https://github.com/RMerl/asuswrt-merlin/wiki/Policy-based-routing
How-To: OpenVPN – selective Routing and Killswitch on Asus with Merlin WRT
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Tag Archives: OpenVPN
Running multiple OpenVPN instances (multi port)
https://www.hugeserver.com/kb/openvpn-multiple-ports/
How To Install and Configure OpenVPN On Your DD-WRT Router
https://www.howtogeek.com/64433/how-to-install-and-configure-openvpn-on-your-dd-wrt-router/
Tutorial: Installing OpenVPN on Ubuntu 16.04
To configure iptables look here:
https://www.digitalocean.com/community/tutorials/how-to-set-up-an-openvpn-server-on-ubuntu-16-04
Site-To-Site Routed VPN Between Two or More Routers using dd-wrt/openvpn
Install OpenVPN Access Server
https://www.linode.com/docs/networking/vpn/install-openvpn-access-server-on-linux