mirror of
https://github.com/systemd/systemd
synced 2024-09-06 08:46:19 +00:00
701 lines
37 KiB
XML
701 lines
37 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
|
|
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
|
|
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
|
|
|
|
<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. (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.)</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 index='false'>/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 service manager, 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 applicable, the daemon should notify the service manager about startup completion
|
|
or status updates via the
|
|
<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
|
|
interface, in particular <varname>READY=1</varname> and <varname>STATUS=…</varname>.
|
|
</para></listitem>
|
|
|
|
<listitem><para>If <constant>SIGTERM</constant> is received, shut down the daemon and exit cleanly.
|
|
A <varname>STOPPING=1</varname> notification should be sent via
|
|
<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
|
|
</para></listitem>
|
|
|
|
<listitem><para>If <constant>SIGHUP</constant> is received, reload the configuration files, if this
|
|
applies. This should be combined with notifications via
|
|
<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>:
|
|
<varname>RELOADING=1</varname> and <varname>READY=1</varname>.
|
|
</para></listitem>
|
|
|
|
<listitem><para>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 <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 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
|
|
<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 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
|
|
<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> with
|
|
<varname>FDSTORE=1</varname>..</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. If log levels are necessary, these
|
|
can be encoded by prefixing individual log lines with strings like <literal><4></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>
|
|
|
|
<listitem><para>As new-style daemons are invoked without a controlling TTY (but as their own session
|
|
leaders) care should be taken to always specify <constant>O_NOCTTY</constant> on
|
|
<citerefentry><refentrytitle>open</refentrytitle><manvolnum>2</manvolnum></citerefentry>
|
|
calls that possibly reference a TTY device node, so that no controlling TTY is accidentally
|
|
acquired.</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 use
|
|
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 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.</para>
|
|
|
|
<para>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
|
|
<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
|
|
[Install] 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 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
|
|
<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>/<constant>IPV6_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 service manager 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
|
|
[Install] 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 < 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</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 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.</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><simplelist type="inline">
|
|
<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>file-hierarchy</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
|
|
</simplelist></para>
|
|
</refsect1>
|
|
|
|
</refentry>
|