How to use KVM from the command line on Debian or Ubuntu

There are different ways to manage virtual machines (VMs) running on KVM hypervisor. For example, virt-manager is a popular GUI-based front-end for VM management. However, if you would like to use KVM on a headless server, GUI-based solutions will not be ideal. That is when virsh comes in handy. virsh is a command-line tool for managing guest VMs. Underneath it, virsh relies on libvirtd service which can control several different hypervisors including KVM, Xen, QEMU, LXC and OpenVZ.

A command-line management interface such as virsh is also useful when you would like to automate the provisioning and management of VMs. Also, the fact that virsh supports multiple hypervisors means you can manage different hypervisors via the same virsh interface.

In this tutorial, I will demonstrate how to run KVM from the command line by using virsh on Debian or Ubuntu.

Step One: Verify Hardware Virtualization Support

As a first step, verify that the host CPU is equipped with hardware virtualization extensions (e.g., Intel VT or AMD-V), which are required for KVM. The following command will do.

$ egrep '(vmx|svm)' --color /proc/cpuinfo
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts mmx fxsr sse sse2 ss syscall nx rdtscp lm constant_tsc up arch_perfmon pebs bts nopl xtopology tsc_reliable nonstop_tsc aperfmperf pni pclmulqdq vmx ssse3 cx16 pcid sse4_1 sse4_2 x2apic popcnt aes xsave avx f16c rdrand hypervisor lahf_lm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi ept vpid fsgsbase smep

If the output does not contain vmx or svm flag, it means the host CPU does not have hardware virtualization support. Thus you cannot use KVM on the host. After verifying that the host CPU comes with vmx or svm, proceed to install KVM next.

Step Two: Install KVM

Using apt-get, install KVM and related user-space tools.

$ sudo apt-get install qemu-kvm libvirt-bin

During installation, libvirtd group will be created, and your userID will be automatically added to the group. This will allows you to manage VMs as a non-root regular user. You can verify that by using id command, which will show your group IDs:

$ id <your-userID>

If for some reason, libvirtd is not found in your groupID list, you can manually add yourself to libvirtd group as follows.

$ sudo adduser [youruserID] libvirtd

Reload updated group membership info as follows. Upon asked for a password, enter your login password.

$ exec su -l $USER

At this point, you should be able to run virsh as a regular user. As a test, try the command below, which will show a list of available VMs (currently none). If you do not encounter a permission error, it means everything is okay so far.

$ virsh list
 Id    Name                           State
----------------------------------------------------

Step Three: Configure Bridged Networking

One way to enable VMs to access external networks is via a Linux bridge which is provisioned on your Linux host. This is called bridged networking. Here is how to create and configure a Linux bridge br0 for bridged networking with KVM.

First, install a necessary package, and create a Linux bridge from the command line.

$ sudo apt-get install bridge-utils
$ sudo brctl addbr br0

The next step is to configure Linux bridge in /etc/network/interfaces, so that the bridge is configured automatically upon boot. To use /etc/network/interfaces, you need to disable Network Manager on your system (if you are using it). Follow the instruction here to disable Network Manager.

After disabling Network Manager, go ahead and configure Linux bridge br0 in /etc/network/interfaces as follows.

#auto eth0
#iface eth0 inet dhcp

auto br0
iface br0 inet dhcp
        bridge_ports eth0
        bridge_stp off
        bridge_fd 0
        bridge_maxwait 0

Here I assume that eth0 is the primary network interface that can access external networks. Also, I assume that eth0 is getting its IP address via DHCP. Note that there is no configuration for eth0 in /etc/network/interface. The Linux bridge br0 takes up the configuration of eth0 as eth0 is enslaved to bridge br0.

Restart network service, and verify that Linux bridge is configured successfully. If successful, br0 should be assigned the eth0's DHCP IP address, and eth0 should have no IP address assigned.

