libvirt/docs/daemons.rst

===============
Libvirt Daemons
===============

.. contents::

A libvirt deployment for accessing one of the stateful drivers will require
one or more daemons to be deployed on the virtualization host. There are a
number of ways the daemons can be configured which will be outlined in this
page.

Architectural options
=====================

Monolithic vs modular daemons
-----------------------------

Traditionally libvirt provided a single monolithic daemon called ``libvirtd``
which exposed support for all the stateful drivers, both primary hypervisor
drivers and secondary supporting drivers. It also enables secure remote access
from clients running off host.

Work is underway for the monolithic daemon to be replaced by a new set of
modular daemons ``virt${DRIVER}d``, each one servicing a single stateful
driver. A further ``virtproxyd`` daemon will provide secure remote access, as
well as backcompatibility for clients using the UNIX socket path of the
monolithic daemon.

The change to modular daemons should not affect API functionality used by
management applications. It will, however, have an impact on host provisioning
tools since there are new systemd services and configuration files to be
managed.

Currently both monolithic and modular daemons are built by default, but the RPC
client still prefers connecting to the monolithic daemon. It is intended to
switch the RPC client to prefer the modular daemons in the near future. At
least 1 year after this switch (but not more than 2 years), the monolithic
daemon will be deleted entirely.

Operating modes
---------------

The libvirt daemons, whether monolithic or modular, can often operate in two
modes

* *System mode* - the daemon is running as the root user account, enabling
  access to its full range of functionality. A read-write connection to
  daemons in system mode **typically implies privileges equivalent to having
  a root shell**. Suitable `authentication mechanisms <auth.html>`__ **must
  be enabled** to secure it against untrustworthy clients/users.

* *Session mode* - the daemon is running as any non-root user account,
  providing access to a more restricted range of functionality. Only client
  apps/users running under **the same UID are permitted to connect**, thus a
  connection does not imply any elevation of privileges.

  Not all drivers support session mode and as such the corresponding
  modular daemon may not support running in this mode


Monolithic driver daemon
========================

The monolithic daemon is known as ``libvirtd`` and has historically been the
default in libvirt. It is configured via the file ``/etc/libvirt/libvirtd.conf``


Monolithic sockets
------------------

When running in system mode, ``libvirtd`` exposes three UNIX domain sockets, and
optionally, one or two TCP sockets:

* ``/var/run/libvirt/libvirt-sock`` - the primary socket for accessing libvirt
  APIs, with full read-write privileges. A connection to this socket gives the
  client privileges that are equivalent to having a root shell. This is the
  socket that most management applications connect to by default.

* ``/var/run/libvirt/libvirt-sock-ro`` - the secondary socket for accessing
  libvirt APIs, with limited read-only privileges. A connection to this socket
  gives the ability to query the existence of objects and monitor some aspects
  of their operation. This is the socket that most management applications
  connect to when requesting read only mode. Typically this is what a
  monitoring app would use.

* ``/var/run/libvirt/libvirt-admin-sock`` - the administrative socket for
  controlling operation of the daemon itself (as opposed to drivers it is
  running). This can be used to dynamically reconfigure some aspects of the
  daemon and monitor/control connected clients.

* ``TCP 16509`` - the non-TLS socket for remotely accessing the libvirt APIs,
  with full read-write privileges. A connection to this socket gives the
  client privileges that are equivalent to having a root shell. Since it does
  not use TLS, an `authentication mechanism <auth.html>`__ that provides
  encryption must be used. Only the GSSAPI/Kerberos mechanism is capable of
  satisfying this requirement. In general applications should not use this
  socket except for debugging in a development/test environment.

* ``TCP 16514`` - the TLS socket for remotely accessing the libvirt APIs,
  with full read-write privileges. A connection to this socket gives the
  client privileges that are equivalent to having a root shell. Access control
  can be enforced either through validation of `x509 certificates
  <tlscerts.html>`__, and/or by enabling an `authentication mechanism
  <auth.html>`__.

NB, some distros will use ``/run`` instead of ``/var/run``.

When running in session mode, ``libvirtd`` exposes two UNIX domain sockets:

* ``$XDG_RUNTIME_DIR/libvirt/libvirt-sock`` - the primary socket for accessing
  libvirt APIs, with full read-write privileges. A connection to this socket
  does not alter the privileges that the client already has. This is the
  socket that most management applications connect to by default.

