NAT & multiple default routes

["Patrick Doyle" <wpd@delcomsys.com>]

Shortly, I will have two independent DSL connections to the internet.  In
theory, one will be my primary connection and the second one will provide
emergency backup service.  In practice, since I have the two pipes, I would
like to go ahead and use both of them as much as possible.  What I would
like to do is to arranage that connections from my NAT'ed hosts be
distributed between the two independent pipes.  Can anybody give me an idea
of where I might start searching to learn how/if this can be done?

On the one hand, I could go down the route of setting up my OpenBSD firewall
as a router, running routed or whatever other daemon(s) made sense.  But
that doesn't feel like the right thing to do, especially since I have no
expectatation that my DSL service provider have anything at the other end of
the pipes that will talk to my daemon.

Ideally, what I would like to see happen is that my firewall would track the
load on each WAN interface and dynamically assign new NAT connections to the
interface that was least loaded at the time the connection was initiated.
In the best possible scenario, I would receive some sort of automatic
notification in the event that one link went down.

Perhaps OpenBSD already does this.  If so, would somebody give me a few
keywords of where to start looking for such a feature?

Perhaps OpenBSD doesn't do this, but there is no reason why it couldn't be
added.  If so, would anybody other than I be interested in such a feature?

Perhaps OpenBSD doesn't do this because there are some very sound technical
reasons why it just won't work.  If so, would you mind telling me the
reasons so I don't waste my time on it?

In the worst possible case, I will simply arrange that half of the users use
one DLS line and the other half use the other.  (Or, possibly that _I_ use
one DSL line and everybody else uses the other :-)

TIA...

--wpd