And there's:
Code:
reply-to
The reply-to option is similar to route-to, but routes packets that
pass in the opposite direction (replies) to the specified inter-
face. Opposite direction is only defined in the context of a state
entry, and reply-to is useful only in rules that create state. It
can be used on systems with multiple external connections to route
all outgoing packets of a connection through the interface the
incoming connection arrived through (symmetric routing enforce-
ment).