Introduction to Radius

The Remote Authentication Dial-In User Service (RADIUS) is a client/server security protocol created by Livingston Enterprises, Inc . RADIUS is an Internet draft standard protocol.
See RFCs 2865 and 2866
User profiles are stored in a central location, known as the RADIUS server. RADIUS clients (Network Access Server ) communicate with the RADIUS server to authenticate users. The server specifies back to the client what the authenticated user is authorized to do. RADIUS server is usually a daemon process running on a UNIX or Windows NT machine. A RADIUS server can act as a proxy client to other RADIUS servers or other kinds of authentication servers. Although the term RADIUS  refers to the network protocol that the client and server use to communicate, it is often used to refer to the entire client/server system.
Communication between a network access server (NAS) and a RADIUS server is based on the User Datagram Protocol (UDP). Generally, the RADIUS protocol is considered a connectionless service. Issues related to server availability, retransmission, and timeouts are handled by the RADIUS-enabled devices rather than the transmission protocol.
The RADIUS server uses the User Datagram Protocol (UDP) and the following UDP ports:
  If different ports are assigned to these services in the /etc/services  file, RADIUS uses those ports in preference to the default ports listed above. You can also specify different UDP ports by using the radiusd -p  portnumber  command on UNIX hosts. Port 1812 is reserved for RADIUS authentication and port 1813 is reserved for RADIUS accounting

How it works

This figure shows the interaction between a dial-in user and the RADIUS client and server.


1- User initiates PPP authentication to the NAS.
2- NAS prompts for username and password (if PAP) or challenge (if CHAP).
3- User replies.
4-  RADIUS client sends username and encrypted password to the RADIUS server.
5- RADIUS server responds with Accept, Reject, or Challenge.
The RADIUS client acts upon services and services parameters bundled with Accept or Reject.

Authentication and Authorization

The RADIUS server can support a variety of methods to authenticate a user. When it is provided with the username and original password given by the user, it can support PPP, PAP or CHAP, UNIX login, and other authentication mechanisms. Typically, a user login consists of a query (Access-Request) from the NAS to the RADIUS server and a corresponding response (Access-Accept or Access-Reject) from the server. The Access-Request packet contains the username, encrypted password, NAS IP address, and port.  The format of the request also provides information about the type of session that the user wants to initiate. For example, if the query is presented in character mode, the inference is "Service-Type = Exec-User," but if the request is presented in PPP packet mode, the inference is "Service Type = Framed User" and "Framed Type = PPP."

When the RADIUS server receives the Access-Request from the NAS, it searches a database for the username listed. If the username does not exist in the database, either a default profile is loaded or the RADIUS server immediately sends an Access-Reject message. This Access-Reject message can be accompanied by a text message indicating the reason for the refusal.

In RADIUS, authentication and authorization are coupled together. If the username is found and the password is correct, the RADIUS server returns an Access-Accept response, including a list of attribute-value pairs that describe the parameters to be used for this session. Typical parameters include service type (shell or framed), protocol type, IP address to assign the user (static or dynamic), access list to apply, or a static route to install in the NAS routing table. The configuration information in the RADIUS server defines what will be installed on the NAS. The figure below illustrates the RADIUS authentication and authorization sequence.


The accounting features of the RADIUS protocol can be used independently of RADIUS authentication or authorization. The RADIUS accounting functions allow data to be sent at the start and end of sessions, indicating the amount of resources (such as time, packets, bytes, and so on) used during the session. An Internet service provider (ISP) might use RADIUS access control and accounting software to meet special security and billing needs.

Transactions between the client and RADIUS server are authenticated through the use of a shared secret, which is never sent over the network. In addition, user passwords are sent encrypted between the client and RADIUS server to eliminate the possibility that someone snooping on an insecure network could determine a user's password.


RADIUS offers the following features:

  In large networks, security information can be scattered throughout the network on different devices. RADIUS allows user information to be stored on one host, minimizing the risk of security loopholes. All authentication and access to network services is managed by the host functioning as the RADIUS server.

Using modifiable "stubs," RADIUS can be adapted to work with existing security systems and protocols. You adapt the RADIUS server to your network, rather than adjusting your network to work with RADIUS.

Simplified management
The RADIUS server stores security information in text files at a central location; you add new users to the database or modify existing user information by editing these text files.
Extensive auditing capabilities
RADIUS provides extensive accounting trail capabilities, referred to as RADIUS accounting . Information collected in a log file can be analyzed for security purposes or used for billing.


  RADIUS 2.1  provides the following enhancements to improve RADIUS functionality:


Proxy RADIUS enables your RADIUS server to forward authentication requests from a network access server (NAS) to a remote RADIUS server and to pass the reply back to the NAS. This feature enables cooperating Internet service providers (ISPs) to handle dial-in service requests from each other's users. Corporate users can easily forward packets from local to remote networks.

RADIUS now supports ActivCard authentication on the following platforms supported by ActivCard 2.1: AIX, HP-UX, Solaris, and Sun-OS. ActivCard authenticates users by means of dynamic passwords generated by a handheld token using the public Digital Encryption Standard (DES) algorithm. The RADIUS server can forward all requests specified by the user profiles to the ActivCard server.

Virtual ports

Improved messages Enhanced debugging

  You can turn on RADIUS debugging by sending a SIGUSR1 signal to radiusd . Sending a SIGUSR2 signal to radiusd  turns debugging off. The RADIUS server logs a short summary message of radiusd  activity when either signal is sent and when radiusd  is exited.


       RADIUS Installation and Configuration

Current Users of RADIUS

Any company with a centralized MIS department managing a large corporate network is concerned with security issues.
1- RADIUS is being used to secure several university networks that provide dial-in IP connectivity to students and faculty.
2- Several Internet service providers use RADIUS to provide security to users accessing their networks from multiple POPs (Points Of Presence). UNIX security systems are typically used in these environments.

3- Radius is now used to authenticate users for services ftp, pop etc....