Azurerm VPN Gateway/Site Plan Errors: A Terraform Deep Dive

When working with Terraform and Azure, you might encounter scenarios where the planned changes don't align with the expected outcome. This article delves into a specific issue involving the azurerm_vpn_gateway_connection and azurerm_vpn_site resources, exploring the root causes and potential solutions. If you're grappling with unexpected resource replacements or errors during Terraform apply, this guide is for you.

Understanding the Initial Problem: Mismatched Plans and Reality

In the realm of Infrastructure as Code (IaC), Terraform stands out as a powerful tool for managing cloud resources. However, discrepancies between the planned and actual states can lead to frustrating errors. Let's break down the initial problem encountered with Azure VPN Gateway and Site resources.

The core issue revolves around Terraform's plan showing "0 to add, 2 to change, 0 to destroy" when adjusting the azurerm_vpn_site.name. Ideally, this change should be a straightforward update. However, upon running terraform apply, an error surfaces:

Error: updating Vpn Site (Subscription: "xxxxxxxxxxxxxxxxx"
Resource Group Name: "xxxxxxxxxxxxxxxxxxxxxx"
Vpn Site Name: "xxx"): polling after VpnSitesCreateOrUpdate: polling failed: the Azure API returned the following error:

Status: "DeleteVpnsiteWithExistingConnectionsNotAllowed"
Code: ""
Message: "There are existing connections for this vpnsite. Please delete connections and try again."
Activity Id: ""

This error indicates that Azure prevents deleting a VPN site with existing connections, highlighting a critical discrepancy between Terraform's planned actions and Azure's constraints. This situation underscores the importance of understanding resource dependencies and potential cascading effects of seemingly minor changes.

Diving Deep: Why Does This Happen?

The error message, "There are existing connections for this vpnsite. Please delete connections and try again," is the key to unraveling this issue. When the name of an azurerm_vpn_site changes, it impacts the associated azurerm_vpn_gateway_connection, particularly the vpn_site_link_id. Terraform, in its initial plan, might not recognize this cascading effect, leading to an attempt to update the VPN site without properly handling the dependent connections.

The vpn_site_link_id within the azurerm_vpn_gateway_connection resource is intrinsically linked to the azurerm_vpn_site's links. When the VPN site's name changes, the links are effectively recreated, leading to a mismatch. This mismatch forces Terraform to attempt a replacement of the connection, which Azure prevents due to the active connections.

The Ripple Effect: How One Change Leads to Another

The sequence of events further complicates the situation. After the initial failed apply, running terraform apply again reveals a more accurate picture:

Plan: 1 to add, 1 to change, 1 to destroy.

This updated plan reflects the necessary replacement of the azurerm_vpn_gateway_connection due to the vpn_site_link_id change. The remote_vpn_site_id within the azurerm_vpn_gateway_connection is affected by the azurerm_vpn_site.id, further solidifying the dependency chain.

This behavior highlights a crucial aspect of Terraform: understanding the implicit dependencies between resources. A seemingly minor change in one resource can trigger a cascade of changes in dependent resources, potentially leading to unexpected replacements or errors.

Analyzing the Terraform Configuration

To better understand the issue, let's dissect the provided Terraform configuration files. This will help us pinpoint the exact resources and attributes involved in the incorrect planning behavior.

Examining the azurerm_vpn_gateway_connection Resource

The azurerm_vpn_gateway_connection resource block defines the connection between the VPN gateway and the VPN site. Key attributes in this context include:

• name: The name of the VPN gateway connection.
• vpn_gateway_id: The ID of the VPN gateway.
• remote_vpn_site_id: The ID of the remote VPN site, which is dynamically linked to the azurerm_vpn_site.id.
• vpn_link: A dynamic block that defines the properties of the VPN link, including name, vpn_site_link_id, bandwidth_mbps, and IPsec policy settings.