* ``$XDG_RUNTIME_DIR/libvirt/libvirt-admin-sock`` - the administrative socket
  for controlling operation of the daemon itself (as opposed to drivers it is
  running). This can be used to dynamically reconfigure some aspects of the
  daemon and monitor/control connected clients.

Notice that the session mode does not have a separate read-only socket. Since
the clients must be running as the same user as the daemon itself, there is
not any security benefit from attempting to enforce a read-only mode.

``$XDG_RUNTIME_DIR`` commonly points to a per-user private location on tmpfs,
such as ``/run/user/$UID``.


Monolithic Systemd Integration
------------------------------

When the ``libvirtd`` daemon is managed by ``systemd`` a number of desirable
features are available, most notably socket activation.

Libvirt ships a number of unit files for controlling ``libvirtd``:

* ``libvirtd.service`` - the main unit file for launching the ``libvirtd``
  daemon in system mode. The command line arguments passed can be configured by
  editing ``/etc/sysconfig/libvirtd``. This is typically only needed to control
  the use of the auto shutdown timeout value. It is recommended that this
  service unit be configured to start on boot. This is because various
  libvirt drivers support autostart of their objects. If it is known that
  autostart is not required, this unit can be left to start on demand.

* ``libvirtd.socket`` - the unit file corresponding to the main read-write
  UNIX socket ``/var/run/libvirt/libvirt-sock``. This socket is recommended to
  be started on boot by default.

* ``libvirtd-ro.socket`` - the unit file corresponding to the main read-write
  UNIX socket ``/var/run/libvirt/libvirt-sock-ro``. This socket is recommended
  to be started on boot by default.

* ``libvirtd-admin.socket`` - the unit file corresponding to the administrative
  UNIX socket ``/var/run/libvirt/libvirt-admin-sock``. This socket is
  recommended to be started on boot by default.

* ``libvirtd-tcp.socket`` - the unit file corresponding to the TCP 16509 port
  for non-TLS remote access. This socket should not be configured to start on
  boot until the administrator has configured a suitable authentication
  mechanism.

* ``libvirtd-tls.socket`` - the unit file corresponding to the TCP 16509 port
  for TLS remote access. This socket should not be configured to start on boot
  until the administrator has deployed x509 certificates and optionally
  configured a suitable authentication mechanism.

NB, some distros will use ``/etc/default`` instead of ``/etc/sysconfig``.

The socket unit files are newly introduced in 5.6.0. On newly installed hosts
the UNIX socket units should be enabled by default. When upgrading an existing
host from a previous version of libvirt, the socket unit files will be masked
if ``libvirtd`` is currently configured to use the ``--listen`` argument, since
the ``--listen`` argument is mutually exclusive with use of socket activation.

When systemd socket activation is used a number of configuration settings in
``libvirtd.conf`` are no longer honoured. Instead these settings must be
controlled via the system unit files

* ``listen_tcp`` - TCP socket usage is enabled by starting the
  ``libvirtd-tcp.socket`` unit file.

* ``listen_tls`` - TLS socket usage is enabled by starting the
  ``libvirtd-tls.socket`` unit file.

* ``tcp_port`` - Port for the non-TLS TCP socket, controlled via the
  ``ListenStream`` parameter in the ``libvirtd-tcp.socket`` unit file.

* ``tls_port`` - Port for the TLS TCP socket, controlled via the
  ``ListenStream`` parameter in the ``libvirtd-tls.socket`` unit file.

* ``listen_addr`` - IP address to listen on, independently controlled via the
  ``ListenStream`` parameter in the ``libvirtd-tcp.socket``  or
  ``libvirtd-tls.socket`` unit files.

