Routing Question

[Doug Hardie]

I have several servers with one ethernet interface.  Currently it is connected via a WAN to the internet.  We are in the midst of switching to a different provider.  I would like to be able to operate with both temporarily until all the users/services get switched.  The new circuit is in and working.  I would like somehow to configure the system (I have pf in use) to be able to detect the packets that come from a specific router and route the return packets back through it.  The other network would be the default.  PF's route_to will return the packets to the proper router, but I have not been able to figure out which ones those would be.  The source IP address can be any on either network and its highly likely that we will see packets from the same source network on both at the same time.  The only distinction I see in the input packets between the two paths is the MAC address of the router.  I don't see any way in pf or the system to use that to affect the return path though.