daemon - Writing and packaging system daemons
A daemon is a service process that runs in the background and supervises the
system or provides functionality to other processes. Traditionally, daemons
are implemented following a scheme originating in SysV Unix. Modern daemons
should follow a simpler yet more powerful scheme (here called
"new-style" daemons), as implemented by systemd
manual page covers both schemes, and in particular includes recommendations
for daemons that shall be included in the systemd init system.
When a traditional SysV daemon starts, it should execute the following steps as
part of the initialization. Note that these steps are unnecessary for
new-style daemons (see below), and should only be implemented if compatibility
with SysV is essential.
1.Close all open file descriptors except
standard input, output, and error (i.e. the first three file descriptors 0, 1,
2). This ensures that no accidentally passed file descriptor stays around in
the daemon process. On Linux, this is best implemented by iterating through
/proc/self/fd, with a fallback of iterating from file descriptor 3 to the
value returned by getrlimit() for RLIMIT_NOFILE.
2.Reset all signal handlers to their default.
This is best done by iterating through the available signals up to the limit
of _NSIG and resetting them to SIG_DFL.
3.Reset the signal mask using
4.Sanitize the environment block, removing or
resetting environment variables that might negatively impact daemon
5.Call fork(), to create a background
6.In the child, call setsid() to
detach from any terminal and create an independent session.
7.In the child, call fork() again, to
ensure that the daemon can never re-acquire a terminal again.
8.Call exit() in the first child, so
that only the second child (the actual daemon process) stays around. This
ensures that the daemon process is re-parented to init/PID 1, as all daemons
9.In the daemon process, connect /dev/null to
standard input, output, and error.
10.In the daemon process, reset the umask to
0, so that the file modes passed to open(), mkdir() and suchlike
directly control the access mode of the created files and directories.
11.In the daemon process, change the current
directory to the root directory (/), in order to avoid that the daemon
involuntarily blocks mount points from being unmounted.
12.In the daemon process, write the daemon PID
(as returned by getpid()) to a PID file, for example /run/foobar.pid
(for a hypothetical daemon "foobar") to ensure that the daemon
cannot be started more than once. This must be implemented in race-free
fashion so that the PID file is only updated when it is verified at the same
time that the PID previously stored in the PID file no longer exists or
belongs to a foreign process.
13.In the daemon process, drop privileges, if
possible and applicable.
14.From the daemon process, notify the
original process started that initialization is complete. This can be
implemented via an unnamed pipe or similar communication channel that is
created before the first fork() and hence available in both the
original and the daemon process.
15.Call exit() in the original process.
The process that invoked the daemon must be able to rely on that this
exit() happens after initialization is complete and all external
communication channels are established and accessible.
The BSD daemon()
function should not be used, as it implements only a
subset of these steps.
A daemon that needs to provide compatibility with SysV systems should implement
the scheme pointed out above. However, it is recommended to make this behavior
optional and configurable via a command line argument to ease debugging as
well as to simplify integration into systems using systemd.
Modern services for Linux should be implemented as new-style daemons. This makes
it easier to supervise and control them at runtime and simplifies their
For developing a new-style daemon, none of the initialization steps recommended
for SysV daemons need to be implemented. New-style init systems such as
systemd make all of them redundant. Moreover, since some of these steps
interfere with process monitoring, file descriptor passing and other
functionality of the init system, it is recommended not to execute them when
run as new-style service.
Note that new-style init systems guarantee execution of daemon processes in a
clean process context: it is guaranteed that the environment block is
sanitized, that the signal handlers and mask is reset and that no left-over
file descriptors are passed. Daemons will be executed in their own session,
with standard input connected to /dev/null and standard output/error connected
to the systemd-journald.service
(8) logging service, unless otherwise
configured. The umask is reset.
It is recommended for new-style daemons to implement the following:
1.If SIGTERM is received, shut down
the daemon and exit cleanly.
2.If SIGHUP is received, reload the
configuration files, if this applies.
3.Provide a correct exit code from the main
daemon process, as this is used by the init system to detect service errors
and problems. It is recommended to follow the exit code scheme as defined in
the LSB recommendations for SysV init scripts.
4.If possible and applicable, expose the
daemon's control interface via the D-Bus IPC system and grab a bus name as
last step of initialization.
5.For integration in systemd, provide a
.service unit file that carries information about starting, stopping and
otherwise maintaining the daemon. See systemd.service(5) for
6.As much as possible, rely on the init
system's functionality to limit the access of the daemon to files, services
and other resources, i.e. in the case of systemd, rely on systemd's resource
limit control instead of implementing your own, rely on systemd's privilege
dropping code instead of implementing it in the daemon, and similar. See
systemd.exec(5) for the available controls.
