From the openvpn howto ( http://openvpn.net/index.php/open-source/documentation/howto.html#redirect )
Add the following directive to the server configuration file:
push "redirect-gateway def1" If your VPN setup is over a wireless
network, where all clients and the server are on the same wireless
subnet, add the local flag:
push "redirect-gateway local def1" Pushing the redirect-gateway option
to clients will cause all IP network traffic originating on client
machines to pass through the OpenVPN server. The server will need to
be configured to deal with this traffic somehow, such as by NATing it
to the internet, or routing it through the server site's HTTP proxy.
On Linux, you could use a command such as this to NAT the VPN client
traffic to the internet:
iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE
This command assumes that the VPN subnet is 10.8.0.0/24 (taken from
the server directive in the OpenVPN server configuration) and that the
local ethernet interface is eth0.
When redirect-gateway is used, OpenVPN clients will route DNS queries
through the VPN, and the VPN server will need handle them. This can be
accomplished by pushing a DNS server address to connecting clients
which will replace their normal DNS server settings during the time
that the VPN is active. For example:
push "dhcp-option DNS 10.8.0.1" will configure Windows clients (or
non-Windows clients with some extra server-side scripting) to use
10.8.0.1 as their DNS server. Any address which is reachable from clients may be used as the DNS server address.