Well… I tried to read some articles about this, but it was too difficult for me to understand. The radius server has to be configured somehow everytime it gets a request for authorization to check (i guess at the nas client) if the user is already connected and get a result. If the result is negative (meaning that no one is connected with that user/pass), it should grant the access… if the result is positive (meaning that someone is already connected with that user/pass) it should deny access.
I was reading several articles at freeradius, tried to change something, but I ended messing up something and had to rebuild my zeroshell server once again in VMware. 🙁