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

   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

    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.

    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 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

    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[1].

    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

    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 details.

    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 sd_notify(3) interface.

   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 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 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, 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 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 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 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 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

   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 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 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


   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

    4. Do not disable DefaultDependencies=, unless you really know what
       you do and your unit is involved in early boot or late system

    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.

   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:

            [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$with_systemdsystemunitdir" != "xno"],
             [AC_SUBST([systemdsystemunitdir], [$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:


   Finally, unit files should be installed in the system with an automake
   excerpt like the following:

       if HAVE_SYSTEMD
       systemdsystemunit_DATA = \
         foobar.socket \

   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

   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:

       %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 || :

   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

    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 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

    2. Apple MacOS X Daemon Requirements


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