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External authentication in an ACR task using an Azure-managed identity

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In an ACR task, you can enable a managed identity for Azure resources. The task can use the identity to access other Azure resources, without needing to provide or manage credentials.

In this article, you learn how to enable a managed identity in a task that accesses secrets stored in an Azure key vault.

To create the Azure resources, this article requires that you run the Azure CLI version 2.0.68 or later. Run az --version to find the version. If you need to install or upgrade, see Install Azure CLI.

Scenario overview

The example task reads Docker Hub credentials stored in an Azure key vault. The credentials are for a Docker Hub account with write (push) permissions to a private Docker Hub repository. To read the credentials, you configure the task with a managed identity and assign appropriate permissions to it. The task associated with the identity builds an image, and signs into Docker Hub to push the image to the private repo.

This example shows steps using either a user-assigned or system-assigned managed identity. Your choice of identity depends on your organization's needs.

In a real-world scenario, a company might publish images to a private repo in Docker Hub as part of a build process.

Prerequisites

You need an Azure container registry in which you run the task. In this article, this registry is named myregistry. Replace with your own registry name in later steps.

If you don't already have an Azure container registry, see Quickstart: Create a private container registry using the Azure CLI. You don't need to push images to the registry yet.

You also need a private repository in Docker Hub, and a Docker Hub account with permissions to write to the repo. In this example, this repo is named hubuser/hubrepo.

Create a key vault and store secrets

First, if you need to, create a resource group named myResourceGroup in the eastus location with the following az group create command:

az group create --name myResourceGroup --location eastus

Use the az keyvault create command to create a key vault. Be sure to specify a unique key vault name.

az keyvault create --name mykeyvault --resource-group myResourceGroup --location eastus

Store the required Docker Hub credentials in the key vault using the az keyvault secret set command. In these commands, the values are passed in environment variables:

# Store Docker Hub user name
az keyvault secret set \
  --name UserName \
  --value $USERNAME \
  --vault-name mykeyvault

# Store Docker Hub password
az keyvault secret set \
  --name Password \
  --value $PASSWORD \
  --vault-name mykeyvault

In a real-world scenario, secrets would likely be set and maintained in a separate process.

Define task steps in YAML file

The steps for this example task are defined in a YAML file. Create a file named dockerhubtask.yaml in a local working directory and paste the following contents. Be sure to replace the key vault name in the file with the name of your key vault.

version: v1.1.0
# Replace mykeyvault with the name of your key vault
secrets:
  - id: username
    keyvault: https://mykeyvault.vault.azure.net/secrets/UserName
  - id: password
    keyvault: https://mykeyvault.vault.azure.net/secrets/Password
steps:
# Log in to Docker Hub
  - cmd: bash echo '{{.Secrets.password}}' | docker login --username '{{.Secrets.username}}' --password-stdin 
# Build image
  - build: -t {{.Values.PrivateRepo}}:$ID https://github.com/Azure-Samples/acr-tasks.git -f hello-world.dockerfile
# Push image to private repo in Docker Hub
  - push:
    - {{.Values.PrivateRepo}}:$ID

The task steps do the following:

  • Manage secret credentials to authenticate with Docker Hub.
  • Authenticate with Docker Hub by passing the secrets to the docker login command.
  • Build an image using a sample Dockerfile in the Azure-Samples/acr-tasks repo.
  • Push the image to the private Docker Hub repository.

Option 1: Create task with user-assigned identity

The steps in this section create a task and enable a user-assigned identity. If you want to enable a system-assigned identity instead, see Option 2: Create task with system-assigned identity.

Create a user-assigned identity

Create an identity named myACRTasksId in your subscription using the az identity create command. You can use the same resource group you used previously to create a container registry, or a different one.

az identity create \
  --resource-group myResourceGroup \
  --name myACRTasksId

To configure the user-assigned identity in the following steps, use the az identity show command to store the identity's resource ID, principal ID, and client ID in variables.

# Get resource ID of the user-assigned identity
resourceID=$(az identity show \
  --resource-group myResourceGroup \
  --name myACRTasksId \
  --query id --output tsv)

# Get principal ID of the task's user-assigned identity
principalID=$(az identity show \
  --resource-group myResourceGroup \
  --name myACRTasksId \
  --query principalId --output tsv)

# Get client ID of the user-assigned identity
clientID=$(az identity show \
  --resource-group myResourceGroup \
  --name myACRTasksId \
  --query clientId --output tsv)

Create task

Create the task dockerhubtask by executing the following az acr task create command. The task runs without a source code context, and the command references the file dockerhubtask.yaml in the working directory. The --assign-identity parameter passes the resource ID of the user-assigned identity.

az acr task create \
  --name dockerhubtask \
  --registry myregistry \
  --context /dev/null \
  --file dockerhubtask.yaml \
  --assign-identity $resourceID

In the command output, the identity section shows the identity of type UserAssigned is set in the task:

[...]
"identity": {
    "principalId": null,
    "tenantId": null,
    "type": "UserAssigned",
    "userAssignedIdentities": {
      "/subscriptions/xxxxxxxx-d12e-4760-9ab6-xxxxxxxxxxxx/resourcegroups/myResourceGroup/providers/Microsoft.ManagedIdentity/userAssignedIdentities/myACRTasksId": {
        "clientId": "xxxxxxxx-f17e-4768-bb4e-xxxxxxxxxxxx",
        "principalId": "xxxxxxxx-1335-433d-bb6c-xxxxxxxxxxxx"
      }
[...]

