Chapter 11 IPsec VPN for FortiOS 5.0 : FortiClient dialup-client configurations : Adding XAuth authentication
  
Adding XAuth authentication
Extended Authentication (XAuth) increases security by requiring additional user authentication in a separate exchange at the end of the VPN phase 1 negotiation. The FortiGate unit challenges the user for a user name and password. It then forwards the user’s credentials to an external RADIUS or LDAP server for verification.
Implementation of XAuth requires configuration at both the FortiGate unit and the FortiClient application. For information about configuring a FortiGate unit as an XAuth server, see “Using the FortiGate unit as an XAuth server”. The following procedure explains how to configure the FortiClient application.
Note that XAuth is not compatible with IKE version 2.
For more information on configuring XAuth authentication, see the FortiClient Administration Guide.