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content/en/docs/concepts/containers/_index.md
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title: "Containers"
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content/en/docs/concepts/containers/container-environment-variables.md
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- thockin
title: Container Environment Variables
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This page describes the resources available to Containers in the Container environment.
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## Container environment
The Kubernetes Container environment provides several important resources to Containers:
* A filesystem, which is a combination of an [image](/docs/concepts/containers/images/) and one or more [volumes](/docs/concepts/storage/volumes/).
* Information about the Container itself.
* Information about other objects in the cluster.
### Container information
The *hostname* of a Container is the name of the Pod in which the Container is running.
It is available through the `hostname` command or the
[`gethostname`](http://man7.org/linux/man-pages/man2/gethostname.2.html)
function call in libc.
The Pod name and namespace are available as environment variables through the
[downward API](/docs/tasks/inject-data-application/downward-api-volume-expose-pod-information/).
User defined environment variables from the Pod definition are also available to the Container,
as are any environment variables specified statically in the Docker image.
### Cluster information
A list of all services that were running when a Container was created is available to that Container as environment variables.
Those environment variables match the syntax of Docker links.
For a service named *foo* that maps to a Container named *bar*,
the following variables are defined:
```shell
FOO_SERVICE_HOST=<the host the service is running on>
FOO_SERVICE_PORT=<the port the service is running on>
```
Services have dedicated IP addresses and are available to the Container via DNS,
if [DNS addon](http://releases.k8s.io/{{< param "githubbranch" >}}/cluster/addons/dns/) is enabled. 
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* Learn more about [Container lifecycle hooks](/docs/concepts/containers/container-lifecycle-hooks/).
* Get hands-on experience
[attaching handlers to Container lifecycle events](/docs/tasks/configure-pod-container/attach-handler-lifecycle-event/).
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content/en/docs/concepts/containers/container-lifecycle-hooks.md
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title: Container Lifecycle Hooks
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This page describes how kubelet managed Containers can use the Container lifecycle hook framework
to run code triggered by events during their management lifecycle.
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## Overview
Analogous to many programming language frameworks that have component lifecycle hooks, such as Angular,
Kubernetes provides Containers with lifecycle hooks.
The hooks enable Containers to be aware of events in their management lifecycle
and run code implemented in a handler when the corresponding lifecycle hook is executed.
## Container hooks
There are two hooks that are exposed to Containers:
`PostStart`
This hook executes immediately after a container is created.
However, there is no guarantee that the hook will execute before the container ENTRYPOINT.
No parameters are passed to the handler.
`PreStop`
This hook is called immediately before a container is terminated.
It is blocking, meaning it is synchronous,
so it must complete before the call to delete the container can be sent.
No parameters are passed to the handler.
A more detailed description of the termination behavior can be found in
[Termination of Pods](/docs/concepts/workloads/pods/pod/#termination-of-pods).
### Hook handler implementations
Containers can access a hook by implementing and registering a handler for that hook.
There are two types of hook handlers that can be implemented for Containers:
* Exec - Executes a specific command, such as `pre-stop.sh`, inside the cgroups and namespaces of the Container.
Resources consumed by the command are counted against the Container.
* HTTP - Executes an HTTP request against a specific endpoint on the Container.
### Hook handler execution
When a Container lifecycle management hook is called,
the Kubernetes management system executes the handler in the Container registered for that hook. 
Hook handler calls are synchronous within the context of the Pod containing the Container.
This means that for a `PostStart` hook,
the Container ENTRYPOINT and hook fire asynchronously.
However, if the hook takes too long to run or hangs,
the Container cannot reach a `running` state.
The behavior is similar for a `PreStop` hook.
If the hook hangs during execution,
the Pod phase stays in a `Terminating` state and is killed after `terminationGracePeriodSeconds` of pod ends.
If a `PostStart` or `PreStop` hook fails,
it kills the Container.
Users should make their hook handlers as lightweight as possible.
There are cases, however, when long running commands make sense,
such as when saving state prior to stopping a Container.
### Hook delivery guarantees
Hook delivery is intended to be *at least once*,
which means that a hook may be called multiple times for any given event,
such as for `PostStart` or `PreStop`.
