OpenVPN Server Site-to-Site and RoadWarrior access

It looks to me like the server is pushing the "redirect-gateway" option down to the client. This causes the client to use the VPN as its default gateway. Comment out the line in the server config 'push "redirect-gateway def1"'.

Woah there-- just saw your edit. You client can't be using the same IP addresses as the LAN it's connecting to. That's not going to work. One end or the other needs to use different IP addresses.

Edit:

Assuming routing is configured properly on your Windows Server 2003 machine (per the www.itsatechworld.com page you referenced), you should be able to PING the Windows Server 2003 machine and Windows Vista machine by their LAN IPs via the VPN. If you can, then you've got routing right on the Windows Server 2003 and DD-WRT machines and you can proceed. If not, then you need to start tracking down why either (1) PING traffic coming off the OpenVPN tunnel isn't getting to the destination, or (b) why PING replies from the destination host aren't coming back. You may end up putting something like Wireshark on your Windows Vista machine to see if the PING requests are even getting there (since PING can't tell you if your request is being received and the reply is just being lost).

Once you've got IP connectivity across the VPN working fine. I'd recommend installing the DNS and WINS services on your Windows Server 2003 VPN server computer and configuring the server computer and the Windows Vista home computer to use that machine for WINS and DNS. You can either add your ISP's DNS as a "forwarded" on the Windows Server 2003 machine, or leave the stock "root hints" configured to allow it to resolve Internet names. In your OpenVPN server configuration, add the following line right after the 'push "dhcp-option DNS 192.168.1.1" line:

push "dhcp-option WINS 192.168.1.1"

This is going to get the remote clients taking to the WINS and DNS servers on your Windows Server 2003 machine, and should get you both DNS and NetBIOS name resolution.

If you're not using an Active Directory domain at home, you'll probably want to setup a standard forward lookup zone on the Windows Server 2003 DNS server for your Windows Server 2003 and Windows Vista machines to register into. You'll want to grant clients permission to dynamically update records (albeit insecurely) when you create this zone. You should add the option "DNS domain name" (option 15) to your DHCP scope at home so that your client computers there pick up the right DNS domain name suffix. (If you're using DD-WRT for DNS then I can't tell you how to do that. I'm an OpenWRT guy, and I manage my WRT54G from the command line. I'd recommend running DHCP from the Windows Server 2003 machine anyway, but I just like that DHCP server more.)

If you are using an Active Directory domain you'll already have a forward lookup zone created in DNS. Since your remote VPN clients aren't members of your domain, though, they won't be able to register in DNS under the stock security settings that Windows Server sets on the DNS zone (at least, if you let it create the zone during DCPROMO). It's insecure, but if you want to allow them to register you could either (a - less secure) chang the permission on the zone to allow insecure registrations, or (b - more secure but still insecure) create A and PTR records for them and modify the permission on each of those records to allow anyone to update them.

It sounds like this is a home networking thing, and it's really a good learning opportunity for a lot of things-- IP routing, VPNs, name resolution. Perhaps you're looking for it to "just work" and not as a learning opportunity, in which case I can only offer my apologies and say that these things just aren't "turnkey" yet.

To keep this as easy as possible, make sure that the two offices are on different subnets, e.g. 192.168.10.0/24 for the first one and 192.168.20.0/24 for the second one. Then use OpenVPN to build the VPN connection, using a completely separate subnet (e.g. 192.168.254.0/24). Then add routes for the office of the other network to thew ADSL router, pointing at the machine running the OpenVPN Server/Client.

Now you have the following situation:

1. Traffic from network 1 for network 2 will go the ADSL router (the default gateway), which routes them to the OpenVPN machine, and from there they go through the tunnel.
2. Traffic from network 2 for network 1 will do the same
3. Return traffic will also follow the routes.
4. Computers in need of an IP address use broadcasting, which cannot go across network boundaries (unless you have a DHCP proxy in place). This guarantess that computers on network 1 will always receive their IP settings from the DHCP server on network 1.

I would recommend bridged networking for the OpenVPN instances, but, since they are not located on your default gateways, the extra routes are still needed.