Creating a KVM Virtual Machine

Copy a virtual machine image from a virtual machine template:

root@host:~# cp <path to vm template file> <path to new vm image file>

or create a new image file:

root@host:~# qemu-img create -f <type> -o <options> <path to new vm image file> <size>

E.g. a QCOW2 image file with a size of 2GB:

root@host:~# qemu-img create -f qcow2 -o lazy_refcounts=on slave1.qcow2 2G

Create a virtual machine description file:

vm-template.xml

<domain type='kvm'>
  <name>VMNAME</name>
  <memory unit='KiB'>MEMORY</memory>
  <vcpu placement='static'>CPUS</vcpu>
  <os>
    <type arch='x86_64' machine='pc-i440fx-2.1'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <cpu mode='custom' match='exact'/>
  <clock offset='utc'>
    <timer name='rtc' tickpolicy='catchup'/>
    <timer name='pit' tickpolicy='delay'/>
    <timer name='hpet' present='no'/>
  </clock>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <pm>
    <suspend-to-mem enabled='no'/>
    <suspend-to-disk enabled='no'/>
  </pm>
  <devices>
    <emulator>/usr/bin/kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='PATHTOVMIMAGE'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>
    <controller type='pci' index='0' model='pci-root'/>
    <controller type='usb' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
    </controller>
    <controller type='virtio-serial' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </controller>
    <interface type='network'>
      <source network='NETWORK'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
    <console type='pty'>
      <target type='virtio' port='0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <input type='keyboard' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport='yes'/>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>
    </memballoon>
  </devices>
</domain>

Replace VMNAME, MEMORY, CPUS, PATHTOVMIMAGE and NETWORK, with the appropriate values and save the virtual machine description file as <VMNAME>.xml.

With the virtual machine description file, define a virtual machine:

root@host:~# virsh define <VMNAME>.xml
root@host:~# virsh autostart <VMNAME>

Networking:

Private bridge, with access to networks outside of the KVM host for the virtual machines via NAT on the KVM host:

Create a virtual network description file:

network-template.xml

<network connections='1'>
  <name>default</name>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr0' stp='on' delay='0'/>
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.2' end='192.168.122.254'/>
    </dhcp>
  </ip>
</network>

Replace the IP addresses and the netmask with the appropriate values for the network environment and save the virtual network description file as network-default.xml

With the virtual network description file, define a virtual network:

root@host:~# virsh net-define network-default.xml
root@host:~# virsh net-autostart default
root@host:~# virsh autostart default

Optional: Add entries for each virtual machine to the file /etc/hosts on the KVM server if network access from the KVM server to the virtual machine is required.

Gather the MAC address for each virtual machine interface:

root@host:~# virsh dumpxml <VMNAME> | egrep "<mac "
      <mac address='52:54:00:5a:3d:f0'/>

For each virtual machine insert a static entry to the virtual network. This will be used to map each virtual machines MAC address to a IP address via DHCP:
```
root@host:~# virsh net-update default add ip-dhcp-host "<host mac='MAC' name='HOSTNAME' ip='IPADDRESS' />" --live --config
```
Replace MAC, HOSTNAME and IPADDRESS with the appropriate values for each virtual machine.

Restart the libvirt service on the KVM host:

root@host:~# systemctl restart libvirtd.service

This will have created a virtual network in the form of a Linux bridge:

root@host:~# brctl show
bridge name     bridge id           STP enabled     interfaces
virbr0          8000.fe54005a3df0   yes

It will also have inserted appropriate IPtables NAT rules into the FORWARD chain:

root@host:~# iptables -nL
[...]
Chain FORWARD (policy DROP)
target     prot opt source               destination         
ACCEPT     all  --  0.0.0.0/0            192.168.122.0/24     ctstate RELATED,ESTABLISHED
ACCEPT     all  --  192.168.122.0/24     0.0.0.0/0           
ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0           
REJECT     all  --  0.0.0.0/0            0.0.0.0/0            reject-with icmp-port-unreachable
REJECT     all  --  0.0.0.0/0            0.0.0.0/0            reject-with icmp-port-unreachable
[...]

In order to forward packets from and to the virtual machines, IP forwarding must also be enabled on the KVM host:

root@host:~# sysctl net.ipv4.ip_forward 
net.ipv4.ip_forward = 1

Start the virtual machine:
```
root@host:~# virsh start <VMNAME>
```
If the virtual machine was copied from a template and virtual machine networking is done with a private bridge and NAT, the hostname inside the virtual machine is currently still the wrong one in order to match the definition for a successful DHCP request. On the first boot of the virtual machine, the hostname needs to be changed via a login on the virtual machine console. See Debian - Necessary Changes After Cloning A System for the necessary steps. Afterwards the virtual machine should get an IP address assigned via DHCP.