FreeRADIUS Beginner's Guide

4.2 (6 reviews total)
By Dirk van der Walt
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  1. 1. Introduction to AAA and RADIUS

About this book

The Open Source pioneers have proved during the past few decades that their code and projects can indeed be more solid and popular than commercial alternatives. With data networks always expanding in size and complexity FreeRADIUS is at the forefront of controlling access to and tracking network usage. Although many vendors have tried to produce better products, FreeRADIUS has proved over time why it is the champion RADIUS server. This book will reveal everything you need to know to get started with using FreeRADIUS.

FreeRADIUS has always been a back-room boy. It's not easy to measure the size or number of deployments world-wide but all indications show that it can outnumber any commercial alternatives available. This essential server is part of ISPs, universities, and many corporate networks, helping to control access and measure usage. It is a solid, flexible, and powerful piece of software, but can be a mystery to a newcomer.

FreeRADIUS Beginner's Guide is a friend of newcomers to RADIUS and FreeRADIUS. It covers the most popular Linux distributions of today, CentOS, SUSE, and Ubuntu, and discusses all the important aspects of FreeRADIUS deployment: Installing, configuring and testing; security concerns and limitations; LDAP and Active Directory integration.

It contains plenty of practical exercises that will help you with everything from installation to the more advanced configurations like LDAP and Active Directory integration. It will help you understand authentication, authorization and accounting in FreeRADIUS. It uses many practical step-by-step examples, which are discussed in detail to lead you to a thorough understanding of the FreeRADIUS server as well as the RADIUS protocol. A quiz at the end of each chapter validates your understanding.

Not only can FreeRADIUS be used to monitor and limit the network usage of individual users; but large deployments are possible with realms and fail-over functionality. FreeRADIUS can work alone or be part of a chain where the server is a proxy for other institution's users forwarding requests to their servers. FreeRADIUS features one of the most versatile and comprehensive Extensible Authentication Protocol (EAP) implementations. EAP is an essential requirement to implement enterprise WiFi security. FreeRADIUS Beginner's Guide covers all of these aspects.

Publication date:
September 2011

Authentication, Authorization, and Accounting

Users gain access to data networks and network resources through various devices. This happens through a wide range of hardware. Ethernet switches

, Wi-Fi access points, and VPN servers all offer network access.

When these devices are used to control access to a network, for example a Wi-Fi access point with WPA2 Enterprise security implemented or an Ethernet switch with 802.1x (EAP) port-based authentication enabled, they are referred to as a Network Access Server (NAS).

All these devices need to exercise some form of control to ensure proper security and usage. This requirement is commonly described as Authentication, Authorization, and Accounting (AAA). AAA is also sometimes referred to as the Triple A Framework. AAA is a high-level architecture model, which can be used for specific implementations.

AAA is specified through various RFCs. Generic AAA Architecture is specified in RFC 2903. There are also RFCs that cover different AAA aspects.


RADIUS is an acronym for Remote Access Dial In User Service. RADIUS was part of an AAA solution delivered by Livingston Enterprises to Merit Network in 1991. Merit Network is a non-profit Internet provider, which required a creative way to manage dial-in access to various Points-Of-Presence (POPs) across it's network.

The solution supplied by Livingston Enterprises had a central user store used for authentication. This could be used by numerous RAS (dial-in) servers. Authorization and accounting could also be done whereby AAA was satisfied. Another key aspect of the Livingston solution included proxying to allow scaling.

The RADIUS protocol was then subsequently published in 1997 as RFCs, some changes applied, and today we have RFC2865, which covers the RADIUS protocol, and RFC2866, which covers RADIUS accounting. There are also additional RFCs which cover enhancements on certain RADIUS aspects. Having RFCs to work from allows any person or vendor to implement the RADIUS protocol on their equipment or software. This resulted in widespread adoption of the RADIUS protocol to handle AAA on TCP/IP networks. You will find the word RADIUS is used loosely to either mean the RADIUS protocol or the entire RADIUS client/server system. The meaning should be clear from the context in which it is used.

Supporting the RADIUS protocol and standards became the de facto requirement for NAS vendors. RADIUS is used in a wide variety of places, from cellular network providers having millions of users to a small WISP start-up providing the local neighborhood with Internet connectivity to enterprise networks that implement Network Access Control (NAC) using 802.1x to ring fence their network. RADIUS is found in all these places and more!

ISPs and network administrators should be familiar with RADIUS since it is used by various devices that control access to TCP/IP networks. Here are a couple of examples:

The next section will summarize the RADIUS protocol as specified in RFC2865.

This section explores the RADIUS protocol on a technical level as published in RFC2865. RADIUS accounting is excluded. This is published as RFC2866 and explored in its own section.

The RADIUS protocol is a client/server protocol, which makes use of UDP to communicate. Using UDP instead of TCP indicates that communication is not strict on state. A typical flow of data between the client and server consists of a single request from the client followed by a single reply from the server. This makes RADIUS a very lightweight protocol and helps with its efficiency across slow network links.

