Table of contents
Does Docker run on Linux, macOS, and Windows?
You can run both Linux and Windows programs and executables in Docker containers. The Docker platform runs natively on Linux (on x86-64, ARM and many other CPU architectures) and on Windows (x86-64).
Docker Inc. builds products that let you build and run containers on Linux, Windows and macOS.
What does Docker technology add to just plain LXC?
Docker technology is not a replacement forLXC. "LXC" refers to capabilities ofthe Linux kernel (specifically namespaces and control groups) which allowsandboxing processes from one another, and controlling their resourceallocations. On top of this low-level foundation of kernel features, Dockeroffers a high-level tool with several powerful functionalities:
Portable deployment across machines. Docker defines a format for bundlingan application and all its dependencies into a single object called a container. This container can betransferred to any Docker-enabled machine. The container can be executed there with theguarantee that the execution environment exposed to the application is thesame in development, testing, and production. LXC implements process sandboxing, which is an important pre-requisitefor portable deployment, but is not sufficient for portable deployment.If you sent me a copy of your application installed in a custom LXCconfiguration, it would almost certainly not run on my machine the way it doeson yours. The app you sent me is tied to your machine's specific configuration:networking, storage, logging, etc. Docker defines an abstraction forthese machine-specific settings. The exact same Docker container canrun - unchanged - on many different machines, with many differentconfigurations.
Application-centric. Docker is optimized for the deployment ofapplications, as opposed to machines. This is reflected in its API, userinterface, design philosophy and documentation. By contrast, the
lxchelperscripts focus on containers as lightweight machines - basically servers thatboot faster and need less RAM. We think there's more to containers than justthat.Automatic build. Docker includesa tool for developers to automaticallyassemble a container from their sourcecode, with full control over applicationdependencies, build tools, packaging etc. They are free to use
make,maven,chef,puppet,salt,Debian packages, RPMs, source tarballs, or anycombination of the above, regardless of the configuration of the machines.Versioning. Docker includes git-like capabilities for tracking successiveversions of a container, inspecting the diff between versions, committing newversions, rolling back etc. The history also includes how a container wasassembled and by whom, so you get full traceability from the production serverall the way back to the upstream developer. Docker also implements incrementaluploads and downloads, similar to
git pull, so new versions of a containercan be transferred by only sending diffs.Component re-use. Any container can be used as aparent image tocreate more specialized components. This can be done manually or as part of anautomated build. For example you can prepare the ideal Python environment, anduse it as a base for 10 different applications. Your ideal PostgreSQL setup canbe re-used for all your future projects. And so on.
Sharing. Docker has access to a public registryon DockerHub where thousands ofpeople have uploaded useful images: anything from Redis, CouchDB, PostgreSQL toIRC bouncers to Rails app servers to Hadoop to base images for various Linuxdistros. The registry also includes an official "standardlibrary" of useful containers maintained by the Docker team. The registry itselfis open-source, so anyone can deploy their own registry to store and transferprivate containers, for internal server deployments for example.
Tool ecosystem. Docker defines an API for automating and customizing thecreation and deployment of containers. There are a huge number of toolsintegrating with Docker to extend its capabilities. PaaS-like deployment(Dokku, Deis, Flynn), multi-node orchestration (Maestro, Salt, Mesos, OpenstackNova), management dashboards (docker-ui, Openstack Horizon, Shipyard),configuration management (Chef, Puppet), continuous integration (Jenkins,Strider, Travis), etc. Docker is rapidly establishing itself as the standardfor container-based tooling.
What is different between a Docker container and a VM?
There's a great StackOverflow answershowing 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 itscontainer until you explicitly delete the container. The file system for thecontainer 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 asHeroku run on container technology, at a scale of hundreds ofthousands or even millions of containers.
How do I connect Docker containers?
Currently the recommended way to connect containers is via the Docker networkfeature. You can see details ofhow to work with Docker networks.
How do I run more than one process in a Docker container?
This approach is discouraged for most use cases. For maximum efficiency andisolation, each container should address one specific area of concern. However,if you need to run multiple services within a single container, seeRun multiple services in a container.
How do I report a security issue with Docker?
You can learn about the project's security policyhere and report security issues to thismailbox.
Why do I need to sign my commits to Docker with the DCO?
Readour 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 thedocker-dev mailing list.
Virtually all programs depend on third-party libraries. Most frequently, theyuse dynamic linking and some kind of package dependency, so that whenmultiple programs need the same library, it is installed only once.
Some programs, however, bundle their third-party libraries, because theyrely on very specific versions of those libraries.
When creating a Docker image, is it better to use the bundled libraries, orshould you build those programs so that they use the default system librariesinstead?
The key point about system libraries is not about saving disk or memory space.It is about security. All major distributions handle security seriously, byhaving dedicated security teams, following up closely with publishedvulnerabilities, and disclosing advisories themselves. (Look at theDebianSecurity Informationfor an example of those procedures.) Upstream developers, however, do not alwaysimplement similar practices.
Before setting up a Docker image to compile a program from source, if you wantto use bundled libraries, you should check if the upstream authors provide aconvenient way to announce security vulnerabilities, and if they update theirbundled 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 thechannels 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 OpenSSLlibrary vulnerable to theHeartbleed 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: DialogThese errors don't stop the image from being built but inform you that theinstallation process tried to open a dialog box, but couldn't. Generally,these errors are safe to ignore.
Some people circumvent these errors by changing the DEBIAN_FRONTENDenvironment variable inside the Dockerfile using:
ENV DEBIAN_FRONTEND=noninteractiveThis prevents the installer from opening dialog boxes during installation whichstops the errors.
While this may sound like a good idea, it may have side effects. TheDEBIAN_FRONTEND environment variable is inherited by all images andcontainers built from your image, effectively changing their behavior. Peopleusing those images run into problems when installing softwareinteractively, because installers do not show any dialog boxes.
Because of this, and because setting DEBIAN_FRONTEND to noninteractive ismainly a 'cosmetic' change, we discourage changing it.
If you really need to change its setting, make sure to change it back to itsdefault valueafterwards.
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 yourlocalhost. As a result, requests coming to the container from outside aredropped. To correct this problem, change the service's configuration on yourlocalhost so that the service accepts requests from all IPs. If you aren't surehow 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 virtualmachine. This means that either the virtual machine that works underneathdocker-machine is not running or that the client doesn't correctly point atit.
To verify that the docker machine is running you can use the docker-machine lscommand and start it with docker-machine start if needed.
$ docker-machine lsNAME ACTIVE DRIVER STATE URL SWARM DOCKER ERRORSdefault - virtualbox Stopped Unknown$ docker-machine start defaultYou need to tell Docker to talk to that machine. You can do this with thedocker-machine env command. For example,
$ eval "$(docker-machine env default)"$ docker psWhere can I find more answers?
You can find more answers on:
