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lxc: linux Container library

(C) Copyright IBM Corp. 2007, 2008

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

This library is free software; you can redistribute it and/or
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<!DOCTYPE refentry PUBLIC @docdtd@ [

<!ENTITY seealso SYSTEM "@builddir@/see_also.sgml">
]>

<refentry>

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


  <refmeta>
    <refentrytitle>lxc</refentrytitle>
    <manvolnum>7</manvolnum>
    <refmiscinfo>
      Version @PACKAGE_VERSION@
    </refmiscinfo>
  </refmeta>

  <refnamediv>
    <refname>lxc</refname>

    <refpurpose>
      linux containers
    </refpurpose>
  </refnamediv>

  <refsect1>
    <title>Quick start</title>
    <para>
      You are in a hurry, and you don't want to read this man page. Ok,
      without warranty, here are the commands to launch a shell inside
      a container with a predefined configuration template, it may
      work.
      <command>@BINDIR@/lxc-execute -n foo -f
      @DOCDIR@/examples/lxc-macvlan.conf /bin/bash</command>
    </para>
  </refsect1>

  <refsect1>
    <title>Overview</title>
    <para>
      The container technology is actively being pushed into the
      mainstream linux kernel. It provides the resource management
      through the control groups aka process containers and resource
      isolation through the namespaces.
    </para>

    <para>
      The linux containers, <command>lxc</command>, aims to use these
      new functionalities to provide a userspace container object
      which provides full resource isolation and resource control for
      an applications or a system.
    </para>

    <para>
      The first objective of this project is to make the life easier
      for the kernel developers involved in the containers project and
      especially to continue working on the Checkpoint/Restart new
      features. The <command>lxc</command> is small enough to easily
      manage a container with simple command lines and complete enough
      to be used for other purposes.
    </para>
  </refsect1>

  <refsect1>
    <title>Requirements</title>
    <para>
      The <command>lxc</command> relies on a set of functionalities
      provided by the kernel which needs to be active. Depending of
      the missing functionalities the <command>lxc</command> will
      work with a restricted number of functionalities or will simply
      fail.
    </para>

    <para>
      The following list gives the kernel features to be enabled in
      the kernel to have the full features container:
    </para>
      <programlisting>
	    * General setup
	      * Control Group support
	        -> Namespace cgroup subsystem
	        -> Freezer cgroup subsystem
	        -> Cpuset support
	        -> Simple CPU accounting cgroup subsystem
	        -> Resource counters
	          -> Memory resource controllers for Control Groups
	      * Group CPU scheduler
	        -> Basis for grouping tasks (Control Groups)
	      * Namespaces support
	        -> UTS namespace
	        -> IPC namespace
	        -> User namespace
	        -> Pid namespace
	        -> Network namespace
	    * Device Drivers
	      * Character devices
	        -> Support multiple instances of devpts
	      * Network device support
	        -> MAC-VLAN support
	        -> Virtual ethernet pair device
	    * Networking
	      * Networking options
	        -> 802.1d Ethernet Bridging
	    * Security options
	      -> File POSIX Capabilities
      </programlisting>

      <para>

	The kernel version >= 2.6.32 shipped with the distros, will
	work with <command>lxc</command>, this one will have less
	functionalities but enough to be interesting.

	The helper script <command>lxc-checkconfig</command> will give
	you information about your kernel configuration.
      </para>

      <para>
	  The control group can be mounted anywhere, eg:
	  <command>mount -t cgroup cgroup /cgroup</command>.

	  It is however recommended to use cgmanager, cgroup-lite or systemd
	  to mount the cgroup hierarchy under /sys/fs/cgroup.

      </para>

  </refsect1>

  <refsect1>
    <title>Functional specification</title>
    <para>
      A container is an object isolating some resources of the host,
      for the application or system running in it.
    </para>
    <para>
      The application / system will be launched inside a
      container specified by a configuration that is either
      initially created or passed as parameter of the starting commands.
    </para>

    <para>How to run an application in a container ?</para>
    <para>
      Before running an application, you should know what are the
      resources you want to isolate. The default configuration is to
      isolate the pids, the sysv ipc and the mount points. If you want
      to run a simple shell inside a container, a basic configuration
      is needed, especially if you want to share the rootfs. If you
      want to run an application like <command>sshd</command>, you
      should provide a new network stack and a new hostname. If you
      want to avoid conflicts with some files
      eg. <filename>/var/run/httpd.pid</filename>, you should
      remount <filename>/var/run</filename> with an empty
      directory. If you want to avoid the conflicts in all the cases,
      you can specify a rootfs for the container. The rootfs can be a
      directory tree, previously bind mounted with the initial rootfs,
      so you can still use your distro but with your
      own <filename>/etc</filename> and <filename>/home</filename>
    </para>
    <para>
      Here is an example of directory tree
      for <command>sshd</command>:
      <programlisting>	
[root@lxc sshd]$ tree -d rootfs
	
rootfs	
|-- bin	
|-- dev	
|   |-- pts
|   `-- shm
|       `-- network
|-- etc	
|   `-- ssh
|-- lib	
|-- proc
|-- root
|-- sbin
|-- sys	
|-- usr	
`-- var	
    |-- empty
    |   `-- sshd
    |-- lib
    |   `-- empty
    |       `-- sshd
    `-- run
        `-- sshd
      </programlisting>

      and the mount points file associated with it:
      <programlisting>
	[root@lxc sshd]$ cat fstab

