RADIUS
维基百科上的介绍：
Remote Authentication Dial-In User Service (RADIUS) is a networking protocol 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.
简单的来说，RADUIS是一种网络协议，提供认证、授权以及计费(AAA)的集中式管理。
FreeRADIUS
FreeRADIUS is a modular, high performance free RADIUS suite developed and distributed under the GNU General Public License, version 2, and is free for download and use. The FreeRADIUS Suite includes a RADIUS server, a BSD-licensed RADIUS client library, a PAM library, an Apache module, and numerous additional RADIUS related utilities and development libraries.
In most cases, the word "FreeRADIUS" refers to the free open source RADIUS server from this suite.
FreeRADIUS is the most popular open source RADIUS server and the most widely deployed RADIUS server in the world. It supports all common authentication protocols, and the server comes with a PHP-based web user administration tool called dialupadmin. It is the basis for many commercial RADIUS products and services, such as embedded systems, RADIUS appliances that support Network Access Control, and WiMAX. It supplies the AAA needs of many Fortune-500 companies, telcos, and Tier 1 ISPs. It is also widely used in the academic community, including eduroam. The server is fast, feature-rich, modular, and scalable.
简单的来说，FreeRadius是一个开源免费的RADIUS协议的实现，服务器核心所包含的模块支持LDAP，MySQL，PostgreSQL，Oracle和许多其他数据库。