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NAME | DESCRIPTION | ACTIVATION | INTEGRATION WITH SYSTEMD | PORTING EXISTING DAEMONS | PLACING DAEMON DATA | SEE ALSO | NOTES | COLOPHON |
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DAEMON(7) daemon DAEMON(7)
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(1). This manual page covers
both schemes, and in particular includes recommendations for
daemons that shall be included in the systemd init system.
SysV Daemons
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 sigprocmask().
4. Sanitize the environment block, removing or resetting
environment variables that might negatively impact daemon
runtime.
5. Call fork(), to create a background process.
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. (This is relevant if
the program — and all its dependencies — does not carefully
specify `O_NOCTTY` on each and every single `open()` call
that might potentially open a TTY device node.)
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 should be.
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.
New-Style Daemons
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 implementation.
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 service manager, 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 applicable, the daemon should notify the service manager
about startup completion or status updates via the
sd_notify(3) interface, in particular READY=1 and STATUS=....
2. If SIGTERM is received, shut down the daemon and exit
cleanly. A STOPPING=1 notification should be sent via
sd_notify(3).
3. If SIGHUP is received, reload the configuration files, if
this applies. This should be combined with notifications via
sd_notify(3): RELOADING=1 and READY=1.
4. Provide a correct exit code from the main daemon process, as
this is used by the service manager 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[1].
5. 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.
6. 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 details.
7. As much as possible, rely on the service manager'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 so on. See
systemd.exec(5) for the available controls.
8. 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.
9. 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.
10. If the service opens sockets or other files on it own, and
those file descriptors shall survive a restart, the daemon
should store them in the service manager via sd_notify(3)
with FDSTORE=1.
11. 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. 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).
12. As new-style daemons are invoked without a controlling TTY
(but as their own session leaders) care should be taken to
always specify O_NOCTTY on open(2) calls that possibly
reference a TTY device node, so that no controlling TTY is
accidentally acquired.
These recommendations are similar but not identical to the Apple
MacOS X Daemon Requirements[2].
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.
Activation on Boot
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[1]. 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 use 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, and systemd.special(7) for details about the two
boot targets.
Socket-Based Activation
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
use 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 service
manager. Based on per-daemon configuration, the service manager
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 service manager 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 binary.
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
WantedBy=sockets.target directive in the [Install] section to
automatically add such a dependency on installation of a socket
unit. Unless DefaultDependencies=no 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.
Bus-Based Activation
When the D-Bus IPC system is used for communication with clients,
new-style daemons should use 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
SystemdService= 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 mechanisms simultaneously.
Device-Based Activation
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= property. See
systemd.device(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 of systemctl(1) instead of manipulating the
udev ruleset.
Path-Based Activation
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 systemd.path(5).
Timer-Based Activation
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(5).
Other Forms of Activation
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 Linux IP_FREEBIND/IPV6_FREEBIND 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 service manager shall not negatively
impact the amount of CPU or I/O bandwidth available to other
processes, it should be configured with CPUSchedulingPolicy=idle
and/or IOSchedulingClass=idle. 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.
Writing systemd Unit Files
When writing systemd unit files, it is recommended to consider
the following suggestions:
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 details.
2. If your daemon registers a D-Bus name on the bus, make sure
to use Type=dbus in the service file if possible.
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 system-independent.
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.
Installing systemd Service Files
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 (for 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 enable 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 source configuration:
PKG_PROG_PKG_CONFIG()
AC_ARG_WITH([systemdsystemunitdir],
[AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files])],,
[with_systemdsystemunitdir=auto])
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])])
with_systemdsystemunitdir=no],
[with_systemdsystemunitdir="$def_systemdsystemunitdir"])])
AS_IF([test "x$with_systemdsystemunitdir" != "xno"],
[AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])])
AM_CONDITIONAL([HAVE_SYSTEMD], [test "x$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 automake(1)-based projects:
AM_DISTCHECK_CONFIGURE_FLAGS = \
--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)
Finally, unit files should be installed in the system with an
automake excerpt like the following:
if HAVE_SYSTEMD
systemdsystemunit_DATA = \
foobar.socket \
foobar.service
endif
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:
BuildRequires: systemd
%{?systemd_requires}
And as scriptlets, further down:
%post
%systemd_post foobar.service foobar.socket
%preun
%systemd_preun foobar.service foobar.socket
%postun
%systemd_postun
If the service shall be restarted during upgrades, replace the
"%postun" scriptlet above with the following:
%postun
%systemd_postun_with_restart foobar.service
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
arguments.
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 || :
fi
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 is a 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 here.
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 debugging.
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 service manager 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 details.
4. If the daemon exposes interfaces via D-Bus, write and install
a D-Bus activation file for the service, see above for
details.
It is recommended to follow the general guidelines for placing
package files, as discussed in file-hierarchy(7).
systemd(1), sd-daemon(3), sd_listen_fds(3), sd_notify(3),
daemon(3), systemd.service(5), file-hierarchy(7)
1. LSB recommendations for SysV init scripts
http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html
2. Apple MacOS X Daemon Requirements
https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have
a bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 257~devel DAEMON(7)
Pages that refer to this page: systemd(1), daemon(3), sd-daemon(3), sd_listen_fds(3), sd_notify(3), sd_watchdog_enabled(3), systemd.preset(5), systemd.directives(7), systemd.index(7)
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