How to Implement Virtual Private Network (VPN) Reverse Split Tunneling with Policy-Based Routing on Your Dedicated Server

Setting up a Virtual Private Network (VPN) with reverse split tunneling and policy-based routing on a dedicated server requires a good understanding of networking concepts and administrative access to the server. Please note that implementing this configuration may vary depending on the specific VPN server software and operating system you're using. Below, I'll outline a general guide for setting up this configuration using OpenVPN on a Linux-based dedicated server.

Prerequisites:

1. Access to a Dedicated Server: Ensure you have SSH access to your dedicated server.
2. Linux Distribution: The instructions below are for a Linux-based server. This guide assumes you're using a distribution like Ubuntu or CentOS.
3. OpenVPN Installed: Make sure you have OpenVPN installed on your server. You can install it using the package manager of your Linux distribution.

Step-by-Step Guide:

1. Install Required Packages:bashCopy codesudo apt-get update
   sudo apt-get install openvpn iptables
   
2. Enable IP Forwarding:bashCopy codesudo sysctl -w net.ipv4.ip_forward=1
   To make this change permanent, edit /etc/sysctl.conf and set net.ipv4.ip_forward = 1.
3. Configure OpenVPN Server:
   • Create a configuration file for OpenVPN, e.g., /etc/openvpn/server.conf.
   • In this file, ensure you have the following lines:plaintextCopy codepush "route <subnet_to_route> <subnet_mask>"
     push "redirect-gateway def1 bypass-dhcp"
     Replace <subnet_to_route> and <subnet_mask> with the specific subnets you want to route through the VPN. This allows traffic destined for these subnets to be sent over the VPN.
4. Restart OpenVPN:bashCopy codesudo systemctl restart openvpn
   
5. Policy-Based Routing:
   • Use iptables to implement policy-based routing. Create a script (e.g., /etc/openvpn/routing.sh) with the following content:bashCopy code#!/bin/sh
     ip route add <subnet_to_route> via <gateway> dev <interface>
     Make the script executable:bashCopy codesudo chmod +x /etc/openvpn/routing.sh
     Replace <subnet_to_route>, <gateway>, and <interface> with your specific values.
6. Modify OpenVPN Server Config:Edit your OpenVPN server config (/etc/openvpn/server.conf) to include the following line:plaintextCopy codeup /etc/openvpn/routing.sh
   This will run the routing script every time the OpenVPN server starts.
7. Restart OpenVPN Server:bashCopy codesudo systemctl restart openvpn
   

Notes:

• Ensure that the client configuration specifies redirect-gateway def1 to force all client traffic through the VPN.
• Make sure to adapt the steps to your specific environment and requirements. This is a general guide and may need adjustments depending on your server's setup.
• Always be cautious when making network configuration changes, as they can potentially disrupt your server's connectivity. It's recommended to perform these changes in a controlled environment and have a backup plan in case something goes wrong.

Remember to consult the documentation specific to your operating system and OpenVPN version, as there might be differences in configuration options or file paths.