The DUNE library and headers are licensed under version 2 of the GNU General
Public License, with a special exception for linking and compiling against
DUNE, the so-called "runtime exception." The license is intended to be
similar to the GNU Lesser General Public License, which by itself isn't
suitable for a template library.

The exact wording of the exception reads as follows:

As a special exception, you may use the DUNE library without
restriction.  Specifically, if other files instantiate templates or
use macros or inline functions from one or more of the DUNE source
files, or you compile one or more of the DUNE source files and link
them with other files to produce an executable, this does not by
itself cause the resulting executable to be covered by the GNU
General Public License.  This exception does not however invalidate
any other reasons why the executable file might be covered by the
GNU General Public License.

This license clones the one of the libstdc++ library.

On Debian systems, the full text of the GNU General Public License version 2
can be found in the file  `/usr/share/common-licenses/GPL-2'.