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Adding info to faq per customer problem.
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@ -1,15 +1,17 @@
page_title: FAQ page_title: FAQ page_description: Most frequently asked questions.
page_description: Most frequently asked questions.
page_keywords: faq, questions, documentation, docker 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? ### How much does Docker cost?
Docker is 100% free. It is open source, so you can use it without Docker is 100% free. It is open source, so you can use it without paying.
paying.
### What open source license are you using? ### 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? ### Does Docker run on Mac OS X or Windows?
Docker currently runs only on Linux, but you can use VirtualBox to run Docker currently runs only on Linux, but you can use VirtualBox to run Docker in
Docker in a virtual machine on your box, and get the best of both worlds. a virtual machine on your box, and get the best of both worlds. Check out the
Check out the [*Mac OS X*](../installation/mac/#macosx) and [*Microsoft [*Mac OS X*](../installation/mac/#macosx) and [*Microsoft
Windows*](../installation/windows/#windows) installation guides. The small Windows*](../installation/windows/#windows) installation guides. The small Linux
Linux distribution boot2docker can be run inside virtual machines on these distribution boot2docker can be run inside virtual machines on these two
two operating systems. operating systems.
{{ include "no-remote-sudo.md" }} {{ include "no-remote-sudo.md" }}
### How do containers compare to virtual machines? ### How do containers compare to virtual machines?
They are complementary. VMs are best used to allocate chunks of They are complementary. VMs are best used to allocate chunks of hardware
hardware resources. Containers operate at the process level, which resources. Containers operate at the process level, which makes them very
makes them very lightweight and perfect as a unit of software lightweight and perfect as a unit of software delivery.
delivery.
### What does Docker add to just plain LXC? ### What does Docker add to just plain LXC?
Docker is not a replacement for LXC. "LXC" refers to capabilities of Docker is not a replacement for LXC. "LXC" refers to capabilities of the Linux
the Linux kernel (specifically namespaces and control groups) which kernel (specifically namespaces and control groups) which allow sandboxing
allow sandboxing processes from one another, and controlling their processes from one another, and controlling their resource allocations. On top
resource allocations. On top of this low-level foundation of kernel of this low-level foundation of kernel features, Docker offers a high-level tool
features, Docker offers a high-level tool with several powerful with several powerful functionalities:
functionalities:
- *Portable deployment across machines.* - *Portable deployment across machines.* Docker defines a format for bundling
Docker defines a format for bundling an application and all an application and all its dependencies into a single object which can be
its dependencies into a single object which can be transferred transferred to any Docker-enabled machine, and executed there with the
to any Docker-enabled machine, and executed there with the guarantee that the execution environment exposed to the application will be the
guarantee that the execution environment exposed to the same. LXC implements process sandboxing, which is an important pre-requisite
application will be the same. LXC implements process for portable deployment, but that alone is not enough for portable deployment.
sandboxing, which is an important pre-requisite for portable If you sent me a copy of your application installed in a custom LXC
deployment, but that alone is not enough for portable configuration, it would almost certainly not run on my machine the way it does
deployment. If you sent me a copy of your application on yours, because it is tied to your machine's specific configuration:
installed in a custom LXC configuration, it would almost networking, storage, logging, distro, etc. Docker defines an abstraction for
certainly not run on my machine the way it does on yours, these machine-specific settings, so that the exact same Docker container can
because it is tied to your machine's specific configuration: run - unchanged - on many different machines, with many different
networking, storage, logging, distro, etc. Docker defines an configurations.
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.* - *Application-centric.* Docker is optimized for the deployment of
Docker is optimized for the deployment of applications, as applications, as opposed to machines. This is reflected in its API, user
opposed to machines. This is reflected in its API, user interface, design philosophy and documentation. By contrast, the `lxc` helper
interface, design philosophy and documentation. By contrast, scripts focus on containers as lightweight machines - basically servers that
the `lxc` helper scripts focus on boot faster and need less RAM. We think there's more to containers than just
containers as lightweight machines - basically servers that that.
boot faster and need less RAM. We think there's more to
containers than just that.