7.If D-Bus is used, make your daemon
bus-activatable by supplying a D-Bus service activation configuration file.
This has multiple advantages: your daemon may be started lazily on-demand; it
may be started in parallel to other daemons requiring it — which
maximizes parallelization and boot-up speed; your daemon can be restarted on
failure without losing any bus requests, as the bus queues requests for
activatable services. See below for details.
8.If your daemon provides services to other
local processes or remote clients via a socket, it should be made
socket-activatable following the scheme pointed out below. Like D-Bus
activation, this enables on-demand starting of services as well as it allows
improved parallelization of service start-up. Also, for state-less protocols
(such as syslog, DNS), a daemon implementing socket-based activation can be
restarted without losing a single request. See below for details.
9.If applicable, a daemon should notify the
init system about startup completion or status updates via the
10.Instead of using the syslog() call
to log directly to the system syslog service, a new-style daemon may choose to
simply log to standard error via fprintf(), which is then forwarded to
syslog by the init system. If log levels are necessary, these can be encoded
by prefixing individual log lines with strings like "<4>" (for
log level 4 "WARNING" in the syslog priority scheme), following a
similar style as the Linux kernel's printk() level system. For details,
see sd-daemon(3) and systemd.exec(5).
These recommendations are similar but not identical to the Apple MacOS X
New-style init systems provide multiple additional mechanisms to activate
services, as detailed below. It is common that services are configured to be
activated via more than one mechanism at the same time. An example for
systemd: bluetoothd.service might get activated either when Bluetooth hardware
is plugged in, or when an application accesses its programming interfaces via
D-Bus. Or, a print server daemon might get activated when traffic arrives at
an IPP port, or when a printer is plugged in, or when a file is queued in the
printer spool directory. Even for services that are intended to be started on
system bootup unconditionally, it is a good idea to implement some of the
various activation schemes outlined below, in order to maximize
parallelization. If a daemon implements a D-Bus service or listening socket,
implementing the full bus and socket activation scheme allows starting of the
daemon with its clients in parallel (which speeds up boot-up), since all its
communication channels are established already, and no request is lost because
client requests will be queued by the bus system (in case of D-Bus) or the
kernel (in case of sockets) until the activation is completed.
Old-style daemons are usually activated exclusively on boot (and manually by the
administrator) via SysV init scripts, as detailed in the LSB Linux Standard
Base Core Specification
. This method of activation is supported
ubiquitously on Linux init systems, both old-style and new-style systems.
Among other issues, SysV init scripts have the disadvantage of involving shell
scripts in the boot process. New-style init systems generally employ updated
versions of activation, both during boot-up and during runtime and using more
minimal service description files.
In systemd, if the developer or administrator wants to make sure that a service
or other unit is activated automatically on boot, it is recommended to place a
symlink to the unit file in the .wants/ directory of either multi-user.target
or graphical.target, which are normally used as boot targets at system
startup. See systemd.unit
(5) for details about the .wants/ directories,
(7) for details about the two boot targets.
In order to maximize the possible parallelization and robustness and simplify
configuration and development, it is recommended for all new-style daemons
that communicate via listening sockets to employ socket-based activation. In a
socket-based activation scheme, the creation and binding of the listening
socket as primary communication channel of daemons to local (and sometimes
remote) clients is moved out of the daemon code and into the init system.
Based on per-daemon configuration, the init system installs the sockets and
then hands them off to the spawned process as soon as the respective daemon is
to be started. Optionally, activation of the service can be delayed until the
first inbound traffic arrives at the socket to implement on-demand activation
of daemons. However, the primary advantage of this scheme is that all
providers and all consumers of the sockets can be started in parallel as soon
as all sockets are established. In addition to that, daemons can be restarted
with losing only a minimal number of client transactions, or even any client
request at all (the latter is particularly true for state-less protocols, such
as DNS or syslog), because the socket stays bound and accessible during the
restart, and all requests are queued while the daemon cannot process them.
New-style daemons which support socket activation must be able to receive their
sockets from the init system instead of creating and binding them themselves.
For details about the programming interfaces for this scheme provided by
systemd, see sd_listen_fds
(3) and sd-daemon
(3). For details
about porting existing daemons to socket-based activation, see below. With
minimal effort, it is possible to implement socket-based activation in
addition to traditional internal socket creation in the same codebase in order
to support both new-style and old-style init systems from the same daemon
systemd implements socket-based activation via .socket units, which are
described in systemd.socket
(5). When configuring socket units for
socket-based activation, it is essential that all listening sockets are pulled
in by the special target unit sockets.target. It is recommended to place a
directive in the "[Install]" section
to automatically add such a dependency on installation of a socket unit.
is set, the necessary ordering
dependencies are implicitly created for all socket units. For more information
about sockets.target, see systemd.special
(7). It is not necessary or
recommended to place any additional dependencies on socket units (for example
from multi-user.target or suchlike) when one is installed in sockets.target.
