subuid.5.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
   Copyright (c) 2013 Eric W. Biederman
   All rights reserved.
  
   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions
   are met:
   1. Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
   2. Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in the
      documentation and/or other materials provided with the distribution.
   3. The name of the copyright holders or contributors may not be used to
      endorse or promote products derived from this software without
      specific prior written permission.
  
   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
   PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT
   HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!-- SHADOW-CONFIG-HERE -->
]>
<refentry id='subuid.5'>
  <refentryinfo>
    <author>
      <firstname>Eric</firstname>
      <surname>Biederman</surname>
      <contrib>Creation, 2013</contrib>
    </author>
    <author>
      <firstname>Iker</firstname>
      <surname>Pedrosa</surname>
      <contrib>Developer, 2021</contrib>
    </author>
  </refentryinfo>
  <refmeta>
    <refentrytitle>subuid</refentrytitle>
    <manvolnum>5</manvolnum>
    <refmiscinfo class="sectdesc">File Formats and Conversions</refmiscinfo>
    <refmiscinfo class="source">shadow-utils</refmiscinfo>
    <refmiscinfo class="version">&SHADOW_UTILS_VERSION;</refmiscinfo>
  </refmeta>
  <refnamediv id='name'>
    <refname>subuid</refname>
    <refpurpose>the configuration for subordinate user ids</refpurpose>
  </refnamediv>

  <refsect1 id='description'>
    <title>DESCRIPTION</title>
    <para>
      Subuid authorizes a user id to map ranges of user ids from its namespace
      into child namespaces.
    </para>
    <para>
      The delegation of the subordinate uids can be configured via the
      <replaceable>subid</replaceable> field in
      <filename>/etc/nsswitch.conf</filename> file. Only one value can be set
      as the delegation source. Setting this field to
      <replaceable>files</replaceable> configures the delegation of uids to
      <filename>/etc/subuid</filename>. Setting any other value treats
      the delegation as a plugin following with a name of the form
      <replaceable>libsubid_$value.so</replaceable>. If the value or plugin is
      missing, then the subordinate uid delegation falls back to
      <replaceable>files</replaceable>.
    </para>
    <para>
      Note, that <command>useradd</command> will only create entries in
      <filename>/etc/subuid</filename> if subid delegation is managed via subid
      files.
    </para>
  </refsect1>

  <refsect1 id='local-subordinate-delegation'>
    <title>LOCAL SUBORDINATE DELEGATION</title>
    <para>
      Each line in <filename>/etc/subuid</filename> contains
      a user name and a range of subordinate user ids that user
      is allowed to use.

      This is specified with three fields delimited by colons
      (<quote>:</quote>).
      These fields are:
    </para>
    <itemizedlist mark='bullet'>
      <listitem>
	<para>login name or UID</para>
      </listitem>
      <listitem>
	<para>numerical subordinate user ID</para>
      </listitem>
      <listitem>
	<para>numerical subordinate user ID count</para>
      </listitem>
    </itemizedlist>

    <para>
      This file specifies the user IDs that ordinary users can use, with
      the <command>newuidmap</command> command, to configure uid mapping
      in a user namespace.
    </para>

    <para>
      Multiple ranges may be specified per user.
    </para>

    <para>
      When large number of entries (10000-100000 or more) are defined in
      <filename>/etc/subuid</filename>, parsing performance penalty will
      become noticeable. In this case it is recommended to use UIDs
      instead of login names. Benchmarks have shown speed-ups up to 20x.
    </para>

  </refsect1>

  <refsect1 id='files'>
    <title>FILES</title>
    <variablelist>
      <varlistentry>
	<term><filename>/etc/subuid</filename></term>
	<listitem>
	  <para>Per user subordinate user IDs.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><filename>/etc/subuid-</filename></term>
	<listitem>
	  <para>Backup file for /etc/subuid.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id='see_also'>
    <title>SEE ALSO</title>
    <para>
      <citerefentry>
	<refentrytitle>login.defs</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>newgidmap</refentrytitle><manvolnum>1</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>newuidmap</refentrytitle><manvolnum>1</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>newusers</refentrytitle><manvolnum>1</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>subgid</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>useradd</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>userdel</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>usermod</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>user_namespaces</refentrytitle><manvolnum>7</manvolnum>
      </citerefentry>.
    </para>
  </refsect1>
</refentry>