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Using instance with two NICs to route traffic between two subnets fails on private cloud.

The goal is route traffic from one subnet to another using a computer in the middle with two NIC's. I have successfully created the scenario on both my own computer with VirtualBox and on the cloud with Rackspace's trail cloud. The images I am using for our private cloud and virtualbox are identical and rackspace had the same version of os, just not identical.

When trying the same setup our private cloud, the middle instance is able to ping machines on both subnets. The machines on separate sunsets are able to ping the NIC on the same subnet for the middle instance, but fail when trying to ping the NIC on the other subnet. Running TCP dump on the machines, I can see that the NIC on the different subnet receives the pings and sends a response. But that response never makes it back and I can't find out where it is going. None of the NICs show any dropped packets and I have turned off all the Filtering and Firewalls that i know of that could be messing with the pings.