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>, and
    <filename>/usr/lib/</filename>, in order of precedence.
    Each configuration file in these configuration directories shall be named in
    the style of <filename><replaceable>filename</replaceable>.conf</filename>.
    Files in <filename>/etc/</filename> override files with the same name in
    <filename>/run/</filename> and <filename>/usr/lib/</filename>. Files in
    <filename>/run/</filename> override files with the same name in
    <filename>/usr/lib/</filename>.</para>

    <para>Packages should install their configuration files in
    <filename>/usr/lib/</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. 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. 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>. 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. 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 which of the subdirectories they reside in. 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. 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>