Licenses on Unix


# Intro #


The world of licenses is the legal world, a world where the literal
meaning of words is important and where all the crevasses are exploited.

I'm not a lawyer, nor have I studied laws, and whatever I say will be
based on what I understood from my reading.

In this episode we're going to do a small overview of the topic of
licenses on Unix. But beware, a "small" overview in the legal world is
quite heavy!


## What's a license ##


Let's first define what's a software license.

From wikipedia:

> A software license is a legal instrument (usually by way of contract law,
> with or without printed material) governing the use or redistribution
> of software.

The software license can be distributed on paper or in digital form and
usually contains rules granting or removing rights, imposing restrictions,
that the users of the software should agree to or otherwise would commit
copyright or license infringement. Licenses can also contain some lines
about liability and responsibility from both party, the software provider
and the users.

In sum it's a contract.

Legality falls under the laws of the country and so they might vary from
one place to the other. All countries can have their own jurisdictions.

For instance the Marshall Islands have 0 copyright laws.

Beyond that, there are some international treaties that are signed by some
countries to mutually respect copyrighted materials such as the treaty
of the Berne convention and the World Intellectual Property Organization.

Let's not discuss too much the legality and more about some interesting
stories and stuffs related to Unix that you might fancy.


## The first licenses used on Unix-like systems - History ##


In the 50s till the 70s-80s it was common for computer programs to
be shared with their source code, it was the norm. Everyone had the
permission and the ability to modify the software for their specific
usage.

There were even organizations of users and suppliers that got formed
around sharing the softwares such as SHARE and DECUS.

This changed around the end of the 60s when the competition in the
software world got tighter. Usually it was the hardware manufacturer that
shipped the software with the hardware it was producing but independent
software distributors started selling the software untied to the hardware.

A court case filled in 1969, started in 1975, and finally dropped in 1982:
US vs IBM, stated that bundling hardware with software was anticompetitive
and brought monopoly to the marketplace (Tell that to the DRMs of today).

And so some of the software industry of the 70s and early 80s began
distributing binaries and proprietary packages - Taking measures to
limit the copying and spreading of their softwares so that they could
keep their marketplace.

While the software is distributed this way, as binaries that is, it
cannot be studied nor changed.

Unix wise, before 1975, all the Research Unix that Bell Labs produced
were for internal use only.

It was only in 1975 that the first source license for UNIX V5 was sold
to the University of Illinois Department of Computer Science, UIUC.

This was a great move for the university of Illinois because they started
working on implementing ARPANET on those Unix machines, creating the
RFC 681, for NETWORK UNIX, with FTP and telnet clients. The first Unix
later to included them was BSD.

At the time, Bell Labs wanted to sell licenses but for non-university
users it wanted to sell them at $20K which was very expensive.

The V6 version of UNIX did turn that around and was the first commercially
successful UNIX. It remained the most widely used version into the 80s.

V7 superseded it later in 1978, but V6 still remained the most used.

As we've said, at those time the source code was available to anyone
who got the software, and so it was a great tool to teach in universities.

The license they provided with the software didn't explicitly said
anything special about sharing the source, licenses for softwares didn't
mean much yet.

One of the university who got hold of UNIX V5 in 1974 was the university
of Berkely. Now you might wonder, "that's before 1975", yes it is, because
no license was sold to Berkeley, at least not yet. It all happened because
a teacher named Bob Fabry was teaching a course about OS principles and
took UNIX as an example. It was more like an informal notice written by
the developers themselves.

One year later, Ken Thompson, one of the creator of UNIX, visited the
university and helped install UNIX V6 on the machines they had.

Some students got quite found of the system and began improving it and
adding softwares they liked. Bill Joy was one of those students.

Because other universities became interested in the work they were
doing with UNIX at Berkeley, in 1977, Bill Joy and friends compiled a
distribution of all the addons over V6 and distributed it under the name
BSD: Berkeley Software Distribution.

Finally in 1980 the US established in its law that computer softwares were
copyrightable. Before that softwares were not considered nor intellectual
property nor copyrightable, they were open ideas for everyone.

Actually, this process didn't really start in 1980 because in 1976 there
were already copyright acts related to softwares it was just that they
weren't all agreed on yet. Let's say it started in 1976 and got settled
in 1980 because they wanted to make sure that softwares embodied an
"author's original creation"

So from this point onwards you could claim that a software was yours
instead of distributing binaries or do nasty tricks to deter your users
from sharing it. Now you have the laws behind you, at least in the US.

