This tutorial describes how to set up a dedicated network bridge for KVM slave (host) CentOS 7 server. The tutorial presumes the network interface is eth0 and the network bridge will be called br0.

Who is this guide for?

This tutorial is only intended for skilled network administrators and requires root-level access.

EDITOR'S NOTE: Any errors made in this process can be very serious-often resulting in corrupted network configuration and inaccessible server.

Exercise great care and make backups as often as possible when making changes to your server's network interface configuration.

What is Network Bridge?

A network bridge is a computer networking device that creates a single, aggregate network from multiple communication networks or network segment.

--Wikipedia, "Bridging (networking)"

A network bridge, works by intentionally exposing the connection (or "Bridge") between two network interfaces. By exposing this networking (intentionally, and, internally), the other VMs (guests) on the host are able create network connections in concert with the host's interfaces.

How to Create Network Bridge for KVM Hypervisor (Using CentOS 7)

Prerequisite:

Bridging requires the bridge-utils package to be installed on the server.

Check if bridge-utils is installed by running this command on the terminal, as root user:

[root@dev1 ~]# rpm -q bridge-utils

If you get an output - it's installed, if not, it needs installing:

[root@dev1 ~]# yum install bridge-utils -y

Installing the Network Bridge and Making it Permanent

As root, run the following command to view the contents of the primary network interface: 

[root@dev1 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0

The contents should be similar to:

TYPE="Ethernet"
PROXY_METHOD="none"
BROWSER_ONLY="no"
BOOTPROTO="none"
DEFROUTE="yes"
IPV4_FAILURE_FATAL="no"
IPV6INIT="yes"
IPV6_AUTOCONF="yes"
IPV6_DEFROUTE="yes"
IPV6_FAILURE_FATAL="no"
IPV6_ADDR_GEN_MODE="stable-privacy"
NAME="eth0"
UUID="8d6f722c-b945-4083-b50e-9661bf62ae5f"
DEVICE="enp1s0f0"
ONBOOT="yes"
IPADDR="192.0.0.4"
PREFIX="23"
GATEWAY="192.0.0.1"
DNS1="8.8.8.8"
IPV6_PRIVACY="no"

First, we need to create a backup of the current ifcfg-eth0 in case we need to revert back.

1. Create the backup by running the following command:

   [root@dev1 ~]# cp /etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network-scripts/backup-ifcfg-eth0

2. Now, we will create the bridge file:

   [root@dev1 ~]# nano -w /etc/sysconfig/network-scripts/ifcfg-br0

3. Copy parts of ifcfg-eth0 to it and edit the config file as per the example below obviously replacing the parts unique to your server:

   TYPE=Bridge
   BOOTPROTO=none
   DEFROUTE=yes
   IPV4_FAILURE_FATAL=no
   IPV6INIT=no
   IPV6_AUTOCONF=yes
   IPV6_DEFROUTE=yes
   IPV6_PEERDNS=yes
   IPV6_PEERROUTES=yes
   IPV6_FAILURE_FATAL=no
   NAME=bridge0
   DEVICE=br0
   ONBOOT=yes
   IPADDR=192.0.0.4
   PREFIX=23
   GATEWAY=192.0.0.1
   DNS1=8.8.8.8
   DNS2=8.8.4.4
   IPV6_PRIVACY=no
   IPV6_PEERDNS=yes
   IPV6_PEERROUTES=yes
   HWADDR=ac:1f:6b:94:55:22

4. Save that file and edit ifcfg-eth0:

   nano -w /etc/sysconfig/network-scripts/ifcfg-eth0

5. Remove the networking parts and specify the bridge:

   TYPE=Ethernet
   NAME=eth0
   UUID=8d6f722c-b945-4083-b50e-9661bf62ae5f
   DEVICE=eth0
   ONBOOT=yes
   BRIDGE=br0
   HWADDR=ac:1f:6b:94:55:22

6. You know have a network Bridge set up.

7. Restart the network:

   [root@dev1 ~]# /etc/init.d/network restart

8. Once it's restarted you see the new bridge with ifconfig command (for RHEL 7 and later, replace ifconfig with ip addr)

   [root@dev1 ~]# ifconfig
   br0       Link encap:Ethernet  HWaddr ac:1f:6b:94:55:22
             inet addr:192.0.0.4  Bcast:192.0.0.255  Mask:255.255.255.0
             inet6 addr: fe80::227:eff:fe09:cb2/64 Scope:Link
             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
             RX packets:48 errors:0 dropped:0 overruns:0 frame:0
             TX packets:67 errors:0 dropped:0 overruns:0 carrier:0
             collisions:0 txqueuelen:0
             RX bytes:2984 (2.9 KiB)  TX bytes:13154 (12.8 KiB)
    
   eth0      Link encap:Ethernet  HWaddr 00:27:0E:09:0C:B2
             inet6 addr: fe80::227:eff:fe09:cb2/64 Scope:Link
             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
             RX packets:31613 errors:0 dropped:0 overruns:0 frame:0
             TX packets:9564 errors:0 dropped:0 overruns:0 carrier:0
             collisions:0 txqueuelen:100
             RX bytes:2981335 (2.8 MiB)  TX bytes:2880868 (2.7 MiB)
             Memory:d0700000-d0720000

The above output shows now all the IP Address information, which previously was configured with eth0, is now configured at the br0 interface-which is the expected result. Any subsequent guests created on this host will give similar output as eth0 interface. The network bridge is the only interface with exposed public IPv4 or IPv6 details in the output of ifconfig or ip addr.

You have successfully installed the Network Bridge on KVM hypervisor host server.