Recently i faced the problem of connecting two LANs using PPTP and an intermediate PPTP server. I had to use this approach  because the network setup was such that none of the LAN routers (DD-WRT) could contact the other but both could contact an intermediate server.

In order to achieve this PPTP clients must be always assigned the same IP and when connected, routes to each LAN must be added to the routing table of the intermediate server.

Network setup

PPTP Clients and server

I installed a PPTP server on a CentOS server using this HOWTO and i assigned static IPs for the two routers. To assign static IPs on PPTP clients you must enter the desired IP in the chap-secrets file.

chap-secrets file on the intermediate server:

# Secrets for authentication using CHAP
# client server secret IP addresses

LAN1 *   LAN1pass 172.16.4.200
LAN2 *   LAN2pass 172.16.4.201

This way each client (router) gets always the same IP. Make sure that these static IPs are not in the PPTP client IP range.

Then setup DD-WRT PPTP clients to connect to the intermediate server:

intermediate.example.com

The final step is to add a route to each router's LAN when it connects.
There is a file for running commands when PPP goes up or down. On CentOS it is located at /etc/ppp/ip-up.local or /etc/ppp/ip-up

Adding the following lines does the trick:

#!/bin/bash

case "$5" in
        172.16.4.200)
                /sbin/route add -net 172.16.3.64/26 gw 172.16.4.200
                ;;
        172.16.4.201)
                /sbin/route add -net 172.16.3.128/26 gw 172.16.4.201
                ;;
        *)
esac

Make the script executable: chmod 755 ip-up.local

Arguments available in ip-up and ip-up.local scripts