In 1983, a hacker at the MIT AI lab, Richard Stallman was a bit frustrated
with how the software industry and users were closing themselves. And
so he announced a project called GNU, which aim was to release a free
operating system. In 1985, he released the GNU Manifesto, which was an
explanation of the GNU philosophy, the base definition of free software,
and the "copyleft" ideas.

But still in the 80s UNIX was the corporate norm, it was what
organisations liked.

And what do organisations do when something becomes popular, they try
to regularize it.

It needed regulations because everyone was customizing their own versions
and brand of Unix-like system. It was creating too many rivalries and
doing more harms than good.

There were mainly two groups that appeared trying to do that:

* In 1984 there was the X/Open Company, which was a consortium of several
European UNIX systems manufacturers. Their aim was to create a single
specifications for all UNIX. The members were big players.

* In 1988 there was the OSF, Open Software Foundation, a not-for-profit
organization in the U.S. also to create open standard for implementing
UNIX systems.

The two of them joined way later in 1996 to form the Open Group, with
500 members. They hold a sort of public approved certificate saying what
is UNIX and what isn't, the term is trademarked by them.

Anything that has the term UNIX is licensed under their system to have
this name, which comes with royalty fees.

And this Open Group brand fee isn't cheap...

To register your product to the trademark license you have to pay at least
$2.5K plus a $1K annual fee and more payments for a bunch of other things.

It's good to know that in 1997, the Open Group got hold of the
responsibility of the X Window System too.

Now back in time, in 1989, after all the legislation and standardisation
happening, the guys at Berkeley were still distributing their softwares,
the BSD, it had already reached version 4.3

However, they were paying fees to AT&T, the parent company to the Bell
lab, that got very expensive.

Yep, mobile companies are all tycoons.

Now if you remember the University of Illinois made the specs for Unix
networking and it got implemented first in BSD. Many people requested
this networking code without the hassle of licensing and so that's
what the BSD did, they crafted a custom license - the BSD license -
to distribute their Net/1 code.

The BSD license is very permissive and allows the source code to be
modified, reused and to be sold, which was perfect for the industry.

In 1991, they got fed up with AT&T and rewrote all the standard Unix
utilities under the BSD license so that they would not be tied to any
copyright.

They named it 386BSD.

But you don't get away from a titan without a scar, AT&T sued them
for infringing copyrighted material, they filled the lawsuit in 1992:
USL v. BSDi.

This dealt a blow to the BSD communities and slowed their development.

BSD wasn't a good choice for corporates now.

A year before that lawsuit, in 1991, a student named Linus Torvalds
was working on a kernel named Linux, a UNIX clone, which he decided
to release as free under the license Stallman created, the GPL - GNU
General Public License. Together joined with the userland utilities that
the GNU project had it created the GNU+Linux operating system.

It had no ambiguity with the license and attracted the attention of
volunteer programmers and spread because of the open community and sharing
of ideas, much like the first days of Unix with university students.

Then, one year after it had started, in 1993, the case USL v. BSDi lawsuit
was settled out of court and the FreeBSD and NetBSD projects emerged.

Once the BSD code base was released it got widely adopted as the base
of many proprietary Unix variants, as it did before, all of that because
of how easy and permissive the BSD license is with commercial use.

Still today several operating systems choose to use softwares under
the BSD license for the same reasons, macOS and iOS being two well
known examples.

In 1995, the Apache HTTP Server, commonly referred to as Apache, was
released under the Apache License 1.0 which is also a permissive BSD-like
license that allows commercial use.

It was used to undergird the advent of the world wide web.

In 1997, Eric Raymond published The Cathedral and the Bazaar, which was
more or less putting into words what the hacker community loved about
in the sharing of softwares.

This book was one factor that incited Netscape to release their internet
suite software under a free license. Which is why we have the whole
Mozilla ecosystem today, and a key event that influenced others to
release free and open source softwares after that.

The Mozilla license is GPL compatible, very permissive but at the same
time doesn't allow to change the software without sharing the source code,
this is the idea of "copyleft" that the GNU project envisioned.

In 2002 a company named Santa Cruz Operation, SCO, released the source
code of some Ancient UNIX relicensed under a BSD license.

SCO was the "first Unix company", founded in 1979 and filled bankruptcy
in 2007, it used to port many UNIX softwares to many platforms.

