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Free software is software which grants recipients the freedom to modify and redistribute the software. This would normally be prohibited by copyright law, so with free software, the copyright holder must give recipients the explicit permission to do these things. This grant of rights is called a license, and if the above noted freedoms are included in the grant, the license is a free software license.
Put another way, a free software license is a license which grants permissions to the recipient to remove any ownership issues which would otherwise prevent the software from being free software.
FSF-approved free software licenses
Free Software Foundation, the group that maintains The Free Software Definition, maintains a list of free software licenses.[http://www.fsf.org/licensing/licenses/index_html] The list distinguishes between free software licenses that are compatible or incompatible with the FSF license of choice, the GNU General Public License, which is a copyleft license. The list also contains licenses which the FSF considers non-free for various reasons. Note that the open source license list differs slightly, but in almost all cases the definitions apply to the same licenses.
* Academic Free License, version 1.1, 2.1
* Affero General Public License
* Apache License, Version 1.0, 1.1, 2.0
* Apple Public Source License, version 2
* Arphic Public License
* Artistic License 2.0
* Berkeley Database License (aka the Sleepycat Software Product License)
* Boost Software License
* CeCILL version 2
* Clarified Artistic License
* Common Development and Distribution License
* Common Public License, version 1.0
* Cryptix General License
* EU DataGrid Software License
* Eclipse Public License, version 1.0
* eCos license version 2.0
* Eiffel Forum License, version 2
* Expat License
* GNU General Public License
* GNU Lesser General Public License
* IBM Public License, Version 1.0
* Intel Open Source License (as published by OSI)
* Interbase Public License, Version 1.0
* Jabber Open Source License, version 1.0
* LaTeX Project Public License 1.2, 1.3a
* License of the run-time units of the GNU Ada compiler
* License of Guile
* License of Netscape Javascript
* License of Vim, Version 6.1 or later
* License of ZLib (aka "libpng license")
* License of the iMatix Standard Function Library
* License of xinetd
* Lucent Public License Version, also known ask the Plan 9 License, version 1.02
* MIT License
* Modified BSD license, first used by FreeBSD and OpenBSD
* Mozilla Public License
* Netizen Open Source License, Version 1.0
* Netscape Public License
* Nokia Open Source License
* Old OpenLDAP License, Version 2.3
* Open Software License, version 1.0
* OpenLDAP License, Version 2.7
* OpenSSL license
* Original BSD license, still used by NetBSD
* PHP License, version 3.0 only
* Phorum License, Version 2.0
* Public Domain
* Python Software Foundation License
* Q Public License, version 1.0
* Standard ML of New Jersey Copyright License
* Sun Industry Standards Source License, version 1.0
* Sun Public License
* The Condor Public License
* The license of Ruby[http://www.ruby-lang.org/en/LICENSE.txt]
* Vita Nuova Liberal Source License
* W3C Software Notice and License
* X11 License
* XFree86 1.1 License
* Zend License, version 2.0
* Zope Public License version 2.0
* Zope Public License, version 1 only
Freedom preserving restrictions
To preserve the freedoms to use, study, modify, and redistribute the software, most free software licences include restrictions and requirements which apply to distributors. There is constant debate within the free software community about where to draw the line between restrictions that preserve freedom and restrictions that reduce freedom.
Copyleft
Since the mid 80s, free software licences written by Richard Stallman pioneered a concept known as copyleft. Copyleft provisions say that modified versions of the software must be distributed under the same terms under which the unmodified software was received. Thus, additions and improvements to copylefted free software must also be distributed as free software. This is sometimes referred to as "share and share alike", or "quid pro quo".
Patent retaliation
Most newly written free software licences from the late 90s onward include some form of patent retaliation. This means that a persons' rights under the licence (such as to redistribute) are terminated under certain conditions. For example, the Apple Public Source Licence terminates the users' rights if the user sues Apple for patent litigation.
DRM and Tivoisation
As of late 2006, no free software licenses contain explicit wordings to prohibit additional restrictions being enforced by Digital Restrictions Management (DRM). Current discussion drafts of version 3 of the GNU General Public License include wordings to prevent "Tivoization".
Attribution, disclaimers, notices
Most free software licenses require that when a modified version of the software is distributed, it does not claim to be an unmodified version. Some licences also require that the copyright holders be credited.
An example is that version 2 of the GNU GPL requires that if an interactive program prints warranty or license information, modified versions cannot be distributed if they remove those notices.
Unacceptable restrictions
While protecting the four freedoms allows adding certain restrictions, restrictions on private use of the software ("use restrictions") are generally unacceptable. Examples include prohibiting the software to be used for military purposes, for comparison or benchmarking, or in commercial organisations.
BSD philosophy
Many users and developers of BSD-based operating systems have a different position on licensing. The main difference is the belief that the copyleft licenses, particularly the GNU General Public License (GPL), are too complicated and have restrictions which are undesirable. The GPL requires derivative work to be released according to the GPL while the BSD licence does not. Essentially, the BSD license's only requirement is to acknowledge the original authors, and poses no restrictions on how the source code may be used. As a result, BSD code can find its way into proprietary software that only acknowledge the source. For instance, the IP stack in Microsoft Windows and Mac OS X are derived from BSD-licensed software.
GPL supporters claim that mandating derivative works remain free fosters the growth of free software and requires equal participation by all users. Developers who use GPL code have to share their improvements with the community, supporting the growth of the software they received.
Supporters of the BSD license argue that it is more free than the GPL because it grants the right to do anything with the source code, second only to software in the public domain. The nature of BSD has encouraged the inclusion of well-developed standard code into proprietary software. In response, GPL supporters claim that this is more a form of power than a freedom.[http://www.gnu.org/philosophy/freedom-or-power.html] The right to make closed-source code is therefore not included in the Free Software Foundation's "four freedoms of free software"; using, studying, copying, and distributing modifications of the code.
Code licensed under the BSD license can be relicensed under the GPL (is "GPL-compatible") without securing the consent of all original authors. Code under the GPL cannot be relicensed under the BSD license without securing the consent of all copyright holders, as the BSD license is not copyleft and therefore GPL is "BSD-incompatible". Existing free software BSDs tend to avoid including software licensed under the GPL in the core operating system, or the base system, except as a last resort when alternatives are non-existent or vastly less capable, such as with GCC. The OpenBSD project has acted to remove GPL-licensed tools in favour of BSD-licensed alternatives, some newly written and some adapted from older code.
Debian
The Debian project uses the criteria laid out in its Debian Free Software Guidelines (DFSG). The only notable cases where Debian and Free Software Foundation disagree are over the Artistic License and the GNU Free Documentation License. Debian accept the original Artistic License as being a free software license, but FSF disagree. This has very little impact however since the Artistic License is almost always used in a dual-license setup, along with the GNU General Public License.
Regarding the GNU Free Documentation License, Debian argues that the DFSG applies to documentation as it does on software, and so documentation licenses must be examined against these free software guidelines. FSF say that documentation is qualitatively different from software and is subject to different requirements. The end result of a long discussion and the eventual vote in Debian[http://www.debian.org/vote/2006/vote_001] is that the works licensed under the GFDL are considered free as long as they do not contain unmodifiable sections (Invariant Sections.
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