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This patch allows the user to select any list of namespaces (network, pid, mount, uts, ipc, user) that lxc-attach should use when attaching to the container; all other namespaces will not be attached to. This allows the user to for example attach to just the network namespace and use the host's (and not the container's) network tools to reconfigure the network of the container. Signed-off-by: Christian Seiler <christian@iwakd.de> Cc: Daniel Lezcano <daniel.lezcano@free.fr> Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
279 lines
9.1 KiB
Plaintext
279 lines
9.1 KiB
Plaintext
<!--
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lxc: linux Container library
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(C) Copyright IBM Corp. 2007, 2008
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Authors:
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Daniel Lezcano <dlezcano at fr.ibm.com>
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This library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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This library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with this library; if not, write to the Free Software
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Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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-->
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<!DOCTYPE refentry PUBLIC "-//Davenport//DTD DocBook V3.0//EN" [
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<!ENTITY commonoptions SYSTEM "@builddir@/common_options.sgml">
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<!ENTITY seealso SYSTEM "@builddir@/see_also.sgml">
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]>
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<refentry>
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<docinfo><date>@LXC_GENERATE_DATE@</date></docinfo>
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<refmeta>
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<refentrytitle>lxc-attach</refentrytitle>
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<manvolnum>1</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>lxc-attach</refname>
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<refpurpose>
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start a process inside a running container.
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</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<cmdsynopsis>
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<command>lxc-attach</command>
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<arg choice="req">-n <replaceable>name</replaceable></arg>
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<arg choice="opt">-a <replaceable>arch</replaceable></arg>
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<arg choice="opt">-e</arg>
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<arg choice="opt">-s <replaceable>namespaces</replaceable></arg>
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<arg choice="opt">-- <replaceable>command</replaceable></arg>
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</cmdsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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<command>lxc-attach</command> runs the specified
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<replaceable>command</replaceable> inside the container
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specified by <replaceable>name</replaceable>. The container
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has to be running already.
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</para>
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<para>
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If no <replaceable>command</replaceable> is specified, the
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current default shell of the user running
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<command>lxc-attach</command> will be looked up inside the
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container and executed. This will fail if no such user exists
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inside the container or the container does not have a working
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nsswitch mechanism.
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</para>
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</refsect1>
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<refsect1>
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<title>Options</title>
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<variablelist>
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<varlistentry>
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<term>
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<option>-a, --arch <replaceable>arch</replaceable></option>
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</term>
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<listitem>
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<para>
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Specify the architecture which the kernel should appear to be
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running as to the command executed. This option will accept the
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same settings as the <option>lxc.arch</option> option in
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container configuration files, see
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<citerefentry>
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<refentrytitle><filename>lxc.conf</filename></refentrytitle>
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<manvolnum>5</manvolnum>
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</citerefentry>. By default, the current archictecture of the
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running container will be used.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-e, --elevated-privileges</option>
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</term>
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<listitem>
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<para>
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Do not drop privileges when running
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<replaceable>command</replaceable> inside the container. If
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this option is specified, the new process will
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<emphasis>not</emphasis> be added to the container's cgroup(s)
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and it will not drop its capabilities before executing.
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</para>
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<para>
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<emphasis>Warning:</emphasis> This may leak privileges into the
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container if the command starts subprocesses that remain active
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after the main process that was attached is terminated. The
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(re-)starting of daemons inside the container is problematic,
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especially if the daemon starts a lot of subprocesses such as
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<command>cron</command> or <command>sshd</command>.
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<emphasis>Use with great care.</emphasis>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-s, --namespaces <replaceable>namespaces</replaceable></option>
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</term>
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<listitem>
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<para>
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Specify the namespaces to attach to, as a pipe-separated liste,
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e.g. <replaceable>NETWORK|IPC</replaceable>. Allowed values are
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<replaceable>MOUNT</replaceable>, <replaceable>PID</replaceable>,
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<replaceable>UTSNAME</replaceable>, <replaceable>IPC</replaceable>,
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<replaceable>USER </replaceable> and
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<replaceable>NETWORK</replaceable>. This allows one to change
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the context of the process to e.g. the network namespace of the
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container while retaining the other namespaces as those of the
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host.
