» Azure Virtual Machine Image Builders

Packer can create Azure virtual machine images through variety of ways depending on the strategy that you want to use for building the images. Packer supports the following builders for Azure images at the moment:

  • azure-arm - Uses Azure Resource Manager (ARM) to launch a virtual machine (VM) from which a new image is captured after provisioning. If in doubt, use this builder; it is the easiest builder to get started with.

  • azure-chroot - Uses ARM to create a managed disk that is attached to an existing Azure VM that Packer is running on. Provisioning leverages Chroot environment. After provisioning, the disk is detached an image is created from this disk. This is an advanced builder and should not be used by newcomers. However, it is also the fastest way to build a VM image in Azure.

» Authentication for Azure

The Packer Azure builders provide a couple of ways to authenticate to Azure. The following methods are available and are explained below:

  • Azure Active Directory interactive login. Interactive login is available for the Public and US Gov clouds only.
  • Azure Managed Identity
  • Azure Active Directory Service Principal

No matter which method you choose, the identity you use will need the appropriate permissions on Azure resources for Packer to operate. The minimal set of permissions is highly dependent on the builder and its configuration. An easy way to get started is to assign the identity the Contributor role at the subscription level.

» Azure Active Directory interactive login

If your organization allows it, you can use a command line interactive login method based on oAuth 'device code flow'. Packer will select this method when you only specify a subscription_id in your builder configuration. When you run Packer, it will ask you to visit a web site and input a code. This web site will then authenticate you, satisfying any two-factor authentication policies that your organization might have. The tokens are cached under the .azure/packer directory in your home directory and will be reused if they are still valid on subsequent runs.

» Azure Managed Identity

Azure provides the option to assign an identity to a virtual machine (Azure documentation). Packer can use a system assigned identity for a VM where Packer is running to orchestrate Azure API's. This is the default behavior and requires no configuration properties to be set. It does, however, require that you run Packer on an Azure VM.

To enable this method, let Azure assign a system-assigned identity to your VM. Then, grant your VM access to the appropriate resources. To get started, try assigning the Contributor role at the subscription level to your VM. Then, when you discover your exact scenario, scope the permissions appropriately or isolate Packer builds in a separate subscription.

» Azure Active Directory Service Principal

Azure Active Directory models service accounts as 'Service Principal' (SP) objects. An SP represents an application accessing your Azure resources. It is identified by a client ID (aka application ID) and can use a password or a certificate to authenticate. To use a Service Principal, specify the subscription_id and client_id, as well as either client_secret, client_cert_path or client_jwt. Each of these last three represent a different way to authenticate the SP to AAD:

  • client_secret - allows the user to provide a password/secret registered for the AAD SP.
  • client_cert_path - allows usage of a certificate to be used to authenticate as the specified AAD SP.
  • client_jwt - For advanced scenario's where the used cannot provide Packer the full certificate, they can provide a JWT bearer token for client auth (RFC 7523, Sec. 2.2). These bearer tokens are created and signed using a certificate registered in AAD and have a user-chosen expiry time, limiting the validity of the token. This is also the underlying mechanism used to authenticate when using client_cert_path.

To create a service principal, you can follow the Azure documentation on this subject.