setup bridging br0 eth0 on Centos or RHEL for KVM virtual machines

There were several places online I could've visited to help me get bridging going on my system...They all seemed to be written for a home user though.

I recently setup a new CentOS 5.4 workstation on a machine here at work, and have been planning on moving everything over to that from my Mac Management station. Currently using a Macbook Pro as a management station but I'd like to free it up to do other things...

Also I wanted to do some KVM virtualizing because thats how RedHat does it, and thats our vendor...so...

For starters, I needed to let any VM's I created have bridged access to the network... This was probably the most difficult part, because it was just a mixed-bag when it came to how other people had done it. Here's how I did it, and it works for me.

The interface is an Intel gigabit card (wasn't supported until 5.3 out of the box!), so 1 physical interface formerly known as eth0.

`yum install bridge-utils` will install the proper utilities to enable bridging, and should also enable the kernel module on most distros.

I guess the tricky thing is, when you use bridging, you're actually kind of saying "ok bye eth0, we're going to replace you with br0 and then let br0 hand out access to any other interfaces" - This is hard for some people to grasp...

So goto /etc/sysconfig/network-scripts and `cp ifcfg-eth0 ifcfg-br0`

Then edit ifcfg-br0 to contain the following information:
DEVICE=br0
BOOTPROTO=static
ONBOOT=yes
NETMASK=255.255.255.0
IPADDR=x.x.x.x
GATEWAY= x.x.x.1
TYPE=Bridge
USERCTL=no
IPV6INIT=no
PEERDNS=yes



Then edit ifcfg-eth0, it should just have
DEVICE=eth0
ONBOOT=yes
BRIDGE=br0

-
You should then reboot. your main interface will have become br0, and eth0 will just be sitting there...waiting...for something.

Anyway,
You can use brctl show to show the current bridges on your system, and it gets more complicated from there on out.

You can then assign eth0 to virtual machines in the Virt Manager and the vm will use that as its bridge access. ;)