Bridged Networking Setup For KVM Virtualization

A bridged networking setup is typically used in KVM(Kernel-based virtual machine) to allow the virtual machines to access the LAN/WAN over the IP address on the physical host. This guide is focused on a CentOS or similar derivative configuration setup. This is the second part of a multi-part series in setting up KVM services on a server. If you havent already please view our guide on KVM installation on CentOS

First Install the bridged networking package:

yum -y install bridge-utils

The package bridge-utils contains the utilities for configuring  Ethernet bridges.

Next Reconfigure the network interface:

In this example we are going to reconfigure eth1 to be a bridged interface:

First go to the network-scripts directory:

cd /etc/sysconfig/network-scripts/

Rename the network interface:

mv ifcfg-eth1 ifcfg-bridge1

Edit ifcfg-bridge1:

Before it was this:

DEVICE=eth1
TYPE=Ethernet
ONBOOT=yes
NM_CONTROLLED=no
BOOTPROTO=static
HWADDR=00:1C:23:E1:54:58
IPADDR=192.168.1.80
PREFIX=24
GATEWAY=192.168.1.1
DNS1=8.8.8.8
DNS2=8.8.4.4
DEFROUTE=yes

We are going to change it to:

DEVICE=bridge1
TYPE=Bridge
ONBOOT=yes
NM_CONTROLLED=no
BOOTPROTO=static
IPADDR=192.168.1.80
PREFIX=24
GATEWAY=192.168.1.1
DNS1=8.8.8.8
DNS2=8.8.4.4
DEFROUTE=yes

DEVICE was changed to a new name, type was set to ‘Bridge’ and the HWADDR was removed.

We are going to create a new ifcfg-eth1 for eth1 now that will contain the following:

DEVICE=eth1
ONBOOT=yes
HWADDR=00:1C:23:E1:54:58
BRIDGE=bridge1

DEVICE sets the device name, we copied the HWADDR when we removed it from the bridge1 file and we set BRIDGE=bridge1.

Lastly Load the new settings:

Go ahead and restart the network to bring up the bridged interface and eth1 again

CentOS 6:

/etc/init.d/network restart

CentOS 7:

systemctl restart network

Type the following to confirm the new settings are working correctly:

ifconfig -a

You should see:

bridge1 Link encap:Ethernet HWaddr 00:1C:23:E1:54:58
 inet addr:192.168.1.80 Bcast:192.168.1.255 Mask:255.255.255.0
 inet6 addr: fe80::21c:23ff:fee1:5458/64 Scope:Link
 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
 RX packets:92 errors:0 dropped:0 overruns:0 frame:0
 TX packets:30 errors:0 dropped:0 overruns:0 carrier:0
 collisions:0 txqueuelen:0
 RX bytes:11935 (11.6 KiB) TX bytes:3563 (3.4 KiB)

eth1 Link encap:Ethernet HWaddr 00:1C:23:E1:54:58
 inet6 addr: fe80::21c:23ff:fee1:5458/64 Scope:Link
 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
 RX packets:87 errors:0 dropped:0 overruns:0 frame:0
 TX packets:25 errors:0 dropped:0 overruns:0 carrier:0
 collisions:0 txqueuelen:1000
 RX bytes:13117 (12.8 KiB) TX bytes:3325 (3.2 KiB)
 Interrupt:17

That is it for this portion on setting up networking for KVM.

Part 1: KVM Installation on CentOS
Part 2: Bridged Networking Setup For KVM Virtualization
Part 3: Creating A New Virtual Machine with KVM