LibVirt

Common tips for LibVirt

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"

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.