NT pptp behind firewall

[James T. Moore]

In addition to tcp port 1723, pptp also needs GRE (General Routing
Encapsulation)
which is IP protocol 47.

You might be able to get it work with this line but I haven't tried it:

IPTABLES -t nat -A PREROUTING -p 47 -i eth0 -j DNAT --to  192.168.1.101

I've seen a lot of information about Microsoft's implementation of PPTP not
being secure....
for more info go to: http://www.counterpane.com/pptp-faq.html

Since you are already have a Linux box you may want to check out the PopTop
pptp server
for Linux  at http://poptop.lineo.com