SBS 2008: Connected via VPN but can’t access anything

GRE is not always forwarded by routers, this is one of the benefits of using a SSL VPN as it is more compatible.

Example of SSL VPN: http://openvpn.net/

PPTP uses a TCP connection on port 1723 which you have already routed to your server, then over that a GRE connection is negotiated for the routing.

Make sure any PPTP server on the router is disabled to make sure it is not receiving the GRE traffic instead of forwarding it.

As long as you have permitted (via necessary NAT and ACL) TCP/1723 to the SBS 2008 server there is no need to explicitly configure GRE as long as the ASA's PPTP Application Layer Gateway (ALG) is enabled.

Cisco used to call these ALG's "fixup" now they are called "inspect" in the ASA.

A vanilla ASA comes with a bare bones modular policy framework (MPF) making use of a global policy -- unfortunately the PPTP ALG is not enabled much to the chagrin of many a Windows administrator.

From the CLI

Add inspect pptp to your traffic class (likely defined inspection_default traffic class in the global_policy).

From ASDM

Configuration (Top) -> Firewall (Lower Left) -> Service Policy Rules (Middle Left)

Select the traffic class which is likely inspection_default -> Click Edit

Rule Actions (Tab) -> Check PPTP -> OK -> Apply

Save Configuration.