One fun use of that release was on the Nintentdo Game Boy Advance which
was made to run V5 UNIX using a PDP-11 emulator.

It is also a great way to learn about operating system theories.

...And this is where we are now.

You might be questioning yourself a lot about licenses.

Just like BSD has relicensed itself, and SCO relicensed their UNIX,
how is relicensing possible? What do those licenses really mean?
What are the ones used today? What should I know?


## Licenses today ##


### First Thoughts ###


According to the Free software foundation there are only two big
categories of software licenses, they are either proprietary or free
and open source.

That's a very narrow view of the matter, a black and white fallacy,
as we'll delve into later on.

But for the moment let's just presume it is true.

According to this entity's chosen distinction, the difference between a
free and open source software and a proprietary one lies in the rights
granted to the users.

A free and open source license being one that allows both modifying and
reusing the software, which as a premise implies that the source will
be somehow available.

Then, again according to their distinction, a proprietary software is
everything else.

But what about softwares without licenses, an unlicensed software.

Is it outside of the copyright protection, is it in the public domain?
What if that software is an internal trade-secret software used inside
the company?

There are no correct answer to this, it depends first on the country
and second on the court choice during the legal appeal.

In sum: it's complicated.

In the US, a software distributed without license is fully copyright
protected and thus you can't legally use it until the right has been
granted.

That's as long as there is an owner to the software, which is usually
before the copyright expires. Some countries like the United States have
a limited lifetime for copyright. When the copyright or patent expires
it falls under the public domain.

A patent being a right to a certain type of invention, which is again
a vague term. There are other legal mechanism too such as legislation
and trademark.

What about softwares distributed on Github without license, you may
ask. Well, yes, beware, that this also applies here.

Under most international treaties, such as the Berne convention, an
author automatically obtains the exclusive copyright to anything they
have written, even rights over derivative works.

So remember to always add a license to your projects when pushing them.

However it's also not as simple as that, someone using a software without
license can win a court appeal and the copyright can be waived away. The
legal border is thin because releasing anything without license is not
like releasing with a license.

The only true way to relinquish a software into the public domain is to
do it explicitly via a license such as the CC0, creative common zero or
the WTFPL, what the fuck public license.

That way you're assured to have completely lost ownership, trademark,
and patent to the software.

You must be explicit about whatever you do so that you stay in accordance
with the country's laws.


### In a license ###


So what's inside a license, what varies from one to the other?

Licenses are contracts that or extend the legislation of the citizen
of a certain country over the software or widen their rights over the
software - rights that would otherwise be disallowed by default copyright.

So yeah, you've got the base laws of the country and the license over it.

Many licenses only enumerate those basics rights that the user already has
under law. But keep in mind that the rights you have as a citizen cannot
be taken away if they conflict with a license, that's simply not possible.

Free software licenses are said to be more or less non-contractual
extensions as they aren't an agreement between parties but that remains
to be contested as they are still an agreement. We'll see later on in
this podcast, someone has to be held accountable for infringement.

For instance, citizens in the US that owns a software have the
explicit right to use the software with a computer even if that software
accidentally makes copy of itself. You are legally entitled to own copies
of a software you own.

There's also a first-sale doctrine in the US that grants middleman,
resellers the right if they own something to resell or rent it even
if they are copyrighted material as long as they own it, which is
not the case with proprietary software. This is called "distribution
right". However it's vague how it applies to digital goods as an item
can be copied indefinitely.

In the European Union the laws differ where a copyright holder cannot
oppose the resale of a digitally sold software.

Where that is more of a play-on-words is when an end-user isn't the owner
of the software. That is the precise definition of a proprietary software,
a software you are borrowing and not owning.

The software technology might be patented for instance.

The intellectual property rights remain with the entity that provided
the software and thus a proprietary software doesn't give you the same
right as a citizen as a software you own.

In that case the user has to sign an EULA, end-user license agreement,
with the digital good owner to be able to use it, otherwise without
accepting the license the user may not be able to use the software at all.

An EULA is yet another contract between the end-user and the licensor
that establishes what rights the user has.

Most of the time it is about liability, holding the software
distributor non-liable for damage or the user liable for infringement,
and restrictions, which can range from forbidding reverse engineering,
benchmarking, to limiting the number of users and the number of devices
the software can be used on, etc..

Free softwares also have EULA that is part of the license, we'll see
those in a bit.

