Which server is commonly used as the backend for 802.1X authentication?

• A. TACACS+
• B. RADIUS
• C. DNS
• D. LDAP

Answer: (B)

In 802.1X, the network access device (such as a switch or wireless AP) acts as the authenticator and forwards the user’s credentials to an authentication server. The standard protocol used for this communication is RADIUS. RADIUS is designed specifically for network access control, handling the exchange of EAP messages and returning allow/deny decisions (and any related authorization) to the authenticator. It also integrates with directory services (like LDAP or Active Directory) behind the scenes to verify user credentials, but the transport between the authenticator and the authentication server is RADIUS.

TACACS+ is primarily used for device administration rather than user access control on the network. LDAP is a directory service that stores user accounts and attributes, and while a RADIUS server can query LDAP/AD to verify credentials, LDAP itself is not the protocol used between the authenticator and the authentication server. DNS is unrelated to the authentication process.

So, the backend server commonly used for 802.1X authentication is RADIUS.