$ sudo /etc/init.d/networking restart
$ ifconfig

Step Four: Create a VM from the Command Line

With KVM, the configuration of a VM is stored in a domain XML file. Thus, the first step to create a VM is to prepare its domain XML file.

The following is a sample domain XML file of a VM. You can use and customize it as needed.

<domain type='kvm'>
  <name>alice</name>
  <uuid>f5b8c05b-9c7a-3211-49b9-2bd635f7e2aa</uuid>
  <memory>1048576</memory>
  <currentMemory>1048576</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type>hvm</type>
    <boot dev='cdrom'/>
  </os>
  <features>
    <acpi/>
  </features>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/kvm</emulator>
    <disk type="file" device="disk">
      <driver name="qemu" type="raw"/>
      <source file="/home/dev/images/alice.img"/>
      <target dev="vda" bus="virtio"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x04" function="0x0"/>
    </disk>
    <disk type="file" device="cdrom">
      <driver name="qemu" type="raw"/>
      <source file="/home/dev/iso/ubuntu-13.10-server-amd64.iso"/>
      <target dev="hdc" bus="ide"/>
      <readonly/>
      <address type="drive" controller="0" bus="1" target="0" unit="0"/>
    </disk>
    <controller type="ide" index="0">
      <address type="pci" domain="0x0000" bus="0x00" slot="0x01" function="0x1"/>
    </controller>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport="yes" listen='127.0.0.1'/>
    <console type='pty'>
      <target port='0'/>
    </console>
  </devices>
</domain>

The above domain XML file defines the following VM.

  • 1GB memory, one vCPU and one hard drive.
  • Disk image: /home/dev/images/alice.img.
  • Boot from CD-ROM (/home/dev/iso/ubuntu-13.10-server-amd64.iso).
  • Networking: bridged networking via br0.

The UUID string inside <uuid></uuid> can be randomly generated. To get a random UUID, you can use uuid command-line tool.

$ sudo apt-get install uuid
$ uuid

Another way to create a domain XML file is to dump the domain information of an existing VM as follows.

$ virsh dumpxml alice > bob.xml

Step Five: Start VM from the Command Line

Before starting a VM, you need to create its initial disk image. For that, you can use qemu-img command, which comes with qemu-kvm package you installed.

$ qemu-img create -f qcow2 /home/dev/images/alice.img 5G

The advantage of using qcow2 (as opposed to raw) as a disk image format is that a qcow2-type disk image is not created as a full size (5GB) initially, but grows as the disk gets populated.

Now you are ready to start a VM using the domain XML file you created earlier. The following command will start a VM.

$ virsh create alice.xml

Domain alice created from alice.xml

Verify that a new domain has been created successfully.

$ virsh list
 Id    Name                           State
----------------------------------------------------
 3     alice                          running

Also, verify that the virtual interface for the VM (e.g., vnet0) is successfully added to the Linux bridge br0 that you created earlier.

$ sudo brctl show

Step Six: Remote Access a VM

To access the console of a running VM remotely, you can use any VNC client.

First, find out the VNC port number for the VM as follows.

$ sudo netstat -nap | grep kvm

In this example, the VNC port number for alice VM is 5900.

Then launch a VNC client, and connect to a VNC server running at <KVM-host-IP>:5900.

Manage VMs with virsh

The following lists common usages of virsh command.

To create a new guest domain and start a VM:

$ virsh create alice.xml

To stop a VM and destroy a guest domain:

$ virsh destroy alice

To shutdown a VM (without destroying a domain):

$ virsh shutdown alice

To suspend a VM:

$ virsh suspend alice

To resume a suspended VM:

$ virsh resume alice

To access login console of a running VM:

$ virsh console alice

To autostart a VM upon host booting:

$ virsh autostart alice

To get domain information of a VM:

$ virsh dominfo alice

You can also manage VMs from within a virsh session. To create and enter a new virsh session, simply run:

$ virsh

At the virsh prompt, you can use any virsh commands.

