1-3RADIUS uses UDP port 1812 for authentication and 1813 for accounting. RADIUS defines the RADIUSpacket format and message transfer mechanism.RADIUS was originally designed for dial-in user access. With the diversification of access methods,RADIUS has been extended to support more access methods, for example, Ethernet access and ADSLaccess. It uses authentication and authorization in providing access services and uses accounting tocollect and record usage information of network resources.Client/Server Modelz Client: The RADIUS client runs on the NASs located throughout the network. It passes userinformation to designated RADIUS servers and acts on the responses (for example, rejects oraccepts user access requests).z Server: The RADIUS server runs on the computer or workstation at the network center andmaintains information related to user authentication and network service access. It listens toconnection requests, authenticates users, and returns the processing results (for example,rejecting or accepting the user access request) to the clients.In general, the RADIUS server maintains three databases, namely, Users, Clients, and Dictionary, asshown in Figure 1-2:Figure 1-2 RADIUS server componentsz Users: Stores user information such as the usernames, passwords, applied protocols, and IPaddresses.z Clients: Stores information about RADIUS clients, such as the shared keys and IP addresses.z Dictionary: Stores information about the meanings of RADIUS protocol attributes and their values.Security and Authentication MechanismsInformation exchanged between a RADIUS client and the RADIUS server is authenticated with ashared key, which is never transmitted over the network. This enhances the information exchangesecurity. In addition, to prevent user passwords from being intercepted in non-secure networks,RADIUS encrypts passwords before transmitting them.A RADIUS server supports multiple user authentication methods. Moreover, a RADIUS server can actas the client of another AAA server to provide authentication proxy services.Basic Message Exchange Process of RADIUSFigure 1-3 illustrates the interaction of the host, the RADIUS client, and the RADIUS server.
