doc: add OIDC setup instructions for azurerm provider [skip-ci]

      # specific OIDC role ARN for prod

      aws-prod-oidc-role-arn: "arn:aws:iam::222222222222:role/cicd-role"

```

## Azure authentication with OIDC

The Terraform template supports authenticating to Azure using OpenID Connect (OIDC)

without requiring a dedicated variant.

This approach leverages [GitLab's ID tokens](https://docs.gitlab.com/ci/secrets/id_token_authentication/)

and the [Azure Provider's OIDC support](https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/guides/service_principal_oidc).

### Prerequisites

Before using OIDC authentication with Azure, you must:

1. Configure [Workload Identity Federation between GitLab and Azure](https://docs.gitlab.com/ci/cloud_services/azure/)

   (following the GitLab guide)

2. Create a Service Principal in Azure with appropriate permissions

3. Configure the federated credential on your Service Principal to trust your GitLab instance

### Configuration

#### Step 1: Configure GitLab to generate the ID token

Add the following to your `.gitlab-ci.yml` to generate an ID token for Azure authentication:

```yaml

.tf-base:

  id_tokens:

    ARM_OIDC_TOKEN:

      aud: api://AzureADTokenExchange  # Prefered over GitLab instance URL[2]

```

The `.tf-base` job is the base job that all Terraform jobs inherit from,

so this configuration will apply to all Terraform operations.

:bulb: Audience `api://AzureADTokenExchange` is the [recommended value][2]

that tells the Microsoft API to accept the aud claim in the incoming token.

[2]: https://learn.microsoft.com/en-us/entra/workload-id/workload-identity-federation-create-trust?pivots=identity-wif-apps-methods-azcli#:~:text=600%20characters.%20The-,recommended%20value,-is%20%22api%3A//AzureADTokenExchange

#### Step 2: Configure the Azure Provider in your Terraform code

In your Terraform configuration files,

configure the `azurerm` provider to use OIDC authentication:

```terraform

provider "azurerm" {

  features {}

  # OIDC authentication settings - use_oidc MUST be set to true

  use_oidc        = true                # Required for OIDC authentication

  client_id       = var.client_id       # Your Service Principal's Application (client) ID

  tenant_id       = var.tenant_id       # Your Azure AD tenant ID

  subscription_id = var.subscription_id # Your Azure subscription ID

  # Optional: only required when your Service Principal lacks permissions to register Resource Providers

  resource_provider_registrations = "none"

}

```

:warning: **Important**: The `use_oidc = true` parameter is **required** for OIDC authentication to work.

Without it, the provider will not attempt to authenticate using the OIDC token.

See the [Azure Provider OIDC documentation](https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/guides/service_principal_oidc)

for more details.

#### Step 3: Set the required Terraform variables

Define the required variables as GitLab CI/CD variables:

* `TF_VAR_client_id`: Your Azure Service Principal's Application (client) ID

* `TF_VAR_tenant_id`: Your Azure AD tenant ID

* `TF_VAR_subscription_id`: Your Azure subscription ID

:bulb: **Tip**: Use [GitLab CI/CD variables](https://docs.gitlab.com/ci/variables/#for-a-project) to manage these values.

Mark them as **masked** to prevent them from appearing in logs.

### Multiple environments with different credentials

If you need to use different Service Principals for different environments

(e.g., separate credentials for production),

use environment-specific `.tfvars` files.

The template will automatically apply the appropriate `.tfvars` file based on the deployment environment.

In your `.gitlab-ci.yml`, you only need to configure the ID token (see example in the next section).

### Complete example

Here's a complete example combining all the pieces:

**`.gitlab-ci.yml`:**

```yaml

include:

  - component: $CI_SERVER_FQDN/to-be-continuous/terraform/gitlab-ci-terraform@8.1.2

    inputs:

      image: docker.io/hashicorp/terraform:latest

      staging-enabled: true

      prod-enabled: true

.tf-base:

  id_tokens:

    ARM_OIDC_TOKEN:

      aud: api://AzureADTokenExchange

```

**`production.env.tfvars`:**

```hcl

client_id       = "44444444-4444-4444-4444-444444444444"

tenant_id       = "55555555-5555-5555-5555-555555555555"

subscription_id = "66666666-6666-6666-6666-666666666666"

```

**`staging.env.tfvars`:**

```hcl

client_id       = "11111111-1111-1111-1111-111111111111"

tenant_id       = "22222222-2222-2222-2222-222222222222"

subscription_id = "33333333-3333-3333-3333-333333333333"

```

**`main.tf`:**

```terraform

terraform {

  required_providers {

    azurerm = {

      source  = "hashicorp/azurerm"

      version = "~> 3.0"

    }

  }

}

provider "azurerm" {

  features {}

  use_oidc        = true  # Required for OIDC authentication

  client_id       = var.client_id

  tenant_id       = var.tenant_id

  subscription_id = var.subscription_id

  # Only set this if your Service Principal cannot register Resource Providers

  resource_provider_registrations = "none"

}

```

**`variables.tf`:**

```terraform

variable "client_id" {

  description = "Azure Service Principal client ID"

  type        = string

}

variable "tenant_id" {

  description = "Azure AD tenant ID"

  type        = string

}

variable "subscription_id" {

  description = "Azure subscription ID"

  type        = string

}

```

### Troubleshooting

#### Authentication fails with `AADSTS700016: Application not found`

This indicate that de Service Principal for the registered Application

is not properly configured.

Verify that:

- Your Service Principal's Application ID (`client_id`) is correct

- The federated credential is properly configured on the Service Principal

- The `aud` claim in your ID token configuration matches what's configured in Azure

#### Error `The client '...' with object id '...' does not have authorization to perform action`

Ensure your Service Principal has been granted the appropriate Azure RBAC roles

on the target subscription or resource groups.

#### Authentication fails with `AADSTS7002131: No matching federated identity record found`

This error indicates that the federated identity credential configured in Azure

doesn't match the JWT token claims that GitLab is presenting.

Verify that:

- The federated credential's **Subject** field matches your GitLab project path pattern

  (e.g., `project_path:mygroup/myproject:ref_type:branch:ref:main` for the main branch,

  or use a wildcard like `project_path:mygroup/myproject:ref_type:branch:ref:*`

  to match all branches)

- The **Audience** configured in Azure matches the `aud` value

  in your `.gitlab-ci.yml` ID token configuration

#### Provider initialization is slow or fails with resource provider errors

Try adding `resource_provider_registrations = "none"` to your provider configuration

if your Service Principal doesn't have permissions to register Resource Providers.

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