In the AAA (Authentication, Authorization, and Accounting) framework, the role of the Network Access Server (NAS) is to act as a gateway that enforces access to network services and sends accounting information to the AAA server. The NAS initially requests authentication information from the user and then passes that information to the AAA server. It also enforces the access policies as provided by the AAA server after authentication and provides accounting data to the AAA server based on user activity.
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Technical literature on AAA protocols which often includes a description of the roles and responsibilities of a Network Access Server.
Network security resources that discuss the NAS function within the AAA framework.