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>