> No because GPL does not require you to open your network to anyone.

I don't know what you meant by this, but MYEUHD is correct: you can't release software under GPL and then dictate the terms under which binaries may be distributed, beyond the relevant copyleft restrictions already imposed by the GPL.

I'm not convinced that Signal attempt any such prohibition though. There's a discussion over here [0] on this question. Someone rightly points out Stallman's Freedom 2.

[0] https://github.com/signalapp/Signal-Android/issues/282#issue...

Firefox distributes the source via an open source license, but if you want to distribute binaries, you can't use trademarks not covered by the source license without Mozilla's permission.

Ah, right. So they don't try to prohibit distributing 'unofficial' binaries, they just insist that if you do so, you don't call it 'Signal'.

As you say, Mozilla have done the same thing, and it's not hard to see why, though some FOSS folks (Debian in particular) took issue with this [0][1]

[0] https://lwn.net/Articles/676799/

[1] https://en.wikipedia.org/wiki/Mozilla_software_rebranded_by_...

Not only can you not call it signal, you may also not connect it to the signal network so you have to host your own which can't message anyone.

Here's some relevant piece by m0xie, a maintainer of Signal : https://github.com/libresignal/libresignal/issues/37#issueco...

He and his colleagues are hostile towards federated protocols. This culminated in an article called "The ecosystem is moving" to which Daniel Gultsch (maintainer of conversations.im) answered here: https://gultsch.de/objection.html

The license that Mozilla uses has such restrictions. But this isn't the case with the GPL.

I don't think that's quite right. The GPLv2 makes no mention of trademarks [0], and the GPLv3 mentions trademarks only to clarify that you may be protective of them, independent of the terms of the GPLv3.

If you license under GPL, you're still free to be protective of your trademarks.

This makes sense given that although Mozilla like to use their own licence, they've been careful over the years to either release under the GPL as well, or to use a licence of their own which has been constructed to be GPL-compatible [2]. Clearly, the GPL didn't force Mozilla to be permissive with their trademark.

[0] https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt

[1] https://www.gnu.org/licenses/gpl-3.0.txt

[2] https://en.wikipedia.org/wiki/Firefox#Licensing

There was a Noise messaging app on copperhead os that connected to everyone though, I'm not sure how it worked.