When the D-Bus IPC system is used for communication with clients, new-style
daemons should employ bus activation so that they are automatically activated
when a client application accesses their IPC interfaces. This is configured in
D-Bus service files (not to be confused with systemd service unit files!). To
ensure that D-Bus uses systemd to start-up and maintain the daemon, use the
directive in these service files to configure the
matching systemd service for a D-Bus service. e.g.: For a D-Bus service whose
D-Bus activation file is named org.freedesktop.RealtimeKit.service, make sure
to set SystemdService=rtkit-daemon.service
in that file to bind it to
the systemd service rtkit-daemon.service. This is needed to make sure that the
daemon is started in a race-free fashion when activated via multiple
Often, daemons that manage a particular type of hardware should be activated
only when the hardware of the respective kind is plugged in or otherwise
becomes available. In a new-style init system, it is possible to bind
activation to hardware plug/unplug events. In systemd, kernel devices
appearing in the sysfs/udev device tree can be exposed as units if they are
tagged with the string "systemd". Like any other kind of unit, they
may then pull in other units when activated (i.e. plugged in) and thus
implement device-based activation. systemd dependencies may be encoded in the
udev database via the SYSTEMD_WANTS=
(5) for details. Often, it is nicer to pull in services
from devices only indirectly via dedicated targets. Example: Instead of
pulling in bluetoothd.service from all the various bluetooth dongles and other
hardware available, pull in bluetooth.target from them and bluetoothd.service
from that target. This provides for nicer abstraction and gives administrators
the option to enable bluetoothd.service via controlling a
bluetooth.target.wants/ symlink uniformly with a command like enable
(1) instead of manipulating the udev ruleset.
Often, runtime of daemons processing spool files or directories (such as a
printing system) can be delayed until these file system objects change state,
or become non-empty. New-style init systems provide a way to bind service
activation to file system changes. systemd implements this scheme via
path-based activation configured in .path units, as outlined in
Some daemons that implement clean-up jobs that are intended to be executed in
regular intervals benefit from timer-based activation. In systemd, this is
implemented via .timer units, as described in systemd.timer
Other forms of activation have been suggested and implemented in some systems.
However, there are often simpler or better alternatives, or they can be put
together of combinations of the schemes above. Example: Sometimes, it appears
useful to start daemons or .socket units when a specific IP address is
configured on a network interface, because network sockets shall be bound to
the address. However, an alternative to implement this is by utilizing the
socket option, as accessible via FreeBind=yes
in systemd socket files (see systemd.socket
(5) for details). This
option, when enabled, allows sockets to be bound to a non-local, not
configured IP address, and hence allows bindings to a particular IP address
before it actually becomes available, making such an explicit dependency to
the configured address redundant. Another often suggested trigger for service
activation is low system load. However, here too, a more convincing approach
might be to make proper use of features of the operating system, in
particular, the CPU or I/O scheduler of Linux. Instead of scheduling jobs from
userspace based on monitoring the OS scheduler, it is advisable to leave the
scheduling of processes to the OS scheduler itself. systemd provides
fine-grained access to the CPU and I/O schedulers. If a process executed by
the init system shall not negatively impact the amount of CPU or I/O bandwidth
available to other processes, it should be configured with
Optionally, this may be combined with timer-based activation to schedule
background jobs during runtime and with minimal impact on the system, and
remove it from the boot phase itself.
When writing systemd unit files, it is recommended to consider the following
1.If possible, do not use the
Type=forking setting in service files. But if you do, make sure to set
the PID file path using PIDFile=. See systemd.service(5) for
2.If your daemon registers a D-Bus name on
the bus, make sure to use Type=dbus in the service file if
3.Make sure to set a good human-readable
description string with Description=.
4.Do not disable DefaultDependencies=,
unless you really know what you do and your unit is involved in early boot or
late system shutdown.
5.Normally, little if any dependencies should
need to be defined explicitly. However, if you do configure explicit
dependencies, only refer to unit names listed on systemd.special(7) or
names introduced by your own package to keep the unit file operating
6.Make sure to include an
"[Install]" section including installation information for the unit
file. See systemd.unit(5) for details. To activate your service on
boot, make sure to add a WantedBy=multi-user.target or
WantedBy=graphical.target directive. To activate your socket on boot,
make sure to add WantedBy=sockets.target. Usually, you also want to
make sure that when your service is installed, your socket is installed too,
hence add Also=foo.socket in your service file foo.service, for a
hypothetical program foo.