The vpn_link block is particularly relevant here. The vpn_site_link_id is derived from the azurerm_vpn_site.this.link[0].id, creating a direct dependency between the connection and the VPN site's links. The shared_key is also a critical attribute, often sourced from a secure location like Azure Key Vault.

resource "azurerm_vpn_gateway_connection" "this" {
  name               = var.vpn_site.connection_name
  vpn_gateway_id     = var.vpn_s2s_gw_id
  remote_vpn_site_id = azurerm_vpn_site.this.id

  dynamic "vpn_link" {
    for_each = var.vpn_site.link
    content {
      name             = "${vpn_link.value.name}-link"
      vpn_site_link_id = azurerm_vpn_site.this.link[0].id
      bandwidth_mbps   = vpn_link.value.speed_in_mbps
      bgp_enabled      = vpn_link.value.bgp_enabled
      shared_key       = data.azurerm_key_vault_secret.shared_key.value
      protocol         = var.protocol

      ipsec_policy {
        dh_group                 = vpn_link.value.ipsec_policy.dh_group
        ike_encryption_algorithm = vpn_link.value.ipsec_policy.ike_encryption_algorithm
        ike_integrity_algorithm  = vpn_link.value.ipsec_policy.ike_integrity_algorithm
        encryption_algorithm     = vpn_link.value.ipsec_policy.encryption_algorithm
        integrity_algorithm      = vpn_link.value.ipsec_policy.integrity_algorithm
        pfs_group                = vpn_link.value.ipsec_policy.pfs_group
        sa_data_size_kb          = vpn_link.value.ipsec_policy.sa_data_size_kb
        sa_lifetime_sec          = vpn_link.value.ipsec_policy.sa_lifetime_sec
      }
    }
  }
}

Analyzing the azurerm_vpn_site Resource

The azurerm_vpn_site resource defines the VPN site itself, including its name, location, and links. Key attributes include:

• name: The name of the VPN site.
• resource_group_name: The name of the resource group.
• location: The Azure region.
• virtual_wan_id: The ID of the Virtual WAN.
• link: A dynamic block that defines the properties of the VPN site links, including name, ip_address, fqdn, and BGP settings.

The link block is crucial for understanding the cascading effect. The name attribute within this block directly impacts the vpn_site_link_id in the azurerm_vpn_gateway_connection.

resource "azurerm_vpn_site" "this" {
  name                = var.vpn_site.name
  resource_group_name = var.resource_group_name
  location            = data.azurerm_resource_group.this.location
  virtual_wan_id      = var.virtual_wan_id
  address_cidrs = var.vpn_site.address_cidrs

  dynamic "link" {
    for_each = var.vpn_site.link
    content {
      name                = link.value.name
      ip_address          = link.value.ip_address
      fqdn                = link.value.fqdn
      provider_name       = link.value.provider_name
      speed_in_mbps       = link.value.speed_in_mbps

      dynamic "bgp" {
        for_each = link.value.bgp_enabled && link.value.bgp_asn != null ? [1] : []
        content {
          asn             = link.value.bgp_asn
          peering_address = link.value.bgp_peering_address
        }
      }
    }
  }

  tags = var.tags
}

Identifying the Root Cause Through Configuration Analysis

By analyzing these configurations, we can pinpoint the root cause: changing the azurerm_vpn_site.name triggers a change in the azurerm_vpn_site.link IDs. This, in turn, forces a replacement of the azurerm_vpn_gateway_connection due to the dependency on vpn_site_link_id. Azure's restriction on deleting VPN sites with active connections exacerbates the issue, leading to the initial error.

Devising Solutions and Best Practices

Now that we've identified the problem and its root cause, let's explore potential solutions and best practices to avoid such scenarios in the future.

Solution 1: Detach and Reattach Connections

The most straightforward solution is to explicitly detach the VPN gateway connections before modifying the VPN site's name and then reattach them afterward. This involves a multi-step process:

1. Terraform Apply (Destroy): Apply a configuration that removes the azurerm_vpn_gateway_connection resources. This will gracefully disconnect the VPN connections.
2. Terraform Apply (Modify VPN Site): Apply the configuration changes to the azurerm_vpn_site, including the name change.
3. Terraform Apply (Create): Apply a configuration that recreates the azurerm_vpn_gateway_connection resources, establishing the connections with the updated VPN site.

