Subject: Re: Identifying my version of a GPL program
From: Bernard Lang <>
Date: Wed, 20 Dec 2000 22:59:50 +0100

This is not quite the question I asked, but still an interesting answer.

The problem is that most client users will not pay attention to what
version they are uding.  So Yoyodyne has to check for them,
and someone might decide to make code that bypasses the check.


On Wed, Dec 20, 2000 at 12:48:46PM -0500, Gordon, Seth wrote:
> This looks like a variation on the question: "How can a Quake server confirm
> that a client connecting to it is running the original and official version
> of Quake, and not running a modified version that lets the player cheat?"
> The answer is: "It can't".
> The GPL, in section 2a, says that if you distribute a modified version of a
> GPL program, "You must cause the modified files to carry prominent notices
> stating that you changed the files..."  So, from a legal point of view,
> anyone who passes off a modified version as if it were Yoyodyne's own
> version is violating the GPL; you don't have to bring trademark law or any
> side contracts into the picture.
> Also, Yoyodyne could publish the MD5 hash of the binary on its own Web site.
> Clients could run a separate MD5 program on the binary and compare the
> result with the published value, to confirm that their copy of the program
> really is Yoyodyne's version.
> --sethg

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