shadow/man/subuid.5.xml

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<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 Configuration Files</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>