CentOS / Redhat: KVM Bridged Network Configuration

With bridged networking you can share actual network device with KVM machines. This is required for servers with multiple network cards and gives you good performance. You can choose to put multiple segments into one bridged network or to divide it into different networks interconnected by routers.

Our Sample Setup

The following describes the networking used by our setup:

  +------+       +-----+
LAN      --> | eth0 |  <==> | br0 |       -> KVM VMs connected to LAN for SAN/NFS
  +------+       +-----+
                               10.10.x.y/24
  +------+       +-----+
Internet --> | eth1 |  <==> | br1 |       -> KVM VMs connected to the Inernet
  +------+       +-----+
                               123.1.2.0/28

Where,

• All other clients can reached to all VMs via br1 which is connected to public interface. br1 is our default gateway.
• br0 is connected to private LAN to access other servers, services and storage devices such as SAN/NAS or NFS servers. br0 route is configured via route-br0 static networking configuration file.

Turn Off NetworkManager

The NetworkManager (GUI) tool can create problems with bridged based networking so disable it as follows, enter:
# chkconfig NetworkManager off
# chkconfig network on
# service NetworkManager stop
Edit /etc/sysconfig/network, enter:
# vi /etc/sysconfig/network
Update file as follows:

 
NETWORKING=yes
HOSTNAME=kvm42.nixcraft.net
GATEWAY=br1
 

Save and close the file.

br0: Configure Bridging for eth0

Edit /etc/sysconfig/network-scripts/ifcfg-eth0, enter:
# vi /etc/sysconfig/network-scripts/ifcfg-eth0
Update it as follows:

 
 
DEVICE=eth0
ONBOOT=yes
ETHTOOL_OPTS="autoneg off speed 100 duplex full"
BRIDGE=br0
HWADDR=b8:ac:6f:65:31:e5
 

Save and close the file. Create /etc/sysconfig/network-scripts/ifcfg-br0, enter:
# vi /etc/sysconfig/network-scripts/ifcfg-br0
Update it as follows (note options are case sensitive i.e. Bridge and bridge are two different options)

 
DEVICE=br0
TYPE=Bridge
BOOTPROTO=static
ONBOOT=yes
IPADDR=10.10.21.70
NETMASK=255.255.255.192
DELAY=0
 

Save and close the file. Finally, create static routing file /etc/sysconfig/network-scripts/route-br0, enter :
# vi /etc/sysconfig/network-scripts/route-br0
Edit it as follows:

10.0.0.0/8 via 10.10.21.122 dev br0

Save and close the file. Delete old /etc/sysconfig/network-scripts/route-eth0 (if exists):
# rm /etc/sysconfig/network-scripts/route-eth0

br1: Configure Bridging for eth1

Edit /etc/sysconfig/network-scripts/ifcfg-eth1, enter:
# vi /etc/sysconfig/network-scripts/ifcfg-eth1
Update it as follows:

 
 
DEVICE=eth1
ONBOOT=yes
ETHTOOL_OPTS="autoneg off speed 100 duplex full"
BRIDGE=br1
HWADDR=00:30:49:8c:48:ad
 

Please note that test server is set to 100Mbps full duplex. Save and close the file. Edit /etc/sysconfig/network-scripts/ifcfg-br1, enter:
# vi /etc/sysconfig/network-scripts/ifcfg-br1
Define public IP address including gateway as follows:

 
DEVICE=br1
BOOTPROTO=static
ONBOOT=yes
IPADDR=123.1.2.3
NETMASK=255.255.255.248
GATEWAY=123.1.2.200
TYPE=Bridge
DELAY=0
 

Save and close the file.

Restart Network Service

Type the following command:
# service network restart
Make sure everything is working fine:
# brctl show
Sample outputs:

bridge name bridge id  STP enabled interfaces
br0  8000.0030488e31ac no  eth0
br1  8000.0030488e31ad no  eth1
virbr0  8000.000000000000 yes

Verify IPs and routing tables:
# ip addr show br0
# ip addr show br1
# ip route
# ping cyberciti.biz
# host google.com