Remote Authentication Dial In User Service (RADIUS) is a protocol used by SonicWall security appliances to authenticate users who are attempting to access the network. See Using RADIUS for Authentication. The RADIUS server contains a database with user information, and checks a user’s credentials using authentication schemes such as Password Authentication Protocol (PAP), Challenge-handshake authentication protocol (CHAP), Microsoft CHAP (MSCHAP), or MSCHAPv2.