At the build installation time (e.g. make install
during package build),
packages are recommended to install their systemd unit files in the directory
returned by pkg-config systemd --variable=systemdsystemunitdir
system services) or pkg-config systemd --variable=systemduserunitdir
(for user services). This will make the services available in the system on
explicit request but not activate them automatically during boot. Optionally,
during package installation (e.g. rpm -i
by the administrator),
symlinks should be created in the systemd configuration directories via the
command of the systemctl
(1) tool to activate them
automatically on boot.
Packages using autoconf
(1) are recommended to use a configure script
excerpt like the following to determine the unit installation path during
[AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files])],,
AS_IF([test "x$with_systemdsystemunitdir" = "xyes" -o "x$with_systemdsystemunitdir" = "xauto"], [
def_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)
AS_IF([test "x$def_systemdsystemunitdir" = "x"],
[AS_IF([test "x$with_systemdsystemunitdir" = "xyes"],
[AC_MSG_ERROR([systemd support requested but pkg-config unable to query systemd package])])
AS_IF([test "x$with_systemdsystemunitdir" != "xno"],
AM_CONDITIONAL([HAVE_SYSTEMD], [test "x$with_systemdsystemunitdir" != "xno"])
This snippet allows automatic installation of the unit files on systemd
machines, and optionally allows their installation even on machines lacking
systemd. (Modification of this snippet for the user unit directory is left as
an exercise for the reader.)
Additionally, to ensure that make distcheck
continues to work, it is
recommended to add the following to the top-level Makefile.am file in
DISTCHECK_CONFIGURE_FLAGS = \
Finally, unit files should be installed in the system with an automake excerpt
like the following:
systemdsystemunit_DATA = \
In the rpm
(8) .spec file, use snippets like the following to
enable/disable the service during installation/deinstallation. This makes use
of the RPM macros shipped along systemd. Consult the packaging guidelines of
your distribution for details and the equivalent for other package managers.
At the top of the file:
And as scriptlets, further down:
%systemd_post foobar.service foobar.socket
%systemd_preun foobar.service foobar.socket
If the service shall be restarted during upgrades, replace the
"%postun" scriptlet above with the following:
Note that "%systemd_post" and "%systemd_preun" expect the
names of all units that are installed/removed as arguments, separated by
spaces. "%systemd_postun" expects no arguments.
"%systemd_postun_with_restart" expects the units to restart as
To facilitate upgrades from a package version that shipped only SysV init
scripts to a package version that ships both a SysV init script and a native
systemd service file, use a fragment like the following:
%triggerun -- foobar < 0.47.11-1
if /sbin/chkconfig --level 5 foobar ; then
/bin/systemctl --no-reload enable foobar.service foobar.socket >/dev/null 2>&1 || :
Where 0.47.11-1 is the first package version that includes the native unit file.
This fragment will ensure that the first time the unit file is installed, it
will be enabled if and only if the SysV init script is enabled, thus making
sure that the enable status is not changed. Note that chkconfig
command specific to Fedora which can be used to check whether a SysV init
script is enabled. Other operating systems will have to use different commands
Since new-style init systems such as systemd are compatible with traditional
SysV init systems, it is not strictly necessary to port existing daemons to
the new style. However, doing so offers additional functionality to the
daemons as well as simplifying integration into new-style init systems.
To port an existing SysV compatible daemon, the following steps are recommended:
1.If not already implemented, add an optional
command line switch to the daemon to disable daemonization. This is useful not
only for using the daemon in new-style init systems, but also to ease
2.If the daemon offers interfaces to other
software running on the local system via local AF_UNIX sockets,
consider implementing socket-based activation (see above). Usually, a minimal
patch is sufficient to implement this: Extend the socket creation in the
daemon code so that sd_listen_fds(3) is checked for already passed
sockets first. If sockets are passed (i.e. when sd_listen_fds() returns
a positive value), skip the socket creation step and use the passed sockets.
Secondly, ensure that the file system socket nodes for local AF_UNIX
sockets used in the socket-based activation are not removed when the daemon
shuts down, if sockets have been passed. Third, if the daemon normally closes
all remaining open file descriptors as part of its initialization, the sockets
passed from the init system must be spared. Since new-style init systems
guarantee that no left-over file descriptors are passed to executed processes,
it might be a good choice to simply skip the closing of all remaining open
file descriptors if sockets are passed.
3.Write and install a systemd unit file for
the service (and the sockets if socket-based activation is used, as well as a
path unit file, if the daemon processes a spool directory), see above for
4.If the daemon exposes interfaces via D-Bus,
write and install a D-Bus activation file for the service, see above for
It is recommended to follow the general guidelines for placing package files, as
discussed in file-hierarchy
- LSB recommendations for SysV init scripts
- Apple MacOS X Daemon Requirements