In my actual configuration, OpenVPN provides a remote DNS and only routes traffic to the internal LAN. Every public traffic is kept out of the VPN. Yesterday, I wanted to route every traffic through my VPN ; to masquerade my 3G IP address :-D
Anyway, there’s only a small change to apply to the OpenVPN client configuration file:
# Route all traffic through the VPN
Restart your OpenVPN client and that’s all folks!
Source: OpenVPN – DD-WRT Wiki