Adding upstream version 241.upstream/241upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+  SPDX-License-Identifier: LGPL-2.1+
+-->
+
+<refentry id="daemon">
+
+  <refentryinfo>
+    <title>daemon</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>daemon</refentrytitle>
+    <manvolnum>7</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>daemon</refname>
+    <refpurpose>Writing and packaging system daemons</refpurpose>
+  </refnamediv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para>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
+    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
+    This manual page covers both schemes, and in particular includes
+    recommendations for daemons that shall be included in the systemd
+    init system.</para>
+
+    <refsect2>
+      <title>SysV Daemons</title>
+
+      <para>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.</para>
+
+      <orderedlist>
+        <listitem><para>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
+        <filename>/proc/self/fd</filename>, with a fallback of
+        iterating from file descriptor 3 to the value returned by
+        <function>getrlimit()</function> for
+        <constant>RLIMIT_NOFILE</constant>. </para></listitem>
+
+        <listitem><para>Reset all signal handlers to their default.
+        This is best done by iterating through the available signals
+        up to the limit of <constant>_NSIG</constant> and resetting
+        them to <constant>SIG_DFL</constant>.</para></listitem>
+
+        <listitem><para>Reset the signal mask
+        using
+        <function>sigprocmask()</function>.</para></listitem>
+
+        <listitem><para>Sanitize the environment block, removing or
+        resetting environment variables that might negatively impact
+        daemon runtime.</para></listitem>
+
+        <listitem><para>Call <function>fork()</function>, to create a
+        background process.</para></listitem>
+
+        <listitem><para>In the child, call
+        <function>setsid()</function> to detach from any terminal and
+        create an independent session.</para></listitem>
+
+        <listitem><para>In the child, call <function>fork()</function>
+        again, to ensure that the daemon can never re-acquire a
+        terminal again.</para></listitem>
+
+        <listitem><para>Call <function>exit()</function> 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.</para></listitem>
+
+        <listitem><para>In the daemon process, connect
+        <filename>/dev/null</filename> to standard input, output, and
+        error.</para></listitem>
+
+        <listitem><para>In the daemon process, reset the umask to 0,
+        so that the file modes passed to <function>open()</function>,
+        <function>mkdir()</function> and suchlike directly control the
+        access mode of the created files and
+        directories.</para></listitem>
+
+        <listitem><para>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.</para></listitem>
+
+        <listitem><para>In the daemon process, write the daemon PID
+        (as returned by <function>getpid()</function>) to a PID file,
+        for example <filename>/run/foobar.pid</filename> (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.</para></listitem>
+
+        <listitem><para>In the daemon process, drop privileges, if
+        possible and applicable.</para></listitem>
+
+        <listitem><para>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
+        <function>fork()</function> and hence available in both the
+        original and the daemon process.</para></listitem>
+
+        <listitem><para>Call <function>exit()</function> in the
+        original process. The process that invoked the daemon must be
+        able to rely on that this <function>exit()</function> happens
+        after initialization is complete and all external
+        communication channels are established and
+        accessible.</para></listitem>
+      </orderedlist>
+
+      <para>The BSD <function>daemon()</function> function should not
+      be used, as it implements only a subset of these steps.</para>
+
+      <para>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.</para>
+    </refsect2>
+
+    <refsect2>
+      <title>New-Style Daemons</title>
+
+      <para>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.</para>
+
+      <para>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.</para>
+
+      <para>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 <filename>/dev/null</filename> and standard output/error connected to the
+      <citerefentry><refentrytitle>systemd-journald.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+      logging service, unless otherwise configured. The umask is reset.
