systemd/man/standard-conf.xml

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  <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). 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. 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 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 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>
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