Docker Engine frequently asked questions (FAQ)

Estimated reading time: 11 minutes

If you don’t see your question here, feel free to submit new ones to docs@docker.com. Or, you can fork the repo 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

Does Docker run on Linux, macOS, or Windows?

The Docker Engine client runs natively on Linux, macOS, and Windows. By default, these clients connect to a local Docker daemon running in a virtual environment managed by Docker, which provides the required features to run Linux-based containers within OS X or Windows, or Windows-based containers on Windows.

If your version of macOS or Windows does not include the required virtualization technology, you can use Docker Machine to work around these limitations.

You can run Windows-based containers on Windows Server 2016 and Windows 10. Windows-based containers require a Windows kernel to run, in the same way that Linux-based containers require a Linux kernel to run. You can even run Windows-based containers on a Windows virtual machine running on an macOS or Linux host. Docker Machine is not necessary if you run macOS 10.10.3 Yosemite, Windows Server 2016, or Windows 10.

How do containers compare to virtual machines?

Containers and virtual machines (VMs) are complementary. VMs excel at providing extreme isolation (for example with hostile tenant applications where you need the ultimate break out prevention). Containers operate at the process level, which makes them very lightweight and perfect as a unit of software delivery. While VMs take minutes to boot, containers can often be started in less than a second.

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 called a container. This container can be transferred to any Docker-enabled machine. The container can be executed there with the guarantee that the execution environment exposed to the application will be the same in development, tesing, and production. LXC implements process sandboxing, which is an important pre-requisite for portable deployment, but is not sufficient 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. The app you sent me is tied to your machine’s specific configuration: networking, storage, logging, etc. Docker defines an abstraction for these machine-specific settings. 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, 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” 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 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 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.

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.

Also useful for more flexible service portability is the Ambassador linking pattern.

How do I run more than one process in a Docker container?

Any capable process supervisor such as 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.

What platforms does Docker run on?

Linux:

  • Any distribution running version 3.10+ of the Linux kernel
  • Specific instructions are available for most Linux distributions, including RHEL, Ubuntu, SuSE, and many others.

Microsoft Windows:

  • Windows Server 2016
  • Windows 10

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 and report security issues to this mailbox.

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

Please read our blog post 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.

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 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 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 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:

Looking for something else to read? Checkout the User Guide.

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