Remote Authentication Dial-In User Service (RADIUS) is a networking protocol, operating on port 1812 that provides centralized Authentication, Authorization, and Accounting (AAA or Triple A) management for users who connect and use a network service. RADIUS was developed by Livingston Enterprises, Inc. in 1991 as an access server authentication and accounting protocol and later brought into the Internet Engineering Task Force (IETF) standards.
Because of the broad support and the ubiquitous nature of the RADIUS protocol, it is often used by Internet service providers (ISPs) and enterprises to manage access to the Internet or internal networks, wireless networks, and integrated e-mail services. These networks may incorporate modems, digital subscriber line (DSL), access points, virtual private networks (VPNs), network ports, web servers, etc.
RADIUS is a client/server protocol that runs in the application layer, and can use either TCP or UDP as transport. Network access servers, the gateways that control access to a network, usually contain a RADIUS client component that communicates with the RADIUS server. RADIUS is often the back-end of choice for 802.1X authentication as well.
The RADIUS server is usually a background process running on a UNIX or Microsoft Windows server.