Many users when setting up PPTP pass though to support Microsoft Server VPN Authentication only build a rule for one to one NAT on Port 1723 / the other port request is Port 47

PPTP uses IP protocol 47, designed for "General Routing Encapsulation" or GRE packets. A common mistake in configuring firewalls for use with PPTP is to open PPTP port 1723 (allowing connections to be established) but forget to forward GRE protocol type 47 (denying port data from passing through the tunnel). Some operating systems include "PPTP ping" utilities (pptpsrv and pptpclnt in Windows 2000) that verify both PPTP ports are opened.