Virtualization With KVM On A CentOS 6.4 Server

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Submitted by falko (Contact Author) (Forums) on Tue, 2013-04-16 13:54. :: CentOS | KVM | Virtualization

Virtualization With KVM On A CentOS 6.4 Server

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Author: Falko Timme <ft [at] falkotimme [dot] com>
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Last edited 03/22/2013

This guide explains how you can install and use KVM for creating and running virtual machines on a CentOS 6.4 server. I will show how to create image-based virtual machines and also virtual machines that use a logical volume (LVM). KVM is short for Kernel-based Virtual Machine and makes use of hardware virtualization, i.e., you need a CPU that supports hardware virtualization, e.g. Intel VT or AMD-V.

I do not issue any guarantee that this will work for you!


1 Preliminary Note

I'm using a CentOS 6.4 server with the hostname and the IP address here as my KVM host.

I had SELinux disabled on my CentOS 6.4 system. I didn't test with SELinux on; it might work, but if not, you better switch off SELinux as well:

vi /etc/selinux/config

Set SELINUX=disabled...

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#     enforcing - SELinux security policy is enforced.
#     permissive - SELinux prints warnings instead of enforcing.
#     disabled - No SELinux policy is loaded.
# SELINUXTYPE= can take one of these two values:
#     targeted - Targeted processes are protected,
#     mls - Multi Level Security protection.

... and reboot:


We also need a desktop system where we install virt-manager so that we can connect to the graphical console of the virtual machines that we install. I'm using a Fedora 17 desktop here.


2 Installing KVM

CentOS 6.4 KVM Host:

First check if your CPU supports hardware virtualization - if this is the case, the command

egrep '(vmx|svm)' --color=always /proc/cpuinfo

should display something, e.g. like this:

[root@server1 ~]# egrep '(vmx|svm)' --color=always /proc/cpuinfo
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall
 nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy misalignsse
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall
 nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy misalignsse
[root@server1 ~]#

If nothing is displayed, then your processor doesn't support hardware virtualization, and you must stop here.

Now we import the GPG keys for software packages:

rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY*

To install KVM and virtinst (a tool to create virtual machines), we run

yum install kvm libvirt python-virtinst qemu-kvm

Then start the libvirt daemon:

/etc/init.d/libvirtd start

To check if KVM has successfully been installed, run

virsh -c qemu:///system list

It should display something like this:

[root@server1 ~]# virsh -c qemu:///system list
 Id Name                 State

[root@server1 ~]#

If it displays an error instead, then something went wrong.

Next we need to set up a network bridge on our server so that our virtual machines can be accessed from other hosts as if they were physical systems in the network.

To do this, we install the package bridge-utils...

yum install bridge-utils

... and configure a bridge. Create the file /etc/sysconfig/network-scripts/ifcfg-br0 (please use the IPADDR, PREFIX, GATEWAY, DNS1 and DNS2 values from the /etc/sysconfig/network-scripts/ifcfg-eth0 file); make sure you use TYPE=Bridge, not TYPE=Ethernet:

vi /etc/sysconfig/network-scripts/ifcfg-br0

NAME="System br0"

Modify /etc/sysconfig/network-scripts/ifcfg-eth0 as follows (comment out BOOTPROTO, IPADDR, PREFIX, GATEWAY, DNS1, and DNS2 and add BRIDGE=br0):

vi /etc/sysconfig/network-scripts/ifcfg-eth0

NAME="System eth0"

Restart the network...

/etc/init.d/network restart

... and run


It should now show the network bridge (br0):

[root@server1 ~]# ifconfig
br0       Link encap:Ethernet  HWaddr 00:1E:90:F3:F0:02
          inet addr:  Bcast:  Mask:
          inet6 addr: fe80::21e:90ff:fef3:f002/64 Scope:Link
          RX packets:8 errors:0 dropped:0 overruns:0 frame:0
          TX packets:27 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:460 (460.0 b)  TX bytes:2298 (2.2 KiB)

eth0      Link encap:Ethernet  HWaddr 00:1E:90:F3:F0:02
          inet6 addr: fe80::21e:90ff:fef3:f002/64 Scope:Link
          RX packets:18455 errors:0 dropped:0 overruns:0 frame:0
          TX packets:11861 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:26163057 (24.9 MiB)  TX bytes:1100370 (1.0 MiB)
          Interrupt:25 Base address:0xe000

lo        Link encap:Local Loopback
          inet addr:  Mask:
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:5 errors:0 dropped:0 overruns:0 frame:0
          TX packets:5 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:2456 (2.3 KiB)  TX bytes:2456 (2.3 KiB)

virbr0    Link encap:Ethernet  HWaddr 52:54:00:AC:AC:8F
          inet addr:  Bcast:  Mask:
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

[root@server1 ~]#


3 Installing virt-viewer Or virt-manager On Your Fedora 17 Desktop

Fedora 17 Desktop:

We need a means of connecting to the graphical console of our guests - we can use virt-manager for this. I'm assuming that you're using a Fedora 17 desktop.

Become root...


... and run...

yum install virt-manager libvirt qemu-system-x86 openssh-askpass

... to install virt-manager.

