Openvpn – Want to route all internet traffic through VPN

I am afraid you will need to do some reading on IP routing. A good starting point could be here. Now to your specific problem: you need to replace the "def1" with the address of your IP tunnel, so that all traffic is by default routed into the tunnel.

However, please note that this will route really ALL traffic through the tunnel. If you want to only route traffic for specific target networks through the tunnel you will need lines similar to this:

push "route 192.168.10.0 255.255.255.0"

In this example, 192.168.10.0 stands for the network address of the subnet, and 255.255.255.0 stands for the subnet mask. If you don't know what these terms mean, then you should start reading.

From the OpenVPN HowTo Documentation

Implementation

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.