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lxc: linux Container library

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Authors:
Daniel Lezcano <daniel.lezcano at free.fr>

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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, --name <replaceable>name</replaceable></arg>
      <arg choice="opt">-f, --rcfile <replaceable>config_file</replaceable></arg>
      <arg choice="opt">-a, --arch <replaceable>arch</replaceable></arg>
      <arg choice="opt">-e, --elevated-privileges <replaceable>privileges</replaceable></arg>
      <arg choice="opt">-s, --namespaces <replaceable>namespaces</replaceable></arg>
      <arg choice="opt">-R, --remount-sys-proc</arg>
      <arg choice="opt">--keep-env</arg>
      <arg choice="opt">--clear-env</arg>
      <arg choice="opt">-L, --pty-log <replaceable>file</replaceable></arg>
      <arg choice="opt">-v, --set-var <replaceable>variable</replaceable></arg>
      <arg choice="opt">--keep-var <replaceable>variable</replaceable></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>
    <para>
    Previous versions of <command>lxc-attach</command> simply attached to the
    specified namespaces of a container and ran a shell or the specified command
    without first allocating a pseudo terminal. This made them vulnerable to
    input faking via a TIOCSTI <command>ioctl</command> call after switching
    between userspace execution contexts with different privilege levels. Newer
    versions of <command>lxc-attach</command> will try to allocate a pseudo
    terminal master/slave pair on the host and attach any standard file
    descriptors which refer to a terminal to the slave side of the pseudo
    terminal before executing a shell or command. Note, that if none of the
    standard file descriptors refer to a terminal <command>lxc-attach</command>
    will not try to allocate a pseudo terminal. Instead it will simply attach
    to the containers namespaces and run a shell or the specified command.
    </para>

  </refsect1>

  <refsect1>

    <title>Options</title>

    <variablelist>

      <varlistentry>
	<term>
	  <option>-f, --rcfile <replaceable>config_file</replaceable></option>
	</term>
	<listitem>
	  <para>
	    Specify the configuration file to configure the virtualization
	    and isolation functionalities for the container.
	  </para>
	  <para>
	   This configuration file if present will be used even if there is
	   already a configuration file present in the previously created
	   container (via lxc-create).
	  </para>
	</listitem>
      </varlistentry>

      <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 <replaceable>privileges</replaceable>
    </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>
      You may specify privileges, in case you do not want to elevate all of
      them, as a pipe-separated list, e.g.
      <replaceable>CGROUP|LSM</replaceable>. Allowed values are
      <replaceable>CGROUP</replaceable>, <replaceable>CAP</replaceable> and
      <replaceable>LSM</replaceable> representing cgroup, capabilities and
      restriction privileges respectively. (The pipe symbol needs to be escaped,
      e.g. <replaceable>CGROUP\|LSM</replaceable> or quoted, e.g.
      <replaceable>"CGROUP|LSM"</replaceable>.)
    </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. (The pipe symbol needs to be escaped, e.g.
            <replaceable>MOUNT\|PID</replaceable> or quoted, e.g.
            <replaceable>"MOUNT|PID"</replaceable>.)
	  </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>

      <varlistentry>
	<term>
	  <option>--keep-env</option>
	</term>
	<listitem>
	  <para>
	    Keep the current environment for attached programs. This is
	    the current default behaviour (as of version 0.9), but is
	    is likely to change in the future, since this may leak
	    undesirable information into the container. If you rely on
	    the environment being available for the attached program,
	    please use this option to be future-proof. In addition to
	    current environment variables, container=lxc will be set.
	  </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <option>--clear-env</option>
	</term>
	<listitem>
	  <para>
	    Clear the environment before attaching, so no undesired
	    environment variables leak into the container. The variable
	    container=lxc will be the only environment with which the
	    attached program starts.
	  </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <option>-L, --pty-log <replaceable>file</replaceable></option>
	</term>
	<listitem>
	  <para>
	Specify a file where the output of <command>lxc-attach</command> will be
	logged.
	  </para>
	  <para>
	    <emphasis>Important:</emphasis>  When a standard file descriptor
	    does not refer to a pty output produced on it will not be logged.
	  </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <option>-v, --set-var <replaceable>variable</replaceable></option>
	</term>
	<listitem>
	  <para>
	    Set an additional environment variable that is seen by the
	    attached program in the container. It is specified in the
	    form of "VAR=VALUE", and can be specified multiple times.
	  </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <option>--keep-var <replaceable>variable</replaceable></option>
	</term>
	<listitem>
	  <para>
	    Keep a specified environment variable. It can only be
	    specified in conjunction
	    with <replaceable>--clear-env</replaceable>, and can be
	    specified multiple times.
	  </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 kernel of version 3.8 or higher, or 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 of version 3.7 and prior.
    </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 supported by kernel 3.8 or higher
      with enabling user namespace.
    </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 information
      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, essentially 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>
    <para>
      Previous versions of <command>lxc-attach</command> suffered a bug whereby
      a user could attach to a containers namespace without being placed in a
      writeable cgroup for some critical subsystems. Newer versions of
      <command>lxc-attach</command> will check whether a user is in a writeable
      cgroup for those critical subsystems. <command>lxc-attach</command> might
      thus fail unexpectedly for some users (E.g. on systems where an
      unprivileged user is not placed in a writeable cgroup in critical
      subsystems on login.). However, this behavior is correct and more secure.
    </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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