Again two ADSL lines, routing problems

Hello.
I have a setup where a FreeBSD box is connected to two ADSL routers: default gateway is set to the first and, in case of failure, is moved to the other one. This works perfectly for outgoing connections: in the event of the switch, I'll have to reconnect, but that's acceptable.

The problem is in the incoming connections: if I get one on the "backup" router, this will reach the server, which will however answer through its "default" router. Thus the remote client will see packets coming back from a different host and things won't work.
Just to be clear, the packets travel as follows (with source and dest IP in brackets):
Client (x.x.x.x) -> Backup router (y.y.y.y)
Backup router (x.x.x.x) -> Server (z.z.z.z)
Server (z.z.z.z) -> Default router (x.x.x.x)
Default router (v.v.v.v) -> Client (x.x.x.x)

So the client (x.x.x.x) connects to y.y.y.y (the backup ADSL public IP), but gets answers from v.v.v.v (the master ADSL public IP).


AFAIK there is no solution to this, but I tought I'd ask before giving my official opinion to my customer.
Perhaps there's some sort of hack we could use, that through ipfw/natd/other diverting daemon/whatever delivers answers based on the MAC address of the incoming connections (if the MAC address belongs to the backup router, use that for answers)... does anyone know?

bye & Thanks
av.
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