Code:
lxc(7) lxc(7)
NAME
lxc - linux containers
OVERVIEW
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 con‐
tainer object which provides full resource isolation and resource con‐
trol 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.
REQUIREMENTS
The kernel version >= 3.10 shipped with the distros, will work with
lxc, this one will have less functionalities but enough to be interest‐
ing.
lxc relies on a set of functionalities provided by the kernel. The
helper script lxc-checkconfig will give you information about your ker‐
nel configuration, required, and missing features.
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 a pa‐
rameter 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 re‐
mount /var/run with an empty directory. If you want to avoid the con‐
flicts 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 run‐
ning an application. Why? Because you don't have to care about the re‐
sources to be isolated, everything needs to be isolated, the other re‐
sources are specified as being isolated but without configuration be‐
cause 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 informa‐
tion. 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 com‐
mand. 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 pa‐
rameter.
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 com‐
mands. If the container was not created before starting the applica‐
tion, 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 applica‐
tion 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 speci‐
fied 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 con‐
tainer, 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 use‐
ful:
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 $i
done
MONITORING CONTAINER
It is sometime useful to track the states of a container, for example
to monitor it or just to wait for a specific state in a script.
lxc-monitor command will monitor one or several containers. The parame‐
ter of this command accepts a regular expression for example:
lxc-monitor -n "foo|bar"
will monitor the states of containers named 'foo' and 'bar', and:
lxc-monitor -n ".*"
will monitor all the containers.
For a container 'foo' starting, doing some work and exiting, the output
will be in the form:
'foo' changed state to [STARTING]
'foo' changed state to [RUNNING]
'foo' changed state to [STOPPING]
'foo' changed state to [STOPPED]
lxc-wait command will wait for a specific state change and exit. This
is useful for scripting to synchronize the launch of a container or the
end. The parameter is an ORed combination of different states. The fol‐
lowing example shows how to wait for a container if it successfully
started as a daemon.
# launch lxc-wait in background
lxc-wait -n foo -s STOPPED &
LXC_WAIT_PID=$!
# 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.
SEE ALSO
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)
AUTHOR
Daniel Lezcano <daniel.lezcano@free.fr>
Christian Brauner <christian.brauner@ubuntu.com>
Serge Hallyn <serge@hallyn.com>
Stéphane Graber <stgraber@ubuntu.com>
Version 3.2.1 2019-10-09 lxc(7)
Iced Blended Vanilla Crème Ubuntu
Originally Posted by 1fallen
I think deadflowr's advice would be something to research.
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