RADIUS (Remote Authentication Dial-In User Service)

What is RADIUS (Remote Authentication Dial-In User Service)?

RADIUS (Remote Authentication Dial-In User Service) 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 enables a company to maintain user profiles in a central database that all remote servers can share. Having a central database provides better security, enabling a company to set up a policy that can be applied at a single administered network point. A central database also makes it easier to track usage for billing for the network access or internet service provider and for keeping network statistics. Created by former networking vendor Livingston Enterprises in 1991, RADIUS is a de facto industry standard used by leading networking product companies. The Internet Engineering Task Force adopted the RADIUS protocol as a draft standard in 2000, as documented in RFC 2865.

RADIUS was originally designed to support large numbers of users connecting remotely to internet service providers (ISPs) or corporate networks via modem pools or other point-to-point serial line links. RADIUS is now commonly used for remote access across different types of networks, including wireless networks, Ethernet networks and other types of remote user access through the internet.

How does RADIUS authentication work?

In the RADIUS protocol, remote network users connect to their networks through a network access server (NAS). The NAS queries the authentication server to get authentication, authorization and configuration information about the remote user.

Unlike other client-server applications, where the client is often an individual user, RADIUS clients are the NAS systems used to access a network and the authentication server is the RADIUS server.

RADIUS server and client architecture
In the RADIUS protocol, access servers act asclients of the RADIUS server.

The RADIUS protocol provides centralized authentication services to the servers through which remote users connect to the network. Types of remote user access authentication servers can include:

  • Dial-in servers, which mediate access to corporate or ISP networks through modem pools.
  • Virtual private network servers, which accept requests from remote users to set up secure connections to a private network.
  • Wireless access points, which accept requests from wireless clients to connect to a network.
  • Managed network access switches that implement the 802.1x authenticated access protocol to mediate access to networks by remote users.
RADIUS client server authentication
End users interact only indirectly, through a network access server, with the RADIUS server when authenticating with a remote network.

When an end user opens a connection with a remote network, the NAS initiates a RADIUS exchange with the authentication server.

When a remote user initiates a connection through a NAS, the request can include the remote user ID, password and IP address. The NAS then sends a request for authentication to the RADIUS server.

How are RADIUS servers used?

RADIUS authenticates using two approaches:

  • Password Authentication Protocol (PAP). The RADIUS client forwards the remote user's user ID and password to the RADIUS authentication server. If the credentials are correct, the server authenticates the user and the RADIUS client enables the remote user to connect to the network.
  • Challenge Handshake Authentication Protocol (CHAP). Also known as a three-way handshake, CHAP authentication relies on the client and server using an encrypted shared secret. Compared to PAP, CHAP authentication is considered more secure because it encrypts authentication exchanges and it can be configured to do repeated mid-session authentications.

A RADIUS proxy client can be configured to forward RADIUS authentication requests to other RADIUS servers. RADIUS proxies enable centralized authentication in large or geographically dispersed networks.

The RADIUS protocol is a mature authentication protocol that is incorporated in many networking products and is integrated with directory service software for authorization and accounting. For example, RADIUS is implemented in Microsoft's Network Policy Server, which integrates with Microsoft Active Directory.

Find out more about how RADIUS is incorporated into key network authentication protocols, such as the Extensible Authentication Protocol used for protecting wireless networks. And learn how multifactor authentication can be used to enhance RADIUS security.

This was last updated in September 2021

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