	/lib /home/root/sshd/rootfs/lib none ro,bind 0 0
	/bin /home/root/sshd/rootfs/bin none ro,bind 0 0
	/usr /home/root/sshd/rootfs/usr none ro,bind 0 0
	/sbin /home/root/sshd/rootfs/sbin none ro,bind 0 0
      </programlisting>
    </para>

    <para>How to run a system in a container ?</para>

    <para>Running a system inside a container is paradoxically easier
    than running an application. Why ? Because you don't have to care
    about the resources to be isolated, everything need to be
    isolated, the other resources are specified as being isolated but
    without configuration because the container will set them
    up. eg. the ipv4 address will be setup by the system container
    init scripts. Here is an example of the mount points file:

      <programlisting>
	[root@lxc debian]$ cat fstab

	/dev	/home/root/debian/rootfs/dev none bind 0 0
	/dev/pts /home/root/debian/rootfs/dev/pts  none bind 0 0
      </programlisting>

      More information can be added to the container to facilitate the
      configuration. For example, make accessible from the container
      the resolv.conf file belonging to the host.

      <programlisting>
	/etc/resolv.conf /home/root/debian/rootfs/etc/resolv.conf none bind 0 0
      </programlisting>
    </para>

    <refsect2>
      <title>Container life cycle</title>
      <para>
	When the container is created, it contains the configuration
	information. When a process is launched, the container will be
	starting and running. When the last process running inside the
	container exits, the container is stopped.
      </para>
      <para>
	In case of failure when the container is initialized, it will
	pass through the aborting state.
      </para>

      <programlisting>
<![CDATA[
   ---------
  | STOPPED |<---------------
   ---------                 |
       |                     |
     start                   |
       |                     |
       V                     |
   ----------                |
  | STARTING |--error-       |
   ----------         |      |
       |              |      |
       V              V      |
   ---------    ----------   |
  | RUNNING |  | ABORTING |  |
   ---------    ----------   |
       |              |      |
  no process          |      |
       |              |      |
       V              |      |
   ----------         |      |
  | STOPPING |<-------       |
   ----------                |
       |                     |
        ---------------------
]]>
      </programlisting>
    </refsect2>

    <refsect2>
      <title>Configuration</title>
      <para>The container is configured through a configuration
      file, the format of the configuration file is described in
      <citerefentry>
	<refentrytitle><filename>lxc.conf</filename></refentrytitle>
	<manvolnum>5</manvolnum>
      </citerefentry>
      </para>
    </refsect2>

    <refsect2>
      <title>Creating / Destroying container
	(persistent container)</title>
      <para>
	A persistent container object can be
	created via the <command>lxc-create</command>
	command. It takes a container name as parameter and
	optional configuration file and template.
	The name is used by the different
	commands to refer to this
	container. The <command>lxc-destroy</command> command will
	destroy the container object.
	<programlisting>
	  lxc-create -n foo
	  lxc-destroy -n foo
	</programlisting>
      </para>
    </refsect2>

    <refsect2>
	<title>Volatile container</title>
	<para>It is not mandatory to create a container object
	before to start it.
	The container can be directly started with a
	configuration file as parameter.
	</para>
    </refsect2>

    <refsect2>
      <title>Starting / Stopping container</title>
      <para>When the container has been created, it is ready to run an
      application / system.
      This is the purpose of the <command>lxc-execute</command> and
      <command>lxc-start</command> commands.
      If the container was not created before
      starting the application, the container will use the
      configuration file passed as parameter to the command,
      and if there is no such parameter either, then
      it will use a default isolation.
      If the application is ended, the container will be stopped also,
      but if needed the <command>lxc-stop</command> command can
      be used to kill the still running application.
      </para>

      <para>
	Running an application inside a container is not exactly the
	same thing as running a system. For this reason, there are two
	different commands to run an application into a container:
	<programlisting>
	  lxc-execute -n foo [-f config] /bin/bash
	  lxc-start -n foo [-f config] [/bin/bash]
	</programlisting>
      </para>

      <para>
	<command>lxc-execute</command> command will run the
	specified command into the container via an intermediate
	process, <command>lxc-init</command>.
	This lxc-init after launching  the specified command,
	will wait for its end and all other reparented processes.
        (to support daemons in the container).
	In other words, in the
	container, <command>lxc-init</command> has the pid 1 and the
	first process of the application has the pid 2.
      </para>

