Requiring GPL for clients of AGPL Server

Question

I am working on a piece of server software that is licensed under the AGPL. I would like to require that all clients to it be released under a free license (GPL preferable, but I'm open to other licenses).

Is there a way to do this? Is there a standard way to do this?

Answer 1

The short answer is no, especially not if you expect others to build the clients.

If you are building this server software with the intention of others building clients, they are communicating at arms length with each other, and are therefore seperate works and so the requirements of the AGPL do not apply to the clients.

You also could not practically impose a limitation via a Terms of Service. While you could impose a ToS to restrict connections to your particular server, you would be unable to restrict people from grabbing your source code, firing up servers of their own, and not requiring any such limitations for clients that connect.

Your best bet is to build out both the server and the client, make the client GPL licensed, and then make the client so good no one else bothers to make a different one with a different license.

Answer 2

Per the discussion in the comments of the question, I think the only recourse is to make a requirement in the Terms of Service for accessing the server.

The license of the server can't impose additional restrictions on the client as they are another separate piece of software. Even if it could, the AGPL provides for a way of removing those restrictions.

All other non-permissive additional terms are considered "further restrictions" within the meaning of section 10. If the Program as you received it, or any part of it, contains a notice stating that it is governed by this License along with a term that is a further restriction, you may remove that term.

Without an incentive to keep said term, it would be pretty hard to enforce.

Making it a requirement in the Terms of Service comes with its own baggage. You can't keep someone from creating a proprietary custom client and using it on their own server.

At the end of the day, likely most of this won't really matter though. Given that it is a freely licensed game, with a freely licensed "official" client, the chances that someone will not be willing to offer their code under a free license is pretty slim.