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daemon


DESCRIPTION

       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.

        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.

       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/output/error
       connected to /dev/null 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[1].

        4. If possible and applicable, expose the daemon's control interface
        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].


ACTIVATION

       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.

       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
       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 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 IO 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 IO
           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)

       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:

           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:


PORTING EXISTING DAEMONS

       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 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
           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.


PLACING DAEMON DATA

       It is recommended to follow the general guidelines for placing package
       files, as discussed in file-hierarchy(7).


SEE ALSO

       systemd(1), sd-daemon(3), sd_listen_fds(3), sd_notify(3), daemon(3),
       systemd.service(5), file-hierarchy(7)


NOTES

        1. LSB recommendations for SysV init scripts
           http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html

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