> My goal is to combine the bandwidth of two ADSL connections.  Both
> connections have their own gateways and their IP addresses are on
> different subnets.

I'm not sure this *can* work.  How do packets _to_ you work?  In
particular, what address do you use for the ip_src of outgoing traffic?

If bandwidth use is heavily asymmetric, with outgoing dominant, this
may not be an issue in practice, but the "other" upstream will have to
allow you to send "from" a "forged" address on that connection.  (This
is something that providers should never allow by default, but far too
many do, giving us spammers doing asymmetric routing to, as far as the
rest of the world can tell, spew at high-speed rates from dialup
connections.)

If outgoing traffic gets the IP address suitable for the interface it
actually takes, then you have issues with TCP (or other
connection-based protocols).  If you have a lot of connections sharing
the lines, especially if they're short-lived, you may be able to just
let it pick a path at connection setup and use it forever, but that's a
rather special case.

Once you settle these issues, they may drive your choice of method for
actually doing it.

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