bridging for kvm

A basic layer 2 bridging how-to for virtualization like KVM in debian.

Many times, in KVM hosts we need to bridge the host's physical network interface with the virtual network interfaces used by the KVM guests.

Install the bridge utilities

# apt-get install bridge-utils

List network interfaces

# ip a|grep ":\ "
1: lo:  mtu 16436 qdisc noqueue state UNKNOWN 
2: eth0:  mtu 1500 qdisc pfifo_fast state UP qlen 1000

Inspect the ethernet bridge(s)

# brctl show
bridge name bridge id  STP enabled interfaces
# 

None yet.

Create a bridge instance that you can access from an interface named b0.

# brctl addbr b0

Show bridge

# brctl show
bridge name bridge id  STP enabled interfaces
b0  8000.000000000000 no  

You may add the physical interface(s) to the bridge.

# brctl addif b0 eth0

However, do not try it if you are working on a remote host.
See below how to adjust the interfaces file instead.

Delete b0

# brctl delbr b0

Adjust /etc/network/interfaces to create a "persistent" bridge and restart networking.

# vi /etc/network/interfaces
# /etc/init.d/networking restart

An example /etc/network/interfaces file where the host has the IP address 192.0.2.29/25 and the interface to the bridge is called b0.

auto lo
iface lo inet loopback


auto eth0
iface eth0 inet manual

auto b0
iface b0 inet static
 address 192.0.2.29
 netmask 255.255.255.128
 network 192.0.2.0
 broadcast 192.0.2.127
 gateway 192.0.2.10
 dns-nameservers 192.0.2.4
 dns-search ipduh.rocks
        bridge_ports eth0
        bridge_stp on           #spanning tree 
        bridge_waitport 0       #no delay before a port becomes available
        bridge_fd 0             #no forwarding delay
        bridge_maxwait 0

Inspect bridge.

# brctl show
bridge name bridge id         STP enabled interfaces
b0          8000.40167e6d6745 yes         eth0

List network interfaces.

# ip a|grep ":\ "
1: lo:  mtu 16436 qdisc noqueue state UNKNOWN 
2: eth0:  mtu 1500 qdisc pfifo_fast master b0 state UP qlen 1000
4: b0:  mtu 1500 qdisc noqueue state UP 

Add a KVM host (anaxagoras) and inspect the bridge.

# brctl show
bridge name bridge id         STP enabled interfaces
b0          8000.40167e6d6745 yes         anaxagoras
                                          eth0

The system used.

# cat /etc/issue /etc/debian_version 
Debian GNU/Linux 7 \n \l

7.6
# uname -r
3.2.0-4-amd64

L2 bridging for KVM