Virtualization

Ubuntu includes a virtualization suite built on KVM (Kernel-based VM) and QEMU (Quick Emulator). KVM is built into the Linux kernel and separates the host from guest operating systems, enabling near-native performance. QEMU emulates the hardware components found in physical servers.

KVM/QEMU lets you run virtual machines without a third-party product like VirtualBox. You cannot run VirtualBox and KVM/QEMU simultaneously — the CPU cannot support both hypervisors at the same time.

Virtual Machine Manager (virt-manager) is a GUI for VM administration. It can manage both local and remote KVM servers.

KVM assigns VMs addresses on the 192.168.122.0/24 network from an internal DHCP server (192.168.122.2–192.168.122.254). You can SSH into VMs from your workstation, or configure bridging to connect VMs directly to your local network.

Important addresses

KVM creates a private 192.168.122.0/24 network for VMs by default. The host bridge interface virbr0 handles routing, DHCP, and DNS for all VMs on this network.

Address	Role
192.168.122.0/24	Default VM network
192.168.122.1	Host gateway and DNS server (`virbr0`)
192.168.122.2–192.168.122.254	DHCP assignment range
192.168.122.255	Broadcast address

KVM components

The libvirtd daemon manages VMs, virtual networks, storage pools, and virtual interfaces. Tools like virsh and virt-manager communicate with libvirtd through its API.

The default virtual network uses a bridge interface called virbr0 in NAT mode. Each VM gets a virtual NIC connected to virbr0. The host runs dnsmasq on this bridge to serve DHCP addresses (192.168.122.2–192.168.122.254) and DNS (192.168.122.1). Outbound VM traffic is NATted through the host’s physical interface.

libvirt organizes disk images in storage pools. The default pool is at /var/lib/libvirt/images. KVM uses the qcow2 image format, which supports snapshots and thin provisioning.

KVM/QEMU installation

Before installing, verify that your CPU supports virtualization extensions. A result greater than zero confirms support:

egrep -c '(vmx|svm)' /proc/cpuinfo

To install and configure KVM/QEMU:

Install the required packages:

apt install bridge-utils libvirt-clients \
            libvirt-daemon-system qemu-system-x86

Check the service status:
```
systemctl status libvirtd
```
Stop the service for additional configuration:
```
systemctl stop libvirtd
```
Verify that the kvm and libvirt groups were created:
```
grep kvm /etc/group
grep libvirt /etc/group
```
Add your user to both groups. Log out and back in for group membership to take effect:
```
usermod -aG kvm <username>
usermod -aG libvirt <username>
```

Grant the kvm group access to the images directory:

chown :kvm /var/lib/libvirt/images
chmod g+rw /var/lib/libvirt/images

Start the service:
```
systemctl start libvirtd
```
Install the Virtual Machine Manager GUI:
```
apt install ssh-askpass virt-manager
```

Virtual Machine Manager setup

Complete this setup on your workstation after installing KVM/QEMU on the server. Virtual Machine Manager gives you a GUI to create, start, stop, and manage VMs on local or remote KVM hosts:

Open Virtual Machine Manager by searching in Applications or running virt-manager.
Select File > Add Connection.
For a remote host, select SSH and enter the host details. For a local KVM server, select Autoconnect.

Create a storage pool

A storage pool gives Virtual Machine Manager a managed location to find ISO images when creating VMs. Create one the first time you set up a KVM server so you have a consistent place to store installation media:

In Virtual Machine Manager, right-click the connection and select Details.
Select the Storage tab.
Select the green plus icon at the bottom left.
Enter the following:
- Name: ISO
- Target Path: /var/lib/libvirt/images/ISO
Select Finish.

Set permissions on the new directory:

chown root:kvm /var/lib/libvirt/images/ISO
chmod g+rw /var/lib/libvirt/images/ISO

Move your ISO files into the target path so the VM server can use them to create VMs.

Create a VM

Use Virtual Machine Manager to provision a new VM from an ISO image stored in your storage pool:

In Virtual Machine Manager, right-click the server and select New.
Leave Local install media selected and select Forward.
Select Browse, navigate to the /var/lib/libvirt/images/ISO storage pool, select your ISO, and select Choose Volume. Select Forward.
Allocate at least 2048 MB of RAM and 2 CPUs.
Allocate disk space. 20 GB is sufficient for most test machines.
Enter a name for the VM and select Finish.