+      </para>
+
+      <para>It is recommended for new-style daemons to implement the
+      following:</para>
+
+      <orderedlist>
+        <listitem><para>If <constant>SIGTERM</constant> is received,
+        shut down the daemon and exit cleanly.</para></listitem>
+
+        <listitem><para>If <constant>SIGHUP</constant> is received,
+        reload the configuration files, if this
+        applies.</para></listitem>
+
+        <listitem><para>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 <ulink
+        url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
+        recommendations for SysV init
+        scripts</ulink>.</para></listitem>
+
+        <listitem><para>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.</para></listitem>
+
+        <listitem><para>For integration in systemd, provide a
+        <filename>.service</filename> unit file that carries
+        information about starting, stopping and otherwise maintaining
+        the daemon. See
+        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        for details.</para></listitem>
+
+        <listitem><para>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
+        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        for the available controls.</para></listitem>
+
+        <listitem><para>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.</para></listitem>
+
+        <listitem><para>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.</para></listitem>
+
+        <listitem><para>If applicable, a daemon should notify the init
+        system about startup completion or status updates via the
+        <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+        interface.</para></listitem>
+
+        <listitem><para>Instead of using the
+        <function>syslog()</function> call to log directly to the
+        system syslog service, a new-style daemon may choose to simply
+        log to standard error via <function>fprintf()</function>,
+        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
+        <literal>&lt;4&gt;</literal> (for log level 4 "WARNING" in the
+        syslog priority scheme), following a similar style as the
+        Linux kernel's <function>printk()</function> level system. For
+        details, see
+        <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+        and
+        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+      </orderedlist>
+
+      <para>These recommendations are similar but not identical to the
+      <ulink
+      url="https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html">Apple
+      MacOS X Daemon Requirements</ulink>.</para>
+    </refsect2>
+
+  </refsect1>
+  <refsect1>
+    <title>Activation</title>
+
+    <para>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:
+    <filename>bluetoothd.service</filename> 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.</para>
+
+    <refsect2>
+      <title>Activation on Boot</title>
+
+      <para>Old-style daemons are usually activated exclusively on
+      boot (and manually by the administrator) via SysV init scripts,
+      as detailed in the <ulink
+      url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
+      Linux Standard Base Core Specification</ulink>. 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.</para>
+
+      <para>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 <filename>.wants/</filename> directory of
+      either <filename>multi-user.target</filename> or
+      <filename>graphical.target</filename>, which are normally used
+      as boot targets at system startup. See
+      <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      for details about the <filename>.wants/</filename> directories,
+      and
+      <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+      for details about the two boot targets.</para>
+
+    </refsect2>
+
+    <refsect2>
+      <title>Socket-Based Activation</title>
+
+      <para>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.</para>
+
+      <para>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
+      <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+      and
+      <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
+      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.</para>
+
+      <para>systemd implements socket-based activation via
+      <filename>.socket</filename> units, which are described in
+      <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+      When configuring socket units for socket-based activation, it is
+      essential that all listening sockets are pulled in by the
+      special target unit <filename>sockets.target</filename>. It is
+      recommended to place a
+      <varname>WantedBy=sockets.target</varname> directive in the
+      <literal>[Install]</literal> section to automatically add such a
+      dependency on installation of a socket unit. Unless
+      <varname>DefaultDependencies=no</varname> is set, the necessary
+      ordering dependencies are implicitly created for all socket
+      units. For more information about
+      <filename>sockets.target</filename>, see
+      <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
+      It is not necessary or recommended to place any additional
+      dependencies on socket units (for example from
+      <filename>multi-user.target</filename> or suchlike) when one is
+      installed in <filename>sockets.target</filename>.</para>
+    </refsect2>
+
+    <refsect2>
+      <title>Bus-Based Activation</title>
+
+      <para>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 <varname>SystemdService=</varname>
+      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
+      <filename>org.freedesktop.RealtimeKit.service</filename>, make
+      sure to set
+      <varname>SystemdService=rtkit-daemon.service</varname> in that
+      file to bind it to the systemd service
+      <filename>rtkit-daemon.service</filename>. This is needed to
+      make sure that the daemon is started in a race-free fashion when
+      activated via multiple mechanisms simultaneously.</para>
+    </refsect2>
+
+    <refsect2>
+      <title>Device-Based Activation</title>
+
+      <para>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 <literal>systemd</literal>.
