Home / Linux / Everything You Need to Know about Linux Containers, Part II: Working with Linux Containers (LXC)

Everything You Need to Know about Linux Containers, Part II: Working with Linux Containers (LXC)

Part I of this Deep Dive on containers introduces
the idea of kernel control groups, or cgroups, and the way you can isolate,
limit and monitor selected userspace applications. Here,
I dive a bit deeper and focus on the next step of process
isolation—that is, through containers, and more specifically, the Linux
Containers (LXC) framework.

Containers are about as close to bare metal as you can get when running
virtual machines. They impose very little to no overhead when hosting virtual
instances. First introduced in 2008, LXC adopted much of its functionality
from the Solaris Containers (or Solaris Zones) and FreeBSD jails that
preceded it. Instead of creating a full-fledged virtual machine, LXC enables
a virtual environment with its own process and network space. Using
namespaces to enforce process isolation and leveraging the kernel’s very own
control groups (cgroups) functionality, the feature limits, accounts for and isolates CPU, memory, disk I/O and network usage of one or more
processes. Think of this userspace framework as a very advanced form of

Note: LXC uses namespaces to enforce process isolation, alongside the kernel’s very
own cgroups to account for and limit CPU, memory, disk I/O and network usage
across one or more processes.

But what exactly are containers? The short answer is that containers decouple software
applications from the operating system, giving users a clean and minimal
Linux environment while running everything else in one or more isolated
“containers”. The purpose of a container is to launch a limited set
of applications or services (often referred to as microservices) and have
them run within a self-contained sandboxed environment.

Note: the purpose of a container is to launch a limited set of applications or
services and have them run within a self-contained sandboxed environment.


Figure 1. A Comparison of
Applications Running in a Traditional Environment to Containers

This isolation prevents processes running within a given container from
monitoring or affecting processes running in another container. Also, these
containerized services do not influence or disturb the host machine. The idea
of being able to consolidate many services scattered across multiple physical
servers into one is one of the many reasons data centers have chosen to adopt
the technology.

Container features include the following:

  • Security: network services can be run in a container, which limits the
    damage caused by a security breach or violation. An intruder who
    successfully exploits a security hole on one of the applications running in
    that container is restricted to the set of actions possible within that
  • Isolation: containers allow the deployment of one or more applications on
    the same physical machine, even if those applications must operate under
    different domains, each requiring exclusive access to its respective
    resources. For instance, multiple applications running in different
    containers can bind to the same physical network interface by using
    distinct IP addresses associated with each container.
  • Virtualization and transparency: containers provide the system with a
    virtualized environment that can hide or limit the visibility of the
    physical devices or system’s configuration underneath it. The general
    principle behind a container is to avoid changing the environment in which
    applications are running with the exception of addressing security or
    isolation issues.

Using the LXC Utilities

For most modern Linux distributions, the kernel is enabled with cgroups, but
you most likely still will need to install the LXC utilities.

If you’re using Red Hat or CentOS, you’ll need to install the EPEL
repositories first. For other distributions, such as Ubuntu or Debian, simply type:

$ sudo apt-get install lxc

Now, before you start tinkering with those utilities, you need to
configure your environment. And before doing that, you need to verify that your
current user has both a uid and gid entry defined in /etc/subuid
and /etc/subgid:

$ cat /etc/subuid
$ cat /etc/subgid

Create the ~/.config/lxc directory if it doesn’t already exist,
and copy the /etc/lxc/default.conf configuration file to
~/.config/lxc/default.conf. Append the following two lines to
the end of the file:

lxc.id_map = u 0 100000 65536
lxc.id_map = g 0 100000 65536

It should look something like this:

$ cat ~/.config/lxc/default.conf
lxc.network.type = veth
lxc.network.link = lxcbr0
lxc.network.flags = up
lxc.network.hwaddr = 00:16:3e:xx:xx:xx
lxc.id_map = u 0 100000 65536
lxc.id_map = g 0 100000 65536

Append the following to the /etc/lxc/lxc-usernet file (replace
the first column with your user name):

petros veth lxcbr0 10

The quickest way for these settings to take effect is either to reboot the
node or log the user out and then log back in.

Once logged back in, verify that the veth networking driver is currently

$ lsmod|grep veth
veth                   16384  0

If it isn’t, type:

$ sudo modprobe veth

You now can use the LXC utilities to download, run and manage Linux

Next, download a container image and name it
“example-container”. When you type the following command, you’ll
see a long list of supported containers under many Linux
distributions and versions:

$ sudo lxc-create -t download -n example-container

You’ll be given three prompts to pick the distribution, release and
architecture. I chose the following:

Distribution: ubuntu
Release: xenial
Architecture: amd64

Once you make a decision and press Enter, the rootfs will be
downloaded locally and configured. For security reasons, each container does
not ship with an OpenSSH server or user accounts. A default root password
also is not provided. In order to change the root password and log in, you
must run either an lxc-attach or chroot into the container directory path (after
it has been started).

Start the container:

$ sudo lxc-start -n example-container -d

The -d option dæmonizes the container, and it will run in the background. If
you want to observe the boot process, replace the -d with
-F, and it will
run in the foreground, ending at a login prompt.

