnucobpg.html

To learn more about the Minecraft protocol, you can refer to https://wiki.vg/Protocol. In some cases, it may be helpful to look at real server traffic to better understand the flow of information.

Program Overview

This section provides a high-level overview of CobolCraft from a software design viewpoint.

Source Components

The program entrypoint is main.cob. The remaining COBOL sources are located in the src/ directory, including src/server.cob, which contains the bulk of CobolCraft.

These sources are located in the cpp/ directory and get compiled into a shared library (.so on Linux).

TCP sockets are managed by the CBL_GC_SOCKET socket library located in the CBL_GC_SOCKET/ directory.

Packet Blobs

CobolCraft makes use of network data captured from an instance of the official server application via Wireshark. This data is located in the blobs/ directory and is decoded at run-time.

Data Extraction

The official Minecraft (Java Edition) server and client applications contain large amounts of data such as:

• block and item types
• entity types
• biomes

Fortunately, the freely available server .jar offers a command-line interface for extracting this data as JSON. The CobolCraft Makefile has a target that downloads the .jar and extracts the JSON data from it. The JSON files are evaluated at runtime using a custom-built generic JSON parser, such that CobolCraft can inter-operate successfully with the Minecraft client without distributing potentially copyrighted material.

Legal Notices

This project (except 3rd-party contents as stated below) is licensed under the MIT License. See LICENSE for further information.

This project includes the 3rd-party CBL_GC_SOCKET shared library, licensed under the LGPL v3. See CBL_GC_SOCKET/COPYING.lesser for further information. Note that line 939 of CBL_GC_SOCKET/cob_socket.cpp has been modified (compared to the original distribution of that library) to fix improper snprintf usage.

"Minecraft" is a trademark of Mojang Synergies AB. CobolCraft is neither affiliated with nor endorsed by Mojang.