noble (7) daemon.7.gz

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

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

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

        1. If applicable, the daemon should notify the service manager about startup completion or status
           updates via the sd_notify(3) interface, in particular READY=1 and STATUS=....

        2. If SIGTERM is received, shut down the daemon and exit cleanly. A STOPPING=1 notification should be
           sent via sd_notify(3).

        3. If SIGHUP is received, reload the configuration files, if this applies. This should be combined with
           notifications via sd_notify(3): RELOADING=1 and READY=1.

        4. Provide a correct exit code from the main daemon process, as this is used by the service manager to
           detect service errors and problems. It is recommended to follow the exit code scheme as defined in
           the LSB recommendations for SysV init scripts[1].

        5. If possible and applicable, expose the daemon's control interface via the D-Bus IPC system and grab a
           bus name as last step of initialization.

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

        7. As much as possible, rely on the service manager's functionality to limit the access of the daemon to
           files, services, and other resources, i.e. in the case of systemd, rely on systemd's resource limit
           control instead of implementing your own, rely on systemd's privilege dropping code instead of
           implementing it in the daemon, and so on. See systemd.exec(5) for the available controls.

        8. If D-Bus is used, make your daemon bus-activatable by supplying a D-Bus service activation
           configuration file. This has multiple advantages: your daemon may be started lazily on-demand; it may
           be started in parallel to other daemons requiring it — which maximizes parallelization and boot-up
           speed; your daemon can be restarted on failure without losing any bus requests, as the bus queues
           requests for activatable services. See below for details.

        9. If your daemon provides services to other local processes or remote clients via a socket, it should
           be made socket-activatable following the scheme pointed out below. Like D-Bus activation, this
           enables on-demand starting of services as well as it allows improved parallelization of service
           start-up. Also, for state-less protocols (such as syslog, DNS), a daemon implementing socket-based
           activation can be restarted without losing a single request. See below for details.

       10. If the service opens sockets or other files on it own, and those file descriptors shall survive a
           restart, the daemon should store them in the service manager via sd_notify(3) with FDSTORE=1..

       11. Instead of using the syslog() call to log directly to the system syslog service, a new-style daemon
           may choose to simply log to standard error via fprintf(), which is then forwarded to syslog. If log
           levels are necessary, these can be encoded by prefixing individual log lines with strings like "<4>"
           (for log level 4 "WARNING" in the syslog priority scheme), following a similar style as the Linux
           kernel's printk() level system. For details, see sd-daemon(3) and systemd.exec(5).

       12. As new-style daemons are invoked without a controlling TTY (but as their own session leaders) care
           should be taken to always specify O_NOCTTY on open(2) calls that possibly reference a TTY device
           node, so that no controlling TTY is accidentally acquired.

       These recommendations are similar but not identical to the Apple MacOS X Daemon Requirements[2].

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. See systemd.unit(5) for details about the .wants/ directories, and systemd.special(7) for
       details about the two boot targets.

   Socket-Based Activation
       In order to maximize the possible parallelization and robustness and simplify configuration and
       development, it is recommended for all new-style daemons that communicate via listening sockets to use
       socket-based activation. In a socket-based activation scheme, the creation and binding of the listening
       socket as primary communication channel of daemons to local (and sometimes remote) clients is moved out
       of the daemon code and into the service manager. Based on per-daemon configuration, the service manager
       installs the sockets and then hands them off to the spawned process as soon as the respective daemon is
       to be started. Optionally, activation of the service can be delayed until the first inbound traffic
       arrives at the socket to implement on-demand activation of daemons. However, the primary advantage of
       this scheme is that all providers and all consumers of the sockets can be started in parallel as soon as
       all sockets are established. In addition to that, daemons can be restarted with losing only a minimal
       number of client transactions, or even any client request at all (the latter is particularly true for
       state-less protocols, such as DNS or syslog), because the socket stays bound and accessible during the
       restart, and all requests are queued while the daemon cannot process them.

