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Avahi, including the C client library, is licensed under the LGPL.

On my system, (and I believe everywhere) libavahi-client.so.3 links to libdbus-1.so.3. DBus is licensed under a dual license, GPL OR AFL.

The AFL is documented as being incompatible with the GPL.

This answer explains that software licensed under the LGPL is required to be compatible with the GPL, thus Avahi cannot link with DBus under the terms of the AFL.

If Avahi cannot link to DBus under the terms of the AFL, it must be linking to it under the terms of the GPL (this is how dual-licenseing works).

But, if Avahi is linking to DBus under the terms of the GPL, it must be GPL itself.

What I have written above is my understanding, and I am looking for confirmation if my understanding is correct or why I am wrong.

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The LGPL can be "upgraded" to the GPL. Quoting the LGPL v3.0, section 2:

you may convey a copy of the modified version [...] under the GNU GPL, with none of the additional permissions of this License applicable to that copy.

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  • I'm not sure that addresses the OP's point, which seems to me to be that if avahi is linked to d-bus, and d-bus is under GPL/AFL, then unless the avahi developers are the rightsholders for d-bus or the FSF is wrong about either the licence effects of dynamic linking or the GPL-compatibility of AFL, d-bus must be received by the avahi developers under GPL and thus avahi must be distributed under GPL. LGPL is not a choice that's open to them. Note that I'm not saying I endorse the question; I'm merely trying to restate it as I understand it.
    – MadHatter ♦
    May 26, 2021 at 10:52
  • Hmmm.... I wonder if this is like the readline situation where there's a more permissively licensed library with the same API, just not one that's on the poster's system.
    – Philip Kendall
    May 26, 2021 at 10:56
  • I've changed the title of the question to hopefully more accurately reflect the matter of license incompatibility, rather than it being directly a GPL problem.
    – Violet
    May 26, 2021 at 11:03
  • A replacement for DBus with different licensing would allow avahi to be LGPL on a different system, but I don't really have any evidence avahi-client does ever link to such an alternative, and at any rate I still think that would force avahi to be GPL on my system, which may have viral consequences.
    – Violet
    May 26, 2021 at 11:05
  • @Violet The GPL has no effect whatsoever on linking. Its terms apply only when you distribute software.
    – Philip Kendall
    May 26, 2021 at 11:16

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