libvirt: Libvirt Daemons
Description: libvirt, virtualization, virtualization API

# Libvirt Daemons

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 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 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, and/or by enabling an authentication mechanism.

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-only 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.

## 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${DRIVER}d-ro.socket - the unit file corresponding to the main read-only UNIX socket /var/run/libvirt/virt${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.

### Switching to modular daemons

If a host is currently set to use the monolithic libvirtd daemon and needs to be migrated to the modular 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.

1. Stop the current monolithic daemon and its socket units

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

2. 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.

3. 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

4. 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

5. 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

## Checking whether modular/monolithic mode is in use

New distributions are likely to use the modular mode although the upgrade process preserves whichever mode was in use before the upgrade.

To determine whether modular or monolithic mode is in use on a host running systemd as the init system you can take the following steps:

1. Check whether the modular daemon infrastructure is in use

   First check whether the modular daemon you are interested (see Modular driver daemons for a summary of which daemons are provided by libvirt) in is running:

   1. Check .socket for socket activated services

   >     # systemctl is-active virtqemud.socket
   >     active

   1. Check .service for always-running daemons

   >     # systemctl is-active virtqemud.service
   >     active

   If either of the above is active your system is using the modular daemons.

2. Check whether the monolithic daemon is in use

   1. Check libvirtd.socket

   >     # systemctl is-active libvirtd.socket
   >     active

   1. Check libvirtd.service for always-running daemon

   >     # systemctl is-active libvirtd.service
   >     active

   If either of the above is active your system is using the monolithic daemon.

3. To determine which of the above will be in use on the next boot of the system, substitute is-enabled for is-active in the above examples.

## Proxy daemon

### 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-only 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 anonymous 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.

## 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 anonymous 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.

## Changing command line options for daemons

Two ways exist to override the defaults in the provided service files: either a systemd "drop-in" configuration file, or a /etc/sysconfig/$daemon file must be created. For example, to change the command line option for a debug session of libvirtd, create a file /etc/systemd/system/libvirtd.service.d/debug.conf with the following content:

>     [Unit]
>     Description=Virtualization daemon, with override from debug.conf

>     [Service]
>     Environment=G_DEBUG=fatal-warnings
>     Environment=LIBVIRTD_ARGS="--listen --verbose"

After changes to systemd "drop-in" configuration files it is required to run systemctl daemon-reload.