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Network access server
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A network access server (NAS) is a group of components that provides remote users with a point of access to a network.[1][2]
Overview
[edit]A NAS concentrates dial-in and dial-out user communications. An access server may have a mixture of analog and digital interfaces and support hundreds of simultaneous users. A NAS consists of a communications processor that connects asynchronous devices to a LAN or WAN through network and terminal emulation software. It performs both synchronous and asynchronous routing of supported protocols.
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. A 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.
Examples
[edit]The above translates into different implementations for different uses. Here are some examples.
- An Internet service provider which provides network access via common modem or modem-like devices (be it PSTN, DSL, cable or GPRS/UMTS) can have one or more NAS (network access server) devices which accept PPP, PPPoE or PPTP connections, checking credentials and recording accounting data via back-end RADIUS servers, and allowing users access through that connection.
- The captive portal mechanism used by many WiFi providers: a user wants to access the Internet and opens a 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 the RADIUS protocol to connect to an AAA server and passes off the username and password. The RADIUS server searches through its resources and finds that the credentials are valid and notifies the NAS that it should grant the access. The NAS then grants the user access to the Internet.
- Another use of a NAS would be in voice over IP (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 to verify whether a phone number has long distance access or a telephone card has minutes left.
Associated protocols
[edit]Although not required, NASs are almost exclusively used with authentication, authorization, and accounting (AAA) servers. Of the AAA protocols available, RADIUS tends to be the most widely used. The Diameter base protocol extends RADIUS services by providing error handling and inter-domain communications. This protocol is used in networks like the IP Multimedia Subsystem (IMS).
See also
[edit]External links
[edit]References
[edit]- ^ Clark, Martin P. (2003-05-07). Data Networks, IP and the Internet: Protocols, Design and Operation. John Wiley & Sons. p. 542. ISBN 978-0-470-84856-2.
- ^ Held, Gilbert (2000-05-31). Network Design: Principles and Applications. CRC Press. p. 205. ISBN 978-1-4200-9375-9.
- ^ D. Mitton; M. Beadles (July 2000). Network Access Server Requirements Next Generation (NASREQNG) NAS Model. Network Working Group. doi:10.17487/RFC2881. RFC 2881. Informational.
Network access server
View on Grokipediafrom Grokipedia
A Network Access Server (NAS) is a device positioned at the edge of a network that facilitates controlled access to network resources for remote or external users by managing authentication, authorization, and accounting (AAA) processes.[1] It typically accepts multiple simultaneous point-to-point connections, such as dial-up modem links or virtual private network (VPN) tunnels, on one interface while connecting to internal routed networks on another.[1] In RADIUS infrastructures, a NAS operates as a client that forwards user connection requests and accounting data to a central RADIUS server for processing, ensuring secure policy enforcement without handling authentication logic itself.[2]
NAS devices perform critical functions in network security and management, including user identity verification, service authorization based on policies, and usage accounting for billing or auditing purposes.[1] They support dynamic allocation of per-user services, such as bandwidth limits or quality of service (QoS) levels, and integrate with AAA protocols to enable scalability in large environments.[1] Common examples of NAS implementations include wireless access points for Wi-Fi connectivity, 802.1X-capable Ethernet switches for wired LAN access, VPN servers like those in Windows Server Remote Access, and traditional dial-up servers.[2]
The concept of NAS has evolved from early dial-up and terminal access systems in the ARPANET era, where protocols like TACACS provided basic access control, to modern standards supporting tunneling and roaming.[3] Key protocols defining NAS operations include RADIUS for authentication and accounting, as standardized in RFC 2865 and RFC 2866, and its successor Diameter in RFC 7155 for enhanced scalability in next-generation networks. Additionally, extensible frameworks like the Extensible Authentication Protocol (EAP) allow NAS to support diverse authentication methods, such as certificates or biometrics, for secure network entry.[4] These capabilities make NAS essential for enforcing network access policies in enterprise, ISP, and cloud environments, reducing risks from unauthorized access.[1][2]
Introduction
Definition and Purpose
