moby--moby/docs/sources/faq.md

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/dotcloud/docker/blob/master/LICENSE](https://github.com/dotcloud/docker/blob/master/LICENSE)

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

> Not at this time, Docker currently only runs 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](http://index.docker.io) 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](http://docs.docker.io/en/latest/use/working_with_links_names/).

Also of useful when enabling more flexible service portability is the
[Ambassador linking
pattern](http://docs.docker.io/en/latest/use/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](http://docs.docker.io/en/latest/examples/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](http://www.docker.io/security/) and report security issues to
this [mailbox](mailto:security%40docker.com).

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

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

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

Definitely! You can fork [the
repo](http://www.github.com/dotcloud/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](http://www.github.com/dotcloud/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 [*Hello
World*](../examples/hello_world/#hello-world) example.