page_title: FAQ
page_description: Most frequently asked questions.
page_keywords: faq, questions, documentation, docker

# FAQ

## Most frequently asked questions

### How much does Docker cost?

Docker is 100% free. It is open source, so you can use it without
paying.

### What open source license are you using?

We are using the Apache License Version 2.0, see it here:
[https://github.com/docker/docker/blob/master/LICENSE](
https://github.com/docker/docker/blob/master/LICENSE)

### Does Docker run on Mac OS X or Windows?

Docker currently runs only on Linux, but you can use VirtualBox to run
Docker in a virtual machine on your box, and get the best of both worlds.
Check out the [*Mac OS X*](../installation/mac/#macosx) and [*Microsoft
Windows*](../installation/windows/#windows) installation guides. The small
Linux distribution boot2docker can be run inside virtual machines on these
two operating systems.

### How do containers compare to virtual machines?

They are complementary. VMs are best used to allocate chunks of
hardware resources. Containers operate at the process level, which
makes them very lightweight and perfect as a unit of software
delivery.

### What does Docker add to just plain LXC?

Docker is not a replacement for LXC. "LXC" refers to capabilities of
the Linux kernel (specifically namespaces and control groups) which
allow sandboxing processes from one another, and controlling their
resource allocations. On top of this low-level foundation of kernel
features, Docker offers a high-level tool with several powerful
functionalities:

 - *Portable deployment across machines.*
   Docker defines a format for bundling an application and all
   its dependencies into a single object which can be transferred
   to any Docker-enabled machine, and executed there with the
   guarantee that the execution environment exposed to the
   application will be the same. LXC implements process
   sandboxing, which is an important pre-requisite for portable
   deployment, but that alone is not enough for portable
   deployment. If you sent me a copy of your application
   installed in a custom LXC configuration, it would almost
   certainly not run on my machine the way it does on yours,
   because it is tied to your machine's specific configuration:
   networking, storage, logging, distro, etc. Docker defines an
   abstraction for these machine-specific settings, so that the
   exact same Docker container can run - unchanged - on many
   different machines, with many different configurations.

 - *Application-centric.*
   Docker is optimized for the deployment of applications, as
   opposed to machines. This is reflected in its API, user
   interface, design philosophy and documentation. By contrast,
   the `lxc` helper scripts focus on
   containers as lightweight machines - basically servers that
   boot faster and need less RAM. We think there's more to
   containers than just that.

 - *Automatic build.*
   Docker includes [*a tool for developers to automatically
   assemble a container from their source
   code*](../reference/builder/#dockerbuilder), with full control
   over application dependencies, build tools, packaging etc.
   They are free to use `make`, `maven`, `chef`, `puppet`, `salt,` 
   Debian packages, RPMs, source tarballs, or any combination of the
   above, regardless of the configuration of the machines.

 - *Versioning.*
   Docker includes git-like capabilities for tracking successive
   versions of a container, inspecting the diff between versions,
   committing new versions, rolling back etc. The history also
   includes how a container was assembled and by whom, so you get
   full traceability from the production server all the way back
   to the upstream developer. Docker also implements incremental
   uploads and downloads, similar to `git pull`, so new versions
   of a container can be transferred by only sending diffs.

 - *Component re-use.*
   Any container can be used as a [*"base image"*](
   ../terms/image/#base-image-def) to create more specialized components.
   This can be done manually or as part of an automated build. For example
   you can prepare the ideal Python environment, and use it as a base for
   10 different applications. Your ideal Postgresql setup can be re-used for
   all your future projects. And so on.

 - *Sharing.*
   Docker has access to a [public registry](https://hub.docker.com) where
   thousands of people have uploaded useful containers: anything from Redis,
   CouchDB, Postgres to IRC bouncers to Rails app servers to Hadoop to
   base images for various Linux distros. The
   [*registry*](../reference/api/registry_index_spec/#registryindexspec)
   also includes an official "standard library" of useful
   containers maintained by the Docker team. The registry itself
   is open-source, so anyone can deploy their own registry to
   store and transfer private containers, for internal server
   deployments for example.

 - *Tool ecosystem.*
   Docker defines an API for automating and customizing the
   creation and deployment of containers. There are a huge number
   of tools integrating with Docker to extend its capabilities.
   PaaS-like deployment (Dokku, Deis, Flynn), multi-node
   orchestration (Maestro, Salt, Mesos, Openstack Nova),
   management dashboards (docker-ui, Openstack Horizon,
   Shipyard), configuration management (Chef, Puppet), continuous
   integration (Jenkins, Strider, Travis), etc. Docker is rapidly
   establishing itself as the standard for container-based
   tooling.

### What is different between a Docker container and a VM?

There's a great StackOverflow answer [showing the differences](
http://stackoverflow.com/questions/16047306/how-is-docker-io-different-from-a-normal-virtual-machine).

### Do I lose my data when the container exits?

Not at all! Any data that your application writes to disk gets preserved
in its container until you explicitly delete the container. The file
system for the container persists even after the container halts.

