Linux router: bandwidth of client

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http://serverfault.com – I am trying to build a linux router (captive portal) with a local RADIUS server for authentication. I want to log the bandwidth being used by each client on my network. I want to associate bandwidth at a given point in time with MAC address. My question is, how can I get the bandwidth of a current client on my network? I understand that I will need to use IP address to get this information and then look at my ARP table to determine the MAC. I understand RADIUS accounting can do this, but I do not want to use that approach at the moment. Ideally, I would use this information to write a smal (HowTos)