RADIUS server authentication: Old but still relevant

February 14, 2023  |  Anas Baig

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A radius server uses a network protocol for remote user authentication and authorization. It is a client/server protocol that allows a remote user to access a network using a shared secret (usually a password). RADIUS servers are typically located on the perimeter of a network and use port 1812 (UDP) or 1645/1813 (TCP).

RADIUS was originally developed by Livingston Enterprises, Inc. in 1991. It is now an IETF standard (RFC 2865). The following are the most important things to know about RADIUS server authentication.

  •  RADIUS is a remote authentication dial-in user service

It was developed to provide centralized authentication, authorization, and accounting management for networked devices such as routers and switches.

What does dial-in refer to here? Dial-in is a type of authentication that allows a user to connect to a network remotely using a phone line or other connection. RADIUS servers are used to manage user access to a network. They can be used to control who can access the network, what services they can use, and how much bandwidth they can consume.

  •  RADIUS is an alternative to TACACS and is often used in conjunction with TACACS+ for authentication and authorization

The reason for this is that RADIUS is typically used for remote access, while TACACS+ is usually used for device administration. While both protocols can be used for both purposes, RADIUS is usually the preferred protocol for remote access.

  •  A RADIUS server typically uses UDP port 1812 (or TCP port 1645/1813) to communicate with clients

RADIUS servers typically listen on UDP port 1812 (or TCP port 1645/1813). When a RADIUS client sends a request to the server, it includes the secret key in the request. The server uses this key to authenticate the client and authorize the request.

RADIUS is a client/server protocol, which means that each RADIUS client must have a corresponding RADIUS server. A RADIUS client is typically a network device such as a router or switch. A RADIUS server is a computer that runs the RADIUS software and manages user access to the network.

What this means is that for a user to be able to access the network, they must first authenticate with the RADIUS server. The RADIUS server then authorizes the user's access to the network and controls what services they can use.

  •  RADIUS uses a client/server architecture

The RADIUS server is responsible for authenticating users and maintaining their account information, while the RADIUS client is typically a network device that forwards authentication requests to the server. The reason this distinction matters is that it allows the server to be centrally located and managed, while the clients can be distributed throughout the network. This architecture also makes it possible for the server to authenticate users against multiple databases, such as an LDAP server or a local file.

The implications of this are that if the server goes down, the entire network will be unavailable to users. This is why it is important to have redundant RADIUS servers in a production environment.

  •  A RADIUS server can authenticate users against multiple databases

RADIUS supports multiple authentication methods, including PAP, CHAP, MS-CHAP, and EAP. PAP is the simplest authentication method and sends the username and password in clear text. CHAP encrypts the password but sends it over the network in plain text. MS-CHAP encrypts both the username and password. EAP is a more secure authentication method that uses digital certificates.

  •  RADIUS uses UDP for transport

RADIUS uses UDP as its transport protocol. UDP is a connectionless protocol, which means that each packet is sent independently and does not require a connection to be established beforehand. This makes RADIUS very scalable, as it can support a large number of clients without requiring a lot of resources on the server.

It matters that RADIUS uses UDP for transport because UDP is a less reliable protocol than TCP. This means that RADIUS packets can be dropped or lost in transit. However, this is usually not a problem because RADIUS uses retransmission and error checking to ensure that packets are delivered reliably.

  •  The RADIUS server must have a shared secret with the clients

The RADIUS server and clients must have a shared secret, which is used to encrypt and decrypt packets. This shared secret is typically a password or phrase that is known only to the server and clients. Without the shared secret, an attacker would not be able to read or modify the packets being exchanged between the server and clients.

  •  RADIUS uses Access-Request and Access-Accept packets

When a client sends an authentication request to a RADIUS server, it does so using an Access-Request packet. The server then responds with an Access-Accept or Access-Reject packet, depending on whether the authentication was successful. If the authentication was successful, the server will also include an Access-Challenge packet, which contains a challenge that the client must answer to prove its identity.

  •  RADIUS can be used for AAA

RADIUS can be used for AAA, which stands for Authentication, Authorization, and Accounting. Authentication is the process of verifying a user's identity, authorization is the process of determining what resources a user is allowed to access, and accounting is the process of tracking and billing for a user's usage.

AAA is a common security model that is used to control access to network resources.

  •  RADIUS is standardized by the IETF

RADIUS is a standards-based protocol, which means that it is defined by an Internet Engineering Task Force (IETF) specification. The most recent version of the RADIUS specification is RFC 2865, which was published in June 2000.

  •  RADIUS is commonly used by ISPs

RADIUS is commonly used by Internet service providers (ISPs) to authenticate and authorize users who are trying to access the internet. RADIUS is also used by corporate networks to authenticate and authorize users who are trying to access the network.

  •  There are a few different RADIUS implementations

There are a few different RADIUS implementations, including FreeRADIUS, Microsoft NPS, and Cisco ACS. FreeRADIUS is the most popular open-source RADIUS server. Microsoft NPS is the RADIUS server included in Windows Server. Cisco ACS is a commercial RADIUS server from Cisco Systems.


These are the most important things to know about RADIUS server authentication. RADIUS is a critical part of many network security systems, and understanding how it works is essential for anyone who is responsible for managing a network.

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