### How far do Docker containers scale?

Some of the largest server farms in the world today are based on
containers. Large web deployments like Google and Twitter, and platform
providers such as Heroku and dotCloud all run on container technology,
at a scale of hundreds of thousands or even millions of containers
running in parallel.

### How do I connect Docker containers?

Currently the recommended way to link containers is via the link
primitive. You can see details of how to [work with links
here](/userguide/dockerlinks).

Also useful for more flexible service portability is the
[Ambassador linking pattern](/articles/ambassador_pattern_linking/).

### How do I run more than one process in a Docker container?

Any capable process supervisor such as [http://supervisord.org/](
http://supervisord.org/), runit, s6, or daemontools can do the trick.
Docker will start up the process management daemon which will then fork
to run additional processes. As long as the processor manager daemon continues
to run, the container will continue to as well. You can see a more substantial
example [that uses supervisord here](/articles/using_supervisord/).

### What platforms does Docker run on?

Linux:

 - Ubuntu 12.04, 13.04 et al
 - Fedora 19/20+
 - RHEL 6.5+
 - Centos 6+
 - Gentoo
 - ArchLinux
 - openSUSE 12.3+
 - CRUX 3.0+

Cloud:

 - Amazon EC2
 - Google Compute Engine
 - Rackspace

### How do I report a security issue with Docker?

You can learn about the project's security policy
[here](https://www.docker.com/security/) and report security issues to
this [mailbox](mailto:security@docker.com).

### Why do I need to sign my commits to Docker with the DCO?

Please read [our blog post](
http://blog.docker.com/2014/01/docker-code-contributions-require-developer-certificate-of-origin/)
on the introduction of the DCO.

### When building an image, should I prefer system libraries or bundled ones?

*This is a summary of a discussion on the [docker-dev mailing list](
https://groups.google.com/forum/#!topic/docker-dev/L2RBSPDu1L0).*

Virtually all programs depend on third-party libraries. Most frequently,
they will use dynamic linking and some kind of package dependency, so
that when multiple programs need the same library, it is installed only once.

Some programs, however, will bundle their third-party libraries, because
they rely on very specific versions of those libraries. For instance,
Node.js bundles OpenSSL; MongoDB bundles V8 and Boost (among others).

When creating a Docker image, is it better to use the bundled libraries,
or should you build those programs so that they use the default system
libraries instead?

The key point about system libraries is not about saving disk or memory
space. It is about security. All major distributions handle security
seriously, by having dedicated security teams, following up closely
with published vulnerabilities, and disclosing advisories themselves.
(Look at the [Debian Security Information](https://www.debian.org/security/)
for an example of those procedures.) Upstream developers, however,
do not always implement similar practices.

Before setting up a Docker image to compile a program from source,
if you want to use bundled libraries, you should check if the upstream
authors provide a convenient way to announce security vulnerabilities,
and if they update their bundled libraries in a timely manner. If they
don't, you are exposing yourself (and the users of your image) to
security vulnerabilities.

Likewise, before using packages built by others, you should check if the
channels providing those packages implement similar security best practices.
Downloading and installing an "all-in-one" .deb or .rpm sounds great at first,
except if you have no way to figure out that it contains a copy of the
OpenSSL library vulnerable to the [Heartbleed](http://heartbleed.com/) bug.

### Why is `DEBIAN_FRONTEND=noninteractive` discouraged in Dockerfiles?

When building Docker images on Debian and Ubuntu you may have seen errors like:

    unable to initialize frontend: Dialog

These errors don't stop the image from being built but inform you that the
installation process tried to open a dialog box, but was unable to. 
Generally, these errors are safe to ignore.

Some people circumvent these errors by changing the `DEBIAN_FRONTEND` 
environment variable inside the Dockerfile using:

    ENV DEBIAN_FRONTEND=noninteractive

This prevents the installer from opening dialog boxes during installation 
which stops the errors.

While this may sound like a good idea, it *may* have side effects. 
The `DEBIAN_FRONTEND` environment variable will be inherited by all 
images and containers built from your image, effectively changing
their behavior. People using those images will run into problems when
installing software interactively, because installers will not show
any dialog boxes.

Because of this, and because setting `DEBIAN_FRONTEND` to `noninteractive` is
mainly a 'cosmetic' change, we *discourage* changing it.

If you *really* need to change its setting, make sure to change it
back to its [default value](https://www.debian.org/releases/stable/i386/ch05s03.html.en) 
afterwards.

### Can I help by adding some questions and answers?

Definitely! You can fork [the repo](https://github.com/docker/docker) and
edit the documentation sources.

### Where can I find more answers?

You can find more answers on:

- [Docker user mailinglist](https://groups.google.com/d/forum/docker-user)
- [Docker developer mailinglist](https://groups.google.com/d/forum/docker-dev)
- [IRC, docker on freenode](irc://chat.freenode.net#docker)
- [GitHub](https://github.com/docker/docker)
- [Ask questions on Stackoverflow](http://stackoverflow.com/search?q=docker)
- [Join the conversation on Twitter](http://twitter.com/docker)

Looking for something else to read? Checkout the [User
Guide](/userguide/).
