Tutorial: Use a Linux VM system-assigned identity to access Azure Storage via a SAS credential

Managed identities for Azure resources is a feature of Azure Active Directory. Each of the Azure services that support managed identities for Azure resources are subject to their own timeline. Make sure you review the availability status of managed identities for your resource and known issues before you begin.

This tutorial shows you how to use a system-assigned managed identity for a Linux virtual machine (VM) to obtain a storage Shared Access Signature (SAS) credential. Specifically, a Service SAS credential.

Note

The SAS key generated in this tutorial will not be restricted/bound to the VM.

A Service SAS provides the ability to grant limited access to objects in a storage account, for a limited time and a specific service (in our case, the blob service), without exposing an account access key. You can use a SAS credential as usual when doing storage operations, for example when using the Storage SDK. For this tutorial, we demonstrate uploading and downloading a blob using Azure Storage CLI. You will learn how to:

  • Create a storage account
  • Create a blob container in the storage account
  • Grant your VM access to a storage account SAS in Resource Manager
  • Get an access token using your VM's identity, and use it to retrieve the SAS from Resource Manager

Prerequisites

Create a storage account

If you don't already have one, you will now create a storage account. You can also skip this step and grant your VM system-assigned managed identity access to the keys of an existing storage account.

  1. Click the +/Create new service button found on the upper left-hand corner of the Azure portal.

  2. Click Storage, then Storage Account, and a new "Create storage account" panel will display.

  3. Enter a Name for the storage account, which you will use later.

  4. Deployment model and Account kind should be set to "Resource manager" and "General purpose", respectively.

  5. Ensure the Subscription and Resource Group match the ones you specified when you created your VM in the previous step.

  6. Click Create.

    Create new storage account

Create a blob container in the storage account

Later we will upload and download a file to the new storage account. Because files require blob storage, we need to create a blob container in which to store the file.

  1. Navigate back to your newly created storage account.

  2. Click the Containers link in the left panel, under "Blob service."

  3. Click + Container on the top of the page, and a "New container" panel slides out.

  4. Give the container a name, select an access level, then click OK. The name you specified will be used later in the tutorial.

    Create storage container

Grant your VM's system-assigned managed identity access to use a storage SAS

Azure Storage does not natively support Azure AD authentication. However, you can use your VM's system-assigned managed identity to retrieve a storage SAS from the Resource Manager, then use the SAS to access storage. In this step, you grant your VM's system-assigned managed identity access to your storage account SAS.

  1. Navigate back to your newly created storage account.

  2. Click the Access control (IAM) link in the left panel.

  3. Click + Add role assignment on top of the page to add a new role assignment for your VM

  4. Set Role to "Storage Account Contributor", on the right side of the page.

  5. In the next dropdown, set Assign access to the resource "Virtual Machine".

  6. Next, ensure the proper subscription is listed in Subscription dropdown, then set Resource Group to "All resource groups".

  7. Finally, under Select choose your Linux Virtual Machine in the dropdown, then click Save.

    Alt image text

Get an access token using the VM's identity and use it to call Azure Resource Manager

For the remainder of the tutorial, we will work from the VM we created earlier.

To complete these steps, you will need an SSH client. If you are using Windows, you can use the SSH client in the Windows Subsystem for Linux. If you need assistance configuring your SSH client's keys, see How to Use SSH keys with Windows on Azure, or How to create and use an SSH public and private key pair for Linux VMs in Azure.

  1. In the Azure portal, navigate to Virtual Machines, go to your Linux virtual machine, then from the Overview page click Connect at the top. Copy the string to connect to your VM.

  2. Connect to your VM using your SSH client.

  3. Next, you will be prompted to enter in your Password you added when creating the Linux VM. You should then be successfully signed in.

  4. Use CURL to get an access token for Azure Resource Manager.

    The CURL request and response for the access token is below:

    curl 'http://169.254.169.254/metadata/identity/oauth2/token?api-version=2018-02-01&resource=https%3A%2F%2Fmanagement.azure.com%2F' -H Metadata:true    
    

    Note

    In the previous request, the value of the "resource" parameter must be an exact match for what is expected by Azure AD. When using the Azure Resource Manager resource ID, you must include the trailing slash on the URI. In the following response, the access_token element as been shortened for brevity.

    {"access_token":"eyJ0eXAiOiJ...",
    "refresh_token":"",
    "expires_in":"3599",
    "expires_on":"1504130527",
    "not_before":"1504126627",
    "resource":"https://management.azure.com",
    "token_type":"Bearer"} 
    

Get a SAS credential from Azure Resource Manager to make storage calls

Now use CURL to call Resource Manager using the access token we retrieved in the previous section, to create a storage SAS credential. Once we have the SAS credential, we can call storage upload/download operations.

