Can I relicense someone's MIT code under GPL

Question

I've picked up an open source project (an application) which was released under MIT license. I've made many modifications to it and have been working on it and would like to release my version under a GPL license.

I heard that licenses like the MIT and New BSD are more permissive than the GPL, and vaguely remember reading that it's possible to re-license them under the GPL (even though it's not possible to do the same with GPL. GPL can't be re-licensed under MIT or new BSD). Can those with more experience in open source clarify a little bit?

If I release my modified version under GPL, how do I give the original author credit. The license I found says to attach the license file, but if I attach their file, it might be confused as the official license file for my version. How are these issues usually handled?

Answer 1

From Wikipedia:

Two variants of the license, the New BSD License/Modified BSD License, and the Simplified BSD License/FreeBSD License have been verified as GPL-compatible free software licenses by the Free Software Foundation, and have been vetted as open source licenses by the Open Source Initiative, while the original has not been accepted as an open source license and, although it is considered to be a free software license by the FSF, the FSF does not consider it to be compatible with the GPL.

For the MIT license:

The license is also GPL-compatible, meaning that the GPL permits combination and redistribution with software that uses the MIT License.

Answer 2

You cannot change the original copyright or license agreement on the code in any way unless you get permission from the author to do so. If the code is under the MIT license, it stays under the MIT license, unless the original author agrees to the change.

If the original license included an express statement allowing you to relicense the code, then you could do so, but if you read the BSD license or the MIT license, you'll see that they have no such statement.

(By contrast, the GNU licenses are generally used in a way that allows you to use the contribution under a newer version of the same license. Wikipedia and the Free Software Foundation collaborated to use this as a loophole that would allow for relicensing -- but even that loophole isn't open anymore.)

That said, it shouldn't be necessary to change from the MIT license, because the MIT license and the GPL are compatible (as already explained by Vivin)

(I am not a lawyer.)

Answer 3

You can't remove a license without permission, which is not in the license. What you can do is have more than one license on the software.

So, you need to include the license file, but I'd add something about it not applying to the whole product. To be nice, you might want to point out where somebody can get a copy of the MIT-licensed starting point.

You need to include the GPL as well as the original license, and you need to make it clear that each individual file as a whole is covered by the GPL, as well as MIT for portions of it.

You will doubtless get criticized for taking an MIT-licensed project and licensing something based on it under the GPL. Too many people in the Free Software and Open Source communities get too emotional about licenses. I wouldn't let that stop me, but if I made incidental improvements to the MIT-licensed stuff I'd send those improvements back to the original project under the MIT license.

Finally, don't forget the copyright notices. Something like "Portions copyright 2008 by Them" is mandatory, since you can't remove a copyright notice without permission, but you need to put in your own copyright notice.