> i can't add a mapping for an ip address that's not on my local
> network using arp(8).

>  this# arp -s 1.2.3.4 00:50:04:68:c6:11
>  cannot intuit interface index and type for 1.2.3.4

> perhaps one can do such a thing using a "raw" routing socket, but i
> wouldn't expect it.

I expect you can.  I know you can use a routing socket to add an arp
entry for an address that is on a subnet you do have an interface on,
but pointing to some other interface, to one for which it's off-subnet.
(I know this 'cause I've done it.)

I'll try to rip out bits of that program and build something a bit like
arp(8) but for which you specify everything manually, including the
interface.  I'll see what it does for stuff like your example.

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