* ``unix_sock_group`` - UNIX socket group owner, controlled via the
  ``SocketGroup`` parameter in the ``libvirtd.socket`` and
  ``libvirtd-ro.socket`` unit files

* ``unix_sock_ro_perms`` - read-only UNIX socket permissions, controlled via the
  ``SocketMode`` parameter in the ``libvirtd-ro.socket`` unit file

* ``unix_sock_rw_perms`` - read-write UNIX socket permissions, controlled via
  the ``SocketMode`` parameter in the ``libvirtd.socket`` unit file

* ``unix_sock_admin_perms`` - admin UNIX socket permissions, controlled via the
  ``SocketMode`` parameter in the ``libvirtd-admin.socket`` unit file

* ``unix_sock_dir`` - directory in which all UNIX sockets are created
  independently controlled via the ``ListenStream`` parameter in any of the
  ``libvirtd.socket``, ``libvirtd-ro.socket`` and ``libvirtd-admin.socket`` unit
  files.

Systemd releases prior to version 227 lacked support for passing the activation
socket unit names into the service. When using these old versions, the
``tcp_port``, ``tls_port`` and ``unix_sock_dir`` settings in ``libvirtd.conf``
must be changed in lock-step with the equivalent settings in the unit files to
ensure that ``libvirtd`` can identify the sockets.


Modular driver daemons
======================

The modular daemons are named after the driver which they are running, with
the pattern ``virt${DRIVER}d`` and will become the default in future libvirt.
They are configured via the files ``/etc/libvirt/virt${DRIVER}d.conf``

The following modular daemons currently exist for hypervisor drivers

* ``virtqemud`` - the QEMU management daemon, for running virtual machines
  on UNIX platforms, optionally with KVM acceleration, in either system or
  session mode
* ``virtxend`` - the Xen management daemon, for running virtual machines
  on the Xen hypervisor, in system mode only
* ``virtlxcd`` - the Linux Container management daemon, for running LXC guests
  in system mode only
* ``virtbhyved`` - the BHyve management daemon, for running virtual machines
  on FreeBSD with the BHyve hypervisor, in system mode.
* ``virtvboxd`` - the VirtualBox management daemon, for running virtual machines
  on UNIX platforms.

The additional modular daemons service secondary drivers

* ``virtinterfaced`` - the host NIC management daemon, in system mode only
* ``virtnetworkd`` - the virtual network management daemon, in system mode only
* ``virtnodedevd`` - the host physical device management daemon, in system mode
  only
* ``virtnwfilterd`` - the host firewall management daemon, in system mode only
* ``virtsecretd`` - the host secret management daemon, in system or session mode
* ``virtstoraged`` - the host storage management daemon, in system or session
  mode


Modular Sockets
---------------

When running in system mode, ``virt${DRIVER}d`` exposes three UNIX domain
sockets:

* ``/var/run/libvirt/virt${DRIVER}d-sock`` - the primary socket for accessing
  libvirt APIs, with full read-write privileges. For many of the daemons, a
  connection to this socket gives the client privileges that are equivalent to
  having a root shell. This is the socket that most management applications
  connect to by default.

* ``/var/run/libvirt/virt${DRIVER}d-sock-ro`` - the secondary socket for
  accessing libvirt APIs, with limited read-only privileges. A connection to
  this socket gives the ability to query the existence of objects and monitor
  some aspects of their operation. This is the socket that most management
  applications connect to when requesting read only mode. Typically this is
  what a monitoring app would use.

* ``/var/run/libvirt/virt${DRIVER}d-admin-sock`` - the administrative socket for
  controlling operation of the daemon itself (as opposed to drivers it is
  running). This can be used to dynamically reconfigure some aspects of the
  daemon and monitor/control connected clients.

NB, some distros will use ``/run`` instead of ``/var/run``.

When running in session mode, ``virt${DRIVER}d`` exposes two UNIX domain sockets:

* ``$XDG_RUNTIME_DIR/libvirt/virt${DRIVER}d-sock`` - the primary socket for
  accessing libvirt APIs, with full read-write privileges. A connection to this
  socket does not alter the privileges that the client already has. This is the
  socket that most management applications connect to by default.

* ``$XDG_RUNTIME_DIR/libvirt/virt${DRIVER}d-admin-sock`` - the administrative
  socket for controlling operation of the daemon itself (as opposed to drivers
  it is running). This can be used to dynamically reconfigure some aspects of
  the daemon and monitor/control connected clients.

Notice that the session mode does not have a separate read-only socket. Since
the clients must be running as the same user as the daemon itself, there is
not any security benefit from attempting to enforce a read-only mode.

``$XDG_RUNTIME_DIR`` commonly points to a per-user private location on tmpfs,
such as ``/run/user/$UID``.

