CentOS / Redhat: KVM Bridged Network Configuration

by on January 12, 2010 · 21 comments· LAST UPDATED February 1, 2011

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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

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{ 21 comments… read them below or add one }

1 Tapas Mallick June 24, 2010 at 2:58 pm

A small typo needs to be corrected Under “br1: Configure Bridging for eth1″ heading, “vi /etc/sysconfig/network-scripts/ifcfg-eth0″ should be changed with “vi /etc/sysconfig/network-scripts/ifcfg-eth1″

Reply

2 nixCraft June 24, 2010 at 3:01 pm

Thanks for the heads up! The post has been updated.

Reply

3 Slapper October 21, 2010 at 12:24 pm

regarding the Bridging…
watch the custom options ( ETHTOOL_OPTS ) you pass in eth0, eth1 etc !!
you may find network issues in guest machines. I have three guest ( Ubuntu,Centos, Openbsd ) in my Centos host and the 3 guest had problems.Probably has to do with the model of the network card..so leave the default!!!
something like this :
DEVICE=eth0
ONBOOT=yes
BRIDGE=br0
HWADDR=XX:XX:XX:XX:XX:XX

Reply

4 Kevin D March 14, 2011 at 7:42 pm

How would a single eth0 nic grabbing its information from a DHCP server be handled?

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5 Robert June 10, 2011 at 1:44 pm

I think your GATEWAY parameter in /etc/sysconfig/network is incorrect. That parameter should be the IP address. If you want to direct to a device, then you need to use GATEWAYDEV parameter.

http://www.centos.org/docs/5/html/Deployment_Guide-en-US/ch-sysconfig.html

Thanks for the good work on your site. I always learn something new.

Reply

6 Kapil August 4, 2011 at 5:54 am

Dear Vivek,

Can U please elaborate on this entry.

10.0.0.0/8 via 10.10.21.122 dev br0

Reply

7 John Brier December 16, 2011 at 3:51 pm

I also do not know why you would need a static route. This route should be included by default.

The RHEL docs do not mention this.

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8 Ibanez August 28, 2011 at 1:08 am

How if eth1 have 2 class ip ?

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9 Cloud Strife September 16, 2011 at 6:37 am

I got up to the point where you have to edit
/etc/sysconfig/network-scripts/ifcfg-eth0

I dont have that file….
Its a clean install too….

Reply

10 Joel November 16, 2011 at 10:45 pm

Ok how do I remove the bridge?

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11 Floyd Maloto November 30, 2011 at 7:58 pm

Delete ifcfg-brX and restart the network, make sure you restore your route-eth0 file if you’re removing the private network bridge

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12 Satheesh March 14, 2012 at 8:12 am

Actually i am using wireless to connect to my modem (ifcfg-wlan0) from my host machine. if i create bridge as br0, in my host machine, will there be any change in ifcfg-br0 for wireless setup.
Also, whether i need to include type=bridge parameter in ifcfg-wlan0 file of my host machine?

Wheter i need to use eth0 or wlan0 in my kvm for connecting this bridge.

Reply

13 George March 23, 2012 at 5:45 pm

What if I have only one nic which is connected (wired) to a wireless router and internet gateway and I want my guest VMs accessible from the LAN and can access the internet as well?

Appreciating your support.

Reply

14 rtt March 26, 2012 at 10:31 pm

Don’t try this on a remote production machine with a public IP……you’ll lose connectivity. I haven’t found a guide for bridging anything yet that won’t cause a production machine to go down :(

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15 scooper March 28, 2012 at 8:27 pm

My RHEL 5.8 box has one nic with two static IP’s (one VIP) – I have a DHCP server in my environment – do I need bridging?

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16 damien May 23, 2012 at 5:39 pm

How do you map your VM to use br0 ? When adding your VM using virt-manager, which network should my VM card be added to ?

If I add to virbr0, i get mapped to the 192.168.x.x/24 private network.
I can’t seem to create a new network with the same network as br0 cause it doesn’t allow it.
Any help appreciated.

Reply

17 CheapIndianITGuy March 4, 2013 at 10:20 pm

This is so idiotic. Doing things round about way in redhat KVM virualization. Just use Oracle’s Virtual box. So much easier and not at all confusing.

Reply

18 Jay Johnson June 15, 2013 at 6:08 pm

@ CheapIndianITGuy: Why would anyone do this in Oracle virtualbox if their goal was to be an RHCSA or RHCE???? It defeats the purpose of developing a level of proficiency with RedHat KVM (a “known” RH Linux test objective) as well as gaining an overall familiarity of the OS’s functionality. Heck, even a linux enthusiast can gain from learning this method.

Personally, the guy who delves a bit deeper into the woodworks has more than a “job;” he has a “career…”

Reply

19 gohar July 6, 2013 at 12:23 pm

Thanks for the good work friends..
I am also having bit issue in my KVM host machine.

1- I have one eth0 interface in my HOST Machine.
2- KVM Installed on RHEL 6.0 and XP is running as Guest

I try making bridge from KVM GUI interface with eth0 for my KVM guests to use internet . BUt first it hangs up..i forcefully close the GUI of making bridge with eth0 and then I try remove “rm /etc/sysconfig/network-scripts/ifcfg-br0″

File is removed but i didn’t get my eth0 back to normal When i use ifup eth0,it gives me:

:: ERROR ::
SIOCGMIIPHY on ‘eth0′ failed: Resource temporarily unavailable SIOCGMIIPHY

Please i am not very experienced but i need help in this regard, ??

Really thankful friends if you just help me in this

Reply

20 ChiefEngr July 16, 2013 at 8:22 pm

Nice write-up. You apparently know what you are doing, so while I plan to study your post carefully and see if I can solve my own question(s) later tonight, I thought I would post my target topology and perhaps someone can let me know what is and isn’t possible, and perhaps point me in the right direction. Thanks in advance for any help!

My host platform has 4 GigE ports and is running CentOS 6.3 with KVM virtualization. I will have three different VMs running — two will probably be Windows 7 and one will be RHEL. What I want to accomplish is to assign a separate, independent GigE port to each of the VMs. In a picture:

eth0 Host System
eth1 VM1
eth2 VM2
eth3 VM3

The Host system will not use eth1, eth2, or eth3 *at all* and should not respond to any traffic on them. Similarly, each VM is oblivious to the traffic on the three ports that are not assigned to it. And if a VM needs to talk to different VM, the traffic should leave the platform via one port, be routed by the external network(s) and fed back in via a different port (assuming a route exists). [and by 'external network', I mean outside the VM Host, not outside the building]

I know how to use ONE port to give each VM its own connection to the external network with unique MAC and IP Addresses, so I suppose I could implement a similar scheme where each VM gets its own connection via a different physical port, but how do I tell the host itself to keep its hands off that traffic.

I am convinced there is a simple solution to this but I think I keep getting caught up in unnecessary complications (and confusions).

Thanks in advance!

Reply

21 Tarek Esmail November 20, 2013 at 11:54 am

How i can add to NICs to KVM machine 1 for private and the 2 for public ?

Reply

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