Forwarding all incoming traffic to a specific dest IP through OpenVPN tap0 interface

In this article, we will guide you through the process of forwarding all incoming traffic to a specific destination IP through the OpenVPN tap0 interface. This is a useful technique for routing all your network traffic through a VPN tunnel, providing an additional layer of security and privacy.

Prerequisites

Before we begin, ensure that you have the following:

• A working OpenVPN server and client configuration.
• Root or sudo access to your OpenVPN client machine.

Configure the OpenVPN Client

First, we need to configure the OpenVPN client to use the tap0 interface and enable IP forwarding. To do this, follow the steps below:

1. Edit the OpenVPN client configuration file, usually located at /etc/openvpn/client.conf or C:\Program Files\OpenVPN\config\client.ovpn.
2. Add the following lines to the configuration file:

   dev tap
   persist-tun
   persist-key
   cipher AES-256-CBC
   tls-auth ta.key 1
   up /etc/openvpn/update-resolv-conf
   down /etc/openvpn/update-resolv-conf
   script-security 2
   up /etc/openvpn/up.sh
   down /etc/openvpn/down.sh

   The dev tap line specifies that we will use the tap0 interface. The persist-tun and persist-key options ensure that the TUN/TAP device and encryption keys are not closed when the client disconnects from the server. The cipher option sets the encryption algorithm to AES-256-CBC. The tls-auth option enables HMAC authentication for added security. The up and down options update the system's DNS settings when the VPN connection is established or closed. The script-security option enables the execution of custom scripts.

3. Create the up.sh and down.sh scripts to configure the tap0 interface and enable IP forwarding. To create the scripts, run the following commands:

   sudo touch /etc/openvpn/up.sh
   sudo touch /etc/openvpn/down.sh
   sudo chmod +x /etc/openvpn/up.sh
   sudo chmod +x /etc/openvpn/down.sh

   These commands create two empty scripts with execute permissions. Now, open the up.sh script in a text editor and add the following lines:

   #!/bin/sh
   ifconfig tap0 10.8.0.2 netmask 255.255.255.0
   sysctl -w net.ipv4.ip_forward=1
   iptables -t nat -A POSTROUTING -o tap0 -j MASQUERADE
   iptables -A FORWARD -i tap0 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
   iptables -A FORWARD -i eth0 -o tap0 -j ACCEPT

   The ifconfig command sets the IP address of the tap0 interface. The sysctl command enables IP forwarding. The first iptables command masquerades the client's IP address when traffic is forwarded through the tap0 interface. The second iptables command allows incoming traffic through the tap0 interface if it is part of an established connection. The third iptables command allows incoming traffic through the eth0 interface if it is destined for the tap0 interface.

   Now, open the down.sh script in a text editor and add the following lines:

   #!/bin/sh
   ifconfig tap0 down
   sysctl -w net.ipv4.ip_forward=0
   iptables -t nat -F POSTROUTING
   iptables -F FORWARD

   The ifconfig command brings down the tap0 interface. The sysctl command disables IP forwarding. The first iptables command flushes the POSTROUTING chain. The second iptables command flushes the FORWARD chain.

4. Save and close the scripts. Now, restart the OpenVPN client to apply the changes:

   sudo systemctl restart openvpn@client
   # or
   sudo service openvpn restart

   If you are using Windows, run the OpenVPN client GUI as an administrator and click the "Connect" button.

   Test the Configuration

   To test if all incoming traffic is forwarded to the specific destination IP, follow these steps:

   1. Connect to the OpenVPN server.
   2. Run a traceroute command to the destination IP:

      traceroute 
      # For example:
      # traceroute 8.8.8.8

      The output should show that all traffic is routed through the OpenVPN server's IP address.

   3. Disconnect from the OpenVPN server and run the traceroute command again. The output should show that traffic is no longer routed through the OpenVPN server.

   In this article, we have shown you how to forward all incoming traffic to a specific destination IP through the OpenVPN tap0 interface. This technique is useful for securing and anonymizing your network traffic. Remember to always use strong encryption and secure authentication methods when configuring your VPN.

   References

   Title	URL
   OpenVPN Manual	https://openvpn.net/community-resources/reference-manual-for-openvpn-2-4/
   IPTables Manual	https://linux.die.net/man/8/iptables
   Sysctl Manual	https://linux.die.net/man/8/sysctl