While Nmap is free, the license allows for certain proprietary redistributions under specific conditions, though this is rare and usually requires direct negotiation with the Nmap copyright holder (Gordon Lyon/Fyodor). For example, embedding Nmap into a proprietary hardware appliance might technically violate the license terms if the appliance's software is closed-source. Vendors usually work around this by isolating Nmap as a separate, user-accessible GPL component.
Originally, Nmap used a slightly modified version of the GPLv2. While this served the community well, the project's maintainers found that the "derivative works" clause of the GPL was often misinterpreted or exploited by companies incorporating Nmap into proprietary products without contributing back or purchasing a commercial license. nmap license
Fyodor faced a dilemma. On one hand, he wanted to ensure that Nmap remained free and open-source, available to anyone who needed it. On the other hand, he also wanted to allow companies to use Nmap in their commercial products, as long as they complied with the license terms. While Nmap is free, the license allows for