Wiki VPN
OpenVPN: how secure virtual private networks really work
!!!Favorite explanation
ip - How do VPN's forward network traffic? (Layer 3) - Network Engineering Stack Exchange
Routers Support for VPN(OpenVPN) Client
How to set up a router with Surfshark? – Surfshark Customer Support
Routers Supporting VPN Client - Home Network Community
Kill Switch
KillSwitch could be used to block outgoing traffic when the VPN connection drops and crashes.
PF(packet filter) MacOS
Setting up correctly Packet Filter (pf) firewall on any macOS
Prevent outgoing traffic unless OpenVPN connection is active using pf.conf on Mac OS X
Quick and easy pf (packet filter) firewall rules on macOS
A Cheat Sheet For Using pf in OS X Lion and Up
Set Up Firewall to Allow Access Only via VPN(KillSwitch)
ENABLING VPN-ONLY ACCESS TO THE INTERNET WITH WINDOWS FIREWALL
Prevent outgoing traffic unless OpenVPN connection is active using pf.conf on Mac OS X