Before successful communication between the client and server can be established, each has to define a shared secret. This is used to authenticate clients.

RADIUS protocol (RFC2865)

RADIUS packets have a specified format defined in the RFC. Two key components inside a RADIUS packet are:

Let's investigate the composition of a RADIUS datagram.

Knowing the format of a RADIUS packet will greatly assist in understanding the RADIUS protocol. Let us look more closely at the RADIUS packet. We will look at a simple authentication request. A client sends an Access-Request packet to the server. The server answers with an Access-Accept packet to indicate success.

The RADIUS packets shown here are only the payload of a UDP packet. A discussion of the UDP and IP protocols is beyond the scope of this book.

The following screenshot shows the Access-Request packet send from the RADIUS client:

The data packet

The following screenshot shows the RADIUS server responding to this request with an Access-Accept packet:

The data packet

Let's discuss the packets.

AVPs are the workhorse of the RADIUS protocol. AVPs can be categorized as either check or reply attributes. Check attributes are sent from the client to the server. Reply attributes are sent from the server to the client.

Attributes serve as carriers of information between the client and server. They are used by the client to supply information about itself as well as the user connecting through it. They are also used when the server responds to the client. The client can then use this response to control the user's connection based on the AVPs received in the server's response.

The following sections will describe the format of an AVP.

The first octet of the AVP is the type field. The numeric value of this octet is associated with an attribute name so that we humans can also understand. Assignment of these attribute names to numbers is controlled by IANA ( The attribute names are usually descriptive enough to deduce their function, for example User-Name(1), User-Password(2), or NAS-IP-Address(4).

RADIUS also allows extending the protocol; attribute Type 26 (called Vendor-Specific) allows for this. The value of the Vendor-Specific attribute can in turn contain Vendor Specific Attributes (VSAs) which are managed by a vendor.

This section explores the accounting functionality of the RADIUS protocol. Accounting is a means of tracking usage of resources and typically used for billing.

After the initial RFCs defining RADIUS in general and RADIUS accounting, various extensions were proposed to expand RADIUS usage or improve some weaknesses.

There is also an improved RADIUS protocol called Diameter (A word play—twice as good as RADIUS). The uptake of Diameter has been very slow though, and RADIUS still remains the de facto standard for the foreseeable future. A major reason for this is probably the fact that the many enhancements that Diameter was supposed to bring are already covered by the various RADIUS extensions. There is, for instance, the RadSec protocol that transports RADIUS over TCP and TLS. This makes RADIUS scale better in roaming environments.

Although there are more, we will only look at two important extensions likely to be used.


FreeRADIUS is an open source project supplying a very feature-rich implementation of the RADIUS protocol with its various enhancements ( When people refer to FreeRADIUS, they usually talk about the server software. This is the main component of the software suite included in a FreeRADIUS download.

FreeRADIUS has many strengths, which contributed to its popularity. Let us look at some of them:


This chapter is the introduction and foundation on which we will build that. As a rehash on important points discussed, be sure to know the following facts:


Stands for

Short description


Authentication, Authorization, and Accounting

The three components required for proper control of access and usage.


Network Access Server

A device controlling access to the network for example, a VPN server. Acts as the RADIUS client.


Attribute Value Pair

A three-field component inside a RADIUS packet used to contain a specified field and its data.


Vendor-Specific Attributes

An extension of the AVP managed by a specific vendor.

This chapter was a FreeRADIUS starter. The main course begins with the next chapter where we'll be installing FreeRADIUS and starting to use it.

About the Author

  • Dirk van der Walt

    Dirk van der Walt is an Open Source Software Specialist from Pretoria, South Africa. He is a firm believer in the potential of Open Source software. Being a Linux user for almost 10 years it was love at first boot. From then on Dirk has spent his available time sharing his knowledge with others equally passionate about the freedom and affordability Open Source software gives to the community. In 2003 Dirk started coding with Perl as his language of choice and gave his full attention to functional and aesthetic user interface design. He also compiled an on-line Gtk2-Perl study guide to promote the advancement of Perl on the desktop. As Rich Internet Applications (RIA) became more popular, Dirk added the Dojo toolkit and CakePHP to his skills-set to create an AJAX-style front-end to a FreeRADIUS MySQL database. His latest work is YFi Hotspot Manager. Today YFi Hotspot Manager is used in many localities around the globe. With many contributors to the project it proves just how well the Open Source software model can work.

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Latest Reviews

(6 reviews total)
Packt books are always great value
Have read most of this book and found the content to be very good.
The book has good examples but it is for older release. I got my setup working by using the materials and a little Google searching. The LDAP schemas for Ubuntu 16/18 do not match the book information. The concepts were helpful to know what to Google.
FreeRADIUS Beginner's Guide
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