> Further, to my knowledge, you can't tell NetBSD "route packets
> between interfaces 3, 4, 5 and do not route packets received on
> interface 1 and 2",

Not stock NetBSD, maybe, but it's really not hard to add.  A few years
back I added interface flags NOFWFROM and NOFWTO, which when set
prevent packet routing from and to (respectively) the interface(s) the
bits are set on - at least IPv4 packet forwarding; the environment I
wanted them for didn't care about anything else.  While I haven't tried
to roll them forward to -current, unless the packet-forwarding
architecture has changed drastically, it'd be quite simple.

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