Can anybody help?
I have a Windows VPN server running PPTP. I have a dedicated 'VPN' line going to this box, however I would like to put a Linux firewall infront of it for that extra security and to block everything but VPN connections.
I thought IPCOP would be able to provide this solution. However, I have installed it and there seems to be no way of configuring a VPN pass-through and as far as I can see very little support documentation on the net. Maybe I am just being blind, but I cannot find any documentation regarding this problem. I would be willing to install another distro, I just don't know which.
Can anybody help? Please?

well the passthrough isn't really what you're after, you surely just want to port forward those ports to the internal host system if you intend to be able to bring up a tunnel from the internet inbound to that box.

Well I installed IPCOP and I forward GRE and tcp 1723 ports to the VPN. However the GRE packets are lost at IPCOP. Is there anything else I need to change on ipcop to allow GRE packets to be successfully forwarded?