When using ipf's "fastroute" option to redirect a packet out an
interface, what controls the next-hop address used?

For example, if I do

	pass out quick fastroute le1 on le0 from 10.0.0.0/8 to any

to redirect all traffic that would normally go out le0, but which comes
from 10/8, out le1 instead, how do I control the next-hop gateway
address used when sending the packet out le1?  (For example, if the arp
table for le1 is empty, what controls what address will be arped for?)

RTFMs will be welcomed provided they are accompanied by a pointer to
the FM that contains the answer; it doesn't seem to be anywhere in the
manpages I have at hand, including current's basesrc/dist/ipf/man.

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