RADIUS stands for Remote Authentication Dial-In User Service. It is a networking protocol that provides centralized authentication, authorization, and accounting (AAA) management for users who connect and access network resources, such as VPNs, wireless networks, and dial-up connections.

RADIUS operates in a client-server model, where the client (often a network access server or NAS) sends authentication requests to a RADIUS server for verification. The RADIUS server then authenticates the user’s credentials and determines whether they are authorized to access the requested network resources. If the authentication is successful, the RADIUS server sends an acceptance message to the client, allowing the user to connect. If the authentication fails, a rejection message is sent, denying access.

Here are the key components and functions of RADIUS:

1. RADIUS Client: The RADIUS client is typically a network access server (NAS) or a device that controls access to network resources. It forwards authentication requests from users to the RADIUS server for verification.

2. RADIUS Server: The RADIUS server is responsible for authenticating and authorizing users. It stores user credentials, such as usernames and passwords, or connects to external authentication sources, such as Active Directory or LDAP, to validate user credentials. The RADIUS server also maintains user profiles and access policies.

3. Authentication: When a user attempts to access network resources, the RADIUS client sends an authentication request to the RADIUS server. The server verifies the user’s credentials, such as username and password, using its authentication database or external authentication sources.

4. Authorization: After successful authentication, the RADIUS server determines the user’s access privileges based on predefined policies and attributes. It specifies the level of access the user is granted, such as the type of network resources they can access or the duration of the session.

5. Accounting: RADIUS also provides accounting functionality, which involves tracking and logging user activities for auditing and billing purposes. The RADIUS server records information such as the user’s session duration, data usage, and network resources accessed.

6. Integration with other systems: RADIUS can integrate with various systems, such as VPN servers, wireless access points, and network switches, to provide a centralized authentication and authorization mechanism across the network infrastructure.

RADIUS is widely used in enterprise networks, internet service providers (ISPs), and other organizations that require secure and centralized user authentication and authorization. It helps ensure that only authorized users can access network resources and provides a standardized method for managing user access across different network devices and technologies.