Modular Systemd Integration
---------------------------

When the ``virt${DRIVER}d`` daemon is managed by ``systemd`` a number of
desirable features are available, most notably socket activation.

Libvirt ships a number of unit files for controlling ``virt${DRIVER}d``:

* ``virt${DRIVER}d.service`` - the main unit file for launching the
  ``virt${DRIVER}d`` daemon in system mode. The command line arguments passed
  can be configured by editing ``/etc/sysconfig/virt${DRIVER}d``. This is
  typically only needed to control the use of the auto shutdown timeout value.
  It is recommended that this service unit be configured to start on boot.
  This is because various libvirt drivers support autostart of their objects.
  If it is known that autostart is not required, this unit can be left to start
  on demand.

* ``virt${DRIVER}d.socket`` - the unit file corresponding to the main read-write
  UNIX socket ``/var/run/libvirt/virt${DRIVER}d-sock``. This socket is
  recommended to be started on boot by default.

* ``virt${DRIVER}d-ro.socket`` - the unit file corresponding to the main
  read-write UNIX socket ``/var/run/libvirt/virt${DRIVER}d-sock-ro``. This
  socket is recommended to be started on boot by default.

* ``virt${DRIVER}d-admin.socket`` - the unit file corresponding to the
  administrative UNIX socket ``/var/run/libvirt/virt${DRIVER}d-admin-sock``.
  This socket is recommended to be started on boot by default.

NB, some distros will use ``/etc/default`` instead of ``/etc/sysconfig``.

The socket unit files are newly introduced in 5.6.0. On newly installed hosts
the UNIX socket units should be enabled by default. When upgrading an existing
host from a previous version of libvirt, the socket unit files will be masked
if ``virt${DRIVER}d`` is currently configured to use the ``--listen`` argument,
since the ``--listen`` argument is mutually exclusive with use of socket
activation.

When systemd socket activation is used a number of configuration settings in
``virt${DRIVER}d.conf`` are no longer honoured. Instead these settings must be
controlled via the system unit files:

* ``unix_sock_group`` - UNIX socket group owner, controlled via the
  ``SocketGroup`` parameter in the ``virt${DRIVER}d.socket`` and
  ``virt${DRIVER}d-ro.socket`` unit files

* ``unix_sock_ro_perms`` - read-only UNIX socket permissions, controlled via the
  ``SocketMode`` parameter in the ``virt${DRIVER}d-ro.socket`` unit file

* ``unix_sock_rw_perms`` - read-write UNIX socket permissions, controlled via
  the ``SocketMode`` parameter in the ``virt${DRIVER}d.socket`` unit file

* ``unix_sock_admin_perms`` - admin UNIX socket permissions, controlled via the
  ``SocketMode`` parameter in the ``virt${DRIVER}d-admin.socket`` unit file

* ``unix_sock_dir`` - directory in which all UNIX sockets are created
  independently controlled via the ``ListenStream`` parameter in any of the
  ``virt${DRIVER}d.socket``, ``virt${DRIVER}d-ro.socket`` and
  ``virt${DRIVER}d-admin.socket`` unit files.

Systemd releases prior to version 227 lacked support for passing the activation
socket unit names into the service. When using these old versions, the
``unix_sock_dir`` setting in ``virt${DRIVER}d.conf`` must be changed in
lock-step with the equivalent setting in the unit files to ensure that
``virt${DRIVER}d`` can identify the sockets.


Switching to modular daemons
----------------------------

If a host is currently set to use the monolithic ``libvirtd`` daemon and needs
to be migrated to the monolithic daemons a number of services need to be
changed. The steps below outline the process on hosts using the systemd init
service.

While it is technically possible to do this while virtual machines are running,
it is recommended that virtual machines be stopped or live migrated to a new
host first.

#. Stop the current monolithic daemon and its socket units

   ::

      $ systemctl stop libvirtd.service
      $ systemctl stop libvirtd{,-ro,-admin,-tcp,-tls}.socket

#. Disable future start of the monolithic daemon

   ::

      $ systemctl disable libvirtd.service
      $ systemctl disable libvirtd{,-ro,-admin,-tcp,-tls}.socket

   For stronger protection it is valid to use ``mask`` instead of ``disable``
   too.