It is up to the hook implementation to handle this correctly.
Generally, only single deliveries are made.
If, for example, an HTTP hook receiver is down and is unable to take traffic,
there is no attempt to resend.
In some rare cases, however, double delivery may occur.
For instance, if a kubelet restarts in the middle of sending a hook,
the hook might be resent after the kubelet comes back up.
### Debugging Hook handlers
The logs for a Hook handler are not exposed in Pod events.
If a handler fails for some reason, it broadcasts an event.
For `PostStart`, this is the `FailedPostStartHook` event,
and for `PreStop`, this is the `FailedPreStopHook` event.
You can see these events by running `kubectl describe pod <pod_name>`.
Here is some example output of events from running this command:
```
Events:
FirstSeen LastSeen Count From SubobjectPath Type Reason Message
--------- -------- ----- ---- ------------- -------- ------ -------
1m 1m 1 {default-scheduler } Normal Scheduled Successfully assigned test-1730497541-cq1d2 to gke-test-cluster-default-pool-a07e5d30-siqd
1m 1m 1 {kubelet gke-test-cluster-default-pool-a07e5d30-siqd} spec.containers{main} Normal Pulling pulling image "test:1.0"
1m 1m 1 {kubelet gke-test-cluster-default-pool-a07e5d30-siqd} spec.containers{main} Normal Created Created container with docker id 5c6a256a2567; Security:[seccomp=unconfined]
1m 1m 1 {kubelet gke-test-cluster-default-pool-a07e5d30-siqd} spec.containers{main} Normal Pulled Successfully pulled image "test:1.0"
1m 1m 1 {kubelet gke-test-cluster-default-pool-a07e5d30-siqd} spec.containers{main} Normal Started Started container with docker id 5c6a256a2567
38s 38s 1 {kubelet gke-test-cluster-default-pool-a07e5d30-siqd} spec.containers{main} Normal Killing Killing container with docker id 5c6a256a2567: PostStart handler: Error executing in Docker Container: 1
37s 37s 1 {kubelet gke-test-cluster-default-pool-a07e5d30-siqd} spec.containers{main} Normal Killing Killing container with docker id 8df9fdfd7054: PostStart handler: Error executing in Docker Container: 1
38s 37s 2 {kubelet gke-test-cluster-default-pool-a07e5d30-siqd} Warning FailedSync Error syncing pod, skipping: failed to "StartContainer" for "main" with RunContainerError: "PostStart handler: Error executing in Docker Container: 1"
1m 22s 2 {kubelet gke-test-cluster-default-pool-a07e5d30-siqd} spec.containers{main} Warning FailedPostStartHook
```
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* Learn more about the [Container environment](/docs/concepts/containers/container-environment-variables/).
* Get hands-on experience
[attaching handlers to Container lifecycle events](/docs/tasks/configure-pod-container/attach-handler-lifecycle-event/).
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title: Images
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You create your Docker image and push it to a registry before referring to it in a Kubernetes pod.
The `image` property of a container supports the same syntax as the `docker` command does, including private registries and tags.
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## Updating Images
The default pull policy is `IfNotPresent` which causes the Kubelet to skip
pulling an image if it already exists. If you would like to always force a pull,
you can do one of the following:
- set the `imagePullPolicy` of the container to `Always`;
- use `:latest` as the tag for the image to use;
- enable the [AlwaysPullImages](/docs/admin/admission-controllers/#alwayspullimages) admission controller.
If you did not specify tag of your image, it will be assumed as `:latest`, with
pull image policy of `Always` correspondingly.
Note that you should avoid using `:latest` tag, see [Best Practices for Configuration](/docs/concepts/configuration/overview/#container-images) for more information.
## Using a Private Registry
Private registries may require keys to read images from them.
