I have a Linux box operating as router. There's a NIC that's connected to the internet (WAN), a NIC connected to an 8-port GbE switch (LAN), and a NIC connected to a Linksys wireless N-router (WLAN). Routing between everything is working perfectly.

I have security completely disabled on the wireless router, but the WLAN NIC is firewalled such that it will only accept DNS queries and PPTP VPN connections. Currently HTTP/HTTPS traffic and everything else is blocked.

I would like to run something that listens on port 80/443 of the WLAN NIC, and, for non VPN'ed connections, given any HTTP/HTTPS request it will return a single webpage saying "Unauthenticated" and explain how to sign into the VPN. A transparent proxy seems to be what I need, but my searches all seem to direct me to Squid, which is already running on my server and seems overkill for this simple task.

Is there a simpler, lightweight program out there that does just this or should I just suck it up and run two instances of Squid (or figure out how to configure it)?

Or, is this entire VPN thing I'm doing complete nonsense and I should just enable encryption on the wireless router?