librfid is licensed under GNU General Public License, Version 2 as published by the Free Software Foundation (see the file COPYING). This means that as soon as you wish to distribute a program that links to librfid, that other program has to be licensed under the GPL or a compatible license, too. However, an alternative (royalty-based) licensing is possible. If you want to create a proprietary program based on librfid, contact Harald Welte for licensing details.