Libvirt - Nova Virtualisation Driver¶
Libvirt is the most commonly used virtualisation driver in OpenStack. It uses
libvirt, backed by QEMU and when available, KVM. Libvirt is executed in the
nova_libvirt container, or as a daemon running on the host.
Two values are supported for
nova_compute_virt_type with libvirt -
kvm being the default.
For optimal performance,
kvm is preferable, since many aspects of
virtualisation can be offloaded to hardware. If it is not possible to enable
hardware virtualisation (e.g. Virtualisation Technology (VT) BIOS configuration
on Intel systems),
qemu may be used to provide less performant
The default configuration of Kolla Ansible is to run libvirt over TCP, authenticated with SASL. This should not be considered as providing a secure, encrypted channel, since the username/password SASL mechanisms available for TCP are no longer considered cryptographically secure. However, it does at least provide some authentication for the libvirt API. For a more secure encrypted channel, use libvirt TLS.
SASL is enabled according to the
libvirt_enable_sasl flag, which defaults
The username is configured via
libvirt_sasl_authname, and defaults to
nova. The password is configured via
libvirt_sasl_password, and is
generated with other passwords using
and stored in
The list of enabled authentication mechanisms is configured via
libvirt_sasl_mech_list, and defaults to
["SCRAM-SHA-256"] if libvirt
TLS is enabled, or
Host vs containerised libvirt¶
By default, Kolla Ansible deploys libvirt in a
nova_libvirt container. In
some cases it may be preferable to run libvirt as a daemon on the compute hosts
Kolla Ansible does not currently support deploying and configuring
libvirt as a host daemon. However, since the Yoga release, if a libvirt daemon
has already been set up, then Kolla Ansible may be configured to use it. This
may be achieved by setting
When the firewall driver is set to
openvswitch, libvirt will plug VMs
directly into the integration bridge,
br-int. To do this it uses the
ovs-vsctl utility. The search path for this binary is controlled by the
$PATH environment variable (as seen by the libvirt process). There are a
few options to ensure that this binary can be found:
True. This will install a wrapper script to the path:
/usr/bin/ovs-vsctlthat will execute
ovs-vsctlin the context of the
openvswitch_vswitchdcontainer. This option is useful if you do not have openvswitch installed on the host. It also has the advantage that the
ovs-vsctlutility will match the version of the server.
Install openvswitch on the hypervisor. Kolla mounts
/run/openvswitchfrom the host into the
openvswitch_vswitchdcontainer. This means that socket is in the location
ovs-vsctlexpects with its default options.
Migration from container to host¶
kolla-ansible nova-libvirt-cleanup command may be used to clean up the
nova_libvirt container and related items on hosts, once it has
been disabled. This should be run after the compute service has been disabled,
and all active VMs have been migrated away from the host.
By default, the command will fail if there are any VMs running on the host. If
you are sure that it is safe to clean up the
nova_libvirt container with
running VMs, setting
will allow the command to proceed.
nova_libvirt container has several associated Docker volumes:
nova_libvirt_secrets. By default,
these volumes are not cleaned up. If you are sure that the data in these
volumes can be safely removed, setting
true will cause the Docker volumes to be removed.
A future extension could support migration of existing VMs, but this is currently out of scope.
The default configuration of Kolla Ansible is to run libvirt over TCP, with SASL authentication. As long as one takes steps to protect who can access the network this works well. However, in a less trusted environment one may want to use encryption when accessing the libvirt API. To do this we can enable TLS for libvirt and make nova use it. Mutual TLS is configured, providing authentication of clients via certificates. SASL authentication provides a further level of security.
Using libvirt TLS¶
Libvirt TLS can be enabled in Kolla Ansible by setting the following option in
Creation of production-ready TLS certificates is currently out-of-scope for Kolla Ansible. You will need to either use an existing Internal CA or you will need to generate your own offline CA. For the TLS communication to work correctly you will have to supply Kolla Ansible the following pieces of information:
This is the CA’s public certificate that all of the client and server certificates are signed with. Libvirt and nova-compute will need this so they can verify that all the certificates being used were signed by the CA and should be trusted.
serverkey.pem (not used when using a host libvirt daemon)
This is the private key for the server, and is no different than the private key of a TLS certificate. It should be carefully protected, just like the private key of a TLS certificate.
servercert.pem (not used when using a host libvirt daemon)
This is the public certificate for the server. Libvirt will present this certificate to any connection made to the TLS port. This is no different than the public certificate part of a standard TLS certificate/key bundle.
This is the client private key, which nova-compute/libvirt will use when it is connecting to libvirt. Think of this as an SSH private key and protect it in a similar manner.
This is the client certificate that nova-compute/libvirt will present when it is connecting to libvirt. Think of this as the public side of an SSH key.
Kolla Ansible will search for these files for each compute node in the following locations and order on the host where Kolla Ansible is executed:
In most cases you will want to have a unique set of server and client
certificates and keys per hypervisor and with a common CA certificate. In this
case you would place each of the server/client certificate and key PEM files
/etc/kolla/config/nova/nova-libvirt/<hostname>/ and the CA
However, it is possible to make use of wildcard server certificate and a single
client certificate that is shared by all servers. This will allow you to
generate a single client certificate and a single server certificate that is
shared across every hypervisor. In this case you would store everything under
Externally managed certificates¶
One more option for deployers who already have automation to get TLS certs onto
servers is to disable certificate management under
With this option disabled Kolla Ansible will simply assume that certificates and keys are already installed in their correct locations. Deployers will be responsible for making sure that the TLS certificates/keys get placed in to the correct container configuration directories on the servers so that they can get copied into the nova-compute and nova-libvirt containers. With this option disabled you will also be responsible for restarting the nova-compute and nova-libvirt containers when the certs are updated, as kolla-ansible will not be able to tell when the files have changed.
Generating certificates for test and development¶
Since the Yoga release, the
kolla-ansible certificates command generates
certificates for libvirt TLS. A single key and certificate is used for all
hosts, with a Subject Alternative Name (SAN) entry for each compute host