github.com/akashshinde/docker@v1.9.1/docs/misc/faq.md (about) 1 <!--[metadata]> 2 +++ 3 title = "FAQ" 4 description = "Most frequently asked questions." 5 keywords = ["faq, questions, documentation, docker"] 6 [menu.main] 7 parent = "mn_about" 8 weight = 3 9 +++ 10 <![end-metadata]--> 11 12 # Frequently Asked Questions (FAQ) 13 14 If you don't see your question here, feel free to submit new ones to 15 <docs@docker.com>. Or, you can fork [the 16 repo](https://github.com/docker/docker) and contribute them yourself by editing 17 the documentation sources. 18 19 20 ### How much does Docker cost? 21 22 Docker is 100% free. It is open source, so you can use it without paying. 23 24 ### What open source license are you using? 25 26 We are using the Apache License Version 2.0, see it here: 27 [https://github.com/docker/docker/blob/master/LICENSE]( 28 https://github.com/docker/docker/blob/master/LICENSE) 29 30 ### Does Docker run on Mac OS X or Windows? 31 32 Docker currently runs only on Linux, but you can use VirtualBox to run Docker in 33 a virtual machine on your box, and get the best of both worlds. Check out the 34 [*Mac OS X*](../installation/mac.md) and [*Microsoft 35 Windows*](../installation/windows.md) installation guides. The small Linux 36 distribution Docker Machine can be run inside virtual machines on these two 37 operating systems. 38 39 >**Note:** if you are using a remote Docker daemon on a VM through Docker 40 >Machine, then _do not_ type the `sudo` before the `docker` commands shown in 41 >the documentation's examples. 42 43 ### How do containers compare to virtual machines? 44 45 They are complementary. VMs are best used to allocate chunks of hardware 46 resources. Containers operate at the process level, which makes them very 47 lightweight and perfect as a unit of software delivery. 48 49 ### What does Docker add to just plain LXC? 50 51 Docker is not a replacement for LXC. "LXC" refers to capabilities of the Linux 52 kernel (specifically namespaces and control groups) which allow sandboxing 53 processes from one another, and controlling their resource allocations. On top 54 of this low-level foundation of kernel features, Docker offers a high-level tool 55 with several powerful functionalities: 56 57 - *Portable deployment across machines.* Docker defines a format for bundling 58 an application and all its dependencies into a single object which can be 59 transferred to any Docker-enabled machine, and executed there with the 60 guarantee that the execution environment exposed to the application will be the 61 same. LXC implements process sandboxing, which is an important pre-requisite 62 for portable deployment, but that alone is not enough for portable deployment. 63 If you sent me a copy of your application installed in a custom LXC 64 configuration, it would almost certainly not run on my machine the way it does 65 on yours, because it is tied to your machine's specific configuration: 66 networking, storage, logging, distro, etc. Docker defines an abstraction for 67 these machine-specific settings, so that the exact same Docker container can 68 run - unchanged - on many different machines, with many different 69 configurations. 70 71 - *Application-centric.* Docker is optimized for the deployment of 72 applications, as opposed to machines. This is reflected in its API, user 73 interface, design philosophy and documentation. By contrast, the `lxc` helper 74 scripts focus on containers as lightweight machines - basically servers that 75 boot faster and need less RAM. We think there's more to containers than just 76 that. 77 78 - *Automatic build.* Docker includes [*a tool for developers to automatically 79 assemble a container from their source 80 code*](../reference/builder.md), with full control over application 81 dependencies, build tools, packaging etc. They are free to use `make`, `maven`, 82 `chef`, `puppet`, `salt,` Debian packages, RPMs, source tarballs, or any 83 combination of the above, regardless of the configuration of the machines. 84 85 - *Versioning.* Docker includes git-like capabilities for tracking successive 86 versions of a container, inspecting the diff between versions, committing new 87 versions, rolling back etc. The history also includes how a container was 88 assembled and by whom, so you get full traceability from the production server 89 all the way back to the upstream developer. Docker also implements incremental 90 uploads and downloads, similar to `git pull`, so new versions of a container 91 can be transferred by only sending diffs. 