# Apply custom metadata You can apply metadata to your images, containers, or daemons via labels. Labels serve a wide range of uses, such as adding notes or licensing information to an image, or to identify a host. A label is a `` / `` pair. Docker stores the label values as *strings*. You can specify multiple labels but each `` must be unique or the value will be overwritten. If you specify the same `key` several times but with different values, newer labels overwrite previous labels. Docker uses the last `key=value` you supply. >**Note:** Support for daemon-labels was added in Docker 1.4.1. Labels on >containers and images are new in Docker 1.6.0 ## Label keys (namespaces) Docker puts no hard restrictions on the `key` used for a label. However, using simple keys can easily lead to conflicts. For example, you have chosen to categorize your images by CPU architecture using "architecture" labels in your Dockerfiles: LABEL architecture="amd64" LABEL architecture="ARMv7" Another user may apply the same label based on a building's "architecture": LABEL architecture="Art Nouveau" To prevent naming conflicts, Docker recommends using namespaces to label keys using reverse domain notation. Use the following guidelines to name your keys: - All (third-party) tools should prefix their keys with the reverse DNS notation of a domain controlled by the author. For example, `com.example.some-label`. - The `com.docker.*`, `io.docker.*` and `org.dockerproject.*` namespaces are reserved for Docker's internal use. - Keys should only consist of lower-cased alphanumeric characters, dots and dashes (for example, `[a-z0-9-.]`). - Keys should start *and* end with an alpha numeric character. - Keys may not contain consecutive dots or dashes. - Keys *without* namespace (dots) are reserved for CLI use. This allows end- users to add metadata to their containers and images without having to type cumbersome namespaces on the command-line. These are simply guidelines and Docker does not *enforce* them. However, for the benefit of the community, you *should* use namespaces for your label keys. ## Store structured data in labels Label values can contain any data type as long as it can be represented as a string. For example, consider this JSON document: { "Description": "A containerized foobar", "Usage": "docker run --rm example/foobar [args]", "License": "GPL", "Version": "0.0.1-beta", "aBoolean": true, "aNumber" : 0.01234, "aNestedArray": ["a", "b", "c"] } You can store this struct in a label by serializing it to a string first: LABEL com.example.image-specs="{\"Description\":\"A containerized foobar\",\"Usage\":\"docker run --rm example\\/foobar [args]\",\"License\":\"GPL\",\"Version\":\"0.0.1-beta\",\"aBoolean\":true,\"aNumber\":0.01234,\"aNestedArray\":[\"a\",\"b\",\"c\"]}" While it is *possible* to store structured data in label values, Docker treats this data as a 'regular' string. This means that Docker doesn't offer ways to query (filter) based on nested properties. If your tool needs to filter on nested properties, the tool itself needs to implement this functionality. ## Add labels to images To add labels to an image, use the `LABEL` instruction in your Dockerfile: LABEL [.][=] ... The `LABEL` instruction adds a label to your image, optionally with a value. Use surrounding quotes or backslashes for labels that contain white space characters in the ``: LABEL vendor=ACME\ Incorporated LABEL com.example.version.is-beta LABEL com.example.version="0.0.1-beta" LABEL com.example.release-date="2015-02-12" The `LABEL` instruction also supports setting multiple `` / `` pairs in a single instruction: LABEL com.example.version="0.0.1-beta" com.example.release-date="2015-02-12" Long lines can be split up by using a backslash (`\`) as continuation marker: LABEL vendor=ACME\ Incorporated \ com.example.is-beta \ com.example.version="0.0.1-beta" \ com.example.release-date="2015-02-12" Docker recommends you add multiple labels in a single `LABEL` instruction. Using individual instructions for each label can result in an inefficient image. This is because each `LABEL` instruction in a Dockerfile produces a new IMAGE layer. You can view the labels via the `docker inspect` command: $ docker inspect 4fa6e0f0c678 ... "Labels": { "vendor": "ACME Incorporated", "com.example.is-beta": "", "com.example.version": "0.0.1-beta", "com.example.release-date": "2015-02-12" } ... # Inspect labels on container $ docker inspect -f "{{json .Config.Labels }}" 4fa6e0f0c678 {"Vendor":"ACME Incorporated","com.example.is-beta":"","com.example.version":"0.0.1-beta","com.example.release-date":"2015-02-12"} # Inspect labels on images $ docker inspect -f "{{json .ContainerConfig.Labels }}" myimage ## Query labels Besides storing metadata, you can filter images and containers by label. To list all running containers that have the `com.example.is-beta` label: # List all running containers that have a `com.example.is-beta` label $ docker ps --filter "label=com.example.is-beta" List all running containers with the label `color` that have a value `blue`: $ docker ps --filter "label=color=blue" List all images with the label `vendor` that have the value `ACME`: $ docker images --filter "label=vendor=ACME" ## Container labels docker run \ -d \ --label com.example.group="webservers" \ --label com.example.environment="production" \ busybox \ top Please refer to the [Query labels](#query-labels) section above for information on how to query labels set on a container. ## Daemon labels docker daemon \ --dns 8.8.8.8 \ --dns 8.8.4.4 \ -H unix:///var/run/docker.sock \ --label com.example.environment="production" \ --label com.example.storage="ssd" These labels appear as part of the `docker info` output for the daemon: $ docker -D info Containers: 12 Images: 672 Server Version: 1.9.0 Storage Driver: aufs Root Dir: /var/lib/docker/aufs Backing Filesystem: extfs Dirs: 697 Dirperm1 Supported: true Execution Driver: native-0.2 Logging Driver: json-file Kernel Version: 3.19.0-22-generic Operating System: Ubuntu 15.04 CPUs: 24 Total Memory: 62.86 GiB Name: docker ID: I54V:OLXT:HVMM:TPKO:JPHQ:CQCD:JNLC:O3BZ:4ZVJ:43XJ:PFHZ:6N2S Debug mode (server): true File Descriptors: 59 Goroutines: 159 System Time: 2015-09-23T14:04:20.699842089+08:00 EventsListeners: 0 Init SHA1: Init Path: /usr/bin/docker Docker Root Dir: /var/lib/docker Http Proxy: http://test:test@localhost:8080 Https Proxy: https://test:test@localhost:8080 WARNING: No swap limit support Username: svendowideit Registry: [https://index.docker.io/v1/] Labels: com.example.environment=production com.example.storage=ssd