Attributes location

Dear all, Could someone tell me where is the location of the Radius attributes in the freeradius? The attributes are used, when the NAS connect with the server. But after seeing the file that come in freeradius, what I can only see are modules and dictionary. The attributes are listed in the dictionary, but where is the actual location of the source code? I use Linux as my NAS. I want to extend some attributes. Really apreciate your guidance since I don't know where to search again. Many thanks Santy --