[pptp-server] PPTP client connection trough masqueraded firewall

[Kurt Glazemakers]

This is the setup I wanted to use:
	Machine A		  Checkpoint  FW-1
   192.168.1.1/24=======|  |-----------|
|-------------------|	
				|==|		   |========....
INTERNET  ....======| Linux PPTP server |
	Machine B		|  |		   |
|			    |
   192.168.1.2/24=======|  |-----------|
|-------------------|
   			192.168.1.254/24     213.2.45.6

Machine A and machine B needs to connect to the PPTP server and are
using 2 different accounts.

My question is: Is it possible to do this, can the PPTP protocol be
masqueraded ? The TCP port 1723 will be offcourse no problem, but what
about the GRE. Because 1 connection works, but a second fails ...

Thanks,

Kurt