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NAME | OVERVIEW | REQUIREMENTS | FUNCTIONAL SPECIFICATION | SEE ALSO | AUTHOR | COLOPHON |
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LXC(7) LXC(7)
lxc - linux containers
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.
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.
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.
lxc(7), lxc-create(1), lxc-copy(1), lxc-destroy(1), lxc-start(1),
lxc-stop(1), lxc-execute(1), lxc-console(1), lxc-monitor(1),
lxc-wait(1), lxc-cgroup(1), lxc-ls(1), lxc-info(1), lxc-freeze(1),
lxc-unfreeze(1), lxc-attach(1), lxc.conf(5)
Daniel Lezcano <daniel.lezcano@free.fr>
Christian Brauner <christian.brauner@ubuntu.com>
Serge Hallyn <serge@hallyn.com>
Stéphane Graber <stgraber@ubuntu.com>
This page is part of the lxc (Linux containers) project. Information
about the project can be found at ⟨http://linuxcontainers.org/⟩. If
you have a bug report for this manual page, send it to
lxc-devel@lists.linuxcontainers.org. This page was obtained from the
project's upstream Git repository ⟨git://github.com/lxc/lxc⟩ on
2018-02-02. (At that time, the date of the most recent commit that
was found in the repository was 2018-02-01.) If you discover any
rendering problems in this HTML version of the page, or you believe
there is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
Version 2.1.0-devel 2018-02-02 LXC(7)
Pages that refer to this page: lxc-attach(1), lxc-autostart(1), lxc-cgroup(1), lxc-checkconfig(1), lxc-checkpoint(1), lxc-config(1), lxc-console(1), lxc-copy(1), lxc-create(1), lxc-destroy(1), lxc-device(1), lxc-execute(1), lxc-freeze(1), lxc-info(1), lxc-ls(1), lxc-monitor(1), lxc-snapshot(1), lxc-start(1), lxc-stop(1), lxc-top(1), lxc-unfreeze(1), lxc-unshare(1), lxc-update-config(1), lxc-usernsexec(1), lxc-wait(1), lxc.container.conf(5), lxc.system.conf(5), lxc(7)
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