Relicensing

Software relicensing is applied in open-source software development when software licenses of software modules are incompatible and are required to be compatible for a greater combined work. Licenses applied to software as copyrightable works, in source code as binary form, can contain contradictory clauses. This requirements can make it impossible to combine source code or content of several software works to create a new combined one.

Sometimes open-source software projects get stuck in a license incompatibility situation. Often the only feasible way to resolve this situation is re-licensing of all participating software parts. For successful relicensing the agreement of all involved copyright holders, typically the developers, to a changed license is required. While in the free and open-source domain achieving 100% coverage of all authors is often impossible due to the many contributors involved, often it is assumed that a great majority is sufficient. For instance, Mozilla assumed an author coverage of 95% to be sufficient. Others in the FOSS domain, as Eric S. Raymond, came to different conclusions regarding the requirements for relicensing of a whole code base.

An early example of an open-source project who did successfully re-license for license compatibility reasons is the Mozilla project and their Firefox browser. The source code of Netscape's Communicator 4.0 browser was originally released in 1998 under the Netscape Public License/Mozilla Public License but was criticised by the FSF and OSI for being incompatible. Around 2001 Time Warner, exercising its rights under the Netscape Public License, and at the request of the Mozilla Foundation, relicensed all code in Mozilla that was under the Netscape Public License (including code by other contributors) to an MPL 1.1/GPL 2.0/LGPL 2.1 tri-license, thus achieving GPL-compatibility.

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