Howto add 2nd external Network


I am running IPFire with a simple red and green setup with a routed network of 64 external IPs, i.e. my internal systems all have official IP addresses.

The red interface is configured to run on a transfer network and physically connected to my providers end-point router.

Now there a some internal systems that use up a lot of my bandwidth every once in a while and therefore I ordered a 2nd DSL line that will come with a dynamic IP and a FRITZ!box providing standard NATing for internal devices.

I would like to physically connect one of my IPFire boxes free ethernet ports to then FRITZ!box, assign one of the NATed addresses and use this line to route the traffic of my bandwidth eaters through it, if initiated from inside.

This would allow these machines to still stay in the internal network with access to printer, NAS etc. but keep the downstream traffic from the primary interface.

Is such a setup possible and if so - how?