A network access server (NAS) is a specialized device or software system that mediates access to a network for remote users or devices by handling initial connections through methods such as dial-up, virtual private network (VPN), or broadband services like digital subscriber line (DSL) or cable.[5][6] It functions as a remote access server (RAS) or media gateway, establishing point-to-point protocol sessions to connect external clients to internal resources.[7][8] The primary purpose of a NAS is to enforce access control by verifying user identities and allocating network resources, thereby preventing unauthorized entry into protected environments.[5][6] This involves authenticating credentials such as usernames and passwords, or other identifiers like IP addresses, and granting or denying access based on validation results to ensure secure connectivity.[8][7] In this role, the NAS contributes to the broader authentication, authorization, and accounting (AAA) framework by interfacing with external services for policy enforcement.[5] Key characteristics of a NAS include its operation as a gateway between public networks, such as the internet, and private local area networks (LANs), where it supports session establishment without storing user data locally.[5][8] It forwards authentication requests to dedicated validation systems and applies security measures like access control lists (ACLs) to manage traffic flow.[6] Unlike general-purpose servers that handle data storage, processing, or application hosting, a NAS focuses exclusively on access mediation and connection management, lacking built-in capabilities for credential validation or broader computing tasks.[7][8] This specialization enables efficient handling of remote logins while relying on separate infrastructure for deeper security functions.[5]Historical Development
The emergence of network access servers (NAS) in the 1980s was closely tied to the rise of packet-switched networks like X.25, which enabled remote access in enterprise environments through dial-up modems and packet assembler/disassemblers (PADs). PADs served as early precursors to modern NAS by allowing multiple asynchronous terminals to connect to X.25 networks, facilitating data communication over public switched telephone networks (PSTN) for business applications such as connecting remote workers to mainframes.[9][10] In the 1990s, NAS saw significant growth driven by the expansion of the internet, with the introduction of the Point-to-Point Protocol (PPP) in 1994 standardizing IP transmission over serial links, including dial-up connections. This protocol, defined in RFC 1661, replaced older methods like SLIP and became essential for reliable remote access, supporting authentication and error detection in NAS deployments. A milestone product was the Ascend Pipeline, one of the first commercial NAS introduced in 1993 by Ascend Communications, which provided integrated ISDN and frame relay support for high-density remote connections in enterprise and ISP settings.[11][12] Key events further advanced NAS integration, including Cisco's acquisition of StrataCom in 1996 for $4 billion, which enhanced Cisco's capabilities in asynchronous transfer mode (ATM) and frame relay.[13] By the early 2000s, NAS evolved with the shift from analog dial-up to digital technologies like Integrated Services Digital Network (ISDN) and Digital Subscriber Line (DSL), offering higher speeds up to 1.5 Mbps and reducing reliance on traditional modems while maintaining compatibility with PPP for IP-based access.[14] Traditional NAS usage declined post-2010 as broadband technologies like DSL and cable became ubiquitous, rendering dial-up infrastructure obsolete for most consumers and enterprises, with major providers like AOL ending their dial-up services in September 2025.[15][16] However, NAS experienced a resurgence in virtual private network (VPN) contexts amid the 2020 COVID-19 pandemic, as remote work demands spiked VPN adoption by over 150% in some regions, prompting adaptations of NAS hardware and software for secure, scalable remote access over broadband links.[17]Core Functionality
Authentication Processes
The authentication process in a network access server (NAS) begins when a user or device initiates a connection attempt, which may occur over various access methods, including PPP links for dial-up or VPN, or Ethernet/Wi-Fi via 802.1X. The NAS collects credentials from the client, such as usernames and passwords, digital certificates, or biometric data, and verifies them either locally or by forwarding the request to a backend authentication server, typically using RADIUS or Diameter protocols. If valid, the NAS proceeds to authorization; otherwise, it denies access. This ensures identity verification within the AAA framework, preceding authorization and accounting.[18] For PPP-based connections, common methods include the Password Authentication Protocol (PAP) and the Challenge-Handshake Authentication Protocol (CHAP). PAP employs a simple two-way handshake where the client sends the username and password in clear text to the NAS upon link establishment, and the NAS responds with an acknowledgment if valid or a rejection if not; however, its lack of encryption makes it insecure for modern deployments.[19] In contrast, CHAP uses a three-way handshake for enhanced security: the NAS sends a challenge packet containing an identifier and a random value to the client, which responds with a hashed value combining the challenge, identifier, and shared secret; the NAS then verifies the hash against its records without transmitting the password. CHAP's periodic re-challenges further protect against replay attacks.[20] In non-PPP scenarios, such as 802.1X for wired or wireless access, the NAS (acting as an authenticator) uses the Extensible Authentication Protocol (EAP) to facilitate advanced methods. The client and NAS exchange EAP messages over the link layer (e.g., EAPOL for Ethernet), and the NAS relays these to a RADIUS server for processing, supporting methods like EAP-TLS for certificate-based authentication or EAP-PEAP for username/password with TLS tunneling. This enables secure access without establishing a full link initially.