Credentials can be provided in several ways:
- Using Google Container Registry
- Per-cluster
- automatically configured on Google Compute Engine or Google Kubernetes Engine
- all pods can read the project's private registry
- Using AWS EC2 Container Registry (ECR)
- use IAM roles and policies to control access to ECR repositories
- automatically refreshes ECR login credentials
- Using Azure Container Registry (ACR)
- Configuring Nodes to Authenticate to a Private Registry
- all pods can read any configured private registries
- requires node configuration by cluster administrator
- Pre-pulling Images
- all pods can use any images cached on a node
- requires root access to all nodes to setup
- Specifying ImagePullSecrets on a Pod
- only pods which provide own keys can access the private registry
Each option is described in more detail below.
### Using Google Container Registry
Kubernetes has native support for the [Google Container
Registry (GCR)](https://cloud.google.com/tools/container-registry/), when running on Google Compute
Engine (GCE). If you are running your cluster on GCE or Google Kubernetes Engine, simply
use the full image name (e.g. gcr.io/my_project/image:tag).
All pods in a cluster will have read access to images in this registry.
The kubelet will authenticate to GCR using the instance's
Google service account. The service account on the instance
will have a `https://www.googleapis.com/auth/devstorage.read_only`,
so it can pull from the project's GCR, but not push.
### Using AWS EC2 Container Registry
Kubernetes has native support for the [AWS EC2 Container
Registry](https://aws.amazon.com/ecr/), when nodes are AWS EC2 instances.
Simply use the full image name (e.g. `ACCOUNT.dkr.ecr.REGION.amazonaws.com/imagename:tag`)
in the Pod definition.
All users of the cluster who can create pods will be able to run pods that use any of the
images in the ECR registry.
The kubelet will fetch and periodically refresh ECR credentials. It needs the following permissions to do this:
- `ecr:GetAuthorizationToken`
- `ecr:BatchCheckLayerAvailability`
- `ecr:GetDownloadUrlForLayer`
- `ecr:GetRepositoryPolicy`
- `ecr:DescribeRepositories`
- `ecr:ListImages`
- `ecr:BatchGetImage`
Requirements:
- You must be using kubelet version `v1.2.0` or newer. (e.g. run `/usr/bin/kubelet --version=true`).
- If your nodes are in region A and your registry is in a different region B, you need version `v1.3.0` or newer.
- ECR must be offered in your region
Troubleshooting:
- Verify all requirements above.
- Get $REGION (e.g. `us-west-2`) credentials on your workstation. SSH into the host and run Docker manually with those creds. Does it work?
- Verify kubelet is running with `--cloud-provider=aws`.
- Check kubelet logs (e.g. `journalctl -u kubelet`) for log lines like:
- `plugins.go:56] Registering credential provider: aws-ecr-key`
- `provider.go:91] Refreshing cache for provider: *aws_credentials.ecrProvider`
### Using Azure Container Registry (ACR)
When using [Azure Container Registry](https://azure.microsoft.com/en-us/services/container-registry/)
you can authenticate using either an admin user or a service principal.
In either case, authentication is done via standard Docker authentication. These instructions assume the
[azure-cli](https://github.com/azure/azure-cli) command line tool.
You first need to create a registry and generate credentials, complete documentation for this can be found in
the [Azure container registry documentation](https://docs.microsoft.com/en-us/azure/container-registry/container-registry-get-started-azure-cli).
Once you have created your container registry, you will use the following credentials to login:
* `DOCKER_USER` : service principal, or admin username
* `DOCKER_PASSWORD`: service principal password, or admin user password
* `DOCKER_REGISTRY_SERVER`: `${some-registry-name}.azurecr.io`
* `DOCKER_EMAIL`: `${some-email-address}`
Once you have those variables filled in you can
[configure a Kubernetes Secret and use it to deploy a Pod](/docs/concepts/containers/images/#specifying-imagepullsecrets-on-a-pod).
### Configuring Nodes to Authenticate to a Private Repository
**Note:** if you are running on Google Kubernetes Engine, there will already be a `.dockercfg` on each node with credentials for Google Container Registry. You cannot use this approach.
**Note:** if you are running on AWS EC2 and are using the EC2 Container Registry (ECR), the kubelet on each node will
manage and update the ECR login credentials. You cannot use this approach.
**Note:** this approach is suitable if you can control node configuration. It
will not work reliably on GCE, and any other cloud provider that does automatic
node replacement.