92 93 - *Component re-use.* Any container can be used as a [*"base image"*](../reference/glossary.md#image) to create more specialized components. This can 94 be done manually or as part of an automated build. For example you can prepare 95 the ideal Python environment, and use it as a base for 10 different 96 applications. Your ideal PostgreSQL setup can be re-used for all your future 97 projects. And so on. 98 99 - *Sharing.* Docker has access to a public registry [on Docker Hub](https://hub.docker.com/) 100 where thousands of people have uploaded useful images: anything from Redis, 101 CouchDB, PostgreSQL to IRC bouncers to Rails app servers to Hadoop to base 102 images for various Linux distros. The 103 [*registry*](https://docs.docker.com/registry/) also 104 includes an official "standard library" of useful containers maintained by the 105 Docker team. The registry itself is open-source, so anyone can deploy their own 106 registry to store and transfer private containers, for internal server 107 deployments for example. 108 109 - *Tool ecosystem.* Docker defines an API for automating and customizing the 110 creation and deployment of containers. There are a huge number of tools 111 integrating with Docker to extend its capabilities. PaaS-like deployment 112 (Dokku, Deis, Flynn), multi-node orchestration (Maestro, Salt, Mesos, Openstack 113 Nova), management dashboards (docker-ui, Openstack Horizon, Shipyard), 114 configuration management (Chef, Puppet), continuous integration (Jenkins, 115 Strider, Travis), etc. Docker is rapidly establishing itself as the standard 116 for container-based tooling. 117 118 ### What is different between a Docker container and a VM? 119 120 There's a great StackOverflow answer [showing the differences]( 121 http://stackoverflow.com/questions/16047306/how-is-docker-io-different-from-a-normal-virtual-machine). 122 123 ### Do I lose my data when the container exits? 124 125 Not at all! Any data that your application writes to disk gets preserved in its 126 container until you explicitly delete the container. The file system for the 127 container persists even after the container halts. 128 129 ### How far do Docker containers scale? 130 131 Some of the largest server farms in the world today are based on containers. 132 Large web deployments like Google and Twitter, and platform providers such as 133 Heroku and dotCloud all run on container technology, at a scale of hundreds of 134 thousands or even millions of containers running in parallel. 135 136 ### How do I connect Docker containers? 137 138 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](https://docs.docker.com/networking). 139 140 Also useful for more flexible service portability is the [Ambassador linking 141 pattern](../articles/ambassador_pattern_linking.md). 142 143 ### How do I run more than one process in a Docker container? 144 145 Any capable process supervisor such as [http://supervisord.org/]( 146 http://supervisord.org/), runit, s6, or daemontools can do the trick. Docker 147 will start up the process management daemon which will then fork to run 148 additional processes. As long as the processor manager daemon continues to run, 149 the container will continue to as well. You can see a more substantial example 150 [that uses supervisord here](../articles/using_supervisord.md). 151 152 ### What platforms does Docker run on? 153 154 Linux: 155 156 - Ubuntu 12.04, 13.04 et al 157 - Fedora 19/20+ 158 - RHEL 6.5+ 159 - CentOS 6+ 160 - Gentoo 161 - ArchLinux 162 - openSUSE 12.3+ 163 - CRUX 3.0+ 164 165 Cloud: 166 167 - Amazon EC2 168 - Google Compute Engine 169 - Microsoft Azure 170 - Rackspace 171 172 ### How do I report a security issue with Docker? 173 174 You can learn about the project's security policy 175 [here](https://www.docker.com/security/) and report security issues to this 176 [mailbox](mailto:security@docker.com). 177 178 ### Why do I need to sign my commits to Docker with the DCO? 179 180 Please read [our blog post]( 181 http://blog.docker.com/2014/01/docker-code-contributions-require-developer-certificate-of-origin/) on the introduction of the DCO. 182 183 ### When building an image, should I prefer system libraries or bundled ones? 184 185 *This is a summary of a discussion on the [docker-dev mailing list]( 186 https://groups.google.com/forum/#!topic/docker-dev/L2RBSPDu1L0).