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I have a Ubuntu 12.04 LTS machine running KVM/libvirt. It has two physical ethernet interfaces. I have dedicated eth0 as the primary interface and configured bridging on eth1 for my VMs. The KVM host /etc/network/interfaces looks like this:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static

# The secondary network interface
auto br0
iface br0 inet static
    bridge_ports eth1
    bridge_stp off
    bridge_fd 0
    bridge_maxwait 0

My KVM guests have IP addresses on the network. Everything is working as expected.

My question is: if I want to change the network address to a different network altogether, do I simply change it from within the guest OS and then plug my eth1 interface in on the other network...or does the bridging interface need to be reconfigured on the host? If so, how? I am trying to understand the role of the IP assignment on the br0 virtual interface.

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The IP assignment on the br0 interface is so the host system can talk on that interface. Since you're using the separate network card for the host, you can safely turn off the IP address. How to accomplish that with Ubuntu /etc/network/interfaces, I dunno, but I've done it on Centos.

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When you say "turn off the IP address" - can you elaborate as to what you mean? I cant down the interface of course, but I tried to remove the address, net mask, and gateway for eth1 in /etc/network/interfaces and the result is that the KVM guests can no longer get out - despite having no changes to their network configurations. – dean Feb 15 '14 at 1:10
I guess I can use iface eth1 inet manual – dean Feb 15 '14 at 1:32

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