One last trick you might like to consider: if your Linux host is connected to an Ethernet, you can apply accounting rules to all of the data from the segment, not only that which it is transmitted by or destined for it. Your host will passively listen to all of the data on the segment and count it.
You should first turn IP forwarding off on your Linux host so that it doesn't try to route the packets it receives.[1] You can do so by running this command:
[1] This isn't a good thing to do if your Linux machine serves as a router. If you disable IP forwarding, it will cease to route! Do this only on a machine with a single physical network interface.
# echo 0 >/proc/sys/net/ipv4/ip_forwardYou should then enable promiscuous mode on your Ethernet interface using the ifconfig command. Enabling promiscuous mode for an Ethernet device causes it to deliver all packets to the operating system rather than only those with its Ethernet address as the destination. This is only relevant if the device is connected to a broadcast medium (such as unswitched Ethernet). For example, to enable promiscuous mode on interface etH1:
# ifconfig eth1 promiscNow you can establish accounting rules that allow you to collect information about the packets flowing across your Ethernet without involving your Linux accounting host in the route at all.