Setting up a VPN between two DD-WRT routers..

This setup worked for two WRT54GL routers running DD-WRT v24-sp1.

PPTP server (192.168.0.1):
First, one machine needs to be setup as a PPTP server.
1. Go to Services->PPTP to enable the PPTP server.
2. Set the server IP (should be a virtual lAN IP address different than your LAN IP address -- i.e. 192.168.0.2),
3. Set the Client IP block (192.168.0.50-192.168.0.70), and then setup the CHAP Secrets (johndoe* mypassword *).

PPTP client (192.168.1.1) :
1. Enable PPTP client.
2. Set the PPTP Server IP.
3. Set the Remote Subnet (192.168.0.0) and Remote Subnet Mask (255.255.255.0)
4. Set the MPPE Encryption to "mppe required".
5. Set the MTU/MRU to be 1450.
6. Disable NAT mdoe.
7. Set the username and password to the CHAP secret set in the PPTP Server.

You should verify the PPTP connection is established by telnetting into the PPTP client box and attempting to ping the private IP address of the PPTP server (i.e. 192.168.0.2 or 192.168.0.1). If this succeeds, then you may be able to ping the routers but other machines on the network are not able to talk with each other. In this case, you may wish to confirm that the PPTP server has not setup a route of 192.168.1.0. To do it automatically, you need to do the following:

1. Go to Administration->Commands.
2. Add the following firewall commands. Usually what happens is that the /tmp/pptpd_client/ip-up script is created. A delay is inserted before adding the route and then re-executing the ip-up bash script again.

sleep 40
/bin/sh -c 'echo "ip route add 192.168.1.0/24 dev ppp0" >> /tmp/pptpd/ip-up'
/tmp/pptpd_client/ip-up

Click on Save->Firewall after saving.

If you want to reinitiate the PPTP connection, try to click Save/Apply Settings and waiting for the VPN connection to be re-established. If you really want to check things out, you can configure a VPN client on Ubuntu 10.04 through the Network Manager (make sure to click Point-to-Point MPPE Encryption and allow stateful encryption, send PPP echo packets to help keep the connection alive). (Note: If you forget to enable the MPPE encryption/stateful encryption options, you may find that the VPN connection is flaky. It seems as if there are CHAP requests/rejects that keep happening without these two options). This VPN client will help you verify that the PPTP server is responding correctly.

You should also telnet to both DD-WRT routers and verify the routes have been established between the two subnets. You should also cat /tmp/pptpd_client/ip-up on the PPTP server to verify that the IP route was added correctly.