What is RADIUS?

RADIUS (Remote Authentication Dial-In User Service) is a client-server networking protocol.RADIUS is used to provide centralized Authentication, Authorization, and Accounting (AAA) management for computer networks. RADIUS is a UDP-based protocol that uses a shared secret to authenticate users.

The RADIUS protocol uses a RADIUS Server and RADIUS Clients.

RADIUS server: authenticates users against a database of usernames and passwords. It also authorizes access to network resources.

RADIUS client: a device that connects to the network, and sends its credentials to the RADIUS server. The RADIUS server then authenticates the client and provides authorization or access control information back to it.

It is a connection-oriented protocol. The RADIUS server and client exchange messages to establish an authenticated session. This session is used for authorization, accounting, and other tasks.


How RADIUS Server Authentication Works?

RADIUS Server Authentication VPN MFA Flow


The RADIUS server authenticates a user by verifying their credentials against a database. The RADIUS client sends its credentials to the RADIUS server, which then authenticates them against an authentication database. If the credentials are valid, authorization information is returned to the client. This can include access control lists (ACLs), which specify which networks or resources each user has access to.

  1. In the first step, the RADIUS Client tries to authenticate itself using its user credentials.
  2. The Client then sends an Access-Request message to the RADIUS Server, which contains a username and password.
  3. The RADIUS Server uses the information contained in the request to authenticate users against an external database (such as Active Directory).
  4. If the RADIUS Server finds a match, it extracts additional details of that user from its database.
    The RADIUS server checks if there is an access policy or a profile that matches the user credentials. If one is found, MFA (if enabled) will be prompted and an Access-Challenge request sent.
  5. The response to the Access-Challenge request will be provided by entering an OTP or accepting a push notification. The Radius server will validate that response.
  6. If the RADIUS Server can validate the response, it sends an Access-Accept message back to your device.
  7. If the response from the server does not match a policy or is invalid, then RADIUS returns an Access-Reject message and ends the transaction. The user will be denied access to the system.
  8. The Access-Accept message has a Filter ID attribute and a shared secret. If the RADIUS Client does not recognize the Shared Secret, it rejects the message.
  9. If the shared secret matches, the Client reads the value of the Filter ID attribute. The RADIUS Client then connects the user to a particular RADIUS Group using this Filter ID.
  10. The user is now able to connect to the network.


What is AAA?

AAA – Authentication, Authorization, and Accounting


This refers to the confirmation of the user which can be accomplished via presenting identity and credentials (for example: username and password or OTP or digital certificates.)


This refers to the granting of specific types of services or resources based on the authentication process of the user. This helps in giving restricted permissions to the users. These restrictions may be based on the physical location, IP address, or time of access.


This refers to the tracking of consumption of resources by the users. This feature can be used independently of RADIUS authentication or authorization. This may be used for management, planning, billing, etc.

RADIUS Server Authentication methods

The RADIUS server supports various methods to authenticate users. When it is provided with the username and original password given by the user, it can support PAP, CHAP, MS-CHAP, EAP, EAP_TLS, UNIX login, and other authentication mechanisms.

  • PAP – Password Authentication Protocol (PAP) authentication uses the PPP configuration files and PAP database for setting up authentication. The operation of PAP is similar to the UNIX login program, though PAP does not grant shell access to the user.
  • CHAP – Challenge-Handshake Authentication Protocol (CHAP) authentication uses challenge and response, which means that the authenticator challenges the caller (authenticates) to prove its identity. The challenge includes the unique ID generated by the authenticator and a random number. The caller uses the ID, random number, and its CHAP security credentials to generate the response (handshake) to send to the peer.
  • MS-CHAP – is the Microsoft version of the Challenge-Handshake Authentication Protocol (CHAP). It is used as an authentication option in Microsoft’s implementation of the PPTP protocol for VPNs.
  • EAP – Extensible Authentication Protocol (EAP) is an authentication framework, used in wireless networks and point-to-point connections.



A RADIUS Server eliminates the possibility of leaks of your private information to snooping outsiders and enables you to easily control members’ access levels. It can be implemented with minimal impact on the rest of your system.



Further Reading

Leave a Reply

Your email address will not be published.