Several ethernet interfaces + encapsulation over IP

At work I have a Linux (kernel 2.4) box with 5 ethernet interfaces.
Four of them connect to an arbitrary traffic generator, and the
fifth connects to a subset of the site LAN. This is box A.

On another LAN segment I have another box with the same configuration,
except that the work area is too far away to easily connect to the
traffic generator. Because of company politics this has to stay where
it is... Anyway, this is box B.

I would like to be able to capture the traffic (always
layer 2 ethernet) sent into box A, encapsulate it somehow onto the
LAN (which I'll make sure has sufficient capacity), and then transmit
it out the corresponding interface on box B, thusly:

h eth0 eth0 h
j eth1 box A eth4 -//- eth4 box B eth1 j
k eth2 eth2 k
m eth3 eth3 m

The goal is that traffic sent into eth0 on box A would come
out eth0 on box B, and so on. Note that this is layer 2 (bridged)
traffic, not routed (layer 3) traffic...

Is there some simple (or difficult but pre-existing) way to do this?
Or can anyone point me to some tips on kernel hacking / ethernet
interface driver writing?

Thanks in advance for any pointers you can offer.

-TC
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