A nasty way of including the EULA is to do it in a shrink-wrap manner. The
term "shrink-wrap" means that the EULA cannot be read before the product
is actually bought. Which means that if the users don't accept the license
they won't be able to use the software and would have paid for nothing.

So what's in a license really?

* The basic rights of your country
* Who owns it
* What rights and restrictions the end-user has such as modifying,
reselling, benchmarking, sharing it
* Who is held liable
* What responsibility the owner has
* What responsibility the user has


### The scale ###


Depending on the way those previously mentioned liabilities, rights,
and restrictions are set the license is said to be more open and free
or less open and free.

This is a better realistic approach than the mundane dichotomy the free
software foundation discusses.

There's actually a lot of nagging over the definition of what is truly
"free software".

The arguments are about those: how permissive the license is, the
requirement for redistribution, if the license is copyleft/share-alike
or not, if the license allows derivative work, if the license allows
selling the software or using it for advertisement, etc..

We often hear about "Free and open source software" which is a actually
a vague umbrella term for softwares that provide liberty to their control
and are open source.

It's truly confusing because the distinction between open source and
free software is nebulous, it's more philosophical than practical because
they describe almost the same things.

Depending on who defines them they might mean different things. It's a
philosophical war and using "Free and open source software" doesn't make
anyone cry and that is why it's used a majority of the time instead of
simply "open source" or "free software".

Some examples organizations and groups that put those definitions together
are the free software foundation, FSF, the OSI, open source initiative,
the Debian project, the BSD project, etc..

They all have different guidelines and definitions for "free and open
source" software licenses.

But generically they all allow to use the code for any purpose, modify
it, and redistribute it, which is normally forbidden by default copyright.

What they also have in common is how they're perceived by the communities
using it for how they decrease costs, increase security and stability,
privacy, and control.

Free and open source softwares often live surrounded by a community of
contributors.

Now let's talk about some of the philosophies.

The Open Source Initiative is an organization that derives it's license
norms from the Debian Free Software Guidelines.

They hold a list of licenses that they "certify" open-source.

Their definition is:

> An open-source license is a type of license for computer software and
> other products that allows the source code, blueprint or design to be
> used, modified and/or shared under defined terms and conditions.

Most of the other popular free and open source licenses are approved as
open source, for instance:

* Apache License 2.0
* BSD 3-Clause "New" or "Revised" license
* BSD 2-Clause "Simplified" or "FreeBSD" license
* GNU General Public License (GPL)
* GNU Library or "Lesser" General Public License (LGPL)
* MIT license
* Mozilla Public License 2.0
* Common Development and Distribution License
* Eclipse Public License

On the side of the FSF, which also holds a list of licenses agreeing to
their views, they perceive everything that isn't in their definition of
free software as an unfair shadow of what they are trying to achieve by
bringing liberty in the software world.

They define free softwares in terms of liberty and not price.

For them liberty is based around community sharing and respect of
individuals, which lies, in practicality, inside licenses as the concept
of copyleft aka share-alike.

A copyleft is an additional clauses that requires users who modify the
code to also redistribute the modifications under the same license as
the original software.

This way it protects all derivative works from drifting and losing the
original permissions and be used in proprietary programs.

Now what about the BSD side.

The BSD camp has yet another position on licensing.

They despise copyleft licenses and dismiss them as overly complicated
and restrictive.

It is true that copyleft licenses has an insane amount of clauses in their
licenses compared to the usual 4 that you find in the BSD license family.

The BSD license is simple and permissive, it's simply about acknowledging
that there was an original author and to keep this license so that the
author is credited for his or her work.

Some even consider the term "permissive license" is synonym with anything
that is a BSD license.

The BSD license family is liked because it can be use in proprietary
softwares, as we've mentioned before.

The supporters of the BSD license argue that it's more free than copyleft
licenses because it grants the right to do anything with the source as
long as there is attribution to the authors.

But then according to this definition the public domain is more free
than the BSD license.

In the end it's all philosophy.


### License compatibility ###


Because the BSD license only requires you to give credits to the authors
without imposing any limitation on the usage it can be used in proprietary
softwares, as we mentioned before.

That means the license can be modified by adding new clauses to it as
long as they don't conflict with the previous clauses.

For instance MATLAB is based on MathWorks which is distributed under the
BSD license but has an extra clause limiting it's usage to the proprietary
MATLAB which renders all contributions useless outside of MATLAB.