- *Automatic build.* - *Automatic build.* Docker includes [*a tool for developers to automatically
Docker includes [*a tool for developers to automatically assemble a container from their source
assemble a container from their source code*](../reference/builder/#dockerbuilder), with full control over application
code*](../reference/builder/#dockerbuilder), with full control dependencies, build tools, packaging etc. They are free to use `make`, `maven`,
over application dependencies, build tools, packaging etc. `chef`, `puppet`, `salt,` Debian packages, RPMs, source tarballs, or any
They are free to use `make`, `maven`, `chef`, `puppet`, `salt,` combination of the above, regardless of the configuration of the machines.
Debian packages, RPMs, source tarballs, or any combination of the
above, regardless of the configuration of the machines.
- *Versioning.* - *Versioning.* Docker includes git-like capabilities for tracking successive
Docker includes git-like capabilities for tracking successive versions of a container, inspecting the diff between versions, committing new
versions of a container, inspecting the diff between versions, versions, rolling back etc. The history also includes how a container was
committing new versions, rolling back etc. The history also assembled and by whom, so you get full traceability from the production server
includes how a container was assembled and by whom, so you get all the way back to the upstream developer. Docker also implements incremental
full traceability from the production server all the way back uploads and downloads, similar to `git pull`, so new versions of a container
to the upstream developer. Docker also implements incremental can be transferred by only sending diffs.
uploads and downloads, similar to `git pull`, so new versions
of a container can be transferred by only sending diffs.
- *Component re-use.* - *Component re-use.* Any container can be used as a [*"base image"*](
Any container can be used as a [*"base image"*]( ../terms/image/#base-image-def) to create more specialized components. This can
../terms/image/#base-image-def) to create more specialized components. be done manually or as part of an automated build. For example you can prepare
This can be done manually or as part of an automated build. For example the ideal Python environment, and use it as a base for 10 different
you can prepare the ideal Python environment, and use it as a base for applications. Your ideal Postgresql setup can be re-used for all your future
10 different applications. Your ideal Postgresql setup can be re-used for projects. And so on.
all your future projects. And so on.
- *Sharing.* - *Sharing.* Docker has access to a [public registry](https://hub.docker.com)
Docker has access to a [public registry](https://hub.docker.com) where where thousands of people have uploaded useful containers: anything from Redis,
thousands of people have uploaded useful containers: anything from Redis, CouchDB, Postgres to IRC bouncers to Rails app servers to Hadoop to base images
CouchDB, Postgres to IRC bouncers to Rails app servers to Hadoop to for various Linux distros. The
base images for various Linux distros. The [*registry*](../reference/api/registry_index_spec/#registryindexspec) also
[*registry*](../reference/api/registry_index_spec/#registryindexspec) includes an official "standard library" of useful containers maintained by the
also includes an official "standard library" of useful Docker team. The registry itself is open-source, so anyone can deploy their own
containers maintained by the Docker team. The registry itself registry to store and transfer private containers, for internal server
is open-source, so anyone can deploy their own registry to deployments for example.
store and transfer private containers, for internal server
deployments for example.
- *Tool ecosystem.* - *Tool ecosystem.* Docker defines an API for automating and customizing the
Docker defines an API for automating and customizing the creation and deployment of containers. There are a huge number of tools
creation and deployment of containers. There are a huge number integrating with Docker to extend its capabilities. PaaS-like deployment
of tools integrating with Docker to extend its capabilities. (Dokku, Deis, Flynn), multi-node orchestration (Maestro, Salt, Mesos, Openstack
PaaS-like deployment (Dokku, Deis, Flynn), multi-node Nova), management dashboards (docker-ui, Openstack Horizon, Shipyard),
orchestration (Maestro, Salt, Mesos, Openstack Nova), configuration management (Chef, Puppet), continuous integration (Jenkins,
management dashboards (docker-ui, Openstack Horizon, Strider, Travis), etc. Docker is rapidly establishing itself as the standard
Shipyard), configuration management (Chef, Puppet), continuous for container-based tooling.
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? ### What is different between a Docker container and a VM?
There's a great StackOverflow answer [showing the differences]( 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? ### Do I lose my data when the container exits?
Not at all! Any data that your application writes to disk gets preserved Not at all! Any data that your application writes to disk gets preserved in its
in its container until you explicitly delete the container. The file container until you explicitly delete the container. The file system for the
system for the container persists even after the container halts. container persists even after the container halts.
