@@ -1,15 +1,17 @@

-page_title: FAQ

-page_description: Most frequently asked questions.

+page_title: FAQ page_description: Most frequently asked questions.

 page_keywords: faq, questions, documentation, docker

-# FAQ

+# 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.

-## Most frequently asked questions

 ### How much does Docker cost?

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

-paying.

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

 ### What open source license are you using?

@@ -19,213 +21,194 @@ 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.

+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.

 {{ include "no-remote-sudo.md" }}

 ### 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.

+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.

+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).

+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.

+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.

+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).

+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/).

+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/).

+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+

+ - 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

+ - 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).

+[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.

+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.

+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.

+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?

@@ -234,46 +217,48 @@ 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.

+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` 

+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.

+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.

+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) 

+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?

+### 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.

-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)

+

+- [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/).

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