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Open Source Licenses

Many people assume that open source means software free of all restrictions. Indeed, in many cases there is no charge for the software. However, almost all open source software is covered by a license that you must agree to when using the software, just as you do when using a commercial product. Generally this license is much less restrictive than a traditional closed source license; nonetheless, it does put limits on what you can do with the software. Without these limits, no programmer would feel safe releasing the results of his or her hard work into the public domain. When using open source software, make sure you are in accordance with the license. Also be sure that any modifications or changes you make also comply. This is the important part: If your company spends a lot of time customizing an open source program for its own use, you should be aware that you will have some responsibilities under the open source license.

There are two main types of open source licenses: the GNU General Public License and the BSD license. As long as you understand them thoroughly, you should be able to confidently use most open source software without fear of running afoul of any copyright issues. There are some unusual open source licenses coming out for things like artwork created in games and so forth. These "hybrid" licenses are a little murkier to deal with, and you should definitely be careful when using them, because you could be incurring charges or be in violation of their copyright without knowing it.

The goal of both major open source licenses is not so much to protect the existing software, but to control the uses of derivative code from that software. After all, it is usually free and the original developer shouldn't care if you make a million copies of it and distribute them to your friends. It's when you start making changes to the software and want to distribute it that you have to be careful. The two major open source licenses and their similarities and differences are described next.

The GNU General Public License

The GNU General Public License (GPL) is probably the more commonly used open source license. It is championed by the Free Software Foundation, which promotes the creation and proliferation of free software using this license. The actual GNU project works on certain specific software projects and puts their stamp of approval on them. These projects are usually core tools and libraries, such as the Gcc compiler and other major works. Anyone can use the GPL license for software as long as you use it verbatim and without changes or additions. Many developers use it because it has been vetted by a team of lawyers and has withstood the test of time. It is so common that if someone says that something is "GPL'd," generally people understand that to mean that it has been released open source under the GPL license.

The GPL is more complicated than the other major open source license, the BSD license. It has a few more restrictions on the use of the code by the licensee, which makes it more appropriate for companies that are making a commercial product. Generally, if you are licensing something under the GPL, it is understood that it is free software. A vendor, however, may charge for packaging, distribution, and support. This is the area that a lot of companies make money from what is supposedly a free package. Witness the retail packages of various flavors of Linux and commercial versions of the Apache Web servers and Sendmail communication package. However, if you download or load from a CD-ROM something that is covered under the GPL and didn't put a credit card number in somewhere, you can reasonably assume that you don't owe anyone any money for it.

The real beauty of the GPL from a developer's standpoint is that it allows the original author of the program to maintain the copyright and some rights while releasing it for free to the maximum number of people. It also allows for future development, without worry that the original developer could end up competing against a proprietary version of his or her own program.

In its basic form, the GPL allows you to use and distribute the program as much as you want with the following limitations.

  • If you distribute the work, you must include the original author's copyright and the GPL in its entirety. This is so that any future users of your distributions fully understand their rights and responsibilities under the GPL.

  • You must always make a version of the source code of the program available when you distribute it. You can also distribute binaries, but you must also make the source code easily available. This gets back to the goal of the open source concept. If all that is floating around is the binaries of a free program and you have to track down the original designer to get access to the source, the power of free software is greatly diminished. This ensures that every recipient of the software will have the full benefit of being able to see the source code.

  • If you make any changes to the program and release or distribute it, you must also make available the source code of those modifications in the same manner as the original code, that is, freely available and under the GPL. The key phrase here is "and release or distribute it." If you don't release it, then you are not obligated to release the source code. If you are making custom changes to the code for your company, they might be worried about giving out the results of your efforts. As long as you don't release it publicly or intend to sell it, it can remain proprietary.

    However, it usually makes good sense to go ahead and release the new code with the GPL. This not only generates lots of good will with the open source community, but it will also ensure that your changes are compatible with future versions of the program and are fully tested. You can use this logic to convince your company that they can get the experience and free labor of all the other programmers on the project by doing this. It will generally not hurt a company competitively to release this kind of code unless that program is part of the core business of the company, in which case open source software may not make sense anyway. And finally, it won't hurt your reputation and leverage with the other developers on the project and elsewhere in the software community.

Appendix A has the entire text of the GPL. You can get it in different text formats from

The BSD License

The BSD license is the open source license under which the original University of California at Berkley version of UNIX was released. After they won their lawsuit with AT&T over the original license, they released the software into the public domain with the permissive BSD license. The primary difference from the GPL is that the BSD license does not include the requirement of releasing future modifications under the same license. Based on this, several companies went on to release commercial versions of UNIX based on the BSD code base. BSDI is one such company. Some say that this goes against the idea of open source, when a company can take an improved version and charge for it, while others feel that it encourages innovation by giving a commercial incentive. Either way, it spawned a whole family of UNIX versions, including FreeBSD, NetBSD, and OpenBSD, from the free side of the house, and others such as BSDi on the commercial side. Appendix A has the full text of the BSD license. You can also access it at

Now that you understand the background of info-security and open source software, we are going to get into the specifics: installing, configuring, and using actual software packages. The following chapters review programs that can help you secure your network and information in a variety of ways. The chapters are loosely organized into different info-security subjects, and most of the most major areas of information security are covered. Also, many tools can have multiple uses. For example, even though Snort is covered in the chapter on intrusion detection systems, it can be used in forensic work too. And certainly if your interest is in a tool for particular area, you can skip right to that section.

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