Ubuntu Manpage: lxc - linux containers

Provided by: lxc1_2.0.11-0ubuntu1~16.04.3_amd64

NAME

       lxc - linux containers

QUICK START

       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.
       /usr/bin/lxc-execute -n foo -f /usr/share/doc/lxc/examples/lxc-macvlan.conf /bin/bash

OVERVIEW

       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.

       The  linux containers, 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 system.

       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 lxc is
       small enough to easily manage a container with simple command lines and complete enough to  be  used  for
       other purposes.

REQUIREMENTS

       The  lxc relies on a set of functionalities provided by the kernel which needs to be active. Depending of
       the missing functionalities the lxc will work with a restricted number of functionalities or will  simply
       fail.

       The  following  list  gives  the  kernel  features  to be enabled in the kernel to have the full features
       container:

                * 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

       The kernel version >= 2.6.32 shipped with the distros, will work  with  lxc,  this  one  will  have  less
       functionalities  but  enough  to  be  interesting.   The  helper  script  lxc-checkconfig  will  give you
       information about your kernel configuration.

       The control group can be mounted anywhere, eg: mount -t cgroup cgroup /cgroup.  It is however recommended
       to use cgmanager, cgroup-lite or systemd to mount the cgroup hierarchy under /sys/fs/cgroup.

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 parameter of the starting 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 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 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 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:

[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

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.

/etc/resolv.conf /home/root/debian/rootfs/etc/resolv.conf 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 CONTAINER (PERSISTENT CONTAINER)
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 to start 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 is ended,
the container will be stopped also, but if needed the lxc-stop command can be used to kill the still
running application.

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]

lxc-execute command will run the specified command into the 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 the pid 1 and the first process of the application has the pid 2.

lxc-start command will run directly the specified command into 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

CONNECT TO AN AVAILABLE TTY
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.

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 cgroup freezer 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 into a specific container. For this reason, the following commands
may be useful:

lxc-ls
lxc-info -n foo

lxc-ls lists the containers of the system.

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 accept 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 he went to the background.

# 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"

SETTING THE CONTROL GROUP FOR CONTAINER
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.

BUGS

       The lxc is still in development, so the command syntax and the API can change. The version 1.0.0 will  be
       the frozen version.

AUTHOR

       Daniel Lezcano <daniel.lezcano@free.fr>