[21] NAS systems often integrate with external directory services like LDAP or Active Directory for scalable credential validation, especially in enterprise environments. The NAS, acting as a client, queries the directory server—typically via an intermediary RADIUS or TACACS+ server—using bind operations to authenticate against centralized user stores, supporting thousands of entries without local storage overhead. For instance, FreeRADIUS implementations bind to LDAP servers using secure credentials to perform searches and comparisons, enabling seamless AD integration for domain users.[22] This setup centralizes management while the NAS handles the initial credential exchange.[23] Error handling in NAS authentication includes immediate rejection responses for invalid credentials, configurable session timeouts to prevent indefinite waits (e.g., 30-60 seconds for unresponsive clients), and logging of failed attempts for auditing and security analysis. Upon failure, the NAS terminates the session and may retry alternative methods in a predefined list before final denial; timeouts trigger similar fallbacks without exposing sensitive data. Logs capture details like timestamp, client IP, and error codes to facilitate troubleshooting.[18] For performance in high-load scenarios, NAS platforms are designed to manage concurrent authentications efficiently, such as processing up to 250 method lists or scaling PPP requests via adjustable process counts (e.g., 1 to over 2 billion) to handle thousands of simultaneous users without degradation. In Microsoft NPS deployments, increasing concurrent authentications to the domain controller—defaulting to 10 but tunable higher—mitigates bottlenecks during peak usage, ensuring sub-second response times for large-scale access.[24]Authorization and Accounting
In the AAA (Authentication, Authorization, and Accounting) framework employed by network access servers (NAS), authorization follows successful authentication to determine the specific resources and services a user may access, while accounting tracks resource usage for auditing and billing purposes.[25] The NAS acts as the enforcement point, querying a backend AAA server to apply policies that define user privileges based on factors such as identity, time of day, or network conditions.[26] Authorization involves assigning user-specific privileges, including bandwidth limitations, access to particular virtual local area networks (VLANs), or session duration restrictions, all derived from predefined policies stored in the AAA server.[25] These policies are evaluated dynamically, often using attribute-value pairs (AVPs) to communicate and enforce permissions, allowing the NAS to configure the user's connection accordingly—such as allocating IP addresses or applying quality-of-service (QoS) rules.[26] For instance, in enterprise environments, authorization might restrict a guest user to internet access only, excluding internal resources, while integrating with policy decision points for multi-domain scenarios.[26] Accounting mechanisms record detailed session information to monitor usage and ensure accountability, capturing elements like session start and stop times, data volumes transferred (bytes in and out), and assigned IP addresses.[27] The NAS sends these records to the AAA server at session initiation, termination, or via interim updates during active sessions to provide real-time visibility, which is essential for Internet service providers (ISPs) integrating with billing systems.[27] This process supports both batch reporting for efficiency and guaranteed delivery with acknowledgments to prevent data loss, enabling accurate resource planning and fraud detection.[27] Compliance with IETF standards ensures interoperability in AAA implementations, as outlined in RFC 2881 for next-generation NAS requirements and RFC 2989 for evaluating AAA protocols in network access contexts.[25] These standards mandate support for dynamic AVP-based policy application and interim accounting updates, with configurable intervals typically ranging from seconds to minutes in practice, facilitating seamless enforcement and tracking across diverse network deployments.[27]Technical Architecture
Hardware Components
A network access server (NAS) typically employs a rack-mountable chassis designed for high-density deployment in data centers or telecom facilities, featuring modular slots that accommodate line cards for various interfaces. These chassis, such as the Cisco 5814 dial shelf, provide 14 slots dedicated to components like 10 for modem cards and 2-4 for trunk cards, enabling flexible expansion for handling multiple concurrent connections.[28] The architecture supports asynchronous ports for modem-based dial-up access and Ethernet interfaces for local area network integration, with line cards like the Cisco 8-Port Async/Sync EIA-232 Serial module offering up to eight asynchronous connections per card for terminal server applications.[29] Key hardware modules include digital signal processors (DSPs) optimized for voice and data modulation, which perform real-time signal processing tasks such as analog-to-digital conversion and error correction in dial-up scenarios. In systems like Cisco access servers, DSPs integrated into modem cards, such as NextPort DSPs, function as modems or terminal adapters, supporting standards like V.90 for high-speed modulation.[30] Network interface cards (NICs) facilitate wide area network (WAN) connectivity, with trunk cards providing interfaces for T1 (24 channels) or E1 (32 channels) lines to aggregate multiple access lines into the core network.