Wi-Fi router with OpenWrt to sniff for MAC addresses

We need to install a passive security system that keep track of smartphones entering specific areas. Our idea is to use a Wi-Fi router with OpenWrt and develop (or adapt) a module that "sniff" packet exchange before a smartphone connects to it. As far as we know, a smartphone with Wi-Fi enabled routinively looks for networks nearby and doing so it exchange something with the router. Inside these packets there is the mac address of the device and this should be: 1) stored in the memory of the router with a timestamp 2) if there is wired connection with a central server, it should be sent to a web service 3) if there is not a wired connection, the router should wait a specified amount of time and try again to send the mac adress to the central server This software should be installed on multiple routers, so along with the specific data of the smartphone, you should send also router data to identify it. We are asking for commented source code, so you should work just as developers and the Intellectual Property will be on our side.