What is a RADIUS Server? A Complete Guide for ISP Operators

What is RADIUS? (Remote Authentication Dial-In User Service)

RADIUS stands for Remote Authentication Dial-In User Service. It is a networking protocol that provides centralized authentication, authorization, and accounting (AAA) for users who connect to a network. Originally designed in 1991 for dial-up internet access, RADIUS has evolved into the industry standard for managing subscriber connections at ISPs worldwide.

In simple terms, a RADIUS server answers three questions every time a subscriber tries to connect:

Together, these three functions — known as AAA — form the backbone of subscriber management for any ISP using PPPoE, Hotspot, or other access methods. Without a RADIUS server, operators would have no centralized way to control who accesses their network or how usage is billed.

How RADIUS Works for ISPs

The RADIUS authentication flow involves three key components: the subscriber device, the Network Access Server (NAS), and the RADIUS server. Here's how they work together in a typical ISP environment:

1. 1.A subscriber initiates a PPPoE connection from their router or device.
2. 2.The NAS (typically a MikroTik router or BRAS) receives the connection request and forwards the credentials to the RADIUS server.
3. 3.The RADIUS server checks the username and password against its database (e.g., the radcheck table).
4. 4.If authentication succeeds, the RADIUS server sends back an Access-Accept packet with authorization attributes — bandwidth profile, IP assignment, session limits — from the radreply table.
5. 5.The NAS applies these attributes and establishes the subscriber's session.
6. 6.Throughout the session, the NAS sends accounting packets to the RADIUS server, recording session start, interim updates, and session stop events in the radacct table.

The Network Access Serveris the bridge between your subscribers and your RADIUS server. In most ISP setups, this is a MikroTik router configured as a PPPoE server. The NAS doesn't store subscriber credentials itself — it delegates all authentication decisions to the RADIUS server, which makes centralized management possible even across dozens of NAS devices and thousands of subscribers.

FreeRADIUS — The ISP Standard

FreeRADIUSis the most widely deployed RADIUS server in the world. It's open-source, battle-tested, and handles authentication for a significant share of global internet users. For ISP operators, FreeRADIUS is the de facto choice because of its flexibility, performance, and deep integration with SQL databases.

Key reasons ISPs choose FreeRADIUS:

• •Supports all common authentication protocols: PAP, CHAP, MS-CHAP, EAP.
• •SQL-driven configuration — subscriber credentials and attributes are stored in database tables (radcheck, radreply, radgroupcheck, radgroupreply), making integration with ISP billing software straightforward.
• •Handles thousands of concurrent authentications with minimal hardware.
• •Supports Change of Authorization (CoA) and Disconnect Messages for real-time session control.
• •Extensive vendor-specific attribute (VSA) support for MikroTik, Cisco, Huawei, and other NAS vendors.
• •Free and open-source with a large community and comprehensive documentation.

ISPbills uses FreeRADIUS 3.x with a PostgreSQL backend database. This combination provides the reliability and query performance that ISPs need for real-time authentication decisions while keeping subscriber data fully integrated with the billing and CRM platform.

Key Benefits of Using RADIUS in ISP Operations

A properly configured RADIUS server transforms how an ISP operates. Here are the most impactful benefits:

How ISPbills Integrates RADIUS

ISPbills is designed as a complete ISP billing software and CRM with deep, native RADIUS integration. Unlike platforms that bolt on RADIUS as an afterthought, ISPbills treats RADIUS as a core component of its architecture. Here's how:

• •NAS Management — Register and manage all your MikroTik routers and other NAS devices directly from the ISPbills dashboard. Each NAS is configured with its shared secret, IP, and vendor type.
• •radcheck & radreply — When you create a subscriber or change their package, ISPbills automatically writes the correct attributes to the radcheck (credentials) and radreply (bandwidth, IP, policies) tables in the per-tenant PostgreSQL RADIUS database.
• •Change of Authorization (CoA) — ISPbills sends real-time CoA packets to your NAS devices to apply bandwidth changes, suspend users, or disconnect sessions without waiting for re-authentication.
• •Session Tracking — Live PPPoE session data from the radacct table is displayed in the operator dashboard, showing active connections, assigned IPs, session duration, and data usage.
• •Automatic Suspension — When a subscriber's invoice is overdue, ISPbills updates RADIUS attributes to block access. When payment is received (via bKash, Nagad, Stripe, or other gateways), access is restored automatically.
• •Per-Tenant Isolation — Each ISP operator gets their own PostgreSQL RADIUS database, ensuring complete data isolation in multi-tenant deployments.

This tight integration means ISP operators don't need to manually edit RADIUS database tables or write custom scripts. Everything — from subscriber onboarding to package changes to disconnection — is handled through a unified interface that keeps your RADIUS server and billing system in perfect sync.

Getting Started with RADIUS on ISPbills

Setting up RADIUS with ISPbills is straightforward. The platform handles FreeRADIUS configuration, database provisioning, and NAS integration out of the box. As an operator, you simply:

1. 1.Add your NAS devices (MikroTik routers, BRAS, etc.) in the ISPbills dashboard with their IP addresses and shared secrets.
2. 2.Create bandwidth packages with the speed limits and policies you want RADIUS to enforce.
3. 3.Add subscribers and assign them a package — ISPbills writes the correct radcheck and radreply attributes automatically.
4. 4.Configure your MikroTik PPPoE server to point to the ISPbills RADIUS server for authentication.
5. 5.Monitor live sessions, track accounting data, and manage your entire subscriber base from one dashboard.

Whether you're migrating from a legacy system or setting up a new ISP from scratch, ISPbills gives you enterprise-grade RADIUS integration without the complexity. Check out our customer portal guide and network management overview to see how RADIUS fits into the bigger picture.