+      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 <varname>SYSTEMD_WANTS=</varname> property. See
+      <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      for details. Often, it is nicer to pull in services from devices
+      only indirectly via dedicated targets. Example: Instead of
+      pulling in <filename>bluetoothd.service</filename> from all the
+      various bluetooth dongles and other hardware available, pull in
+      bluetooth.target from them and
+      <filename>bluetoothd.service</filename> from that target. This
+      provides for nicer abstraction and gives administrators the
+      option to enable <filename>bluetoothd.service</filename> via
+      controlling a <filename>bluetooth.target.wants/</filename>
+      symlink uniformly with a command like <command>enable</command>
+      of
+      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+      instead of manipulating the udev ruleset.</para>
+    </refsect2>
+
+    <refsect2>
+      <title>Path-Based Activation</title>
+
+      <para>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 <filename>.path</filename>
+      units, as outlined in
+      <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+    </refsect2>
+
+    <refsect2>
+      <title>Timer-Based Activation</title>
+
+      <para>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
+      <filename>.timer</filename> units, as described in
+      <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+    </refsect2>
+
+    <refsect2>
+      <title>Other Forms of Activation</title>
+
+      <para>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 <filename>.socket</filename> 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
+      <constant>IP_FREEBIND</constant> socket option, as accessible
+      via <varname>FreeBind=yes</varname> in systemd socket files (see
+      <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      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
+      <varname>CPUSchedulingPolicy=idle</varname> and/or
+      <varname>IOSchedulingClass=idle</varname>. 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.</para>
+    </refsect2>
+
+  </refsect1>
+  <refsect1>
+    <title>Integration with systemd</title>
+
+    <refsect2>
+      <title>Writing systemd Unit Files</title>
+
+      <para>When writing systemd unit files, it is recommended to
+      consider the following suggestions:</para>
+
+      <orderedlist>
+        <listitem><para>If possible, do not use the
+        <varname>Type=forking</varname> setting in service files. But
+        if you do, make sure to set the PID file path using
+        <varname>PIDFile=</varname>. See
+        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        for details.</para></listitem>
+
+        <listitem><para>If your daemon registers a D-Bus name on the
+        bus, make sure to use <varname>Type=dbus</varname> in the
+        service file if possible.</para></listitem>
+
+        <listitem><para>Make sure to set a good human-readable
+        description string with
+        <varname>Description=</varname>.</para></listitem>
+
+        <listitem><para>Do not disable
+        <varname>DefaultDependencies=</varname>, unless you really
+        know what you do and your unit is involved in early boot or
+        late system shutdown.</para></listitem>
+
+        <listitem><para>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
+        <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+        or names introduced by your own package to keep the unit file
+        operating system-independent.</para></listitem>
+
+        <listitem><para>Make sure to include an
+        <literal>[Install]</literal> section including installation
+        information for the unit file. See
+        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        for details. To activate your service on boot, make sure to
+        add a <varname>WantedBy=multi-user.target</varname> or
+        <varname>WantedBy=graphical.target</varname> directive. To
+        activate your socket on boot, make sure to add
+        <varname>WantedBy=sockets.target</varname>. Usually, you also
+        want to make sure that when your service is installed, your
+        socket is installed too, hence add
+        <varname>Also=foo.socket</varname> in your service file
+        <filename>foo.service</filename>, for a hypothetical program
+        <filename>foo</filename>.</para></listitem>
+
+      </orderedlist>
+    </refsect2>
+
+    <refsect2>
+      <title>Installing systemd Service Files</title>
+
+      <para>At the build installation time (e.g. <command>make
+      install</command> during package build), packages are
+      recommended to install their systemd unit files in the directory
+      returned by <command>pkg-config systemd
+      --variable=systemdsystemunitdir</command> (for system services)
+      or <command>pkg-config systemd
+      --variable=systemduserunitdir</command> (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. <command>rpm
