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I have recently installed Ubuntu 12.04 LTS server and I chose to install Virtual Machine Host as one of the packages during install.

The server has an on-board NIC and a PCI NIC and during install, the installer chose to configure the PCI NIC. I then installed the on-board NIC and swapped the labels (eth0 & eth1) around and gave them static IP addresses. Coincidently, I'm not actively using the PCI NIC (now eth1) yet (no plan to use multiples at this stage).

The problem I have is that the install process created a bridged network adapter but it's not in /etc/network/interfaces, but I can see it when I run ifconfig.

cyntech@multiplex:/etc/network$ ifconfig
eth0      Link encap:Ethernet  HWaddr 94:de:80:07:c7:4f
          inet addr:  Bcast:  Mask:
          inet6 addr: fe80::96de:80ff:fe07:c74f/64 Scope:Link
          RX packets:1403 errors:0 dropped:136 overruns:0 frame:0
          TX packets:316 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:206654 (206.6 KB)  TX bytes:79027 (79.0 KB)

lo        Link encap:Local Loopback
          inet addr:  Mask:
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:88 errors:0 dropped:0 overruns:0 frame:0
          TX packets:88 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:6644 (6.6 KB)  TX bytes:6644 (6.6 KB)

virbr0    Link encap:Ethernet  HWaddr c6:3a:d2:2b:37:a8
          inet addr:  Bcast:  Mask:
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

My /etc/network/interfaces file:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static

auto eth1
iface eth0 inet static

brctl show displays:

cyntech@multiplex:~$ brctl show
bridge name bridge id          STP enabled  interfaces
br0         8000.94de8007c74f  no           eth0
virbr0      8000.000000000000  yes

I can't find anything on where I might find where virbr0 is configured or how to change it as I'm assuming that it would be bridged with eth0, which is now eth1. Or does it not matter which interface is eth0, it just references 'eth0'?

share|improve this question
What does 'brctl show' (or something like that, I'm not near a computer that I check this)? Anyway, if I remember right, that bridge is used for an internal network, not to connect directly to Internet through the bridge. So no, I don't think this is wrong. –  Anders Jun 16 '13 at 17:04
It shows '8000.000000000000' for the bridge ID, it's STP enabled but nothing for interfaces. I've since found that it's the bridge for the default Usermode networking (NAT). I still can't find where it's defined, however. –  Cyntech Jun 17 '13 at 5:15
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