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.</para>
  </refsection>

  <refsection id='conf'>
    <title>Configuration File</title>

    <para>Configuration is also read from a single configuration file in
    <filename>/etc/</filename>. This 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.</para>
  </refsection>
</refsection>