BSD licensed code can also be included in copylefted softwares by added
the clauses related to copyleft. That's what we refer to as compatible
licenses.

The FSF indeed has the BSD license under its list of licenses that are
considered free.

But it doesn't work the other way around, code under GPL cannot be
relicensed under a BSD license without the consent of the copyright
holders.

For this reason the BSD camp avoids at all cost to include anything
under the GPL license in their distribution. They've recently switched
from GCC to the Clang/LLVM compiler.

Another reason is that GPLv3 includes new clauses about patent and
trademark which some individuals try to avoid. Some stay with the
softwares under the v2 some switch to the BSD licenses.

Several major FOSS projects like the Linux kernel, MySQL, BusyBox Blender,
VLC media player decided against adopting the GPLv3.

According to some stats, which are contested, between the 90s and 2000s,
during the dotcom bubble, the GPLv2 was the most widely used license
with a 42.5% usage rate.

In 2015 according the same company who made the first stats accompanied
by Github this time said that it's now the MIT license that is the most
used and GPLv2 being second.

Other stats from Fedora in 2016 shows that the most used licenses in their
packages are under the GPL license, and second place the MIT license.

Those stats shows that depending on where they are used some licenses
are more favored than others.


## Some odd questions ##


Let's answer some odd questions.


- Can an open source software be used for commercial purposes?

Sure as long as the license doesn't limit you for what you want to do
commercially. But beware that commercial is not equal to proprietary
some licenses don't allow you to place extra restrictions on the user.

- Can I call my program "Open Source" or "Free Software" or "X"?

It's probably better to avoid that if your license isn't listed as an
approved on that organization or group website. You have the right to
call it whatever you want but that would confuse people too much.

That applies even if you have just changed a few words or added one
clause, any small change implies it's now a different license.

- Can I use the code that a minor releases as open source?

This falls back to the laws of the country related to liability and
copyright. In a lot of countries the copyright is usually transferred to
the legal parents who will sign and handle everything, just like with
the parents of child stars. So when using the software of a minor you
are actually dealing with his or her parents.

- Can I have many licenses on a single software?

What it's referring to is called multi-licensing and the answer is yes
but that depends.

Multi-licensing is the practice of distributing a software under two or
more different set of terms and conditions.

If someone wants to use the software they can choose whichever license
pleases them.

Where it becomes tricky is when that license is permissive and allows
adding terms to it.

The original distributor is then not allowed to include the changes of
a contributor that changed the license until all the original licenses
are compatible and not contradictory with the original one.

The GPL goes a long way with that not allowing code under GPLv3 to be
used with code under GPLv2 is it doesn't include the "any later version"
statement for instance.

Multi-licensing is common in scenario where a free software company wants
to make business, they release a software under a free license and another
license which allows to create proprietary softwares but has royalty fees.

This is a sort of business model of companies that use free softwares
and want to remain the single-vendor of it.

There are many examples of multi-licensed softwares, MySQL, FreeType
are good ones.

- Can I swap the license along the way?

The short answer is yes, but again that depends.

If you own *all* the code then you can re-release the software under
any license you want. That includes suddenly making a software under
GPL into a proprietary one.

The problem that arises is when you include patches, fixes, and parts
of code you don't own into your _first released free_ software.

Now if you can prove those contributions are benign/not-significant
and that the free and open source license discussed or didn't discuss
the topic of contributions then you might be able to prove the code is
still all yours, the original author.

Otherwise, you will have to contact the other contributors so that they
relinquish their software to you, a legal agreement so that they give
you ownership of the code they wrote.

That's why in some communities, like the FreeBSD and OpenBSD camps and
also the Linux kernel, copyright assignment is considered bad taste so
there's none.

What if someone still uses the prior free and open version?

Then this person is still able to make changes to the software as they
wish, they are able to use it just like the prior license implied,
the new license doesn't apply to the old one.

That is one of the reason sometimes projects get forked.

A good recent example is Oracle that now owns MySQL and so the community
forked it as the MariaDB project.


## Legal Battles ##


There are many examples of legal battle fought over free and open source
licenses. Free software licenses have indeed been tested in court and
their validity hasn't been contested.

BusyBox is one interesting example.

Busybox is a wrapper of many core utilities that is under the GPL.