This approach ensures that Azure's restrictions are respected by breaking the dependency chain during the name change.

Solution 2: Implement lifecycle prevent_destroy

To prevent accidental destruction of the VPN gateway connection, you can implement a lifecycle block with the prevent_destroy = true argument. This will force you to explicitly remove the resource from your configuration before it can be destroyed, providing an extra layer of protection.

resource "azurerm_vpn_gateway_connection" "this" {
  # ... other configurations ...

  lifecycle {
    prevent_destroy = true
  }
}

Solution 3: Utilize Terraform State Management Effectively

Proper Terraform state management is crucial for avoiding unexpected behavior. Ensure your state is stored remotely and securely, using services like Azure Storage or Terraform Cloud. This prevents state corruption and ensures consistency across environments.

Best Practice 1: Thorough Planning and Dependency Analysis

Before making any changes, carefully analyze the potential impact on dependent resources. Use terraform plan to review the proposed changes and identify any resources that might be replaced or destroyed unexpectedly. Pay close attention to resources with interdependencies, such as VPN gateways and connections.

Best Practice 2: Modularize Your Configuration

Break down your Terraform configuration into smaller, modular components. This improves readability, maintainability, and reduces the risk of unintended consequences. For example, you can create separate modules for VPN sites, VPN gateways, and connections.

Best Practice 3: Test Changes in a Non-Production Environment

Always test your Terraform changes in a non-production environment before applying them to production. This allows you to identify and resolve any issues without impacting your live services.

Best Practice 4: Leverage Data Sources for Dynamic Values

Use Terraform data sources to fetch dynamic values, such as resource IDs or names. This reduces hardcoding and makes your configuration more resilient to changes. For example, use the azurerm_key_vault_secret data source to securely retrieve secrets.

Practical Steps to Reproduce and Verify the Solution

To solidify your understanding, let's outline the steps to reproduce the issue and verify the proposed solution.

Steps to Reproduce the Issue

1. Deploy the Initial Configuration: Deploy the Terraform configuration provided earlier, creating the VPN site and gateway connection.
2. Modify the VPN Site Name: Change the name attribute of the azurerm_vpn_site resource in your Terraform configuration.
3. Run terraform plan: Observe the plan output, noting that it might initially show only changes to the VPN site.
4. Run terraform apply: Observe the error related to deleting a VPN site with existing connections.
5. Run terraform apply Again: Observe the updated plan showing the replacement of the VPN gateway connection.

Steps to Verify the Solution (Detach and Reattach)

1. Create a Configuration to Remove Connections: Create a separate Terraform configuration (or modify the existing one) to remove the azurerm_vpn_gateway_connection resources.
2. Apply the Configuration: Run terraform apply with this configuration to disconnect the VPN connections.
3. Modify the VPN Site Name: Change the name attribute of the azurerm_vpn_site resource in your Terraform configuration.
4. Apply the Changes to VPN Site: Run terraform apply to apply the changes to the VPN site.
5. Recreate the Connections: Revert or modify your configuration to include the azurerm_vpn_gateway_connection resources.
6. Apply the Configuration: Run terraform apply to recreate the VPN connections with the updated VPN site.

By following these steps, you can reproduce the issue, understand the error, and verify that the proposed solution effectively addresses the problem.

Conclusion: Mastering Terraform and Azure Resource Management

Navigating the intricacies of Terraform and Azure resource management requires a deep understanding of resource dependencies, potential cascading effects, and Azure's inherent constraints. This article has dissected a specific issue involving azurerm_vpn_gateway_connection and azurerm_vpn_site resources, providing a comprehensive analysis of the root cause and practical solutions.

By implementing the suggested solutions and best practices, you can mitigate the risk of unexpected resource replacements, errors, and downtime. Remember to always plan your changes meticulously, test them thoroughly in a non-production environment, and leverage Terraform's features for state management and dependency analysis.

By mastering these concepts, you'll be well-equipped to build and manage robust, scalable, and reliable infrastructure on Azure with Terraform.

For further reading on Terraform and Azure, consider exploring the official Terraform documentation. This resource provides in-depth information on various Azure resources and their configurations.