The radvd license is based off of the BSD with advertising license. Clause 0, if invoked, would make the resulting work non-free. The advertising clause makes it GPL-incompatible.
There is a variant of this license, called "Inner Net License", which is identical, except that Clause 4 (the advertising clause) has been waived.
A copy of this license was taken from https://github.com/reubenhwk/radvd/blob/master/COPYRIGHT on 2018-07-03.
The author(s) grant permission for redistribution and use in source and binary forms, with or without modification, of the software and documentation provided that the following conditions are met: 0. If you receive a version of the software that is specifically labelled as not being for redistribution (check the version message and/or README), you are not permitted to redistribute that version of the software in any way or form. 1. All terms of all other applicable copyrights and licenses must be followed. 2. Redistributions of source code must retain the authors' copyright notice(s), this list of conditions, and the following disclaimer. 3. Redistributions in binary form must reproduce the authors' copyright notice(s), this list of conditions, and the following disclaimer in the documentation and/or other materials provided with the distribution. 4. All advertising materials mentioning features or use of this software must display the following acknowledgement with the name(s) of the authors as specified in the copyright notice(s) substituted where indicated: This product includes software developed by the authors which are mentioned at the start of the source files and other contributors. 5. Neither the name(s) of the author(s) nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY ITS AUTHORS AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.