This is a work in progress, 20201015

Just a note, the files created by virsh and virt-install are stored in /etc/libvirt/qemu/. However, they should not be hand-edited; only edit with virsh.

First, verify you have the required hardware virtualization support in your CPU:

#Verify the hvm support
egrep --color 'vmx|svm' /proc/cpuinfo

You should see either vmx or svm in the output.

Now, install the basic packages needed, a couple of utilities, but not all the extra crud.

apt install -y --no-install-recommends qemu-kvm libvirt-clients libvirt-daemon-system bridge-utils libguestfs-tools genisoimage virtinst libosinfo-bin virt-top
reboot # brings libraries online

Verify system is working ok

Commands are of the form: virsh --connect qemu:///system command where command is any of the commands accepted by virsh.

sudo su # become root user
virsh --connect qemu:///system list --all

The --connect qemu:///system is the connection used, which is not the default. To set that to the default, run the following one time. This will become the “norm” on the next login:

echo "# set default uri for libvirt" >> ~/.profile
echo "export LIBVIRT_DEFAULT_URI='qemu:///system'" >> ~/.profile

For your network, you need bridges for the outside world.

This is a basic setup that will work for a single interface as per the Debian documentation. It sets up one bridge off of eth0 and gives it a static IP.

interfaces

auto lo
iface lo inet loopback
 
# The primary network interface
auto eth0
 
#make sure we don't get addresses on our raw device
iface eth0 inet manual
iface eth0 inet6 manual
 
#set up bridge and give it a static ip
auto br0
iface br0 inet static
        address 192.168.1.2
        netmask 255.255.255.0
        network 192.168.1.0
        broadcast 192.168.1.255
        gateway 192.168.1.1
        bridge_ports eth0
        bridge_stp off
        bridge_fd 0
        bridge_maxwait 0
        dns-nameservers 8.8.8.8

I'm hoping, if you're reading this article, you know how to set up bonding and vlans. The following is a more real world scenario and sets up three bridges, br_lan, br_dmz and br_wan for LAN, DMZ and public interface respectively. br_wan and br_dmz are set to dummy IP's with a netmask of 255.255.255.255, meaning they can't respond to anything but themselves. br_lan gets its IP from DHCP, so only the LAN interface can be accessed on the hypervisor. Modify it as you want.

interfaces

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
 
source /etc/network/interfaces.d/*
 
# The loopback network interface
auto lo
iface lo inet loopback
 
iface eth0 inet manual
iface eth0 inet6 manual
 
iface eth1 inet manual
iface eth1 inet6 manual
 
auto bond0
iface bond0 inet manual
   bond-mode 4
   bond-miimon 100
   bond_xit_hash_policy layer2+3
   bond_lacp_rate slow
   slaves eth0 eth1
 
 
iface bond0.10 inet manual
   vlan-raw-device bond0.10
 
iface bond0.20 inet manual
   vlan-raw-device bond0.20
 
iface bond0.30 inet manual
   vlan-raw-device bond0.30
 
# the public interface on vlan 10
auto br_wan
iface br_wan inet static
        address 192.168.1.13
        netmask 255.255.255.255
   bridge_ports bond0.10
   bridge_stp off
   bridge_fd 0
   bridge_maxwait 0
 
# the DMZ on vlan 20
auto br_dmz
iface br_dmz inet static
        address 192.168.1.12
        netmask 255.255.255.255
        bridge_ports bond0.20
        bridge_stp off
        bridge_fd 0
        bridge_maxwait 0
 
# the private (LAN) interface on vlan 30
auto br_lan
iface br_lan inet dhcp
        bridge_ports bond0.30
        bridge_stp off
        bridge_fd 0
        bridge_maxwait 0

In order to use a network with vir-lib, you need to define it. The best way is to create a few XML files, then use virsh to define them into the system.

Create one XML file per interface as follows:

br_wan.xml

<network>
  <name>br_wan</name>
  <forward mode="bridge"/>
  <bridge name="br_wan"/>
</network>

Then, import it into the system with virsh, then set it to autostart on boot

# import the network xml file
virsh net-define --file br_wan.xml
# set to autostart on boot
virsh net-autostart br_wan

I'm lazy, so I just created all three, then imported them all at one time.

br_wan.xml

<network>
  <name>br_wan</name>
  <forward mode="bridge"/>
  <bridge name="br_wan"/>
</network>

br_dmz.xml

<network>
  <name>br_dmz</name>
  <forward mode="bridge"/>
  <bridge name="br_dmz"/>
</network>

br_lan.xml

<network>
  <name>br_lan</name>
  <forward mode="bridge"/>
  <bridge name="br_lan"/>
</network>

And imported them all at once.

for interface in `ls *.xml` ; do virsh net-define --file $interface ; done
for interface in `ls *.xml | cut -d'.' -f1` ; do virsh net-autostart $interface ; virsh net-start $interface ; done
virsh net-list

After the last command, you should see your three interfaces defined. That means you can now use them.

In our example, we are going to use LVM2 to grab a piece of the disk for a new virtual. You can also use a file (File Backed Device, or FBD) by running fallocate, or set up access to an iSCSI.

By default, libvirt uses File Backed Devices (FBD's) from a pool defined internally. You can manually override this by defining a new pool, or by creating a file in your location using fallocate. For example, to create a 10G file in /srv/images named test.disk, you would use:

fallocate -l 10G /srv/images/test.disk

and use that when you create the virtual. However, it is better (easier) if you define a pool (or use the default). I have not researched this; see man virt-install.

Just create an LV the way you always do.

lvcreate -L 10G -n test.disk vg0

Ok, if you're using iSCSI, I'm guessing you know how to set it up. Just make sure it is available, then use the correct path when you create the image.