The Duo Authentication Proxy is an on-premises software service that receives authentication requests from your local devices and applications via RADIUS or LDAP, optionally performs primary authentication against your existing LDAP directory or RADIUS authentication server, and then contacts Duo to perform secondary authentication. In addition to providing two-factor authentication, the Duo Authentication Proxy is a required component for importing Active Directory or OpenLDAP users into Duo via sync, Active Directory authentication for Duo Single Sign-On, and can also act as an HTTP proxy itself for other systems that also need to contact Duo's cloud service.
This video explains the basics of the Duo Authentication Proxy and shows how to install and configure the service on a Windows system, using Active Directory (ad_client) and a RADIUS Auto (radius_server_auto) setup.
Learn more about the Duo Authentication Proxy at https://duo.com/docs/authproxy-reference. To learn more about Duo Security, visit https://www.duo.com.