You can if you can enable NAT transversal on the internal VPN server since it will pass through Astaro. That is for IPSEC tunnels. PPTP I don't think cares about NAT and not really a secure tunnel anyway.
You more than likely would need to make the internal server the initiator of the tunnel so you don't have to create an SNAT coming from the other way which can be problematic.
So do you want to basically use lets say an internal windows 2000 2003 rras server. I was thinking about doing that also it would seem that one would simply be able to set up port forwarding.
An internal RAS server would a different thing. I was talking about netnet tunnels. A user->{internal server}net would involve port forwarding and I'm sure has been discussed many times and is also possible. I haven't tried it though.
Let's see what the poster was referring to. I recently tested IPSEC tunnels from behind a NAT router and it works fine. The tunnel might collapse if the NAT router is restarted though.
