How to allow all internet traffic through a specified NIC?

Question

Hello,

I am looking for a way to direct all internet traffic through a specific NIC, here is the scenario:

> I am running a VPN client on a Windows computer. When the user logs in, the VPN client automatically connects (rather, the tunnel is established without the user authenticating), and the user is unable to disconnect the VPN, (e.g. Cloudflare WARP, with switch mode off). The VPN split-tunneling is set to off, so all traffic goes through the VPN adapter/tunnel. <

The VPN is used for secure browsing and remote access.

This works fine because when a user logs into the computer and starts to use it for internet browsing, I can track all traffic to the device and I can content filter the traffic also via the VPN server side. Here is the twist, I would like to force the user to authenticate the VPN when they log into the Windows desktop, that way, they authenticate before accessing the resources or browsing the internet. That I can do (allow them to authenticate before accessing the remote resources), but the challenge is can't block them from using the internet without first authenticating the VPN and establishing the tunnel.

Which, leads me to my question, is there a way I can set internet access via a specific NIC? That way I can set internet traffic to flow via the VPN connection,

1. But if I block the internet on all other NICs, how will the VPN client itself establish the tunnel?
2. I was playing around with Windows firewall but was not getting what I wanted.
3. If I can allow access only to the port that the VPN client needs, and block all other traffic until the tunnel is established

Thanks for your help in advance

Answer 1

Hi,

To direct all internet traffic through a specific network interface card (NIC) in Windows, you can modify the routing table using the "route" command. Here's how you can achieve this:

Identify the interface index: Open the Command Prompt as an administrator and run the following command:

netsh interface ipv4 show interface

Note down the index number of the NIC you want to use for internet traffic.

Open the Command Prompt as an administrator and run the following command to set the default route for internet traffic to the desired NIC:

route -p add 0.0.0.0 mask 0.0.0.0 <gateway IP> if <interface index>

Replace "<gateway IP>" with the IP address of your router or default gateway, and "<interface index>" with the index number of the NIC you want to use.

For example, if the gateway IP is "192.168.1.1" and the interface index is "2", the command would be:

route -p add 0.0.0.0 mask 0.0.0.0 192.168.1.1 if 2

The "-p" flag makes the route persistent, so it survives a system restart.

To ensure that the VPN client can establish the tunnel, you need to allow specific traffic through the other NICs until the tunnel is established. You can configure the Windows Firewall to achieve this.

Open the Windows Firewall with Advanced Security by searching for it in the Start menu.

In the Windows Firewall with Advanced Security window, click on "Inbound Rules" in the left pane.

Click on "New Rule" in the right pane to create a new inbound rule.

Select the "Port" option and click "Next".

Choose the specific TCP or UDP port that the VPN client uses to establish the tunnel. If you're not sure which port it is, you may need to consult the documentation or support resources for your specific VPN client.

Select "Allow the connection" and click "Next".

Select the network profiles for which this rule should apply (e.g., Domain, Private, Public) and click "Next".

Provide a name for the rule and click "Finish" to create the rule.

By allowing traffic only on the specific port required by the VPN client, you can block other internet traffic until the VPN tunnel is established. Once the tunnel is established, all internet traffic will be routed through the specified NIC based on the default route.

Please note that modifying network settings and firewall rules can have significant impacts on network connectivity and security. Make sure to thoroughly test these changes in a controlled environment before implementing them in a production environment.

Best Regards,

Karlie