NPS (Network Policy Server) is the Microsoft implementation of a Remote Authentication Dial-in User Service (RADIUS) server, and as such, it performs connection authentication, authorization, and accounting for many types of network access, including wireless and virtual private network (VPN) connections. NPS also functions as a health evaluation server for NAP (Network Access Protection).
The following illustration shows NPS as a RADIUS server for a variety of access clients and a RADIUS proxy. NPS uses an Active Directory domain for user credential authentication of incoming RADIUS Access-Request messages.
When NPS is used as a RADIUS server, RADIUS messages provide authentication, authorization, and accounting for network access connections in the following way:
- Access servers, such as dial-up network access servers, VPN servers, and wireless access points, receive connection requests from access clients.
- The access server, configured to use RADIUS as the authentication, authorization, and accounting protocol, creates an Access-Request message and sends it to the NPS server.
- The NPS server evaluates the Access-Request message.
- If required, the NPS server sends an Access-Challenge message to the access server. The access server processes the challenge and sends an updated Access-Request to the NPS server.
- The user credentials are checked and the dial-in properties of the user account are obtained by using a secure connection to a domain controller.
- The connection attempt is authorized with both the dial-in properties of the user account and remote access policies.
- If the connection attempt is both authenticated and authorized, the NPS server sends an Access-Accept message to the access server. If the connection attempt is either not authenticated or not authorized, the NPS server sends an Access-Reject message to the access server.
- The access server completes the connection process with the access client and sends an Accounting-Request message to the NPS server, where the message is logged.
- The NPS server sends an Accounting-Response to the access server.
Issues in the functioning of NPS, if not promptly isolated and resolved, might result in the complete collapse of the remote authentication and authorization service provided by the Windows server. 24x7 monitoring of NPS, hence becomes imperative. The eG Enterprise Suite helps administrators in this task.