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+<!--[metadata]>
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+aliases = ["/engine/misc/faq/"]
+title = "FAQ"
+description = "Most frequently asked questions."
+keywords = ["faq, questions, documentation,  docker"]
+[menu.main]
+identifier="engine_faq"
+parent = "engine_use"
+weight = 80
++++
+<![end-metadata]-->
+
+# Frequently Asked Questions (FAQ)
+
+If you don't see your question here, feel free to submit new ones to
+<docs@docker.com>.  Or, you can fork [the
+repo](https://github.com/docker/docker) and contribute them yourself by editing
+the documentation sources.
+
+
+### How much does Engine cost?
+
+Docker Engine 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 Engine currently runs only on Linux, but you can use VirtualBox to run
+Engine in a virtual machine on your box, and get the best of both worlds. Check
+out the [*Mac OS X*](installation/mac.md) and [*Microsoft
+Windows*](installation/windows.md) installation guides. The small Linux
+distribution boot2docker can be set up using the Docker Machine tool to be run
+inside virtual machines on these two operating systems.
+
+>**Note:** if you are using a remote Docker Engine daemon on a VM through Docker
+>Machine, then _do not_ type the `sudo` before the `docker` commands shown in
+>the documentation's examples.
+
+### 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 technology add to just plain LXC?
+
+Docker technology 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.md), 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"*](reference/glossary.md#image) 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 [on Docker Hub](https://hub.docker.com/)
+ where thousands of people have uploaded useful images: anything from Redis,
+ CouchDB, PostgreSQL to IRC bouncers to Rails app servers to Hadoop to base
+ images for various Linux distros. The
+ [*registry*](https://docs.docker.com/registry/) 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 connect containers is via the Docker network feature. You can see details of how to [work with Docker networks here](userguide/networking/work-with-networks.md).
+
+Also useful for more flexible service portability is the [Ambassador linking
+pattern](admin/ambassador_pattern_linking.md).
+
+### 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](admin/using_supervisord.md).
+
+### 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
+ - Microsoft Azure
+ - 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.
+
+### Why do I get `Connection reset by peer` when making a request to a service running in a container?
+
+Typically, this message is returned if the service is already bound to your
+localhost. As a result, requests coming to the container from outside are
+dropped. To correct this problem, change the service's configuration on your
+localhost so that the service accepts requests from all IPs.  If you aren't sure
+how to do this, check the documentation for your OS.
+
+### Why do I get `Cannot connect to the Docker daemon. Is the docker daemon running on this host?` when using docker-machine?
+
+This error points out that the docker client cannot connect to the virtual machine.
+This means that either the virtual machine that works underneath `docker-machine`
+is not running or that the client doesn't correctly point at it.
+
+To verify that the docker machine is running you can use the `docker-machine ls`
+command and start it with `docker-machine start` if needed.
+
+    $ docker-machine ls
+    NAME             ACTIVE   DRIVER       STATE     URL   SWARM                   DOCKER    ERRORS
+    default          -        virtualbox   Stopped                                 Unknown
+
+    $ docker-machine start default
+
+You have to tell Docker to talk to that machine. You can do this with the
+`docker-machine env` command. For example,
+
+    $ eval "$(docker-machine env default)"
+    $ docker ps
+
+### 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/index.md).