You may want to make sure A is working cleanly before connecting to B. I'd use a X-over cable via...
> +--------+ > |A | > | eth0+---+ > | | | > | eth1+---+ > | | > +--------+ Talking to itself, you can eliminate any problems there. Granted, depending upon the complexity of your rules, you may have to change them to do the actual test with $FIRMs box.
