Which type of authentication realm does not require any communication with an external authentication server?

A local realm is designed to manage user authentication directly within the proxy itself, eliminating the need for any external authentication server interactions. This means that user credentials are stored locally, allowing for immediate authentication without the need to connect to or query any outside systems, such as RADIUS, LDAP, or Kerberos servers.

In contrast, realms like RADIUS, LDAP, and Kerberos rely on external servers to validate user credentials. RADIUS requires communication with a RADIUS server for authentication requests, while LDAP requires a connection to a directory service for user information. Kerberos operates in a similar fashion, using a Key Distribution Center (KDC) for ticket granting and user authentication checks. This necessity for external communication makes these realms less efficient in scenarios where immediate, local authentication is desired. Therefore, the local realm stands out as the option that offers self-sufficient authentication capabilities without external dependencies.