Install Apache radius module :
apt-get install libapache2-mod-auth-radius
enable radius module for Apache :
a2enmod auth_radius
open /etc/apache2/apache2.conf and add the following lines to end of file :
AddRadiusAuth IP_OF_RADIUS_SERVER:PORT SECRET 5 AddRadiusCookieValid 60
go to /var/www folder or the folder which you want to protect and create a .htaccess file inside it containing following lines :
AuthType Basic AuthName "AdminseHow Radius Authentication" AuthBasicAuthoritative Off AuthBasicProvider radius AuthRadiusAuthoritative on AuthRadiusActive On Require valid-user
restart Apache :
/etc/init.d/apache2 restart
for more info regarding the configuration options , you can read the following link :
http://freeradius.org/mod_auth_radius/