You may experience an error similar to the following:

$ sudo lxc-start -n example-container -d
lxc-start: tools/lxc_start.c: main: 366 The container
failed to start.
lxc-start: tools/lxc_start.c: main: 368 To get more details,
run the container in foreground mode.
lxc-start: tools/lxc_start.c: main: 370 Additional information
can be obtained by setting the --logfile and --logpriority

If you do, you’ll need to debug it by running the lxc-start service in
the foreground:

$ sudo lxc-start -n example-container -F
lxc-start: conf.c: instantiate_veth: 2685 failed to create veth
 pair (vethQ4NS0B and vethJMHON2): Operation not supported
    lxc-start: conf.c: lxc_create_network: 3029 failed to
    create netdev
    lxc-start: start.c: lxc_spawn: 1103 Failed to create
    the network.
    lxc-start: start.c: __lxc_start: 1358 Failed to spawn
    container "example-container".
    lxc-start: tools/lxc_start.c: main: 366 The container failed
    to start.
    lxc-start: tools/lxc_start.c: main: 370 Additional information
    can be obtained by setting the --logfile and --logpriority

From the example above, you can see that the veth module
probably isn’t
inserted. After inserting it, it resolved the issue.

Anyway, open up a second terminal window and verify the status of the

$ sudo lxc-info -n example-container
Name:           example-container
State:          RUNNING
PID:            1356
CPU use:        0.29 seconds
BlkIO use:      16.80 MiB
Memory use:     29.02 MiB
KMem use:       0 bytes
Link:           vethPRK7YU
 TX bytes:      1.34 KiB
 RX bytes:      2.09 KiB
 Total bytes:   3.43 KiB

Another way to do this is by running the following command to list all
installed containers:

$ sudo lxc-ls -f
example-container RUNNING 0         - -

But there’s a problem—you still can’t log in! Attach directly to the
running container, create your users and change all relevant passwords using
the passwd command:

$ sudo lxc-attach -n example-container
[email protected]:/#
[email protected]:/# useradd petros
[email protected]:/# passwd petros
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

Once the passwords are changed, you’ll be able to log in directly to the
container from a console and without the lxc-attach command:

$ sudo lxc-console -n example-container

If you want to connect to this running container over the network, install
the OpenSSH server:

[email protected]:/# apt-get install openssh-server

Grab the container’s local IP address:

[email protected]:/# ip addr show eth0|grep inet
    inet brd scope global eth0
    inet6 fe80::216:3eff:fed8:53b4/64 scope link

Then from the host machine and in a new console window, type:

$ ssh

Voilà! You now can ssh in to the running container and type your user name and

On the host system, and not within the container, it’s interesting to observe
which LXC processes are initiated and running after launching a container:

$ ps aux|grep lxc|grep -v grep
root       861  0.0  0.0 234772  1368 ?        Ssl  11:01
 ↪0:00 /usr/bin/lxcfs /var/lib/lxcfs/
lxc-dns+  1155  0.0  0.1  52868  2908 ?        S    11:01
 ↪0:00 dnsmasq -u lxc-dnsmasq --strict-order
 ↪--bind-interfaces --pid-file=/run/lxc/dnsmasq.pid
 ↪--listen-address --dhcp-range,
 ↪--dhcp-lease-max=253 --dhcp-no-override
 ↪--except-interface=lo --interface=lxcbr0
root      1196  0.0  0.1  54484  3928 ?        Ss   11:01
 ↪0:00 [lxc monitor] /var/lib/lxc example-container
root      1658  0.0  0.1  54780  3960 pts/1    S+   11:02
 ↪0:00 sudo lxc-attach -n example-container
root      1660  0.0  0.2  54464  4900 pts/1    S+   11:02
 ↪0:00 lxc-attach -n example-container

To stop a container, type (from the host machine):

$ sudo lxc-stop -n example-container

Once stopped, verify the state of the container:

$ sudo lxc-ls -f
example-container STOPPED 0         -      -    -

$ sudo lxc-info -n example-container
Name:           example-container
State:          STOPPED

To destroy a container completely—that is, purge it from the host system—type:

$ sudo lxc-destroy -n example-container
Destroyed container example-container

Once destroyed, verify that it has been removed:

$ sudo lxc-info -n example-container
example-container doesn't exist

$ sudo lxc-ls -f

Note: if you attempt to destroy a running container, the command will fail
and inform you that the container is still running:

$ sudo lxc-destroy -n example-container
example-container is running

A container must be stopped before it is destroyed.

Advanced Configurations

At times, it may be necessary to configure one or more containers to
accomplish one or more tasks. LXC simplifies this by having the administrator
modify the container’s configuration file located in

$ sudo su
# cd /var/lib/lxc
# ls

The container’s parent directory will consist of at least two files: 1)
the container config file and 2) the container’s entire rootfs:

# cd example-container/
# ls
config  rootfs

Say you want to autostart the container labeled example-container
on host system boot up. To do this, you’d need to append the
following lines to the container’s configuration file,

# Enable autostart
lxc.start.auto = 1

After you restart the container or reboot the host system, you should see
something like this:

$ sudo lxc-ls -f
example-container RUNNING 1         - -

Notice how the AUTOSTART field is now set to “1”.

