Freeradius help

Hi all, I have installed a freeradius server and it works perfectly... Now, I want to play with the Session-Timeout attribute. I want to set that value according to the NAS in which the client is connected. For instance, let us assume a user connect himself to a "Group1 NAS" and have X seconds session-timeout. But if he would be connected to a "Group2 NAS", he will have X-10 seconds session-timeout. Finally, what I want is: 1) to run a script that set a session-timeout value according to the NAS IP address. 2) to run a script when the user log out (when the session-timeout attribute is updated) But I don't know how to do so...I thought about launching scripts but if you have a better solution, i'll take it heartily. Thanks a lot.