#. Enable the new daemons for the particular virtualizationd driver desired,
   and any of the secondary drivers to accompany it. The following example
   enables the QEMU driver and all the secondary drivers:

   ::

      $ for drv in qemu interface network nodedev nwfilter secret storage
        do
          systemctl unmask virt${drv}d.service
          systemctl unmask virt${drv}d{,-ro,-admin}.socket
          systemctl enable virt${drv}d.service
          systemctl enable virt${drv}d{,-ro,-admin}.socket
	done

#. Start the sockets for the same set of daemons. There is no need to start the
   services as they will get started when the first socket connection is
   established

   ::

      $ for drv in qemu network nodedev nwfilter secret storage
        do
          systemctl start virt${drv}d{,-ro,-admin}.socket
	done

#. If connections from remote hosts need to be supported the proxy daemon
   must be enabled and started

   ::

      $ systemctl unmask virtproxyd.service
      $ systemctl unmask virtproxyd{,-ro,-admin}.socket
      $ systemctl enable virtproxyd.service
      $ systemctl enable virtproxyd{,-ro,-admin}.socket
      $ systemctl start virtproxyd{,-ro,-admin}.socket

   The UNIX sockets allow for remote access using SSH tunneling. If ``libvirtd``
   had TCP or TLS sockets configured, those should be started too

   ::

      $ systemctl unmask virtproxyd-tls.socket
      $ systemctl enable virtproxyd-tls.socket
      $ systemctl start virtproxyd-tls.socket


Proxy daemon
============

The monolithic daemon is known as ``libvirtd`` and has historically been the
default in libvirt. It is configured via the file ``/etc/libvirt/libvirtd.conf``


Proxy sockets
-------------

When running in system mode, ``virtproxyd`` exposes three UNIX domain sockets,
and optionally, one or two TCP sockets. These sockets are identical to those
provided by the traditional ``libvirtd`` so refer to earlier documentation in
this page.

When running in session mode, ``virtproxyd`` exposes two UNIX domain sockets,
which are again identical to those provided by ``libvirtd``.

Proxy Systemd Integration
-------------------------

When the ``virtproxyd`` daemon is managed by ``systemd`` a number of desirable
features are available, most notably socket activation.

Libvirt ships a number of unit files for controlling ``virtproxyd``:

* ``virtproxyd.service`` - the main unit file for launching the ``virtproxyd``
  daemon in system mode. The command line arguments passed can be configured by
  editing ``/etc/sysconfig/virtproxyd``. This is typically only needed to
  control the use of the auto shutdown timeout value.

* ``virtproxyd.socket`` - the unit file corresponding to the main read-write
  UNIX socket ``/var/run/libvirt/libvirt-sock``. This socket is recommended to
  be started on boot by default.

* ``virtproxyd-ro.socket`` - the unit file corresponding to the main read-write
  UNIX socket ``/var/run/libvirt/libvirt-sock-ro``. This socket is recommended
  to be started on boot by default.

* ``virtproxyd-admin.socket`` - the unit file corresponding to the
  administrative UNIX socket ``/var/run/libvirt/libvirt-admin-sock``. This
  socket is recommended to be started on boot by default.

* ``virtproxyd-tcp.socket`` - the unit file corresponding to the TCP 16509 port
  for non-TLS remote access. This socket should not be configured to start on
  boot until the administrator has configured a suitable authentication
  mechanism.

* ``virtproxyd-tls.socket`` - the unit file corresponding to the TCP 16509 port
  for TLS remote access. This socket should not be configured to start on boot
  until the administrator has deployed x509 certificates and optionally
  configured a suitable authentication mechanism.

NB, some distros will use ``/etc/default`` instead of ``/etc/sysconfig``.

The socket unit files are newly introduced in 5.6.0. On newly installed hosts
the UNIX socket units should be enabled by default. When upgrading an existing
host from a previous version of libvirt, the socket unit files will be masked
if ``virtproxyd`` is currently configured to use the ``--listen`` argument, since
the ``--listen`` argument is mutually exclusive with use of socket activation.

When systemd socket activation is used a number of configuration settings in
``virtproxyd.conf`` are no longer honoured. Instead these settings must be
controlled via the system unit files. Refer to the earlier documentation on
the ``libvirtd`` service socket configuration for further information.


