lxc-shutdown: Make all processes exit before timeout if shutdown works

[mirror_lxc.git] / doc / lxc-attach.sgml.in

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<refentry>

  <docinfo><date>@LXC_GENERATE_DATE@</date></docinfo>

  <refmeta>
    <refentrytitle>lxc-attach</refentrytitle>
    <manvolnum>1</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>lxc-attach</refname>

    <refpurpose>
      start a process inside a running container.
    </refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <cmdsynopsis>
      <command>lxc-attach</command>
      <arg choice="req">-n <replaceable>name</replaceable></arg>
      <arg choice="opt">-a <replaceable>arch</replaceable></arg>
      <arg choice="opt">-e</arg>
      <arg choice="opt">-s <replaceable>namespaces</replaceable></arg>
      <arg choice="opt">-R</arg>
      <arg choice="opt">-- <replaceable>command</replaceable></arg>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>
      <command>lxc-attach</command> runs the specified
      <replaceable>command</replaceable> inside the container
      specified by <replaceable>name</replaceable>. The container
      has to be running already.
    </para>
    <para>
      If no <replaceable>command</replaceable> is specified, the
      current default shell of the user running
      <command>lxc-attach</command> will be looked up inside the
      container and executed. This will fail if no such user exists
      inside the container or the container does not have a working
      nsswitch mechanism.
    </para>

  </refsect1>

  <refsect1>

    <title>Options</title>

    <variablelist>

      <varlistentry>
	<term>
	  <option>-a, --arch <replaceable>arch</replaceable></option>
	</term>
	<listitem>
	  <para>
	    Specify the architecture which the kernel should appear to be
	    running as to the command executed. This option will accept the
	    same settings as the <option>lxc.arch</option> option in
	    container configuration files, see
	    <citerefentry>
	      <refentrytitle><filename>lxc.conf</filename></refentrytitle>
	      <manvolnum>5</manvolnum>
	    </citerefentry>. By default, the current archictecture of the
	    running container will be used.
	  </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <option>-e, --elevated-privileges</option>
	</term>
	<listitem>
	  <para>
	    Do not drop privileges when running
	    <replaceable>command</replaceable> inside the container. If
	    this option is specified, the new process will
	    <emphasis>not</emphasis> be added to the container's cgroup(s)
	    and it will not drop its capabilities before executing.
	  </para>
	  <para>
	    <emphasis>Warning:</emphasis> This may leak privileges into the
	    container if the command starts subprocesses that remain active
	    after the main process that was attached is terminated. The
	    (re-)starting of daemons inside the container is problematic,
	    especially if the daemon starts a lot of subprocesses such as
	    <command>cron</command> or <command>sshd</command>.
	    <emphasis>Use with great care.</emphasis>
	  </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <option>-s, --namespaces <replaceable>namespaces</replaceable></option>
	</term>
	<listitem>
	  <para>
	    Specify the namespaces to attach to, as a pipe-separated list,
	    e.g. <replaceable>NETWORK|IPC</replaceable>. Allowed values are
	    <replaceable>MOUNT</replaceable>, <replaceable>PID</replaceable>,
	    <replaceable>UTSNAME</replaceable>, <replaceable>IPC</replaceable>,
	    <replaceable>USER </replaceable> and
	    <replaceable>NETWORK</replaceable>. This allows one to change
	    the context of the process to e.g. the network namespace of the
	    container while retaining the other namespaces as those of the
	    host.
	  </para>
	  <para>
	    <emphasis>Important:</emphasis> This option implies
	    <option>-e</option>.
	  </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <option>-R, --remount-sys-proc</option>
	</term>
	<listitem>
	  <para>
	    When using <option>-s</option> and the mount namespace is not
	    included, this flag will cause <command>lxc-attach</command>
	    to remount <replaceable>/proc</replaceable> and
	    <replaceable>/sys</replaceable> to reflect the current other
	    namespace contexts.
	  </para>
	  <para>
	    Please see the <emphasis>Notes</emphasis> section for more
	    details.
	  </para>
	  <para>
	    This option will be ignored if one tries to attach to the
	    mount namespace anyway.
	  </para>
	</listitem>
      </varlistentry>

