Remote Authentication Dial-In User Service (RADIUS) is a client/server protocol and software that enables remote access servers to communicate with a central server to authenticate dial-in users and authorize their access to the requested system or service.
RADIUS allows a company to maintain user profiles in a centralized database that is usually shared by all connected remote servers. The Radius server provides enhanced security and enables organizations to set up a policy that can be applied at a single administered network point. The benefit of having centralized service is easier tracking of billing, usage and network statistics.
A RADIUS server implements RFC 2865 and RFC 2866 RADIUS authentication and accounting protocols, which are UDP-based protocols. During the RADIUS authentication phase a network client connects to a network access server (NAS) and provides authentication credentials.