(If you're using an Ubuntu 12.04 desktop, you can install virt-manager as follows:

sudo apt-get install virt-manager



4 Creating A Debian Squeeze Guest (Image-Based) From The Command Line

CentOs 6.4 KVM Host:

Now let's go back to our CentOS 6.4 KVM host.

Take a look at

man virt-install

to learn how to use virt-install.

We will create our image-based virtual machines in the directory /var/lib/libvirt/images/ which was created automatically when we installed KVM in chapter two.

To create a Debian Squeeze guest (in bridging mode) with the name vm10, 512MB of RAM, two virtual CPUs, and the disk image /var/lib/libvirt/images/vm10.img (with a size of 12GB), insert the Debian Squeeze Netinstall CD into the CD drive and run

virt-install --connect qemu:///system -n vm10 -r 512 --vcpus=2 --disk path=/var/lib/libvirt/images/vm10.img,size=12 -c /dev/cdrom --vnc --noautoconsole --os-type linux --os-variant debiansqueeze --accelerate --network=bridge:br0 --hvm

Of course, you can also create an ISO image of the Debian Squeeze Netinstall CD (please create it in the /var/lib/libvirt/images/ directory because later on I will show how to create virtual machines through virt-manager from your Fedora desktop, and virt-manager will look for ISO images in the /var/lib/libvirt/images/ directory)...

dd if=/dev/cdrom of=/var/lib/libvirt/images/debian-6.0.5-amd64-netinst.iso

... and use the ISO image in the virt-install command:

virt-install --connect qemu:///system -n vm10 -r 512 --vcpus=2 --disk path=/var/lib/libvirt/images/vm10.img,size=12 -c /var/lib/libvirt/images/debian-6.0.5-amd64-netinst.iso --vnc --noautoconsole --os-type linux --os-variant debiansqueeze --accelerate --network=bridge:br0 --hvm

The output is as follows:

[root@server1 ~]# virt-install --connect qemu:///system -n vm10 -r 512 --vcpus=2 --disk path=/var/lib/libvirt/images/vm10.img,size=12 -c /var/lib/libvirt/images/debian-6.0.5-amd64-netinst.iso --vnc --noautoconsole --os-type linux --os-variant debiansqueeze --accelerate --network=bridge:br0 --hvm

Starting install...
Allocating 'vm10.img'              |  12 GB     00:00
Creating domain...                 |    0 B     00:00
Domain installation still in progress. You can reconnect to
the console to complete the installation process.
[root@server1 ~]#

Please do not use the comment function to ask for help! If you need help, please use our forum.
Comments will be published after administrator approval.
Submitted by RjBradlow (not registered) on Sat, 2013-11-16 07:18.

Red Hat Enterprise Linux ~ Virtualization Guide

10.2. Bridged networking with libvirt

Bridged networking (also known as physical device sharing) is used for dedicating a physical device to a virtual machine. Bridging is often used for more advanced setups and on servers with multiple network interfaces.

Disable Xen network scripts

If your system was using a Xen bridge, it is recommended to disable the default Xen network bridge by editing /etc/xen/xend-config.sxp and changing the line:

(network-script network-bridge)


(network-script /bin/true)


Disable NetworkManager

NetworkManager does not support bridging. Running NetworkManager will overwrite any manual bridge configuration. Because of this, NetworkManager should be disabled in order to use networking via the network scripts (located in the /etc/sysconfig/network-scripts/ directory):

# chkconfig NetworkManager off

# chkconfig network on

# service NetworkManager stop

# service network start


As an alternative to turning off NetworkManager, add "NM_CONTROLLED=no" to the ifcfg-* scripts used in the examples.

If you do not either set this parameter or disable NetworkManager entirely, any bridge configuration will be overwritten and lost when NetworkManager next starts.


Submitted by V.J. (not registered) on Thu, 2013-07-11 22:06.

I have followed the instructions upto starting the network service.

when I run /etc/init.d/network restart I get the following error:


Shutting down interface eth0:  Device state: 3 (disconnected)
                                                           [  OK  ]
Shutting down loopback interface:                          [  OK  ]
Bringing up loopback interface:                            [  OK  ]
Bringing up interface eth0:  Error: Connection activation failed: Master connection not found or invalid
Bringing up interface br0:  Error: Connection activation failed: Failed to determine connection's virtual interface name

Submitted by httpstergeek (not registered) on Mon, 2013-11-11 17:34.
I experienced the same problem.  Where you able to resolve it?
Submitted by Anonymous (not registered) on Fri, 2013-08-16 22:48.

I usually just set NM_CONTROLLED=NO and then do a 'service network restart'.

Submitted by Shashank (not registered) on Mon, 2014-01-13 21:33.

I faced the same problem but now I got the solution. 

When changing the config of eth0 interface, leave it as is instead of commenting ipaddr, gateway etc etc. 

Submitted by Timothy K (not registered) on Fri, 2013-06-07 14:21.

 I had SELinux disabled on my CentOS 6.4 system. I didn't test with SELinux on; it might work, but if not, you better switch off SELinux as well:

 This is poor advice.  If you are running KVM chances are you are also going to be running a webserver of some kind, likely connected to a public network in some way.  In this setting you don't want your processes running unconstrained on the system.

 Instead of disabling SELinux entirely - set it to 'Permissive'.  This won't enforce any policies.  It will however log any process that violates the policies, so you can refer to it later and adjust your settings as necessary.