IFF_SIMPLEX vs bridge(4)

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I've just run into a case where IFF_SIMPLEX is interacting badly with
bridge(4).

The setup: 4.0/i386.  Two interfaces are in a bridge.  One of them
(vr0) is simplex (IFF_SIMPLEX is set).  An ARP request is received on
the other interface, generated by an external event.  This packet is
bridged through to vr0.  But ether_output on vr0 says "oh, this
interface is simplex and this packet is broadcast, we'll loop back a
copy".  Thus, the barest instant later, vr0 receives a copy of the very
same packet.

This causes the bridge to end up with the sending MAC address learnt on
vr0 rather than the interface it really is on.  This then causes the
ARP reply, which is aimed at that MAC address, to go unbridged
(because, as far as the bridge is concerned, it was received on the
same interface it should go out on and thus shouldn't be bridged
anywhere).

This leads to the obvious communication failure.

Thoughts on what the correct fix is?

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