[28] For reliability in carrier-grade environments, NAS hardware incorporates dual power supplies and hot-swappable components to ensure continuous operation. Power-entry modules (PEMs) in the dial shelf, for instance, operate on -48 VDC input with load-sharing redundancy, while the associated router shelf features hot-swappable 280W AC/DC supplies.[28] These designs support high availability through features like online insertion and removal (OIR) for cards, minimizing downtime during maintenance.[28] Scalability is achieved through high port density and session capacity, with modem cards supporting up to 144 digital modem ports or 192 Voice over IP (VoIP) ports per shelf, allowing hundreds of simultaneous user sessions in aggregated configurations.[28] The evolution of NAS hardware traces from proprietary designs in the 1990s, such as early Cisco AS5300 series with custom modular chassis, to standards-based architectures in the 2000s, including Advanced Telecommunications Computing Architecture (ATCA) shelves that promote interoperability across vendors for telecom applications.[31] By the mid-2000s, ATCA adoption enabled scalable, carrier-grade platforms with standardized backplanes and blades for enhanced port densities in IP-based networks.[32]Software and Configuration Elements
Network access servers (NAS) rely on specialized operating systems to ensure reliable, low-latency handling of user sessions and network traffic. Embedded real-time operating systems (RTOS) like VxWorks provide deterministic performance for dedicated hardware appliances, supporting multitasking and real-time responses essential for high-throughput access control. For instance, VxWorks powers network infrastructure devices, including legacy NAS implementations such as the Shanghai Bell Matix2000, where it manages protocol processing and resource allocation. Linux-based distributions offer greater flexibility for customizable deployments, commonly hosting open-source RADIUS servers like FreeRADIUS to integrate authentication services within broader network environments.[33] In routing-integrated setups, Cisco IOS functions as the core operating system, embedding NAS capabilities directly into routers for seamless VPN and dial-up access management.[34] Configuration of NAS involves multiple interfaces tailored to administrative needs, balancing automation with user-friendliness. Command-line interfaces (CLI) enable scripting and granular control, such as defining access policies or troubleshooting sessions in real time, as seen in Cisco IOS where commands likeaaa new-model initialize authentication frameworks.[35] Graphical user interfaces (GUI), often delivered through web-based consoles, simplify policy setup for non-experts, allowing visual configuration of user profiles and connection rules; Microsoft's Network Policy Server (NPS), running on Windows Server, exemplifies this with its console wizards for RADIUS client registration and network policy creation.[23] Simple Network Management Protocol (SNMP) supports remote monitoring and limited configuration, querying device status or setting traps for events like session failures, integrated across platforms like Cisco devices for centralized oversight.[36]
Core software modules form the backbone of NAS operations, orchestrating user interactions and data flows. Session management daemons oversee connection lifecycles, allocating resources and enforcing timeouts; in Linux environments, tools like xl2tpd handle Layer 2 Tunneling Protocol (L2TP) sessions by establishing virtual tunnels for remote access.[37] Protocol stacks implement standards like Point-to-Point Protocol (PPP) via daemons such as pppd, which negotiate links, encapsulate packets, and integrate with authentication backends for secure handshakes.[38] Scripting extensions allow custom authentication logic, such as embedding Tcl scripts in Cisco IOS to tailor responses based on user attributes or dynamic policy evaluation.[35] These modules collectively ensure scalable handling of concurrent sessions, often processing thousands per device in enterprise settings.
Firmware updates are essential for maintaining NAS integrity, addressing vulnerabilities and incorporating protocol enhancements without disrupting service. They patch security flaws, such as those in TCP/IP stacks, and can be delivered via traditional methods like TFTP in Cisco IOS or through automated processes in software-defined environments.[35] In virtualized NAS deployments, over-the-air (OTA) updates enable remote patching, leveraging cloud-based orchestration to push revisions to instances running on hypervisors, minimizing downtime in NFV architectures.[39] This approach supports rapid response to threats, with vendors recommending regular verification of update integrity using checksums.
Diagnostics in NAS focus on proactive issue resolution, embedding tools for real-time analysis. Log analysis utilities parse event records to trace authentication failures or session drops, accessible via CLI commands like show logging in Cisco IOS for filtering by severity or timestamp.[35] Traffic mirroring copies packets from monitored interfaces to analysis ports, aiding in protocol debugging without inline interference, often configurable through SNMP for selective capture.[36] Performance metrics, including CPU and memory utilization, are exposed via built-in counters or SNMP OIDs, enabling thresholds for alerts; for example, FreeRADIUS logs provide session throughput data to gauge load balancing needs.[33] These tools run atop the hardware platform, integrating with overall system monitoring for holistic visibility.