Logging daemon
==============

The ``virtlogd`` daemon provides a service for managing log files associated
with QEMU virtual machines. The QEMU process is given one or more pipes, the
other end of which are owned by the ``virtlogd`` daemon. It will then write
data on those pipes to log files, while enforcing a maximum file size and
performing log rollover at the size limit.

Since the daemon holds open anoymous pipe file descriptors, it must never be
stopped while any QEMU virtual machines are running. To enable software updates
to be applied, the daemon is capable of re-executing itself while keeping all
file descriptors open. This can be triggered by sending the daemon ``SIGUSR1``

Logging Sockets
---------------

When running in system mode, ``virtlogd`` exposes two UNIX domain sockets:

* ``/var/run/libvirt/virtlogd-sock`` - the primary socket for accessing
  libvirt APIs, with full read-write privileges. Access to the socket is
  restricted to the root user.

* ``/var/run/libvirt/virtlogd-admin-sock`` - the administrative socket for
  controlling operation of the daemon itself (as opposed to drivers it is
  running). This can be used to dynamically reconfigure some aspects of the
  daemon and monitor/control connected clients.

NB, some distros will use ``/run`` instead of ``/var/run``.

When running in session mode, ``virtlogd`` exposes two UNIX domain sockets:

* ``$XDG_RUNTIME_DIR/libvirt/virtlogd-sock`` - the primary socket for
  accessing libvirt APIs, with full read-write privileges. Access to the
  socket is restricted to the unprivileged user running the daemon.

* ``$XDG_RUNTIME_DIR/libvirt/virtlogd-admin-sock`` - the administrative
  socket for controlling operation of the daemon itself (as opposed to drivers
  it is running). This can be used to dynamically reconfigure some aspects of
  the daemon and monitor/control connected clients.

``$XDG_RUNTIME_DIR`` commonly points to a per-user private location on tmpfs,
such as ``/run/user/$UID``.

Logging Systemd Integration
---------------------------

When the ``virtlogd`` daemon is managed by ``systemd`` a number of desirable
features are available, most notably socket activation.

Libvirt ships a number of unit files for controlling ``virtlogd``:

* ``virtlogd.service`` - the main unit file for launching the
  ``virtlogd`` daemon in system mode. The command line arguments passed
  can be configured by editing ``/etc/sysconfig/virtlogd``. This is
  typically only needed to control the use of the auto shutdown timeout value.

* ``virtlogd.socket`` - the unit file corresponding to the main read-write
  UNIX socket ``/var/run/libvirt/virtlogd-sock``. This socket is recommended
  to be started on boot by default.

* ``virtlogd-admin.socket`` - the unit file corresponding to the administrative
  UNIX socket ``/var/run/libvirt/virtlogd-admin-sock``. This socket is
  recommended to be started on boot by default.

NB, some distros will use ``/etc/default`` instead of ``/etc/sysconfig``.

When systemd socket activation is used a number of configuration settings in
``virtlogd.conf`` are no longer honoured. Instead these settings must be
controlled via the system unit files:

* ``unix_sock_group`` - UNIX socket group owner, controlled via the
  ``SocketGroup`` parameter in the ``virtlogd.socket`` and
  ``virtlogd-ro.socket`` unit files

* ``unix_sock_ro_perms`` - read-only UNIX socket permissions, controlled via the
  ``SocketMode`` parameter in the ``virtlogd-ro.socket`` unit file

* ``unix_sock_rw_perms`` - read-write UNIX socket permissions, controlled via
  the ``SocketMode`` parameter in the ``virtlogd.socket`` unit file

* ``unix_sock_admin_perms`` - admin UNIX socket permissions, controlled via the
  ``SocketMode`` parameter in the ``virtlogd-admin.socket`` unit file

* ``unix_sock_dir`` - directory in which all UNIX sockets are created
  independently controlled via the ``ListenStream`` parameter in any of the
  ``virtlogd.socket`` and ``virtlogd-admin.socket`` unit files.

Systemd releases prior to version 227 lacked support for passing the activation
socket unit names into the service. When using these old versions, the
``unix_sock_dir`` setting in ``virtlogd.conf`` must be changed in
lock-step with the equivalent setting in the unit files to ensure that
``virtlogd`` can identify the sockets.