Bridging the network

By default, KVM assigns VMs addresses from its internal DHCP server. Bridging connects VMs directly to your local network so they receive addresses from your main DHCP server and are reachable like any other machine. This is useful when you want other devices on the network to communicate with your VMs.

Bridging requires a wired network interface. It does not work on wireless cards. Back up your Netplan configuration before making changes.

Cloning

Cloning lets you replicate a configured VM without repeating the full installation and setup process. KVM does not have a built-in templating feature, but you can configure a base VM, name it with a -template suffix, and clone it whenever you need a new instance:

Right-click a stopped VM and select Clone.
Confirm that Clone this disk is selected.
Select Clone.

Managing VMs from the command line

Use virsh to manage VMs when Virtual Machine Manager is not available, such as on a headless server or over SSH:

virsh list                    # list running VMs
virsh list --all              # list all VMs
virsh start <vm-name>         # start a VM
virsh shutdown <vm-name>      # shut down a VM gracefully
virsh suspend <vm-name>       # pause a VM
virsh resume <vm-name>        # resume a paused VM
virsh destroy <vm-name>       # force-stop a VM immediately
virsh undefine <vm-name>      # remove a VM definition (disk files remain in /var/lib/libvirt/images)

Set static IP address

Assign a static IP to a VM by adding a DHCP host entry to the default network configuration. This ensures the VM always receives the same address from KVM’s internal DHCP server. See the official docs for reference.

Look up the VM’s MAC address and current leases, then update the network configuration:

virsh net-dhcp-leases default                         # view all DHCP leases on the default network
virsh dumpxml <vm-name> | grep -i '<mac'              # get the VM's MAC address
virsh net-list                                        # list available networks
virsh net-dumpxml default                             # view the default network XML config
virsh net-edit <network-name>                         # edit network config directly

Add or remove a static IP entry:

# add a static IP
virsh net-update default add ip-dhcp-host \
    "<host mac='<mac-addr>' name='<hostname>' ip='<ip-addr>' />" --live --config

# remove a static IP
virsh net-update default delete ip-dhcp-host \
    "<host mac='<mac-addr>' name='<hostname>' ip='<ip-addr>' />" --live --config

Add a network

KVM’s default network is sufficient for most setups, but you can define additional networks to isolate traffic, use different subnets, or test multi-network configurations. Each network gets its own bridge interface and optional DHCP range.

To add a network:

Create an XML file that defines the network. The following example creates a NAT network on 192.168.100.0/24:
```
<network>
  <name>mynetwork</name>                                    # 1
  <bridge name='virbr1'/>                                   # 2
  <forward mode='nat'/>                                     # 3
  <ip address='192.168.100.1' netmask='255.255.255.0'>      # 4
    <dhcp>
      <range start='192.168.100.2' end='192.168.100.254'/>  # 5
    </dhcp>
  </ip>
</network>
```
1. The network name used in virsh commands. mynetwork is a placeholder. Use a descriptive name that reflects the network’s purpose.
2. The virtual bridge interface created on the host. KVM’s default network uses virbr0, so virbr1 is the next available name and avoids conflicts.
3. Routes outbound VM traffic through the host using NAT. NAT lets VMs reach external networks without exposing them directly on your physical network.
4. The gateway address and subnet mask for the network. 192.168.100.0/24 is an RFC 1918 private range that doesn’t conflict with the default KVM network (192.168.122.0/24) or common home and office networks (192.168.1.0/24). The .1 address is the conventional gateway.
5. The DHCP address range assigned to VMs on this network. The range starts at .2 to leave .1 free for the gateway and ends at .254, the last usable address in a /24 subnet.
Define the network from the XML file:
```
virsh net-define mynetwork.xml
```
Start the network:
```
virsh net-start mynetwork
```
Enable autostart so the network starts with the host:
```
virsh net-autostart mynetwork
```
Verify the network is active:
```
virsh net-list --all
```

To connect a VM to the new network, edit the VM’s configuration and change the <source network='default'/> line to reference your new network name:

virsh edit <vm-name>

<interface type='network'>
  <source network='mynetwork'/>
</interface>

Restart the VM for the new network interface to take effect.