Login page for restaurant hotspot access?

[sethbest]

I have a client that has been asking me about a way to monitor internet activities or to get email addresses for people who use his wifi hotspots. He owns a chain of restaurants and essentially wants a system that retrieves people's email addresses when they connect so he can send them promotional emails.

Personally I think this is a terrible idea, but professionally I have to give him my opinion. I was going to suggest that he have a sign in page that the router directs traffic to before it gives full connectivity to the user. This page would have a user agreement that allows for sending promotional emails. I've seen these at airports and some other businesses and I hate them, but this seems to be the least morally objectionable way to do what he's asking rather than implementing a shady web traffic monitoring system.

I'm pretty sure all this would take would be to have the DHCP managed by a server that through the dns sends all traffic to a page that requires registering a login before granting full access to the network. Has anyone done something like this themselves, or have a suggestion for a method or application that aids in something like this?

Thanks.