Locking daemon
==============

The ``virtlockd`` daemon provides a service for holding locks against file
images and devices serving as backing storage for virtual disks. The locks
will be held for as long as there is a QEMU process running with the disk
open.

To ensure continuity of locking, the daemon holds open anoymous file
descriptors, it must never be stopped while any QEMU virtual machines are
running. To enable software updates to be applied, the daemon is capable of
re-executing itself while keeping all file descriptors open. This can be
triggered by sending the daemon ``SIGUSR1``

Locking Sockets
---------------

When running in system mode, ``virtlockd`` exposes two UNIX domain sockets:

* ``/var/run/libvirt/virtlockd-sock`` - the primary socket for accessing
  libvirt APIs, with full read-write privileges. Access to the socket is
  restricted to the root user.

* ``/var/run/libvirt/virtlockd-admin-sock`` - the administrative socket for
  controlling operation of the daemon itself (as opposed to drivers it is
  running). This can be used to dynamically reconfigure some aspects of the
  daemon and monitor/control connected clients.

NB, some distros will use ``/run`` instead of ``/var/run``.

When running in session mode, ``virtlockd`` exposes two UNIX domain sockets:

* ``$XDG_RUNTIME_DIR/libvirt/virtlockd-sock`` - the primary socket for
  accessing libvirt APIs, with full read-write privileges. Access to the
  socket is restricted to the unprivileged user running the daemon.

* ``$XDG_RUNTIME_DIR/libvirt/virtlockd-admin-sock`` - the administrative
  socket for controlling operation of the daemon itself (as opposed to drivers
  it is running). This can be used to dynamically reconfigure some aspects of
  the daemon and monitor/control connected clients.

``$XDG_RUNTIME_DIR`` commonly points to a per-user private location on tmpfs,
such as ``/run/user/$UID``.

Locking Systemd Integration
---------------------------

When the ``virtlockd`` daemon is managed by ``systemd`` a number of desirable
features are available, most notably socket activation.

Libvirt ships a number of unit files for controlling ``virtlockd``:

* ``virtlockd.service`` - the main unit file for launching the
  ``virtlockd`` daemon in system mode. The command line arguments passed
  can be configured by editing ``/etc/sysconfig/virtlockd``. This is
  typically only needed to control the use of the auto shutdown timeout value.

* ``virtlockd.socket`` - the unit file corresponding to the main read-write
  UNIX socket ``/var/run/libvirt/virtlockd-sock``. This socket is recommended
  to be started on boot by default.

* ``virtlockd-admin.socket`` - the unit file corresponding to the administrative
  UNIX socket ``/var/run/libvirt/virtlockd-admin-sock``. This socket is
  recommended to be started on boot by default.

NB, some distros will use ``/etc/default`` instead of ``/etc/sysconfig``.

When systemd socket activation is used a number of configuration settings in
``virtlockd.conf`` are no longer honoured. Instead these settings must be
controlled via the system unit files:

* ``unix_sock_group`` - UNIX socket group owner, controlled via the
  ``SocketGroup`` parameter in the ``virtlockd.socket`` and
  ``virtlockd-ro.socket`` unit files

* ``unix_sock_ro_perms`` - read-only UNIX socket permissions, controlled via the
  ``SocketMode`` parameter in the ``virtlockd-ro.socket`` unit file

* ``unix_sock_rw_perms`` - read-write UNIX socket permissions, controlled via
  the ``SocketMode`` parameter in the ``virtlockd.socket`` unit file

* ``unix_sock_admin_perms`` - admin UNIX socket permissions, controlled via the
  ``SocketMode`` parameter in the ``virtlockd-admin.socket`` unit file

* ``unix_sock_dir`` - directory in which all UNIX sockets are created
  independently controlled via the ``ListenStream`` parameter in any of the
  ``virtlockd.socket`` and ``virtlockd-admin.socket`` unit files.

Systemd releases prior to version 227 lacked support for passing the activation
socket unit names into the service. When using these old versions, the
``unix_sock_dir`` setting in ``virtlockd.conf`` must be changed in
lock-step with the equivalent setting in the unit files to ensure that
``virtlockd`` can identify the sockets.