Monitor the downloaded data of a single LAN host

[Rocky Hotas <rockyhotas%firemail.cc@localhost>]

Hello!
Consider the following setup:

LAN with local hosts <-> gateway running NetBSD <-> internet

Is it possible from the gateway to measure the amount of data fetched
from the internet by a single LAN host over a speficic amount of time?

Is there some networking tool which can provide this measurement?

That LAN host should be identified through its IP address or MAC
address and the measurement should regard in particular the data
downloaded by the host.

Sometimes vnstat is suggested in similar cases, but AFAIU it seems to
measure only the amount of data sent or received from a local NIC,
so it wouldn't be useful (if run on the gateway it would
record the whole LAN activity involving the gateway).

Bye!

Rocky