LibVirt

Common tips for LibVirt

LibVirt is universal virtualization layer - it supports several hypervisors (QEMU+KVM, Xen, ...) and even containers (called LXC however it uses different toolset for these).

Note: because libvirt started as wrapper on Xen it uses (sometimes confusing) Xen terms - for example 'destroy' normally means "stop" or "shutdown" but Not delete... Similarly create means start (fortunately there is now also start command)

Installation under Fedora 40:

$ sudo dnf install libvirt-daemon-kvm libvirt-daemon-config-network libvirt-client-qemu

$ sudo dnf install virt-manager  # pouplar GUI for LibVirt

# ensure that creepyware geoclue2 is masked:
$ sudo systemctl mask --now geoclue
$ sudo rm -f /etc/xdg/autostart/geoclue-demo-agent.desktop

If you plan to use virt-manager remotely via SSH X11 Forwarding you have to install these packages on Server (with virt-manager):

# required
$ sudo dnf install xorg-x11-xauth

# optional - for testing
$ sudo dnf install xclock xterm

Also on your SSHd server enable X11 forwarding, for example, create /etc/ssh/sshd_config.d/99-local.conf with contents:

X11Forwarding yes

And restart SSHd with systemctl restart sshd

Also ensure that your SSH client requests X11 forwarding, here is excerpt from my ~/.ssh/config

Host f40lvm-s350
        HostName 192.168.0.X
        User USERNAME
        IdentityFile ~/.ssh/MY_SSH_KEY
        IdentitiesOnly yes
        HostKeyAlias f40lvm-s350
        ForwardX11 yes

Now try ssh connection from your client using alias, in my example: ssh f40lvm-s350 And try some X11 application, for example xclock (use Ctrl-C in terminal to quit)

But before running virt-manager please read following chapter:

Connect to system

By default when you run any libvirt command it will use your private User connection (called "session"). So all networks and disks and VMs will be available to you only (and data stored under $HOME/.local)

To use always shared global (called system) connection you have to:

1. Add yourself to libvirt group using:

   sudo /usr/sbin/usermod -G libvirt -a $USER

2. Create file ~/.config/libvirt/libvirt.conf with following content:

   # see https://listman.redhat.com/archives/libvirt-users/2018-October/msg00067.html
   uri_default = "qemu:///system"

On Fedora I have to enable and start "libvirtd legacy service" with:

$ sudo systemctl enable --now libvirtd

To avoid virsh client error:

Failed to connect socket to '/var/run/libvirt/virtqemud-sock': No such file or directory

Remember that you should logout and login to ensure that you are member of group libvirt. Then you can for example try listing networks or pools:

$ virsh net-list

 Name      State    Autostart   Persistent
--------------------------------------------
 default   active   yes         yes

$ virsh pool-list

 Name   State   Autostart
---------------------------

If there is no pool you can follow guide from Ubuntu MAAS KVM. Run as you (non-privileged user)

virsh pool-define-as default dir - - - - "/var/lib/libvirt/images"
virsh pool-autostart default
virsh pool-start default
virsh pool-list

   Name      State    Autostart
  -------------------------------
   default   active   yes

virsh pool-dumpxml default

   ...

Logging DNS queries

At least when using NAT network we can pass option log-queries to dnsmasq that is used to provide both DHCP and DNS server for NAT network.

I followed various sources including: https://serverfault.com/a/1017645

Here is diff of default network - using virsh net-edit default or using virsh net-dumpxml default:

diff -u default.xml default-log-queries.xml
--- default.xml	2024-07-01 19:15:27.965334382 +0200
+++ default-log-queries.xml	2024-07-01 19:28:14.214143882 +0200
@@ -1,4 +1,4 @@
-<network connections='1'>
+<network xmlns:dnsmasq='http://libvirt.org/schemas/network/dnsmasq/1.0'>
   <name>default</name>
   <uuid>2c9b8477-9b7c-4ca5-8c20-f1fbcc7df3c3</uuid>
   <forward mode='nat'>
@@ -14,5 +14,8 @@
       <range start='192.168.100.128' end='192.168.100.254'/>
     </dhcp>
   </ip>
+  <dnsmasq:options>
+    <dnsmasq:option value='log-queries'/>
+  </dnsmasq:options>
 </network>

And then you have to restart network using (scary) destroy and start:

virsh net-destroy default
virsh net-start default

You can also peek content of /var/lib/libvirt/dnsmasq/default.conf if there is your option (log-queries in our case).

After restart you can try on Host:

journalctl -u libvirtd -f

And boot any VM that uses NAT network under LibVirt.