 Content featured on the official campaign server must be licensed under the GNU GPL, like the game itself.Even when not counting this community content, the list of contributors to the official version of the game as displayed in-game contains over 550 unique entries (May 2010).Developers of the game also include well-known authors from the free software and open source scene, like the co-founder of the Open Source Initiative and core Linux kernel developer Eric S. Raymond, or Linux kernel programmer Rusty Russell.