daemon

DAEMON(7)                            daemon                            DAEMON(7)



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 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 init system, it is recommended not to
       execute them when run as new-style service.

       Note that new-style init systems guarantee execution of daemon processes
       in a clean process context: it is guaranteed that the environment block
       is sanitized, that the signal handlers and mask is reset and that no
       left-over file descriptors are passed. Daemons will be executed in their
       own session, with standard input connected to /dev/null and standard
       output/error connected to the systemd-journald.service(8) logging
       service, unless otherwise configured. The umask is reset.

       It is recommended for new-style daemons to implement the following:

        1. If SIGTERM is received, shut down the daemon and exit cleanly.

        2. If SIGHUP is received, reload the configuration files, if this
           applies.

        3. Provide a correct exit code from the main daemon process, as this is
           used by the init system to detect service errors and problems. It is
           recommended to follow the exit code scheme as defined in the LSB
           recommendations for SysV init scripts[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
           initialization.

        5. For integration in systemd, provide a .service unit file that carries
           information about starting, stopping and otherwise maintaining the
           daemon. See systemd.service(5) for 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).

       11. 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()` 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 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
       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
       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 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 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

        2. Apple MacOS X Daemon Requirements
           https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html



systemd 246                                                            DAEMON(7)