What is an open source license? Definition and types of license

What is an open source license? These licenses, also known as open source licenses, govern the use of open source software. The source code of such software can be reused, and modified versions redistributed under certain conditions.

GNU GPL, MIT, Apache, with or without copyleft ? How to choose a license? We take a closer look at the most commonly used open source licenses, to help developers choose the one best suited to their project.

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Open source license: definition

What is an open source license?

An open source license is a software license whose source code is "open". In other words, the code is made available to the general public, and can be shared and modified. It's also known as free software or a free license.

This type of license in no way relieves the author of his or her copyright over the license; simply, the author grants users the right to use the license and exploit his or her work.

The Open Source Initiative is an organization that promotes the use of open source software worldwide.

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Why open source?

The original idea was to offer an alternative to proprietary licenses protected by copyright. These rights are held either by a company or by a programmer.

Today, open source relies on a strong community.

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What are the four criteria that define free software?

Software can be said to be free if it offers the following freedoms:

• freedom to use the software,
• freedom to modify the software to meet specific needs,
• freedom to copy and share the software,
• freedom to modify the software and share modified versions.

What's the difference between free and open source software?

In English, the term " free software" is ambiguous in that " free" means both "free" and "gratuitous". It is this dual meaning that has led the developer community to distinguish between free software and open source software.

The term "open source" was coined in 1998 by Christine Peterson to convey this reality in words. The aim is to remind people that software has a cost.

Open source software, like free software, is not free software. It requires resources, whether human, financial or material, in order to :

• develop the software
• modify it to meet the needs of a company or organization,
• keep it up to date,
• resolve problems in the absence of support,
• hosting it on servers.

All this comes at a cost: you can't confuse open source software with free software, even if users have access to the source code.

The different types of open source licenses

There are hundreds of different open source licenses. Some are compatible with each other, others are not. If they are compatible, this means that code from different programs can be merged.

We'll take a look at the most popular license types to help you make sense of them.

Copyleft

Copyleft is the French word for " gauche d'auteur", in reference to copyright. It's the authorization given by an author "to use, study, modify and distribute his or her work", whether it's a text, a work of art or a computer program.

This means that a user of a copylefted work can modify, reuse and distribute it, but is obliged to respect the same freedoms that apply to the original work. In other words, if a developer wishes to modify a program and share it again, the modified program will necessarily be copylefted.

It is not possible to distribute software with a copyleft license under a proprietary license.

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There are several levels of copyleft:

• strict copyleft : the software, whether modified or not, and its components may only be redistributed under the original license;
• standard or low copyleft: the software, whether modified or not, may only be redistributed under the original license, but new components may be added and redistributed under another license, or even under a proprietary license.

The best-known copyleft licenses are :

• MPL,
• DSL,
• ODbL,
• GFDL,
• GNU,
• OLD,
• SIL Open Font License,
• CC-BY-SA,
• CeCILL,
• free art license.

GPL or GNU GPL general public licenses

These are the most widely used free software licenses: more than half of all free software distributed today is licensed under the GNU GLP.

GPL (General Public License ) licenses are recommended if you want to make your code accessible and reusable by as many people as possible. They benefit from a strong copyleft.

Several versions are available: GPL 2, GPL 3, the latest version being the most complete, as it addresses the problem of tivoization ("the creation of a system that includes free software, but uses electronic hardware to prevent users from running modified versions on it" according to Wikipedia).

There are licenses derived from the GPL, also drafted by the Free Software Foundation:

• the LGPL, Lesser General Public License: this license is derived from the GNU GPL and is intended to be less restrictive with proprietary software. It is often applied to libraries.
• the AGPL license, or GPL Affero, derives from version 3 of the GNU GPL. It has one requirement in relation to the latter: "if you run a modified program on a server and let other users communicate with it, your server must also allow them to download the source code corresponding to the modified version in operation" (source: gnu.org).

The MIT license

This license takes its name from the Massachusetts Institute of Technology, MIT. The term is misleading, as MIT actually uses several licenses.

The MIT license is a very permissive license, with few restrictions on code reuse and no copyleft. It is compatible with a large number of licenses, including the GNU GPL.

It includes several license models, notably the Expat license and the X11 license.

The MIT license is similar in many respects to the new BSD license.

The BSD license

The BSD license is a permissive, copyleft-free open source license compatible with the GNU GPL.

It is recommended not to use the original version because of a restrictive advertising clause: copyright notice is mandatory in any advertisement or document supplied with the software. The new version of the BSD license no longer contains this clause.

Among the most popular BSD licenses is the Apache license. This is a permissive license, compatible with the GNU GPL v3, but not v2. It is recommended to use the Apache 2.0 license for large programs, in particular to avoid patent pitfalls.

Table of open source licenses

The most popular open source software

Here are a few examples of open source software by category:

• Framework: Angular (MIT),
• FTP client/file transfer: FileZilla (GNU GPL),
• CMS: Drupal (GNU),
• CRM: SuiteCRM (AGPL),
• Database management: mySQL (GNU),
• Text editor: Notepad2 (GNU),
• ERP: Odoo (GNU GPL),
• Forge: GitLab (MIT),
• Web browser: Mozilla Firefox (GNU GPL),
• Operating system: Red Hat Enterprise Linux,
• HTTP Server: Apache HTTP Server (Apache).

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Final tips: how to choose an open source license?

• Is copyleft a mandatory criterion?
• How reusable do you want your code to be?
• Do you need a license that allows community work with other developers?
• Do you need a permissive license, and if so, how permissive should it be?
• Do you need a large support community?