Understanding Free Software Licenses

Free and open source software powers many of the world’s servers, and numerous high-traffic sites such as YouTube, Wikipedia, and Facebook use free software to keep their sites performing at extraordinary levels.  Some of the most common free software applications for servers are: variants of the GNU/Linux operating system, Apache HTTP Server, MySQL database server, and descendants of the BSD operating system.

Not all free software licenses are the same, so it is important to know what you can and cannot do with each type.

GNU General Public License – One of the most popular licenses, the GPL is the license of choice for Linux and many Linux distributions.  It is a copyleft license, allowing you to modify and redistribute the source code so long as it remains under the same license.

BSD-Style License – This style of license is considered “permissive” because it allows the user to adapt the software into proprietary products and even release derivatives under proprietary licenses.  Therefore, it is not considered a copyleft license.

GNU Lesser General Public License – In order to accommodate those who need some functionality between free software and proprietary software, the LGPL is like a compromise that permits linking of libraries with proprietary components.  The software licensed under LGPL is still copyleft, but others linked to it do not need to be.

MIT License – Another permissive license, the MIT allows reuse with proprietary software, but that software must be distributed with the license.  It is not copyleft.

Apache License – Famous for its use in Apache web server, it is a non-copyleft license that requires a copyright notice but also allows proprietary software to be linked with it.

There is a perception among some that free software licenses are like Public Domain where the developer forfeits the rights to his/her creation, but that is actually not the case.  In nearly all such licenses, copyright is maintained, and violating the terms of the license can still be considered copyright infringement.