# lxc

## NAME



           lxc - linux containers
    


## OVERVIEW



           The container technology is actively being pushed into the
           mainstream Linux kernel. It provides resource management through
           control groups and resource isolation via namespaces.
    
           **lxc** , 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 full system.
    
           **lxc** is small enough to easily manage a container with simple
           command lines and complete enough to be used for other purposes.
    


## REQUIREMENTS



           The kernel version >= 3.10 shipped with the distros, will work
           with **lxc** , this one will have less functionalities but enough to be
           interesting.
    
           **lxc** relies on a set of functionalities provided by the kernel. The
           helper script **lxc-checkconfig** will give you information about your
           kernel configuration, required, and missing features.
    


## FUNCTIONAL SPECIFICATION



           A container is an object isolating some resources of the host, for
           the application or system running in it.
    
           The application / system will be launched inside a container
           specified by a configuration that is either initially created or
           passed as a parameter of the commands.
    
           How to run an application in a container
    
           Before running an application, you should know what are the
           resources you want to isolate. The default configuration is to
           isolate PIDs, the sysv IPC and 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 **sshd** , you should provide a new network stack and
           a new hostname. If you want to avoid conflicts with some files eg.
           _/var/run/httpd.pid_ , you should remount _/var/run_ 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 _/etc_ and _/home_
    
           Here is an example of directory tree for **sshd** :
    
           [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
    
           and the mount points file associated with it:
    
                [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
    
           How to run a system in a container
    
           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 needs 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:
    
                [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
    
       **CONTAINER LIFE CYCLE**
           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.
    
           In case of failure when the container is initialized, it will pass
           through the aborting state.
    
              ---------
             | STOPPED |<---------------
              ---------                 |
                  |                     |
                start                   |
                  |                     |
                  V                     |
              ----------                |
             | STARTING |--error-       |
              ----------         |      |
                  |              |      |
                  V              V      |
              ---------    ----------   |
             | RUNNING |  | ABORTING |  |
              ---------    ----------   |
                  |              |      |
             no process          |      |
                  |              |      |
                  V              |      |
              ----------         |      |
             | STOPPING |<-------       |
              ----------                |
                  |                     |
                   ---------------------
    
       **CONFIGURATION**
           The container is configured through a configuration file, the
           format of the configuration file is described in lxc.conf(5)
    
       **CREATING / DESTROYING CONTAINERS**
           A persistent container object can be created via the **lxc-create**
           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 **lxc-destroy** command will
           destroy the container object.
    
                  lxc-create -n foo
                  lxc-destroy -n foo
    
       **VOLATILE CONTAINER**
           It is not mandatory to create a container object before starting
           it.  The container can be directly started with a configuration
           file as parameter.
    
       **STARTING / STOPPING CONTAINER**
           When the container has been created, it is ready to run an
           application / system. This is the purpose of the **lxc-execute** and
           **lxc-start** 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 ended, the container will be stopped, but if needed
           the **lxc-stop** command can be used to stop the container.
    
           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:
    
                  lxc-execute -n foo [-f config] /bin/bash
                  lxc-start -n foo [-f config] [/bin/bash]
    
           The **lxc-execute** command will run the specified command into a
           container via an intermediate process, **lxc-init**.  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, **lxc-init** has PID 1
           and the first process of the application has PID 2.
    
           The **lxc-start** command will directly run the specified command in
           the container. The PID of the first process is 1. If no command is
           specified **lxc-start** will run the command defined in lxc.init.cmd
           or if not set, _/sbin/init_ .
    
           To summarize, **lxc-execute** is for running an application and **lxc-**
           **start** is better suited for running a system.
    
           If the application is no longer responding, is inaccessible or is
           not able to finish by itself, a wild **lxc-stop** command will kill
           all the processes in the container without pity.
    
                  lxc-stop -n foo -k
    
       **CONNECT TO AN AVAILABLE TTY**
           If the container is configured with ttys, it is possible to access
           it through them. It is up to the container to provide a set of
           available ttys to be used by the following command. When the tty
           is lost, it is possible to reconnect to it without login again.
    
                  lxc-console -n foo -t 3
    
       **FREEZE / UNFREEZE CONTAINER**
           Sometime, it is useful to stop all the processes belonging to a
           container, eg. for job scheduling. The commands:
    
                  lxc-freeze -n foo
    
           will put all the processes in an uninteruptible state and
    
                  lxc-unfreeze -n foo
    
           will resume them.
    
           This feature is enabled if the freezer cgroup v1 controller is
           enabled in the kernel.
    
       **GETTING INFORMATION ABOUT CONTAINER**
           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 in a specific container. For this reason, the following
           commands may be useful:
    
                  lxc-ls -f
                  lxc-info -n foo
    
           **lxc-ls** lists containers.
    
           **lxc-info** gives information for a specific container.
    
           Here is an example on how the combination of these commands allows
           one to list all the containers and retrieve their state.
    
                  for i in $(lxc-ls -1); do
                    lxc-info -n $i
                  done
    
       **MONITORING CONTAINER**
           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.
    
           **lxc-monitor** command will monitor one or several containers. The
           parameter of this command accepts a regular expression for
           example:
    
                  lxc-monitor -n "foo|bar"
    
           will monitor the states of containers named 'foo' and 'bar', and:
    
                  lxc-monitor -n ".*"
    
           will monitor all the containers.
    
           For a container 'foo' starting, doing some work and exiting, the
           output will be in the form:
    
                  'foo' changed state to [STARTING]
                  'foo' changed state to [RUNNING]
                  'foo' changed state to [STOPPING]
                  'foo' changed state to [STOPPED]
    
           **lxc-wait** 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 it successfully started as a daemon.
    
                  # 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"
    
       **CGROUP SETTINGS FOR CONTAINERS**
           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.
    
           **lxc-cgroup** 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.
    
                  lxc-cgroup -n foo cpuset.cpus
    
           will display the content of this subsystem.
    
                  lxc-cgroup -n foo cpu.shares 512
    
           will set the subsystem to the specified value.
    


* * *