Chapter 10 Install and System Administration for FortiOS 5.0 : PPTP and L2TP : Configuring the FortiGate unit for PPTP pass through
  
Configuring the FortiGate unit for PPTP pass through
To forward PPTP packets to a PPTP server on the network behind the FortiGate unit, you need to perform the following configuration tasks on the FortiGate unit:
Define a virtual IP address that points to the PPTP server.
Create a security policy that allows incoming PPTP packets to pass through to the PPTP server.
 
The address range is the external (public) ip address range which requires access to the internal PPTP server through the FortiGate virtual port-forwarding firewall.
IP addresses used in this document are fictional and follow the technical documentation guidelines specific to Fortinet. Real external IP addresses are not used.
See Also
Configuring a virtual IP address
Configuring a port-forwarding security policy
PPTP and L2TP