I have an idea I'd like to float to the smart people of serverFault to pick holes in it.
I'm looking for a way to lock down a 3rd party application in IIS. It's a web service, so there's no login page or anything, it's meant for use in a VPN environment. I'm trying to put it online without a VPN and am thinking of ways to add some sort of security to it. I need to restrict it to certain networks, its a business product, so I can probably get away with saying that you need to be on a private network (ie not public wifi) to use it. My idea is to use IP Address Restriction in IIS, and write an app that the users install and have it update the server with their current IP every few minutes, the server then blocks all except the ones recently updated.
How secure would this be? Is there a major flaw in this idea? Or is there perhaps a better way to do this in IIS?