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<?xml version="1.0"?>
<!DOCTYPE refsection 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
-->
<refsection>
<refsection 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). 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. It is recommended to prefix all filenames with a two-digit number and a
dash, to simplify the ordering of the files.</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>
</refsection>
<refsection 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 either in
<filename>/usr/lib/systemd/</filename> or <filename>/etc/systemd/</filename> and contains commented out
entries showing the defaults as a guide to the administrator. Local overrides can 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 in <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 of the files. This also defined a concept of drop-in priority to allow
distributions 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.</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>
</refsection>
</refsection>
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