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<?xml version="1.0"?>
<!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
Copyright © 2014 Josh Triplett
-->
<refentry xmlns:xi="http://www.w3.org/2001/XInclude">
<refnamediv>
<refname/>
<refpurpose/>
</refnamediv>
<refsect1 id='confd'>
<title>Configuration Directories and Precedence</title>
<para>Configuration files are read from directories in <filename>/etc/</filename>,
<filename>/run/</filename>, <filename>/usr/local/lib/</filename>, and <filename>/usr/lib/</filename>, in
order of precedence, as listed in the SYNOPSIS section above. Files must have the
<literal>.conf</literal> extension. Files in <filename>/etc/</filename> override files with the same name
in <filename>/run/</filename>, <filename>/usr/local/lib/</filename>, and
<filename>/usr/lib/</filename>. Files in <filename>/run/</filename> override files with the same name
under <filename>/usr/</filename>.</para>
<para>All configuration files are sorted by their filename in lexicographic order, regardless of which of
the directories they reside in. If multiple files specify the same option, the entry in the file with the
lexicographically latest name will take precedence. Thus, the configuration in a certain file may either
be replaced completely (by placing a file with the same name in a directory with higher priority), or
individual settings might be changed (by specifying additional settings in a file with a different name
that is ordered later).</para>
<para>Packages should install their configuration files in <filename>/usr/lib/</filename> (distribution
packages) or <filename>/usr/local/lib/</filename> (local installs)
<xi:include xpointer="usr-local-footnote" />.
Files in <filename>/etc/</filename> are reserved for the local administrator, who may use this logic to
override the configuration files installed by vendor packages.</para>
<para>It is recommended to prefix all filenames with a two-digit number and a dash to simplify the
ordering. It is recommended to use the range 10-40 for configuration files in <filename>/usr/</filename>
and the range 60-90 for configuration files in <filename>/etc/</filename> and <filename>/run/</filename>,
to make sure that local and transient configuration files will always take priority over configuration
files shipped by the OS vendor.</para>
<para>If the administrator wants to disable a configuration file supplied by the vendor, the recommended
way is to place a symlink to <filename>/dev/null</filename> in the configuration directory in
<filename>/etc/</filename>, with the same filename as the vendor configuration file. If the vendor
configuration file is included in the initrd image, the image has to be regenerated.</para>
</refsect1>
<refsect1 id='main-conf'>
<title>Configuration Directories and Precedence</title>
<para>The default configuration is set during compilation, so configuration is only needed when it is
necessary to deviate from those defaults. The main configuration file is loaded from one of the
listed directories in order of priority, only the first file found is used:
<filename>/etc/systemd/</filename>,
<filename>/run/systemd/</filename>,
<filename>/usr/local/lib/systemd/</filename> <xi:include xpointer="usr-local-footnote" />,
<filename>/usr/lib/systemd/</filename>.
The vendor version of the file contains commented out entries showing the defaults as a guide to the
administrator. Local overrides can also be created by creating drop-ins, as described below. The main
configuration file can also be edited for this purpose (or a copy in <filename>/etc/</filename> if it's
shipped under <filename>/usr/</filename>), however using drop-ins for local configuration is recommended
over modifications to the main configuration file.</para>
<para>In addition to the main configuration file, drop-in configuration snippets are read from
<filename>/usr/lib/systemd/*.conf.d/</filename>, <filename>/usr/local/lib/systemd/*.conf.d/</filename>,
and <filename>/etc/systemd/*.conf.d/</filename>. Those drop-ins have higher precedence and override the
main configuration file. Files in the <filename>*.conf.d/</filename> configuration subdirectories are
sorted by their filename in lexicographic order, regardless of in which of the subdirectories they
reside. When multiple files specify the same option, for options which accept just a single value, the
entry in the file sorted last takes precedence, and for options which accept a list of values, entries
are collected as they occur in the sorted files.</para>
<para>When packages need to customize the configuration, they can install drop-ins under
<filename>/usr/</filename>. Files in <filename>/etc/</filename> are reserved for the local administrator,
who may use this logic to override the configuration files installed by vendor packages. Drop-ins have to
be used to override package drop-ins, since the main configuration file has lower precedence. It is
recommended to prefix all filenames in those subdirectories with a two-digit number and a dash, to
simplify the ordering. This also defines a concept of drop-in priorities to allow OS vendors to ship
drop-ins within a specific range lower than the range used by users. This should lower the risk of
package drop-ins overriding accidentally drop-ins defined by users. It is recommended to use the range
10-40 for drop-ins in <filename>/usr/</filename> and the range 60-90 for drop-ins in
<filename>/etc/</filename> and <filename>/run/</filename>, to make sure that local and transient drop-ins
take priority over drop-ins shipped by the OS vendor.</para>
<para>To disable a configuration file supplied by the vendor, the recommended way is to place a symlink
to <filename>/dev/null</filename> in the configuration directory in <filename>/etc/</filename>, with the
same filename as the vendor configuration file.</para>
</refsect1>
<refsect1>
<title>Extras</title>
<para><filename>/usr/local/lib/systemd</filename>
<footnote id='usr-local-footnote'>
<para>💣💥🧨💥💥💣 Please note that those configuration files must be available at all times. If
<filename>/usr/local/</filename> is a separate partition, it may not be available during early boot,
and must not be used for configuration.</para>
</footnote>
</para>
</refsect1>
</refentry>
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