> In 2007 the Software Freedom Law Center (SFLC) announced it has filed the
> first ever U.S. copyright infringement lawsuit based on the violation
> of the GPL on behalf of its clients, two of the original developers
> of BusyBox, against MoonSoon Multimedia Inc.

This arose because MoonSoon Multimedia publicly said that they were using
BusyBox but were not releasing the source code of it to comply with GPL
copyleft clauses.

The case was settled and BusyBox won, making MoonSoon Multimedia pay fees
and release the related source code. This was the first time compliance
with GPL was brought to court.


## Tips and tricks ##


Let's give out some tips to choose the right license.


There are websites which make it easy to search amongst licenses and
understand their terms without having to read all the crevasses in
them. However, like all summaries they don't go into all the details.

Checkout: [tldrlegal](https://tldrlegal.com/) and
[choosealicense](https://choosealicense.com).

There's also a tool called [fossa](http://fossa.io/) which helps keep
track of what licenses you are using so that you comply with them.


# Conclusion #


Legality is a deep subject.

There are two books in the show notes that go into more details:
"Understanding Open Source and Free Software Licensing"

Which is a freely available book I haven't read but skimmed through. It
goes over all the different licenses and discusses their ins and outs.

The second one, also free, is "Fundamentals of software licensing".

This podcast was truly insightful for me, it answered a lot of questions
I had and I hope it did for you too.

Cheers.

------

https://en.wikipedia.org/wiki/Software_license
http://itlaw.wikia.com/wiki/Computer_Sof...ct_of_1980
https://en.wikipedia.org/wiki/List_of_co...ht_lengths
https://en.wikipedia.org/wiki/Internatio...t_treaties
https://en.wikipedia.org/wiki/Category:C...by_country
http://www.historyofinformation.com/expa...hp?id=1157
https://en.wikipedia.org/wiki/Version_6_Unix
https://en.wikipedia.org/wiki/Ancient_UNIX
http://www.ece.ubc.ca/~gillies/mail/dbgi...ompson.txt
http://minnie.tuhs.org/cgi-bin/utree.pl?file=V6
https://web.archive.org/web/200902192203...icense.pdf
https://en.wikipedia.org/wiki/End-user_l..._agreement
https://en.wikipedia.org/wiki/File:Softw...panded.svg
https://en.wikipedia.org/wiki/First-sale_doctrine
https://en.wikipedia.org/wiki/Proprietary_software
https://en.wikipedia.org/wiki/Open_sourc...re_license
http://www.openacs.org/about/licensing/o...-licensing
https://opensource.org/licenses
https://en.wikipedia.org/wiki/BSD_licenses
https://en.wikipedia.org/wiki/Creative_Commons_license
https://en.wikipedia.org/wiki/Derivative_work
https://en.wikipedia.org/wiki/Share-alike
https://www.gnu.org/licenses/
https://www.gnu.org/licenses/gpl-faq.en.html
http://www.rosenlaw.com/lj16.htm
https://cr.yp.to/publicdomain.html
http://www.rosenlaw.com/lj16.htm
https://cr.yp.to/publicdomain.html
https://en.wikipedia.org/wiki/Public_domain_software
http://stackoverflow.com/questions/55210...-specified
https://en.wikipedia.org/wiki/World_Inte...ganization
https://torquemag.io/2013/03/busybox/
https://busybox.net/license.html
https://lwn.net/Articles/478258/
https://en.wikipedia.org/wiki/Free_softw...ness_model
https://en.wikipedia.org/wiki/Free_software_license
https://en.wikipedia.org/wiki/Free_Software_Foundation
https://en.wikipedia.org/wiki/Free_and_o...e_software
https://en.wikipedia.org/wiki/Business_m...e_software
http://www.opengroup.org/openbrand/Brandfees.htm
https://www.techdirt.com/articles/201003...8452.shtml
https://en.wikipedia.org/wiki/Multi-licensing
https://en.wikipedia.org/wiki/GPL_linking_exception
https://en.wikipedia.org/wiki/GNU_Genera...-licensing
http://softwareengineering.stackexchange...u-pick-one
http://softwareengineering.stackexchange...ion-change
https://www.quora.com/Can-you-take-an-op...GPL-or-MIT
https://tldrlegal.com/
https://choosealicense.com/
http://fossa.io/

"Understanding Open Source and Free Software Licensing" http://www.oreilly.com/openbook/osfreeso...index.html
"Fundamentals of software licensing" http://euro.ecom.cmu.edu/program/law/08-...entals.pdf