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# LXC

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LXC is the well-known and heavily tested low-level Linux container runtime. It
is in active development since 2008 and has proven itself in critical
production environments world-wide. Some of its core contributors are the same
people that helped to implement various well-known containerization features
inside the Linux kernel.

## System Containers

LXC's main focus is system containers. That is, containers which offer an
environment as close as possible as the one you'd get from a VM but without the
overhead that comes with running a separate kernel and simulating all the
hardware.

This is achieved through a combination of kernel security features such as
namespaces, mandatory access control and control groups.

## Unprivileged Containers

Unprivileged containers are containers that are run without any privilege. This
requires support for user namespaces in the kernel that the container is run
on. LXC was the first runtime to support unprivileged containers after user
namespaces were merged into the mainline kernel.

In essence, user namespaces isolate given sets of UIDs and GIDs. This is
achieved by establishing a mapping between a range of UIDs and GIDs on the host
to a different (unprivileged) range of UIDs and GIDs in the container. The
kernel will translate this mapping in such a way that inside the container all
UIDs and GIDs appear as you would expect from the host whereas on the host
these UIDs and GIDs are in fact unprivileged. For example, a process running as
UID and GID 0 inside the container might appear as UID and GID 100000 on the
host. The implementation and working details can be gathered from the
corresponding user namespace man page.

Since unprivileged containers are a security enhancement they naturally come
with a few restrictions enforced by the kernel. In order to provide a fully
functional unprivileged container LXC interacts with 3 pieces of setuid code:

- lxc-user-nic (setuid helper to create a veth pair and bridge it on the host)
- newuidmap (from the shadow package, sets up a uid map)
- newgidmap (from the shadow package, sets up a gid map)

Everything else is run as your own user or as a uid which your user owns.

In general, LXC's goal is to make use of every security feature available in
the kernel. This means LXC's configuration management will allow experienced
users to intricately tune LXC to their needs.

A more detailed introduction into LXC security can be found under the following link

- https://linuxcontainers.org/lxc/security/

### Removing all Privilege

In principle LXC can be run without any of these tools provided the correct
configuration is applied. However, the usefulness of such containers is usually
quite restricted. Just to highlight the two most common problems:

1. Network: Without relying on a setuid helper to setup appropriate network
   devices for an unprivileged user (see LXC's `lxc-user-nic` binary) the only
   option is to share the network namespace with the host. Although this should
   be secure in principle, sharing the host's network namespace is still one
   step of isolation less and increases the attack vector.
   Furthermore, when host and container share the same network namespace the
   kernel will refuse any sysfs mounts. This usually means that the init binary
   inside of the container will not be able to boot up correctly.

2. User Namespaces: As outlined above, user namespaces are a big security
   enhancement. However, without relying on privileged helpers users who are
   unprivileged on the host are only permitted to map their own UID into
   a container. A standard POSIX system however, requires 65536 UIDs and GIDs
   to be available to guarantee full functionality.

## Configuration

LXC is configured via a simple set of keys. For example,

- `lxc.rootfs`
- `lxc.mount.entry`

LXC namespaces configuration keys by using single dots. This means complex
configuration keys such as `lxc.net.0` expose various subkeys such as
`lxc.net.0.type`, `lxc.net.0.link`, `lxc.net.0.ipv6`, and others for even
more fine-grained configuration.

LXC is used as the default runtime for [LXD](https://github.com/lxc/lxd),
a container hypervisor exposing a well-designed and stable REST-api on top of
it.

## Kernel Requirements

LXC runs on any kernel from 2.6.32 onwards. All it requires is a functional
C compiler. LXC works on all architectures that provide the necessary kernel
features. This includes (but isn't limited to):

- i686
- x86_64
- ppc, ppc64, ppc64le
- s390x
- armvl7, arm64

LXC also supports at least the following C standard libraries:

- glibc
- musl
- bionic (Android's libc)

## Backwards Compatibility

LXC has always focused on strong backwards compatibility. In fact, the API
hasn't been broken from release `1.0.0` onwards. Main LXC is currently at
version `2.*.*`.

## Reporting Security Issues

The LXC project has a good reputation in handling security issues quickly and
efficiently. If you think you've found a potential security issue, please
report it by e-mail to all of the following persons:

- serge.hallyn (at) ubuntu (dot) com
- stgraber (at) ubuntu (dot) com
- christian.brauner (at) ubuntu (dot) com

For further details please have a look at

- https://linuxcontainers.org/lxc/security/

## Becoming Active in LXC development

We always welcome new contributors and are happy to provide guidance when
necessary. LXC follows the kernel coding conventions. This means we only
require that each commit includes a `Signed-off-by` line. The coding style we
use is identical to the one used by the Linux kernel. You can find a detailed
introduction at:

- https://www.kernel.org/doc/html/v4.10/process/coding-style.html

and should also take a look at the [CONTRIBUTING](CONTRIBUTING) file in this
repo.

If you want to become more active it is usually also a good idea to show up in
the LXC IRC channel `#lxc-dev` on `Freenode`. We try to do all development out
in the open and discussion of new features or bugs is done either in
appropriate Github issues or on IRC.

When thinking about making security critical contributions or substantial
changes it is usually a good idea to ping the developers first and ask whether
a PR would be accepted.

## Semantic Versioning

LXC and its related projects strictly adhere to a [semantic
versioning](http://semver.org/) scheme.

## Downloading the current source code

Source for the latest released version can always be downloaded from

- https://linuxcontainers.org/downloads/

You can browse the up to the minute source code and change history online

- https://github.com/lxc/lxc

## Building LXC

Without considering distribution specific details a simple

    ./autogen.sh && ./configure && make && sudo make install

is usually sufficient.

In order to test current git master of LXC it is usually a good idea to compile with

    ./autogen.sh && ./configure && make

in a convenient directory and set `LD_LIBRARY_PATH="${BUILD_DIR}"/lxc/src/lxc/.libs`.

## Getting help

When you find you need help, the LXC projects provides you with several options.

### Discuss Forum

We maintain an discuss forum at

- https://discuss.linuxcontainers.org/

where you can get support.

### IRC

You can find support by joining `#lxcontainers` on `Freenode`.

### Mailing Lists

You can check out one of the two LXC mailing list archives and register if
interested:

- http://lists.linuxcontainers.org/listinfo/lxc-devel
- http://lists.linuxcontainers.org/listinfo/lxc-users