Outbound Traffic Flow with Any Flat Networking

In any set up with FlatNetworking, the host with nova-network on it is responsible for forwarding traffic from the private network configured with the fixed_range=... directive in nova.conf. This host needs to have a bridge interface (e.g., br100) configured and talking to any other nodes that are hosting VMs. With either of the Flat Networking options, the default gateway for the virtual machines is set to the host which is running nova-network.

When a virtual machine sends traffic out to the public networks, it sends it first to its default gateway, which is where nova-network is configured.

 

Figure 9.6. Single adaptor hosts, first route

Next, the host on which nova-network is configured acts as a router and forwards the traffic out to the Internet.

 

Figure 9.7. Single adaptor hosts, second route

	Warning
	If you're using a single interface, then that interface (often eth0) needs to be set into promiscuous mode for the forwarding to happen correctly. This does not appear to be needed if you're running with physical hosts that have and use two interfaces.