The Tool Desk
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What a NAS does
The IETF defines a NAS as a device that provides an access service for a user to a network (RFC 6158, 2011). RFC 5080 (2007) describes it more briefly as “the device providing access to the network.” Both definitions place the NAS on the access edge, where a user or device first connects and where access is granted or refused.
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A NAS may provide a plain network connection or a service layered on top of one. RFC 5080 gives 802.11 wireless and PPP as examples of access technologies a NAS can offer. The same device category therefore covers very different equipment, from a wireless access point to a dial-in or broadband termination box.
NAS versus RADIUS server
Most confusion comes from mixing up two roles. The NAS handles the user-facing connection. The RADIUS server handles the decision about who may connect and under what conditions. The table below separates the roles.
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| Role | Main job | Starts the AAA exchange? | Carries the user’s data traffic? |
|---|---|---|---|
| Network access server (NAS) | Provides the access service and enforces the result | Yes. It sends the RADIUS Access-Request (or the Diameter AA-Request) | It provides the connection the traffic uses |
| RADIUS server | Evaluates authentication and authorization requests and returns accept, reject, or challenge | No. It responds to requests | No. Ordinary user traffic is not routed through it |
| RADIUS proxy | Relays RADIUS requests between a NAS and remote RADIUS servers | No. It forwards requests | No |
So the answer to “Is a NAS the same as a RADIUS server?” is no. In RADIUS terminology the NAS is the client, and the RADIUS server is the AAA endpoint that answers it.
How the access exchange works
The exact packet fields and responsibilities depend on the protocol and the implementation, but the general sequence in a RADIUS deployment is as follows.
- A user or endpoint asks to connect through the NAS.
- The NAS sends an Access-Request containing the user’s identity and authentication information to a configured RADIUS server. In a Diameter deployment, the NAS starts the exchange with an AA-Request instead.
- The server returns Access-Accept, Access-Reject, or Access-Challenge (RFC 2865, 2000). An accept can carry service attributes.
- The NAS grants or denies access and applies any service settings it received.
- If accounting is configured, the NAS reports session details such as session identity, duration, and traffic totals to an accounting server. Accounting is a summary of the session, not a copy of the traffic.
A NAS does not need to store user credentials locally in this model. The credential check happens on the server side, although some deployments do keep local fallback accounts. Check your own configuration before assuming either way.
Where NAS devices appear
Enterprise switches and wireless access points
A switch or wireless access point can act as the NAS for an 802.1X or RADIUS-controlled network. It decides whether a device can join the wired or wireless network based on the server’s answer. Support for this role depends on the device’s 802.1X and RADIUS client features, so verify it in the vendor documentation before planning a deployment.
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Carrier and broadband access
Carrier access equipment, such as ADSL termination systems or DSLAMs, can perform the NAS role for subscriber access and accounting. The subscriber connects through the access equipment, and the operator’s AAA servers decide whether the session is allowed.
L2TP tunneling
RFC 2809 (2000) describes the NAS as the device clients contact to obtain network access. In compulsory L2TP tunneling, a NAS that performs the tunneling is called an L2TP Access Concentrator (LAC).
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Protocol terminology
The name for the same role changes with the protocol family.
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- IEEE 802.1X and EAP: RFC 5080 says the NAS is also known as the Authenticator in 802.1X or Extensible Authentication Protocol (EAP) terminology.
- Diameter: RFC 4005 (2005) defines the Network Access Server Application. Diameter attributes can carry policy settings for the NAS, such as filter rules.
A RADIUS proxy is worth separating from the NAS. A proxy can sit between the NAS and remote RADIUS servers and forward requests, but it is not the device that gives the user access.
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Common confusions to avoid
- NAS storage is a different thing. In storage contexts, NAS usually means network-attached storage. That is unrelated to network access servers.
- The NAS is not always a standalone appliance. It can be a feature of a switch, access point, or access concentrator.
- The RADIUS server does not carry user traffic. It answers access and accounting requests only.
The NAS, in short, is the access-side enforcement point. Its counterpart is the AAA server that decides what the access may be.
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