How do I share Internet connection with LAN mate using OpenVPN?
VPN is not good on local networks. The only way you can share the connection is to set up internet connection sharing, but this gives no control on who can use it. You would be better off using server software (maybe linux on on old box), pass the connection through this, and you can then allow only specific machines external access using the firewall. This also takes a large load off your machine, as you can unload 1 network card.