For this request we'll use the follow HTTP request parameters to create the SAS credential:

{
    "canonicalizedResource":"/blob/<STORAGE ACCOUNT NAME>/<CONTAINER NAME>",
    "signedResource":"c",              // The kind of resource accessible with the SAS, in this case a container (c).
    "signedPermission":"rcw",          // Permissions for this SAS, in this case (r)ead, (c)reate, and (w)rite.  Order is important.
    "signedProtocol":"https",          // Require the SAS be used on https protocol.
    "signedExpiry":"<EXPIRATION TIME>" // UTC expiration time for SAS in ISO 8601 format, for example 2017-09-22T00:06:00Z.
}

These parameters are included in the POST body of the request for the SAS credential. For more information on the parameters for creating a SAS credential, see the List Service SAS REST reference.

Use the following CURL request to get the SAS credential. Be sure to replace the <SUBSCRIPTION ID>, <RESOURCE GROUP>, <STORAGE ACCOUNT NAME>, <CONTAINER NAME>, and <EXPIRATION TIME> parameter values with your own values. Replace the <ACCESS TOKEN> value with the access token you retrieved earlier:

curl https://management.azure.com/subscriptions/<SUBSCRIPTION ID>/resourceGroups/<RESOURCE GROUP>/providers/Microsoft.Storage/storageAccounts/<STORAGE ACCOUNT NAME>/listServiceSas/?api-version=2017-06-01 -X POST -d "{\"canonicalizedResource\":\"/blob/<STORAGE ACCOUNT NAME>/<CONTAINER NAME>\",\"signedResource\":\"c\",\"signedPermission\":\"rcw\",\"signedProtocol\":\"https\",\"signedExpiry\":\"<EXPIRATION TIME>\"}" -H "Authorization: Bearer <ACCESS TOKEN>"

Note

The text in the prior URL is case sensitive, so ensure if you are using upper-lowercase for your Resource Groups to reflect it accordingly. Additionally, it’s important to know that this is a POST request not a GET request.

The CURL response returns the SAS credential:

{"serviceSasToken":"sv=2015-04-05&sr=c&spr=https&st=2017-09-22T00%3A10%3A00Z&se=2017-09-22T02%3A00%3A00Z&sp=rcw&sig=QcVwljccgWcNMbe9roAJbD8J5oEkYoq%2F0cUPlgriBn0%3D"} 

Create a sample blob file to upload to your blob storage container. On a Linux VM you can do this with the following command.

echo "This is a test file." > test.txt

Next, authenticate with the CLI az storage command using the SAS credential, and upload the file to the blob container. For this step, you will need to install the latest Azure CLI on your VM, if you haven't already.

 az storage blob upload --container-name 
                        --file 
                        --name
                        --account-name 
                        --sas-token

Response:

Finished[#############################################################]  100.0000%
{
  "etag": "\"0x8D4F9929765C139\"",
  "lastModified": "2017-09-21T03:58:56+00:00"
}

Additionally, you can download the file using the Azure CLI and authenticating with the SAS credential.

Request:

az storage blob download --container-name
                         --file 
                         --name 
                         --account-name
                         --sas-token

Response:

{
  "content": null,
  "metadata": {},
  "name": "testblob",
  "properties": {
    "appendBlobCommittedBlockCount": null,
    "blobType": "BlockBlob",
    "contentLength": 16,
    "contentRange": "bytes 0-15/16",
    "contentSettings": {
      "cacheControl": null,
      "contentDisposition": null,
      "contentEncoding": null,
      "contentLanguage": null,
      "contentMd5": "Aryr///Rb+D8JQ8IytleDA==",
      "contentType": "text/plain"
    },
    "copy": {
      "completionTime": null,
      "id": null,
      "progress": null,
      "source": null,
      "status": null,
      "statusDescription": null
    },
    "etag": "\"0x8D4F9929765C139\"",
    "lastModified": "2017-09-21T03:58:56+00:00",
    "lease": {
      "duration": null,
      "state": "available",
      "status": "unlocked"
    },
    "pageBlobSequenceNumber": null,
    "serverEncrypted": false
  },
  "snapshot": null
}

Next steps

In this tutorial, you learned how to use a Linux VM system-assigned managed identity to access Azure Storage using a SAS credential. To learn more about Azure Storage SAS see: