tcpdump -i ethX -w capturefile net x.x.x.x/y
Where ethX the card which receives this traffic and
x.x.x.x/y is the cidr of the wireless network. This should capture anything coming or going to this network and save it to "capturefile" file. Add
and port 80 in the end if you want only web traffic.
This looks like a "honeypot" setup. If you are trying to capture http sessions and/or other private information, this is illegal. Even if those users are trying to steal your internet connection.
Unless you've acquired permission from those who use the wireless network, this is probably illegal.
I thought you already had setup the rerouting part. If you're asking how to reroute the specific traffic, it is possible but it depends on the hardware you have and probably I can't help you on that.