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I'm developing a software which has two parts, kernel and plugins, their code are completely separated. Here's the question:

  • Some of the plugins should be GPL based, as they invoked GPL code.
  • The code of kenel is separated from plugins.
  • The kernel and plugins are distributed separately.
  • I only distribut the kernel to users, and its their decision to install GPL-based plugins or not.

Now, can I license the kernel with MIT license, or I must use GPL for it?

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Assuming a plugin architecture where the plugins form a single program with the kernel, all parts of the system (kernel and any plugins) must be available under a GPL-compatible license. The MIT license is GPL-compatible, and is therefore allowed.

For a list of possible licenses, see the GNU list of Various Licenses and Comments about Them. Note that GPLv2 and GPLv3 are incompatible, and other licenses might be compatible with one but not both of these GPL versions. If there are plugins that use GPLv2 code and other plugins that use GPLv3 code, this might be a good reason to choose a license for the kernel that is compatible with both (e.g. GPLv2+ or MIT, but not Apache 2).

The GPL FAQ has a number of questions regarding plugin architectures, starting with When is a program and its plug-ins considered a single combined program? The FAQ explicitly discusses the case of plugins to a GPL program, but the reverse (GPL plugins to a non-GPL program) is equivalent.

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  • You don't mean to imply that the entire system (plugins received under a GPL license, and the kernel itself) can be re-distributed under MIT, do you? – MadHatter supports Monica♦ Aug 20 '18 at 13:16
  • @MadHatter of course not, thank you for spotting this ambiguity. I edited to clarify that all parts must be available under a GPL-compatible license. – amon Aug 20 '18 at 13:55
  • I didn't seriously think you thought so: but thanks for the clarification! +1 from me. – MadHatter supports Monica♦ Aug 20 '18 at 14:16

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