dar is a shell command that backs up directory trees and files, taking care of hard links, Extended Attributes, sparse files, MacOS's file forks, any inode type (including Solaris Door inodes), etc. It has been tested under Linux, Windows, Solaris, FreeBSD, NetBSD, MacOS X and several other systems, it is released under the GNU General Public License (GPL).
It relies on the libdar library and its APplication Interface (API), which is the core part of dar programs; as such, this library is released under the GPL along with dar. Consequently, to use the API, your program must be released under the GPL as well. Some external programs do rely directly on libdar or on the dar command-line tool to provide Graphical User Interfaces (GUI).