# daemon

## NAME



           daemon - Writing and packaging system daemons
    


## 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. (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. Seesystemd.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].


## 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.
    
       **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. Seesystemd.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.
    


## INTEGRATION WITH SYSTEMD



       **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.
    


## 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 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.
    


## PLACING DAEMON DATA



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


## NOTES



           All example codes in this page are licensed under "MIT No
           Attribution" (SPDX-License-Identifier: MIT-0).
    


## NOTES



            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
    


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