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 >= 2.6.27 shipped with the distros, will work with
lxc, this one will have less functionalities but enough to be
interesting. With the kernel 2.6.29,
lxc is fully functional. The
helper script
lxc-checkconfig will give you information about your
kernel configuration.
Before using the
lxc, your system should be configured with the file
capabilities, otherwise you will need to run the
lxc commands as root.
The control group can be mounted anywhere, eg:
mount -t cgroup cgroup
/cgroup. If you want to dedicate a specific cgroup mount point for
lxc, that is to have different cgroups mounted at different places with
different options but let
lxc to use one location, you can bind the
mount point with the
lxc name, eg:
mount -t cgroup lxc
/cgroup4lxc or
mount -t cgroup -ons,cpuset,freezer,devices lxc
/cgroup4lxc
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
/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.
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 allow 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.
SEE ALSO¶
lxc(7),
lxc-create(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)
AUTHOR¶
Daniel Lezcano <daniel.lezcano@free.fr>