     </variablelist>

  </refsect1>

  &commonoptions;

  <refsect1>
    <title>Examples</title>
      <para>
        To spawn a new shell running inside an existing container, use
        <programlisting>
          lxc-attach -n container
        </programlisting>
      </para>
      <para>
        To restart the cron service of a running Debian container, use
        <programlisting>
          lxc-attach -n container -- /etc/init.d/cron restart
        </programlisting>
      </para>
      <para>
        To deactivate the network link eth1 of a running container that
        does not have the NET_ADMIN capability, use either the
        <option>-e</option> option to use increased capabilities,
        assuming the <command>ip</command> tool is installed:
        <programlisting>
          lxc-attach -n container -e -- /sbin/ip link delete eth1
        </programlisting>
        Or, alternatively, use the <option>-s</option> to use the
        tools installed on the host outside the container:
        <programlisting>
          lxc-attach -n container -s NETWORK -- /sbin/ip link delete eth1
        </programlisting>
      </para>
  </refsect1>

  <refsect1>
    <title>Compatibility</title>
    <para>
      Attaching completely (including the pid and mount namespaces) to a
      container requires a patched kernel, please see the lxc website for
      details. <command>lxc-attach</command> will fail in that case if
      used with an unpatched kernel.
    </para>
    <para>
      Nevertheless, it will succeed on an unpatched kernel of version 3.0
      or higher if the <option>-s</option> option is used to restrict the
      namespaces that the process is to be attached to to one or more of
      <replaceable>NETWORK</replaceable>, <replaceable>IPC</replaceable>
      and <replaceable>UTSNAME</replaceable>.
    </para>
    <para>
      Attaching to user namespaces is currently completely unsupported
      by the kernel. <command>lxc-attach</command> should however be able
      to do this once once future kernel versions implement this.
    </para>
  </refsect1>

  <refsect1>
    <title>Notes</title>
    <para>
      The Linux <replaceable>/proc</replaceable> and
      <replaceable>/sys</replaceable> filesystems contain information
      about some quantities that are affected by namespaces, such as
      the directories named after process ids in
      <replaceable>/proc</replaceable> or the network interface infromation
      in <replaceable>/sys/class/net</replaceable>. The namespace of the
      process mounting the pseudo-filesystems determines what information
      is shown, <emphasis>not</emphasis> the namespace of the process
      accessing <replaceable>/proc</replaceable> or
      <replaceable>/sys</replaceable>.
    </para>
    <para>
      If one uses the <option>-s</option> option to only attach to
      the pid namespace of a container, but not its mount namespace
      (which will contain the <replaceable>/proc</replaceable> of the
      container and not the host), the contents of <option>/proc</option>
      will reflect that of the host and not the container. Analogously,
      the same issue occurs when reading the contents of
      <replaceable>/sys/class/net</replaceable> and attaching to just
      the network namespace.
    </para>
    <para>
      To work around this problem, the <option>-R</option> flag provides
      the option to remount <replaceable>/proc</replaceable> and
      <replaceable>/sys</replaceable> in order for them to reflect the
      network/pid namespace context of the attached process. In order
      not to interfere with the host's actual filesystem, the mount
      namespace will be unshared (like <command>lxc-unshare</command>
      does) before this is done, esentially giving the process a new
      mount namespace, which is identical to the hosts's mount namespace
      except for the <replaceable>/proc</replaceable> and
      <replaceable>/sys</replaceable> filesystems.
    </para>
  </refsect1>

  <refsect1>
    <title>Security</title>
    <para>
      The <option>-e</option> and <option>-s</option> options should
      be used with care, as it may break the isolation of the containers
      if used improperly.
    </para>
  </refsect1>

  &seealso;

  <refsect1>
    <title>Author</title>
    <para>Daniel Lezcano <email>daniel.lezcano@free.fr</email></para>
  </refsect1>

</refentry>

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