MRTG and catching data from LAN IP's ,how ??

i have MRTG now runing for a while, made some snity script to collect
all kind of data.
but the only thing i still cant get to work is, how to catch trafic data
from every network interface on my local network.

i can catch data that goes in and out of my openbsd machine, but i want
for each interface in my network seperate data.
darkstat can provide me the data, but i dont manage to make a script to
rip the data from it.

on my 1st router box, freesco, i got it working, but that was using
ipfwadm, and openbsd is using pfctl...
any usefull advice what i do ?
thnxs.
Tony.