Free and open source software is licensed under terms that allow the user to view, modify, and redistribute the source code. Proprietary software usually has license restrictions that prevent this, as is the default copyright in nearly all countries. Because of this, free software licenses are often called “copyleft”, since they have fewer restrictions.
One of the major benefits of open source is that you know exactly what you are getting. There is no hidden code that could do unspecified things to your server. Another benefit is that you can change it to be whatever you want. Google, for example, has their own version of Linux running on their servers.
For a company that has invested money in dedicated servers, another danger of proprietary software is vendor lock-in. If you have purchased software for a server and the vendor goes out of business or decides to no longer support it, you are just out of luck. Using free and open source software, you will always have the code for your software. Even if a business goes under, you can take that code to another one or use in-house programmers to maintain it.
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