Docker stores keys for private registries in the `$HOME/.dockercfg` or `$HOME/.docker/config.json` file. If you put this
in the `$HOME` of user `root` on a kubelet, then docker will use it.
Here are the recommended steps to configuring your nodes to use a private registry. In this
example, run these on your desktop/laptop:
1. Run `docker login [server]` for each set of credentials you want to use. This updates `$HOME/.docker/config.json`.
1. View `$HOME/.docker/config.json` in an editor to ensure it contains just the credentials you want to use.
1. Get a list of your nodes, for example:
- if you want the names: `nodes=$(kubectl get nodes -o jsonpath='{range.items[*].metadata}{.name} {end}')`
- if you want to get the IPs: `nodes=$(kubectl get nodes -o jsonpath='{range .items[*].status.addresses[?(@.type=="ExternalIP")]}{.address} {end}')`
1. Copy your local `.docker/config.json` to the home directory of root on each node.
- for example: `for n in $nodes; do scp ~/.docker/config.json root@$n:/root/.docker/config.json; done`
Verify by creating a pod that uses a private image, e.g.:
```yaml
$ cat <<EOF > /tmp/private-image-test-1.yaml
apiVersion: v1
kind: Pod
metadata:
name: private-image-test-1
spec:
containers:
- name: uses-private-image
image: $PRIVATE_IMAGE_NAME
imagePullPolicy: Always
command: [ "echo", "SUCCESS" ]
EOF
$ kubectl create -f /tmp/private-image-test-1.yaml
pod "private-image-test-1" created
$
```
If everything is working, then, after a few moments, you should see:
```shell
$ kubectl logs private-image-test-1
SUCCESS
```
If it failed, then you will see:
```shell
$ kubectl describe pods/private-image-test-1 | grep "Failed"
Fri, 26 Jun 2015 15:36:13 -0700 Fri, 26 Jun 2015 15:39:13 -0700 19 {kubelet node-i2hq} spec.containers{uses-private-image} failed Failed to pull image "user/privaterepo:v1": Error: image user/privaterepo:v1 not found
```
You must ensure all nodes in the cluster have the same `.docker/config.json`. Otherwise, pods will run on
some nodes and fail to run on others. For example, if you use node autoscaling, then each instance
template needs to include the `.docker/config.json` or mount a drive that contains it.
All pods will have read access to images in any private registry once private
registry keys are added to the `.docker/config.json`.
### Pre-pulling Images
**Note:** if you are running on Google Kubernetes Engine, there will already be a `.dockercfg` on each node with credentials for Google Container Registry. You cannot use this approach.
**Note:** this approach is suitable if you can control node configuration. It
will not work reliably on GCE, and any other cloud provider that does automatic
node replacement.
By default, the kubelet will try to pull each image from the specified registry.
However, if the `imagePullPolicy` property of the container is set to `IfNotPresent` or `Never`,
then a local image is used (preferentially or exclusively, respectively).
If you want to rely on pre-pulled images as a substitute for registry authentication,
you must ensure all nodes in the cluster have the same pre-pulled images.
This can be used to preload certain images for speed or as an alternative to authenticating to a private registry.
All pods will have read access to any pre-pulled images.
### Specifying ImagePullSecrets on a Pod
**Note:** This approach is currently the recommended approach for Google Kubernetes Engine, GCE, and any cloud-providers
where node creation is automated.
Kubernetes supports specifying registry keys on a pod.
#### Creating a Secret with a Docker Config
Run the following command, substituting the appropriate uppercase values:
```shell
$ kubectl create secret docker-registry myregistrykey --docker-server=DOCKER_REGISTRY_SERVER --docker-username=DOCKER_USER --docker-password=DOCKER_PASSWORD --docker-email=DOCKER_EMAIL
secret "myregistrykey" created.
```
If you need access to multiple registries, you can create one secret for each registry.
Kubelet will merge any `imagePullSecrets` into a single virtual `.docker/config.json`
when pulling images for your Pods.
Pods can only reference image pull secrets in their own namespace,
so this process needs to be done one time per namespace.
