The simplest way to extend the functionality of Radius is to use filters. A filter is an external program that communicates with Radius via its standard input and output channels.
11.1.1 Getting Acquainted with Filters 11.1.2 Declaring the Filter 11.1.3 Invoking the Filter from a User Profile 11.1.4 Adding Reply Attributes 11.1.5 Accounting Filters 11.1.6 Invoking the Accounting Filter