The Remote Authentication Dial-In User Service (RADIUS) is a client/server protocol used for centralized authentication, authorization, and accounting. The RADIUS client is typically a network access server (NAS), and the RADIUS server is usually a process running on a UNIX or Windows NT machine. The client passes user information to designated RADIUS servers and acts on the response that is returned. RADIUS servers receive user connection requests, authenticate the user, and then return the configuration information necessary for the client to deliver service to the user. A RADIUS server can act as a proxy client to other RADIUS servers or other kinds of authentication servers.