##### Bypassing kubectl create secrets
If for some reason you need multiple items in a single `.docker/config.json` or need
control not given by the above command, then you can [create a secret using
json or yaml](/docs/user-guide/secrets/#creating-a-secret-manually).
Be sure to:
- set the name of the data item to `.dockerconfigjson`
- base64 encode the docker file and paste that string, unbroken
as the value for field `data[".dockerconfigjson"]`
- set `type` to `kubernetes.io/dockerconfigjson`
Example:
```yaml
apiVersion: v1
kind: Secret
metadata:
name: myregistrykey
namespace: awesomeapps
data:
.dockerconfigjson: UmVhbGx5IHJlYWxseSByZWVlZWVlZWVlZWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGx5eXl5eXl5eXl5eXl5eXl5eXl5eSBsbGxsbGxsbGxsbGxsbG9vb29vb29vb29vb29vb29vb29vb29vb29vb25ubm5ubm5ubm5ubm5ubm5ubm5ubm5ubmdnZ2dnZ2dnZ2dnZ2dnZ2dnZ2cgYXV0aCBrZXlzCg==
type: kubernetes.io/dockerconfigjson
```
If you get the error message `error: no objects passed to create`, it may mean the base64 encoded string is invalid.
If you get an error message like `Secret "myregistrykey" is invalid: data[.dockerconfigjson]: invalid value ...`, it means
the data was successfully un-base64 encoded, but could not be parsed as a `.docker/config.json` file.
#### Referring to an imagePullSecrets on a Pod
Now, you can create pods which reference that secret by adding an `imagePullSecrets`
section to a pod definition.
```yaml
apiVersion: v1
kind: Pod
metadata:
name: foo
namespace: awesomeapps
spec:
containers:
- name: foo
image: janedoe/awesomeapp:v1
imagePullSecrets:
- name: myregistrykey
```
This needs to be done for each pod that is using a private registry.
However, setting of this field can be automated by setting the imagePullSecrets
in a [serviceAccount](/docs/user-guide/service-accounts) resource.
Check [Add ImagePullSecrets to a Service Account](/docs/tasks/configure-pod-container/configure-service-account/#add-imagepullsecrets-to-a-service-account) for detailed instructions.
You can use this in conjunction with a per-node `.docker/config.json`. The credentials
will be merged. This approach will work on Google Kubernetes Engine.
### Use Cases
There are a number of solutions for configuring private registries. Here are some
common use cases and suggested solutions.
1. Cluster running only non-proprietary (e.g. open-source) images. No need to hide images.
- Use public images on the Docker hub.
- No configuration required.
- On GCE/Google Kubernetes Engine, a local mirror is automatically used for improved speed and availability.
1. Cluster running some proprietary images which should be hidden to those outside the company, but
visible to all cluster users.
- Use a hosted private [Docker registry](https://docs.docker.com/registry/).
- It may be hosted on the [Docker Hub](https://hub.docker.com/account/signup/), or elsewhere.
- Manually configure .docker/config.json on each node as described above.
- Or, run an internal private registry behind your firewall with open read access.
- No Kubernetes configuration is required.
- Or, when on GCE/Google Kubernetes Engine, use the project's Google Container Registry.
- It will work better with cluster autoscaling than manual node configuration.
- Or, on a cluster where changing the node configuration is inconvenient, use `imagePullSecrets`.
1. Cluster with a proprietary images, a few of which require stricter access control.
- Ensure [AlwaysPullImages admission controller](/docs/admin/admission-controllers/#alwayspullimages) is active. Otherwise, all Pods potentially have access to all images.
- Move sensitive data into a "Secret" resource, instead of packaging it in an image.
1. A multi-tenant cluster where each tenant needs own private registry.
- Ensure [AlwaysPullImages admission controller](/docs/admin/admission-controllers/#alwayspullimages) is active. Otherwise, all Pods of all tenants potentially have access to all images.
- Run a private registry with authorization required.
- Generate registry credential for each tenant, put into secret, and populate secret to each tenant namespace.
- The tenant adds that secret to imagePullSecrets of each namespace.
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