Admins eHow

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/