SquidGuard is a URL redirector software, which can be used for content control of websites users can access. It is written as a plug-in for Squid and uses blacklists to define sites for which access is redirected. SquidGuard must be installed on a Unix or Linux computer such as a server computer. The software's filtering extends to all computers in an organization, including Windows and Macintosh computers. SquidGuard is free software licensed under the GNU General Public License (GPL) version 2. It is included in many Linux distributions including Debian, openSUSE and Ubuntu.