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</para>
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<para>
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<emphasis>Important:</emphasis> This option implies
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<option>-e</option>.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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&commonoptions;
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<refsect1>
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<title>Examples</title>
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<para>
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To spawn a new shell running inside an existing container, use
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<programlisting>
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lxc-attach -n container
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</programlisting>
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</para>
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<para>
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To restart the cron service of a running Debian container, use
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<programlisting>
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lxc-attach -n container -- /etc/init.d/cron restart
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</programlisting>
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</para>
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<para>
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To deactivate the network link eth1 of a running container that
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does not have the NET_ADMIN capability, use either the
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<option>-e</option> option to use increased capabilities,
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assuming the <command>ip</command> tool is installed:
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<programlisting>
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lxc-attach -n container -e -- /sbin/ip link delete eth1
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</programlisting>
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Or, alternatively, use the <option>-s</option> to use the
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tools installed on the host outside the container:
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<programlisting>
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lxc-attach -n container -s NETWORK -- /sbin/ip link delete eth1
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</programlisting>
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</para>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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Attaching completely (including the pid and mount namespaces) to a
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container requires a patched kernel, please see the lxc website for
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details. <command>lxc-attach</command> will fail in that case if
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used with an unpatched kernel.
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</para>
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<para>
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Nevertheless, it will succeed on an unpatched kernel of version 3.0
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or higher if the <option>-s</option> option is used to restrict the
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namespaces that the process is to be attached to to one or more of
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<replaceable>NETWORK</replaceable>, <replaceable>IPC</replaceable>
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and <replaceable>UTSNAME</replaceable>.
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</para>
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<para>
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Attaching to user namespaces is currently completely unsupported
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by the kernel. <command>lxc-attach</command> should however be able
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to do this once once future kernel versions implement this.
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</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>
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The Linux <replaceable>/proc</replaceable> and
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<replaceable>/sys</replaceable> filesystems contain information
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about some quantities that are affected by namespaces, such as
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the directories named after process ids in
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<replaceable>/proc</replaceable> or the network interface infromation
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in <replaceable>/sys/class/net</replaceable>. The namespace of the
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process mounting the pseudo-filesystems determines what information
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is shown, <emphasis>not</emphasis> the namespace of the process
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accessing <replaceable>/proc</replaceable> or
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<replaceable>/sys</replaceable>.
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</para>
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<para>
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If one uses the <option>-s</option> option to only attach to
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the pid namespace of a container, but not its mount namespace
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(which will contain the <replaceable>/proc</replaceable> of the
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container and not the host), the contents of <option>/proc</option>
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will reflect that of the host and not the container. Analogously,
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the same issue occurs when reading the contents of
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<replaceable>/sys/class/net</replaceable> and attaching to just
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the network namespace.
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</para>
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<para>
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A workaround is to use <command>lxc-unshare</command> to unshare
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the mount namespace after using <command>lxc-attach</command> with
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<replaceable>-s PID</replaceable> and/or <replaceable>-s
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NETWORK</replaceable> and then unmount and then mount again both
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pseudo-filesystems within that new mount namespace, before
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executing a program/script that relies on this information to be
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correct.
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</para>
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</refsect1>
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<refsect1>
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<title>Security</title>
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<para>
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The <option>-e</option> and <option>-s</option> options should
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be used with care, as it may break the isolation of the containers
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if used improperly.
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</para>
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</refsect1>
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&seealso;
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<refsect1>
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<title>Author</title>
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<para>Daniel Lezcano <email>daniel.lezcano@free.fr</email></para>
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</refsect1>
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</refentry>
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