### How far do Docker containers scale? ### How far do Docker containers scale?
Some of the largest server farms in the world today are based on Some of the largest server farms in the world today are based on containers.
containers. Large web deployments like Google and Twitter, and platform Large web deployments like Google and Twitter, and platform providers such as
providers such as Heroku and dotCloud all run on container technology, Heroku and dotCloud all run on container technology, at a scale of hundreds of
at a scale of hundreds of thousands or even millions of containers thousands or even millions of containers running in parallel.
running in parallel.
### How do I connect Docker containers? ### How do I connect Docker containers?
Currently the recommended way to link containers is via the link Currently the recommended way to link containers is via the link primitive. You
primitive. You can see details of how to [work with links can see details of how to [work with links here](/userguide/dockerlinks).
here](/userguide/dockerlinks).
Also useful for more flexible service portability is the Also useful for more flexible service portability is the [Ambassador linking
[Ambassador linking pattern](/articles/ambassador_pattern_linking/). pattern](/articles/ambassador_pattern_linking/).
### How do I run more than one process in a Docker container? ### How do I run more than one process in a Docker container?
Any capable process supervisor such as [http://supervisord.org/]( Any capable process supervisor such as [http://supervisord.org/](
http://supervisord.org/), runit, s6, or daemontools can do the trick. http://supervisord.org/), runit, s6, or daemontools can do the trick. Docker
Docker will start up the process management daemon which will then fork will start up the process management daemon which will then fork to run
to run additional processes. As long as the processor manager daemon continues additional processes. As long as the processor manager daemon continues to run,
to run, the container will continue to as well. You can see a more substantial the container will continue to as well. You can see a more substantial example
example [that uses supervisord here](/articles/using_supervisord/). [that uses supervisord here](/articles/using_supervisord/).
### What platforms does Docker run on? ### What platforms does Docker run on?
Linux: Linux:
- Ubuntu 12.04, 13.04 et al - Ubuntu 12.04, 13.04 et al
- Fedora 19/20+ - Fedora 19/20+
- RHEL 6.5+ - RHEL 6.5+
- Centos 6+ - Centos 6+
- Gentoo - Gentoo
- ArchLinux - ArchLinux
- openSUSE 12.3+ - openSUSE 12.3+
- CRUX 3.0+ - CRUX 3.0+
Cloud: Cloud:
- Amazon EC2 - Amazon EC2
- Google Compute Engine - Google Compute Engine
- Rackspace - Rackspace
### How do I report a security issue with Docker? ### How do I report a security issue with Docker?
You can learn about the project's security policy You can learn about the project's security policy
[here](https://www.docker.com/security/) and report security issues to [here](https://www.docker.com/security/) and report security issues to this
this [mailbox](mailto:security@docker.com). [mailbox](mailto:security@docker.com).
### Why do I need to sign my commits to Docker with the DCO? ### Why do I need to sign my commits to Docker with the DCO?
Please read [our blog post]( Please read [our blog post](
http://blog.docker.com/2014/01/docker-code-contributions-require-developer-certificate-of-origin/) http://blog.docker.com/2014/01/docker-code-contributions-require-developer-
on the introduction of the DCO. certificate-of-origin/) on the introduction of the DCO.
### When building an image, should I prefer system libraries or bundled ones? ### 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]( *This is a summary of a discussion on the [docker-dev mailing list](
https://groups.google.com/forum/#!topic/docker-dev/L2RBSPDu1L0).* https://groups.google.com/forum/#!topic/docker-dev/L2RBSPDu1L0).*
Virtually all programs depend on third-party libraries. Most frequently, Virtually all programs depend on third-party libraries. Most frequently, they
they will use dynamic linking and some kind of package dependency, so will use dynamic linking and some kind of package dependency, so that when
that when multiple programs need the same library, it is installed only once. multiple programs need the same library, it is installed only once.
Some programs, however, will bundle their third-party libraries, because Some programs, however, will bundle their third-party libraries, because they
they rely on very specific versions of those libraries. For instance, rely on very specific versions of those libraries. For instance, Node.js bundles
Node.js bundles OpenSSL; MongoDB bundles V8 and Boost (among others). OpenSSL; MongoDB bundles V8 and Boost (among others).
When creating a Docker image, is it better to use the bundled libraries, When creating a Docker image, is it better to use the bundled libraries, or
or should you build those programs so that they use the default system should you build those programs so that they use the default system libraries
libraries instead? instead?