* 187 188 Virtually all programs depend on third-party libraries. Most frequently, they 189 will use dynamic linking and some kind of package dependency, so that when 190 multiple programs need the same library, it is installed only once. 191 192 Some programs, however, will bundle their third-party libraries, because they 193 rely on very specific versions of those libraries. For instance, Node.js bundles 194 OpenSSL; MongoDB bundles V8 and Boost (among others). 195 196 When creating a Docker image, is it better to use the bundled libraries, or 197 should you build those programs so that they use the default system libraries 198 instead? 199 200 The key point about system libraries is not about saving disk or memory space. 201 It is about security. All major distributions handle security seriously, by 202 having dedicated security teams, following up closely with published 203 vulnerabilities, and disclosing advisories themselves. (Look at the [Debian 204 Security Information](https://www.debian.org/security/) for an example of those 205 procedures.) Upstream developers, however, do not always implement similar 206 practices. 207 208 Before setting up a Docker image to compile a program from source, if you want 209 to use bundled libraries, you should check if the upstream authors provide a 210 convenient way to announce security vulnerabilities, and if they update their 211 bundled libraries in a timely manner. If they don't, you are exposing yourself 212 (and the users of your image) to security vulnerabilities. 213 214 Likewise, before using packages built by others, you should check if the 215 channels providing those packages implement similar security best practices. 216 Downloading and installing an "all-in-one" .deb or .rpm sounds great at first, 217 except if you have no way to figure out that it contains a copy of the OpenSSL 218 library vulnerable to the [Heartbleed](http://heartbleed.com/) bug. 219 220 ### Why is `DEBIAN_FRONTEND=noninteractive` discouraged in Dockerfiles? 221 222 When building Docker images on Debian and Ubuntu you may have seen errors like: 223 224 unable to initialize frontend: Dialog 225 226 These errors don't stop the image from being built but inform you that the 227 installation process tried to open a dialog box, but was unable to. Generally, 228 these errors are safe to ignore. 229 230 Some people circumvent these errors by changing the `DEBIAN_FRONTEND` 231 environment variable inside the Dockerfile using: 232 233 ENV DEBIAN_FRONTEND=noninteractive 234 235 This prevents the installer from opening dialog boxes during installation which 236 stops the errors. 237 238 While this may sound like a good idea, it *may* have side effects. The 239 `DEBIAN_FRONTEND` environment variable will be inherited by all images and 240 containers built from your image, effectively changing their behavior. People 241 using those images will run into problems when installing software 242 interactively, because installers will not show any dialog boxes. 243 244 Because of this, and because setting `DEBIAN_FRONTEND` to `noninteractive` is 245 mainly a 'cosmetic' change, we *discourage* changing it. 246 247 If you *really* need to change its setting, make sure to change it back to its 248 [default value](https://www.debian.org/releases/stable/i386/ch05s03.html.en) 249 afterwards. 250 251 ### Why do I get `Connection reset by peer` when making a request to a service running in a container? 252 253 Typically, this message is returned if the service is already bound to your 254 localhost. As a result, requests coming to the container from outside are 255 dropped. To correct this problem, change the service's configuration on your 256 localhost so that the service accepts requests from all IPs. If you aren't sure 257 how to do this, check the documentation for your OS. 258 259 260 ### Where can I find more answers? 261 262 You can find more answers on: 263 264 265 - [Docker user mailinglist](https://groups.google.com/d/forum/docker-user) 266 - [Docker developer mailinglist](https://groups.google.com/d/forum/docker-dev) 267 - [IRC, docker on freenode](irc://chat.freenode.net#docker) 268 - [GitHub](https://github.com/docker/docker) 269 - [Ask questions on Stackoverflow](http://stackoverflow.com/search?q=docker) 270 - [Join the conversation on Twitter](http://twitter.com/docker) 271 272 Looking for something else to read? Checkout the [User Guide](../userguide/).