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lxc(7) - phpMan
lxc(7) lxc(7)
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 tem‐
plate, 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 appli‐
cations 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 Check‐
point/Restart new features. The lxc is small enough to easily manage a container with sim‐
ple 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 num‐
ber 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, other‐
wise 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 sys‐
tem 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, espe‐
cially 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 con‐
tainer. 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 configura‐
tion 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 exam‐
ple, 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 con‐
tainer 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 sys‐
tem. For this reason, there are two different commands to run an application into a con‐
tainer:
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 com‐
mand. 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 allow to list all the contain‐
ers 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 syn‐
tax 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-con‐
sole(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 AT free.fr>
Version 1.0.6 Sat Apr 29 06:45:43 UTC 2017 lxc(7)
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