      <para>
	<command>lxc-start</command> command will run directly the specified
	command into the container.
	The pid of the first process is 1. If no command is
	specified <command>lxc-start</command> will
	run the command defined in lxc.init_cmd or if not set,
	<filename>/sbin/init</filename> .
      </para>

      <para>
	To summarize, <command>lxc-execute</command> is for running
	an application and <command>lxc-start</command> is better suited for
	running a system.
      </para>

      <para>
	If the application is no longer responding, is inaccessible or is
	not able to finish by itself, a
	wild <command>lxc-stop</command> command will kill all the
	processes in the container without pity.
	<programlisting>
	  lxc-stop -n foo
	</programlisting>
      </para>
    </refsect2>

    <refsect2>
      <title>Connect to an available tty</title>
      <para>
	If the container is configured with the ttys, it is possible
	to access it through them. It is up to the container to
	provide a set of available tty to be used by the following
	command. When the tty is lost, it is possible to reconnect it
	without login again.
	<programlisting>
	  lxc-console -n foo -t 3
	</programlisting>
      </para>
    </refsect2>

    <refsect2>
      <title>Freeze / Unfreeze container</title>
      <para>
	Sometime, it is useful to stop all the processes belonging to
	a container, eg. for job scheduling. The commands:
	<programlisting>
	  lxc-freeze -n foo
	</programlisting>

	will put all the processes in an uninteruptible state and

	<programlisting>
	  lxc-unfreeze -n foo
	</programlisting>

	will resume them.
      </para>

      <para>
	This feature is enabled if the cgroup freezer is enabled in the
	kernel.
      </para>
    </refsect2>

    <refsect2>
      <title>Getting information about container</title>
      <para>When there are a lot of containers, it is hard to follow
      what has been created or destroyed, what is running or what are
      the pids running into a specific container. For this reason, the
      following commands may be useful:
	<programlisting>
	  lxc-ls
	  lxc-info -n foo
	</programlisting>
      </para>
      <para>
	<command>lxc-ls</command> lists the containers of the
	system.
      </para>

      <para>
	<command>lxc-info</command> gives information for a specific
	container.
      </para>

      <para>
	Here is an example on how the combination of these commands
	allows one to list all the containers and retrieve their state.
	<programlisting>
	  for i in $(lxc-ls -1); do
	    lxc-info -n $i
	  done
	</programlisting>

      </para>

    </refsect2>

    <refsect2>
      <title>Monitoring container</title>
      <para>It is sometime useful to track the states of a container,
      for example to monitor it or just to wait for a specific
      state in a script.
      </para>

      <para>
	<command>lxc-monitor</command> command will monitor one or
	several containers. The parameter of this command accept a
	regular expression for example:
	<programlisting>
	  lxc-monitor -n "foo|bar"
	</programlisting>
	will monitor the states of containers named 'foo' and 'bar', and:
	<programlisting>
	  lxc-monitor -n ".*"
	</programlisting>
	will monitor all the containers.
      </para>
      <para>
	For a container 'foo' starting, doing some work and exiting,
	the output will be in the form:
	<programlisting>
	  'foo' changed state to [STARTING]
	  'foo' changed state to [RUNNING]
	  'foo' changed state to [STOPPING]
	  'foo' changed state to [STOPPED]
	</programlisting>
      </para>
      <para>
	<command>lxc-wait</command> command will wait for a specific
	state change and exit. This is useful for scripting to
	synchronize the launch of a container or the end. The
	parameter is an ORed combination of different states. The
	following example shows how to wait for a container if he went
	to the background.

	<programlisting>
<![CDATA[
	  # launch lxc-wait in background
	  lxc-wait -n foo -s STOPPED &
	  LXC_WAIT_PID=$!

	  # this command goes in background
	  lxc-execute -n foo mydaemon &

	  # block until the lxc-wait exits
	  # and lxc-wait exits when the container
	  # is STOPPED
	  wait $LXC_WAIT_PID
	  echo "'foo' is finished"
]]>
	</programlisting>
      </para>
    </refsect2>

    <refsect2>
      <title>Setting the control group for container</title>
      <para>The container is tied with the control groups, when a
	container is started a control group is created and associated
	with it. The control group properties can be read and modified
	when the container is running by using the lxc-cgroup command.
      </para>
      <para>
	<command>lxc-cgroup</command> command is used to set or get a
	control group subsystem which is associated with a
	container. The subsystem name is handled by the user, the
	command won't do any syntax checking on the subsystem name, if
	the subsystem name does not exists, the command will fail.
      </para>
      <para>
	<programlisting>
	  lxc-cgroup -n foo cpuset.cpus
	</programlisting>
	will display the content of this subsystem.
	<programlisting>
	  lxc-cgroup -n foo cpu.shares 512
	</programlisting>
	will set the subsystem to the specified value.
      </para>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>Bugs</title>
    <para>The <command>lxc</command> is still in development, so the
    command syntax and the API can change. The version 1.0.0 will be
    the frozen version.</para>
  </refsect1>

  &seealso;

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

</refentry>

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