This is how to get a stable solution for establishing PPTP tunnels from one or multiple LAN clients to one or multiple WAN servers passing through OpenWRT's network address translation (NAT). Thus this is often referred to as “PPTP pass through”.
PPTP utilizes the Generic Routing Encapsulation (GRE) protocol for its point-to-point tunnel. As a pure IP protocol GRE uses only IP addresses but no port numbers giving the router's NAT a tough time to track such a connection. In its base configuration OpenWRT Backfire is able to NAT a single PPTP connections but not multiple such connections concurrently. It is also unreliable when trying to establish consecutive single PPTP connections from different LAN clients in rapid succession. This limitation can be lifted (as far as I could make out so far) by installing the following package.
<shell> opkg update opkg install kmod-ipt-nathelper-extra </shell>