Network Access Server
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Overview
A Network Access Server (NAS) is a system that provides access to a network. In some cases also known as a Terminal Server or Remote Access Server (RAS).
The NAS is meant to act as a gateway to guard access to a protected resource. This can be anything from a telephone network, to printers, to the Internet.
The client connects to the NAS. The NAS then connects to another resource asking whether the client's supplied credentials are valid. Based on that answer the NAS then allows or disallows access to the protected resource.
The NAS contains no information about what clients can connect or what credentials are valid. All the NAS does is send the credentials the client supplied to a resource which does know how to process the credentials.
Examples
The above translates into different implementations for different uses. Here are some examples.
- The most common use would be for access to the Internet. A user opens their browser. The NAS detects that the user is not currently authorized to have access to the Internet, so the NAS prompts the user for their username and password. The user supplies them and sends them back to the NAS. The NAS then uses RADIUS to connect to an AAA server (in this case, it is running FreeRADIUS) and passes off the username and password to the FreeRADIUS server. The FreeRADIUS server searches through its resources and finds that the credentials are valid and notifies the NAS they are valid. The NAS then grants the user access to the internet.
- Another use of a NAS would be in VoIP. However, instead of using a username and password, many times a phone number or IP Address are used. If the phone number is a valid customer then the call can be completed. Other uses might be if the phone number has long distance access or is a telephone card and has minutes left.
Associated Protocols
Although not required, NAS are almost exclusively used with AAA servers. Of the AAA protocols available, RADIUS tends to be the most widely used. DIAMETER is a new protocol which extends on RADIUS by providing error handling and inter-domain communications which is starting to be implimented in some high end NAS.