Grant identity access to key vault

Run the following az keyvault set-policy command to set an access policy on the key vault. The following example allows the identity to read secrets from the key vault.

az keyvault set-policy --name mykeyvault \
  --resource-group myResourceGroup \
  --object-id $principalID \
  --secret-permissions get

Proceed to Manually run the task.

Option 2: Create task with system-assigned identity

The steps in this section create a task and enable a system-assigned identity. If you want to enable a user-assigned identity instead, see Option 1: Create task with user-assigned identity.

Create task

Create the task dockerhubtask by executing the following az acr task create command. The task runs without a source code context, and the command references the file dockerhubtask.yaml in the working directory. The --assign-identity parameter with no value enables the system-assigned identity on the task.

az acr task create \
  --name dockerhubtask \
  --registry myregistry \
  --context /dev/null \
  --file dockerhubtask.yaml \
  --assign-identity

In the command output, the identity section shows an identity of type SystemAssigned is set in the task. The principalId is the principal ID of the task identity:

[...]
  "identity": {
    "principalId": "xxxxxxxx-2703-42f9-97d0-xxxxxxxxxxxx",
    "tenantId": "xxxxxxxx-86f1-41af-91ab-xxxxxxxxxxxx",
    "type": "SystemAssigned",
    "userAssignedIdentities": null
  },
  "location": "eastus",
[...]

Use the az acr task show command to store the principalId in a variable, to use in later commands. Substitute the name of your task and your registry in the following command:

principalID=$(az acr task show \
  --name <task_name> --registry <registry_name> \
  --query identity.principalId --output tsv)

Grant identity access to key vault

Run the following az keyvault set-policy command to set an access policy on the key vault. The following example allows the identity to read secrets from the key vault.

az keyvault set-policy --name mykeyvault \
  --resource-group myResourceGroup \
  --object-id $principalID \
  --secret-permissions get

Manually run the task

To verify that the task in which you enabled a managed identity runs successfully, manually trigger the task with the az acr task run command. The --set parameter is used to pass the private repo name to the task. In this example, the placeholder repo name is hubuser/hubrepo.

az acr task run --name dockerhubtask --registry myregistry --set PrivateRepo=hubuser/hubrepo

When the task runs successfully, output shows successful authentication to Docker Hub, and the image is successfully built and pushed to the private repo:

Queued a run with ID: cf24
Waiting for an agent...
2019/06/20 18:05:55 Using acb_vol_b1edae11-30de-4f2b-a9c7-7d743e811101 as the home volume
2019/06/20 18:05:58 Creating Docker network: acb_default_network, driver: 'bridge'
2019/06/20 18:05:58 Successfully set up Docker network: acb_default_network
2019/06/20 18:05:58 Setting up Docker configuration...
2019/06/20 18:05:59 Successfully set up Docker configuration
2019/06/20 18:05:59 Logging in to registry: myregistry.azurecr.io
2019/06/20 18:06:00 Successfully logged into myregistry.azurecr.io
2019/06/20 18:06:00 Executing step ID: acb_step_0. Timeout(sec): 600, Working directory: '', Network: 'acb_default_network'
2019/06/20 18:06:00 Launching container with name: acb_step_0
[...]
Login Succeeded
2019/06/20 18:06:02 Successfully executed container: acb_step_0
2019/06/20 18:06:02 Executing step ID: acb_step_1. Timeout(sec): 600, Working directory: '', Network: 'acb_default_network'
2019/06/20 18:06:02 Scanning for dependencies...
2019/06/20 18:06:04 Successfully scanned dependencies
2019/06/20 18:06:04 Launching container with name: acb_step_1
Sending build context to Docker daemon    129kB
[...]
2019/06/20 18:06:07 Successfully pushed image: hubuser/hubrepo:cf24
2019/06/20 18:06:07 Step ID: acb_step_0 marked as successful (elapsed time in seconds: 2.064353)
2019/06/20 18:06:07 Step ID: acb_step_1 marked as successful (elapsed time in seconds: 2.594061)
2019/06/20 18:06:07 Populating digests for step ID: acb_step_1...
2019/06/20 18:06:09 Successfully populated digests for step ID: acb_step_1
2019/06/20 18:06:09 Step ID: acb_step_2 marked as successful (elapsed time in seconds: 2.743923)
2019/06/20 18:06:09 The following dependencies were found:
2019/06/20 18:06:09
- image:
    registry: registry.hub.docker.com
    repository: hubuser/hubrepo
    tag: cf24
    digest: sha256:92c7f9c92844bbbb5d0a101b22f7c2a7949e40f8ea90c8b3bc396879d95e899a
  runtime-dependency:
    registry: registry.hub.docker.com
    repository: library/hello-world
    tag: latest
    digest: sha256:0e11c388b664df8a27a901dce21eb89f11d8292f7fca1b3e3c4321bf7897bffe
  git:
    git-head-revision: b0ffa6043dd893a4c75644c5fed384c82ebb5f9e

Run ID: cf24 was successful after 15s

To confirm the image is pushed, check for the tag (cf24 in this example) in the private Docker Hub repo.

Next steps