The key point about system libraries is not about saving disk or memory The key point about system libraries is not about saving disk or memory space.
space. It is about security. All major distributions handle security It is about security. All major distributions handle security seriously, by
seriously, by having dedicated security teams, following up closely having dedicated security teams, following up closely with published
with published vulnerabilities, and disclosing advisories themselves. vulnerabilities, and disclosing advisories themselves. (Look at the [Debian
(Look at the [Debian Security Information](https://www.debian.org/security/) Security Information](https://www.debian.org/security/) for an example of those
for an example of those procedures.) Upstream developers, however, procedures.) Upstream developers, however, do not always implement similar
do not always implement similar practices. practices.
Before setting up a Docker image to compile a program from source, Before setting up a Docker image to compile a program from source, if you want
if you want to use bundled libraries, you should check if the upstream to use bundled libraries, you should check if the upstream authors provide a
authors provide a convenient way to announce security vulnerabilities, convenient way to announce security vulnerabilities, and if they update their
and if they update their bundled libraries in a timely manner. If they bundled libraries in a timely manner. If they don't, you are exposing yourself
don't, you are exposing yourself (and the users of your image) to (and the users of your image) to security vulnerabilities.
security vulnerabilities.
Likewise, before using packages built by others, you should check if the Likewise, before using packages built by others, you should check if the
channels providing those packages implement similar security best practices. channels providing those packages implement similar security best practices.
Downloading and installing an "all-in-one" .deb or .rpm sounds great at first, 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 except if you have no way to figure out that it contains a copy of the OpenSSL
OpenSSL library vulnerable to the [Heartbleed](http://heartbleed.com/) bug. library vulnerable to the [Heartbleed](http://heartbleed.com/) bug.
### Why is `DEBIAN_FRONTEND=noninteractive` discouraged in Dockerfiles? ### Why is `DEBIAN_FRONTEND=noninteractive` discouraged in Dockerfiles?
@ -234,46 +217,49 @@ When building Docker images on Debian and Ubuntu you may have seen errors like:
unable to initialize frontend: Dialog unable to initialize frontend: Dialog
These errors don't stop the image from being built but inform you that the 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. installation process tried to open a dialog box, but was unable to. Generally,
Generally, these errors are safe to ignore. 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: environment variable inside the Dockerfile using:
ENV DEBIAN_FRONTEND=noninteractive ENV DEBIAN_FRONTEND=noninteractive
This prevents the installer from opening dialog boxes during installation This prevents the installer from opening dialog boxes during installation which
which stops the errors. stops the errors.
While this may sound like a good idea, it *may* have side effects. While this may sound like a good idea, it *may* have side effects. The
The `DEBIAN_FRONTEND` environment variable will be inherited by all `DEBIAN_FRONTEND` environment variable will be inherited by all images and
images and containers built from your image, effectively changing containers built from your image, effectively changing their behavior. People
their behavior. People using those images will run into problems when using those images will run into problems when installing software
installing software interactively, because installers will not show interactively, because installers will not show any dialog boxes.
any dialog boxes.
Because of this, and because setting `DEBIAN_FRONTEND` to `noninteractive` is Because of this, and because setting `DEBIAN_FRONTEND` to `noninteractive` is
mainly a 'cosmetic' change, we *discourage* changing it. mainly a 'cosmetic' change, we *discourage* changing it.
If you *really* need to change its setting, make sure to change it If you *really* need to change its setting, make sure to change it back to its
back to its [default value](https://www.debian.org/releases/stable/i386/ch05s03.html.en) [default value](https://www.debian.org/releases/stable/i386/ch05s03.html.en)
afterwards. 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. If you aren't sure
how to do this, check the documentation for your OS.
Definitely! You can fork [the repo](https://github.com/docker/docker) and
edit the documentation sources.
### Where can I find more answers? ### Where can I find more answers?
You can find more answers on: 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) - [Docker user mailinglist](https://groups.google.com/d/forum/docker-user)
- [IRC, docker on freenode](irc://chat.freenode.net#docker) - [Docker developer mailinglist](https://groups.google.com/d/forum/docker-dev)
- [GitHub](https://github.com/docker/docker) - [IRC, docker on freenode](irc://chat.freenode.net#docker)
- [Ask questions on Stackoverflow](http://stackoverflow.com/search?q=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) - [Join the conversation on Twitter](http://twitter.com/docker)
Looking for something else to read? Checkout the [User Looking for something else to read? Checkout the [User Guide](/userguide/).
Guide](/userguide/).