       New-style daemons which support socket activation must be able to receive their sockets from the service
       manager instead of creating and binding them themselves. For details about the programming interfaces for
       this scheme provided by systemd, see sd_listen_fds(3) and sd-daemon(3). For details about porting
       existing daemons to socket-based activation, see below. With minimal effort, it is possible to implement
       socket-based activation in addition to traditional internal socket creation in the same codebase in order
       to support both new-style and old-style init systems from the same daemon binary.

       systemd implements socket-based activation via .socket units, which are described in systemd.socket(5).
       When configuring socket units for socket-based activation, it is essential that all listening sockets are
       pulled in by the special target unit sockets.target. It is recommended to place a WantedBy=sockets.target
       directive in the [Install] section to automatically add such a dependency on installation of a socket
       unit. Unless DefaultDependencies=no is set, the necessary ordering dependencies are implicitly created
       for all socket units. For more information about sockets.target, see systemd.special(7). It is not
       necessary or recommended to place any additional dependencies on socket units (for example from
       multi-user.target or suchlike) when one is installed in sockets.target.

   Bus-Based Activation
       When the D-Bus IPC system is used for communication with clients, new-style daemons should use bus
       activation so that they are automatically activated when a client application accesses their IPC
       interfaces. This is configured in D-Bus service files (not to be confused with systemd service unit
       files!). To ensure that D-Bus uses systemd to start-up and maintain the daemon, use the SystemdService=
       directive in these service files to configure the matching systemd service for a D-Bus service. e.g.: For
       a D-Bus service whose D-Bus activation file is named org.freedesktop.RealtimeKit.service, make sure to
       set SystemdService=rtkit-daemon.service in that file to bind it to the systemd service
       rtkit-daemon.service. This is needed to make sure that the daemon is started in a race-free fashion when
       activated via multiple mechanisms simultaneously.

   Device-Based Activation
       Often, daemons that manage a particular type of hardware should be activated only when the hardware of
       the respective kind is plugged in or otherwise becomes available. In a new-style init system, it is
       possible to bind activation to hardware plug/unplug events. In systemd, kernel devices appearing in the
       sysfs/udev device tree can be exposed as units if they are tagged with the string "systemd". Like any
       other kind of unit, they may then pull in other units when activated (i.e. plugged in) and thus implement
       device-based activation. systemd dependencies may be encoded in the udev database via the SYSTEMD_WANTS=
       property. See systemd.device(5) for details. Often, it is nicer to pull in services from devices only
       indirectly via dedicated targets. Example: Instead of pulling in bluetoothd.service from all the various
       bluetooth dongles and other hardware available, pull in bluetooth.target from them and bluetoothd.service
       from that target. This provides for nicer abstraction and gives administrators the option to enable
       bluetoothd.service via controlling a bluetooth.target.wants/ symlink uniformly with a command like enable
       of systemctl(1) instead of manipulating the udev ruleset.

   Path-Based Activation
       Often, runtime of daemons processing spool files or directories (such as a printing system) can be
       delayed until these file system objects change state, or become non-empty. New-style init systems provide
       a way to bind service activation to file system changes. systemd implements this scheme via path-based
       activation configured in .path units, as outlined in systemd.path(5).

   Timer-Based Activation
       Some daemons that implement clean-up jobs that are intended to be executed in regular intervals benefit
       from timer-based activation. In systemd, this is implemented via .timer units, as described in
       systemd.timer(5).

   Other Forms of Activation
       Other forms of activation have been suggested and implemented in some systems. However, there are often
       simpler or better alternatives, or they can be put together of combinations of the schemes above.
       Example: Sometimes, it appears useful to start daemons or .socket units when a specific IP address is
       configured on a network interface, because network sockets shall be bound to the address. However, an
       alternative to implement this is by utilizing the Linux IP_FREEBIND/IPV6_FREEBIND socket option, as
       accessible via FreeBind=yes in systemd socket files (see systemd.socket(5) for details). This option,
       when enabled, allows sockets to be bound to a non-local, not configured IP address, and hence allows
       bindings to a particular IP address before it actually becomes available, making such an explicit
       dependency to the configured address redundant. Another often suggested trigger for service activation is
       low system load. However, here too, a more convincing approach might be to make proper use of features of
       the operating system, in particular, the CPU or I/O scheduler of Linux. Instead of scheduling jobs from
       userspace based on monitoring the OS scheduler, it is advisable to leave the scheduling of processes to
       the OS scheduler itself. systemd provides fine-grained access to the CPU and I/O schedulers. If a process
       executed by the service manager shall not negatively impact the amount of CPU or I/O bandwidth available
       to other processes, it should be configured with CPUSchedulingPolicy=idle and/or IOSchedulingClass=idle.
       Optionally, this may be combined with timer-based activation to schedule background jobs during runtime
       and with minimal impact on the system, and remove it from the boot phase itself.

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).

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