Question about NAS-Port attribute when using freeradiusclient

Hello All I am using freeradiusclient in combination with PPP in order to setup RADIUS authentication for PPTP users. Actually, I managed to authenticate users using RADIUS but I noticed that the NAS-Port attribute which is sent to RADIUS server is always 0. Is this normal? Is there any way to generate proper values for NAS-Port? Is this attribute set by radiusclient or is it extracted from somewhere else? (PPP, maybe?) Warm Regards Ali Majdzadeh Kohbanani