Click is distributed under an MIT/BSD-like license called "the Click license". This license is an MIT license, plus a clause (taken from the W3C license) requiring prior written permission to use our names in publicity. It is posted below.
Portions of the Click software are distributed under different licenses. In particular, portions are derived from the Linux kernel, and are thus distributed under the GNU General Public License, version 2. The GPL applies to the source files in drivers/e1000*, to the files etc/linux-*-patch, linuxmodule/clickfs.cc, linuxmodule/proclikefs.c, linuxmodule/sched.cc, and linuxmodule/skbmgr.cc, and possibly to some of the files in elements/linuxmodule.
Typically, element code that uses only Click's interfaces will not be derived from the Linux kernel, so the main Click license will apply, not the GPL.
(c) 1999-2007 Massachusetts Institute of Technology
(c) 2000-2007 Mazu Networks, Inc.
(c) 2001-2007 International Computer Science Institute
(c) 2004-2007 Regents of the University of California
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
The name and trademarks of copyright holders may NOT be used in advertising or publicity pertaining to the Software without specific, written prior permission. Title to copyright in this Software and any associated documentation will at all times remain with copyright holders.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.