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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/use/working_with_links_names/).
Also of useful when enabling more flexible service portability is the
[Ambassador linking pattern](
http://docs.docker.io/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/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](https://www.docker.io/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.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](https://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](https://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.