man/standard-conf.xml

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<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 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 defined during compilation, so a
    configuration file is only needed when it is necessary to deviate
    from those defaults. By default, the configuration file in
    <filename>/etc/systemd/</filename> contains commented out entries
    showing the defaults as a guide to the administrator. This file
    can be edited to create local overrides.
    </para>

    <para>When packages need to customize the configuration, they can install configuration snippets in
    <filename>/usr/lib/systemd/*.conf.d/</filename> or <filename>/usr/local/lib/systemd/*.conf.d/</filename>.
    The main configuration file is read before any of the configuration directories, and has the lowest
    precedence; entries in a file in any configuration directory override entries in the single 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 with
    the lexicographically latest name takes precedence. For options which accept a list of values, entries
    are collected as they occur in files sorted lexicographically.</para>

    <para>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 in those subdirectories with a two-digit number and a dash, to simplify the ordering of the
    files.</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>