man/standard-conf.xml

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  15
  16   <refsect1 id='confd'>
  17     <title>Configuration Directories and Precedence</title>
  18
  19     <para>Configuration files are read from directories in <filename>/etc/</filename>,
  20     <filename>/run/</filename>, <filename>/usr/local/lib/</filename>, and <filename>/usr/lib/</filename>, in
  21     order of precedence, as listed in the SYNOPSIS section above. Files must have the
  22     <literal>.conf</literal> extension. Files in <filename>/etc/</filename> override files with the same name
  23     in <filename>/run/</filename>, <filename>/usr/local/lib/</filename>, and
  24     <filename>/usr/lib/</filename>. Files in <filename>/run/</filename> override files with the same name
  25     under <filename>/usr/</filename>.</para>
  26
  27     <para>All configuration files are sorted by their filename in lexicographic order, regardless of which of
  28     the directories they reside in. If multiple files specify the same option, the entry in the file with the
  29     lexicographically latest name will take precedence. Thus, the configuration in a certain file may either
  30     be replaced completely (by placing a file with the same name in a directory with higher priority), or
  31     individual settings might be changed (by specifying additional settings in a file with a different name
  32     that is ordered later).</para>
  33
  34     <para>Packages should install their configuration files in <filename>/usr/lib/</filename> (distribution
  35     packages) or <filename>/usr/local/lib/</filename> (local installs). Files in <filename>/etc/</filename>
  36     are reserved for the local administrator, who may use this logic to override the configuration files
  37     installed by vendor packages. It is recommended to prefix all filenames with a two-digit number and a
  38     dash, to simplify the ordering of the files. It is recommended to use the range 10-40 for configuration
  39     files in <filename>/usr/</filename> and the range 60-90 for configuration files in
  40     <filename>/etc/</filename> and <filename>/run/</filename>, to make sure that local and transient
  41     configuration files will always take priority over configuration files shipped by the OS vendor.</para>
  42
  43     <para>If the administrator wants to disable a configuration file supplied by the vendor, the recommended
  44     way is to place a symlink to <filename>/dev/null</filename> in the configuration directory in
  45     <filename>/etc/</filename>, with the same filename as the vendor configuration file. If the vendor
  46     configuration file is included in the initrd image, the image has to be regenerated.</para>
  47   </refsect1>
  48
  49   <refsect1 id='main-conf'>
  50     <title>Configuration Directories and Precedence</title>
  51
  52     <para>The default configuration is set during compilation, so configuration is only needed when it is
  53     necessary to deviate from those defaults. The main configuration file is loaded from one of the
  54     listed directories in order of priority, only the first file found is used:
  55     <filename>/etc/systemd/</filename>,
  56     <filename>/run/systemd/</filename>,
  57     <filename>/usr/local/lib/systemd/</filename>,
  58     <filename>/usr/lib/systemd/</filename>.
  59     The vendor version of the file contains commented out entries showing the defaults as a guide to the
  60     administrator. Local overrides can also be created by creating drop-ins, as described below. The main
  61     configuration file can also be edited for this purpose (or a copy in <filename>/etc/</filename> if it's
  62     shipped under <filename>/usr/</filename>), however using drop-ins for local configuration is recommended
  63     over modifications to the main configuration file.</para>
  64
  65     <para>In addition to the main configuration file, drop-in configuration snippets are read from
  66     <filename>/usr/lib/systemd/*.conf.d/</filename>, <filename>/usr/local/lib/systemd/*.conf.d/</filename>,
  67     and <filename>/etc/systemd/*.conf.d/</filename>. Those drop-ins have higher precedence and override the
  68     main configuration file. Files in the <filename>*.conf.d/</filename> configuration subdirectories are
  69     sorted by their filename in lexicographic order, regardless of in which of the subdirectories they
  70     reside. When multiple files specify the same option, for options which accept just a single value, the
  71     entry in the file sorted last takes precedence, and for options which accept a list of values, entries
  72     are collected as they occur in the sorted files.</para>
  73
  74     <para>When packages need to customize the configuration, they can install drop-ins under
  75     <filename>/usr/</filename>. Files in <filename>/etc/</filename> are reserved for the local administrator,
  76     who may use this logic to override the configuration files installed by vendor packages. Drop-ins have to
  77     be used to override package drop-ins, since the main configuration file has lower precedence. It is
  78     recommended to prefix all filenames in those subdirectories with a two-digit number and a dash, to
  79     simplify the ordering of the files. This also defines a concept of drop-in priorities to allow
  80     OS vendors to ship drop-ins within a specific range lower than the range used by users. This should
  81     lower the risk of package drop-ins overriding accidentally drop-ins defined by users. It is recommended
  82     to use the range 10-40 for drop-ins in <filename>/usr/</filename> and the range 60-90 for drop-ins in
  83     <filename>/etc/</filename> and <filename>/run/</filename>, to make sure that local and transient drop-ins
  84     take priority over drop-ins shipped by the OS vendor.</para>
  85
  86     <para>To disable a configuration file supplied by the vendor, the recommended way is to place a symlink
  87     to <filename>/dev/null</filename> in the configuration directory in <filename>/etc/</filename>, with the
  88     same filename as the vendor configuration file.</para>
  89   </refsect1>
  90 </refentry>