Troubleshooting

1. I am getting the error while trying to create a VM:

error: internal error: no supported architecture for os type 'hvm'

You can get this error if your hardware does not have hardware virtualization support (e.g., Intel VT or AMD-V), which is required to run KVM. If you are getting this error even when your CPU comes with Intel VT or AMD-V, here are possible solutions:

First, check if kvm kernel module is missing.

$ lsmod | grep kvm

If kvm kernel module is not loaded, you must load it as follows.

$ sudo modprobe kvm_intel (for Intel processor)
$ sudo modprobe kvm_amd (for AMD processor)

The second solution is adding "--connect qemu:///system" argument to virsh command as follows. This argument may be needed when you are using more than one hypervisor (e.g., VMware, VirtualBox) on the server hardware.

$ virsh --connect qemu:///system create alice.xml

2. I am getting the error while trying to access login console of my VM:

$ virsh console alice
error: internal error: cannot find character device <null>

This error occurs because you did not define a console device in the VM's XML file. Add the following inside the "device" section of the XML file.

    <console type='pty'>
      <target port='0'/>
    </console>

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Dan Nanni is the founder and also a regular contributor of Xmodulo.com. He is a Linux/FOSS enthusiast who loves to get his hands dirty with his Linux box. He likes to procrastinate when he is supposed to be busy and productive. When he is otherwise free, he likes to watch movies and shop for the coolest gadgets.
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12 thoughts on “How to use KVM from the command line on Debian or Ubuntu

  1. Just a note: you don't need to actually edit the XML manually in most cases. I like using virt-install, and tweaking the result if needed.

    • What I like doing is creating one virtual machine, get it the way I want it, dump the XML, and use that XML as a template for generating more virtual machines.
      It's a bit of a pain, but it allows you to generate a lot of systems quickly once you get it going.

  2. Seriously? In an article about using the (ya know, text based?) command line interface you present your examples as JPEG images instead of, um, text? Genius.

      • > Seriously I don't get your point. :-)

        I think he means that since you're doing a tutorial, people would want to duplicate your commands. If you do them in text (and not in an image), people can copy them from your web page into their terminals.

        This is an excellent post; if the commands are easier to duplicate, it'll be even better.

        • Well, all the commands used are already in text. Images are just repeating what's already said. The guy either didn't read the post, or felt like trolling. :-)

  3. Or do it really from the command line, without the need of an XML configuration file:
    kvm \
    -m 512m \
    -device e1000,romfile=,vlan=0 \
    -net tap \
    -usbdevice tablet \
    -hda hda
    "hda" is a file that you previously created and made bootable. If you don't have hda bootable, then you can point to an external kernel, just add a " \" after "-hda hda" and then this:
    -append "root=/dev/sda1 ro quiet init=/bin/systemd" \
    -kernel linux-3.12/arch/x86/boot/bzImage
    (I didn't specify an initrd, because I dislike initrds on well-defined systems. And for my, VMs are well-defined).

  4. Hi,

    great tutorial. Can you tell me, why VNC running in localhost? I cannot connect to host-PC:5900 :(

    # netstat -nap | grep kvm
    tcp 0 0 127.0.0.1:5900 0.0.0.0:* LISTEN 11771/kvm
    unix 2 [ ACC ] STREAM LISTENING 1180344 11771/kvm /var/lib/libvirt/qemu/unifi.monitor
    unix 3 [ ] STREAM CONNECTED 1172338 11771/kvm /var/lib/libvirt/qemu/unifi.monitor

    Thanks.

  5. how can you actually login to a VM with virsh in the command line?

    I used to manage my VM remotely with ssh, but somehow the network settings or the ssh-server of the VM seem to have a problem and I can't access it anymore with ssh (connection is refused). So I need to access it locally from the command line to fix this problem. Host and guest both are debian/ubuntu servers without graphical interface

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