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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">
<!--
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Copyright 2010 Lennart Poettering
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-->
<refentry id="systemd.special">
<refentryinfo>
<title>daemon</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>lennart@poettering.net</email>
</author>
</authorgroup>
</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 systemd. </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,
and should only be implemented if compatbility
with SysV is essential.</para>
<orderedlist>
<listitem><para>Close all open file
descriptors except STDIN, STDOUT,
STDERR (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
getrlimit() for
RLIMIT_NOFILE.</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
_NSIG and resetting them to
SIG_DFL.</para></listitem>
<listitem><para>Reset the signal mask
using sigprocmask().</para></listitem>
<listitem><para>Call fork(),
to create a background
process.</para></listitem>
<listitem><para>In the child, call
setsid() to detach from any terminal
and create an independent
session.</para></listitem>
<listitem><para>In the child, call
fork() again, to ensure the daemon can
never re-aquire a terminal
again.</para></listitem>
<listitem><para>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 reparented to
init/PID 1, as all daemons should
be.</para></listitem>
<listitem><para>In the daemon process,
connect <filename>/dev/null</filename>
to STDIN, STDOUT,
STDERR.</para></listitem>
<listitem><para>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.</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,
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 fork() and available
in both processes.</para></listitem>
<listitem><para>Call exit() in the
original process. The process that
invoked the daemon must be able to
rely that this exit() happens after
initialization is complete and all
external communication channels
established and
accessible.</para></listitem>
</orderedlist>
<para>The BSD daemon() function should not be
used, as it does 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
behaviour 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>It is recommended for new-style daemons
to implement the following:</para>
<orderedlist>
<listitem><para>If SIGTERM is
received, shut down the daemon and
exit cleanly.</para></listitem>
<listitem><para>If SIGHUP 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 LSB
recommendations for SysV init scripts
(http://refspecs.freestandards.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html).</para></listitem>
<listitem><para>As much as possible,
rely on systemd's functionality to
limit the accces of the daemon to
files, services and other
resources. i.e. 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.</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>If D-Bus is used, make
your daemon bus-activatable, via
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.</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 parallization 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.</para></listitem>
<listitem><para>If applicable a daemon
should notify the init system about
startup completion or status
updates via the sd_notify()
interface.</para></listitem>
<listitem><para>Instead of using the
syslog() call to log directly to the
system logger, a new-style daemon may
choose to simply log to STDERR via
fprintf(), which is then forwarded to
syslog by the init system. If log
priorities are necessary these can be
encoded by prefixing individual log
lines with strings like "<4>"
(for log priority 4 "WARNING" in the
syslog priority scheme), following a
similar style as the Linux kernel's
printk() priority system. In fact, using
this style of logging also enables the
init system to optionally direct all
application logging to the kernel log
buffer (kmsg), as accessible via
dmesg.</para></listitem>
</orderedlist>
</refsect2>
<refsect2>
<title>Bus Activation</title>
</refsect2>
<refsect2>
<title>Socket Activation</title>
</refsect2>
<refsect2>
<title>Writing Service Files</title>
</refsect2>
<refsect2>
<title>Installing Service Files</title>
</refsect2>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
|
