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I'm planning to make a GPLv3 or AGPLv3 library to allow the library user to interact with the multiplayer portion of a non-free, closed-source, proprietary game.

The game's EULA explicitly allows the players to customize the game, create and share mods but has some protective restrictions to prevent the game from being pirated, used commercially, or with intentions to make money out of it. It also distributes the game server binaries publically so that people can host their own dedicated servers.

This is what confuses me, I know that I can't use the game's code or link to it and I'm not willing to, the library code will be written from scratch, but can I make a client/server library with a compatible protocol so it can be used to host customized game servers for example? Do I have to add an addendum to the licenses or can I use the plain GPLv3/AGPLv3 licenses as they are?

There are some community-made server Softwares of this game, but I don't know how legal they are. I want to make this project completely legal since the beginning.

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  • Is there a specific part of the license you are worried about? Generally, you can't stop yourself from doing things with a license - the only concern is whether other people will be allowed to do what you think they should be allowed to do with your program.
    – user253751
    Jul 3, 2020 at 16:47

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Can I make a GPLv3 or AGPLv3 software which serves a proprietary protocol?

It depends upon your country, and should ask your lawyer. I am not a lawyer.

In the European Union, reverse engineering a proprietary protocol is AFAIU legally allowed. You could look at what the developers of the nouveau driver for Nvidia cards on Linux did. It is a widely known example of reverse engineering a proprietary (hardware related) protocol.

I'm planning to make a GPLv3 or AGPLv3 library to allow the library user to interact with the multiplayer portion of a non-free, closed-source, proprietary game.

To be safe and avoid losing your precious time, you could inform in advance the corporation selling that game.

I want to make this project completely legal since the beginning.

You are making a very optimistic bet: that legal systems are logically consistent everywhere. I am not a lawyer but I tend to believe that what is legal in California might be illegal in France or vice versa. Regulations on selling alcohol, or tobacco, or medical drugs comes immediately to mind. The CECILL license has been written by lawyers who told me that some obscure wordings of GPL could be illegal in France.

You (or your lawyers) could also take inspiration from the license driving GCC plugins : the GCC runtime library exception.

My recommendation is to avoid writing your own software license (without help from a lawyer). Choose one existing on open source licenses page.

PS. Notice that I current am, and have been in the past employed, by two out of the three institutions (CEA, INRIA, CNRS) authoring CECILL.

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