If, on container boot up, you want the container to bind mount a directory path
living on the host machine, append the following lines to the same file:

# Bind mount system path to local path
lxc.mount.entry = /mnt mnt none bind 0 0

With the above example and when the container gets restarted, you’ll see
the contents of the host’s /mnt directory accessible to the
container’s local /mnt directory.

Privileged vs. Unprivileged Containers

You often may stumble across LXC-related content discussing the idea of a
privileged container and an unprivileged container. But what are those
exactly? The concept is pretty straightforward, and an LXC container can run in
either configuration.

By design, an unprivileged container is considered safer and more secure than
a privileged one. An unprivileged container runs with a mapping of the
container’s root UID to a non-root UID on the host system. This makes it
more difficult for attackers compromising a container to gain root
privileges to the underlying host machine. In short, if attackers manage
to compromise your container through, for example, a known software
vulnerability, they immediately will find themselves with no rights on the
host machine.

Privileged containers can and will expose a system to such attacks. That’s
why it’s good practice to run few containers in privileged mode. Identify
the containers that require privileged access, and be sure to make extra
efforts to update routinely and lock them down in other ways.

And, What about Docker?

I spent a considerable amount of time talking about Linux Containers,
but what about Docker? It is the most deployed container
solution in production. Since its initial launch, Docker has taken the Linux
computing world by storm. Docker is an Apache-licensed open-source
containerization technology designed to automate the repetitive task of
creating and deploying microservices inside containers. Docker treats
containers as if they were extremely lightweight and modular virtual
machines. Initially, Docker was built on top of LXC, but it has since moved
away from that dependency, resulting in a better developer and user
experience. Much like LXC, Docker continues to make use of the kernel cgroup
subsystem. The technology is more than just running containers, it also eases
the process of creating containers, building images, sharing those built
images and versioning them.

Docker primarily focuses on:

  • Portability: Docker provides an image-based deployment model. This type of
    portability allows for an easier way to share an application or set of
    services (with all of their dependencies) across multiple environments.
  • Version control: a single Docker image is made up of a series of combined
    layers. A new layer is created whenever the image is altered. For instance,
    a new layer is created every time a user specifies a command, such as
    run or
    copy. Docker will reuse these layers for new container builds. Layering to
    Docker is its very own method of version control.
  • Rollback: again, every Docker image has layers. If you don’t want to use
    the currently running layer, you can roll back to a previous version.
    This type of agility makes it easier for software developers to integrate
    and deploy their software technology continuously.
  • Rapid deployment: provisioning new hardware often can take days. And, the
    amount of effort and overhead to get it installed and configured is quite
    burdensome. With Docker, you can avoid all of that by reducing the time it
    takes to get an image up and running to a matter of seconds. When you’re
    done with a container, you can destroy it just as easily.

Fundamentally, both Docker and LXC are very similar. They both are userspace
and lightweight virtualization platforms that implement cgroups and
namespaces to manage resource isolation. However, there are a number of
distinct differences between the two.

Process Management

Docker restricts containers to run as a single process. If your application
consists of X number of concurrent processes, Docker will want you to run X
number of containers, each with its own distinct process. This is not the
case with LXC, which runs a container with a conventional init process and,
in turn, can host multiple processes inside that same container. For example,
if you want to host a LAMP (Linux + Apache + MySQL + PHP) server, each
process for each application will need to span across multiple Docker

State Management

Docker is designed to be stateless, meaning it doesn’t support persistent
storage. There are ways around this, but again, it’s only necessary when the
process requires it. When a Docker image is created, it will consist of
read-only layers. This will not change. During runtime, if the process of the
container makes any changes to its internal state, a diff between the
internal state and the current state of the image will be maintained until
either a commit is made to the Docker image (creating a new layer) or until
the container is deleted, resulting in that diff disappearing.


This word tends to be overused when discussing Docker—that’s because
it’s the single-most important advantage Docker has over LXC. Docker does a
much better job of abstracting away the networking, storage and operating
system details from the application. This results in a truly
configuration-independent application, guaranteeing that the environment for
the application always will remain the same, regardless of the machine on
which it is enabled.

Docker is designed to benefit both developers and system administrators. It
has made itself an integral part of many DevOps (developers + operations)
toolchains. Developers can focus on writing code without having to worry
about the system ultimately hosting it. With Docker, there’s no need to
install and configure complex databases or worry about switching between
incompatible language toolchain versions. Docker gives the operations staff
flexibility, often reducing the number of physical systems needed to host
some of the smaller and more basic applications. Docker streamlines software
delivery. New features and bug/security fixes reach the customer quickly
without any hassle, surprises or downtime.


Isolating processes for the sake of infrastructure security and system
stability isn’t as painful as it sounds. The Linux kernel provides all
the necessary facilities to enable simple-to-use userspace applications, such
as LXC (and even Docker), to manage micro-instances of an operating system
with its local services in an isolated and sandboxed environment.

In Part III of this series, I describe container
orchestration with Kubernetes.



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