Configuring site to site VPN using two m0n0wall boxes is a snap. There are a couple of things you will want to know ahead of time. The IP address or FQDN of both firewalls WAN interface. Internal subnet of both networks. The shared key (password). Log into your firewall and head to the VPN/IPSec section. Click the + sign to get started. Assuming you are connecting two boxes across the internet, set the Interface to “WAN”.