+      -i</command> by the administrator), symlinks should be created
+      in the systemd configuration directories via the
+      <command>enable</command> command of the
+      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+      tool to activate them automatically on boot.</para>
+
+      <para>Packages using
+      <citerefentry project='die-net'><refentrytitle>autoconf</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+      are recommended to use a configure script
+      excerpt like the following to determine the
+      unit installation path during source
+      configuration:</para>
+
+      <programlisting>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"])</programlisting>
+
+      <para>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.)</para>
+
+      <para>Additionally, to ensure that
+      <command>make distcheck</command> continues to
+      work, it is recommended to add the following
+      to the top-level <filename>Makefile.am</filename>
+      file in
+      <citerefentry project='die-net'><refentrytitle>automake</refentrytitle><manvolnum>1</manvolnum></citerefentry>-based
+      projects:</para>
+
+      <programlisting>AM_DISTCHECK_CONFIGURE_FLAGS = \
+  --with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)</programlisting>
+
+      <para>Finally, unit files should be installed in the system with an automake excerpt like the following:</para>
+
+      <programlisting>if HAVE_SYSTEMD
+systemdsystemunit_DATA = \
+  foobar.socket \
+  foobar.service
+endif</programlisting>
+
+      <para>In the
+      <citerefentry project='die-net'><refentrytitle>rpm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+      <filename>.spec</filename> 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.</para>
+
+      <para>At the top of the file:</para>
+
+      <programlisting>BuildRequires: systemd
+%{?systemd_requires}</programlisting>
+
+      <para>And as scriptlets, further down:</para>
+
+      <programlisting>%post
+%systemd_post foobar.service foobar.socket
+
+%preun
+%systemd_preun foobar.service foobar.socket
+
+%postun
+%systemd_postun</programlisting>
+
+      <para>If the service shall be restarted during upgrades, replace
+      the <literal>%postun</literal> scriptlet above with the
+      following:</para>
+
+      <programlisting>%postun
+%systemd_postun_with_restart foobar.service</programlisting>
+
+      <para>Note that <literal>%systemd_post</literal> and
+      <literal>%systemd_preun</literal> expect the names of all units
+      that are installed/removed as arguments, separated by spaces.
+      <literal>%systemd_postun</literal> expects no arguments.
+      <literal>%systemd_postun_with_restart</literal> expects the
+      units to restart as arguments.</para>
+
+      <para>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:</para>
+
+      <programlisting>%triggerun -- foobar &lt; 0.47.11-1
+if /sbin/chkconfig --level 5 foobar ; then
+  /bin/systemctl --no-reload enable foobar.service foobar.socket >/dev/null 2>&amp;1 || :
+fi</programlisting>
+
+      <para>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
+      <command>chkconfig</command> 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.</para>
+    </refsect2>
+  </refsect1>
+
+  <refsect1>
+    <title>Porting Existing Daemons</title>
+
+    <para>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.</para>
+
+    <para>To port an existing SysV compatible daemon, the following
+    steps are recommended:</para>
+
+    <orderedlist>
+      <listitem><para>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.</para></listitem>
+
+      <listitem><para>If the daemon offers interfaces to other
+      software running on the local system via local
+      <constant>AF_UNIX</constant> 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
+      <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+      is checked for already passed sockets first. If sockets are
+      passed (i.e. when <function>sd_listen_fds()</function> returns a
+      positive value), skip the socket creation step and use the
+      passed sockets. Secondly, ensure that the file system socket
+      nodes for local <constant>AF_UNIX</constant> 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.</para></listitem>
+
+      <listitem><para>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.</para></listitem>
+
+      <listitem><para>If the daemon exposes interfaces via D-Bus,
+      write and install a D-Bus activation file for the service, see
+      above for details.</para></listitem>
+    </orderedlist>
+  </refsect1>
+
+  <refsect1>
+    <title>Placing Daemon Data</title>
+
+    <para>It is recommended to follow the general guidelines for
+    placing package files, as discussed in
+    <citerefentry><refentrytitle>file-hierarchy</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>See Also</title>
+    <para>
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>file-hierarchy</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>