Unable to change an azure subnet using terraform - azure

Question

I am new to terraform and want to change the subnet on a network and I am getting a weird error.
google got nothing. here's what I am entering (after changing the main.tf and running plan)
terraform apply -replace="azurerm_subnet.subnet1"
Terraform will perform the following actions:
# module.network.azurerm_subnet.subnet[0] will be updated in-place
~ resource "azurerm_subnet" "subnet" {
~ address_prefixes = [
- "10.0.2.0/24",
+ "10.0.4.0/24",
]
id =
"/subscriptions/xxxxxxxxxxxxxxxxxxxxxxxxx/resourceGroups/lab-
resources/providers/Microsoft.Network/virtualNetworks/acctvnet/subnets/subnet1"
name = "subnet1"
# (7 unchanged attributes hidden)
}
Plan: 0 to add, 1 to change, 0 to destroy.
Do you want to perform these actions?
Terraform will perform the actions described above.
Only 'yes' will be accepted to approve.
I enter yes and I get this error:
Error: updating Subnet: (Name "subnet1" / Virtual Network Name "acctvnet" / Resource Group "lab-resources"): network.SubnetsClient#CreateOrUpdate: Failure sending request: StatusCode=400 -- Original Error: Code="InUseSubnetCannotBeUpdated" Message="Subnet subnet1 is in use and cannot be updated." Details=[]
│
│ with module.network.azurerm_subnet.subnet[0],
│ on .terraform/modules/network/main.tf line 15, in resource "azurerm_subnet" "subnet":
│ 15: resource "azurerm_subnet" "subnet" {
│
The VM is off and I do not see what else can be using it.
I also tried using the terraform taint "azurerm_subnet.subnet1"
Any ideas? Is what I am doing not possible?
Here is my main.tf
terraform {
required_version = ">=0.12"
required_providers {
azurerm = {
source = "hashicorp/azurerm"
version = "~>2.0"
}
}
}
provider "azurerm" {
features {}
}
resource "azurerm_resource_group" "lab_autodeploy" {
name = "lab-resources"
location = "East US 2"
}
module "Windowsservers" {
source = "Azure/compute/azurerm"
resource_group_name = azurerm_resource_group.lab_autodeploy.name
is_windows_image = true
vm_hostname = "new_ddc" // line can be removed if only one VM module per resource group
size = "Standard_F2"
admin_password = "$omePassw0rd"
vm_os_simple = "WindowsServer"
public_ip_dns = ["srv"] // change to a unique name per datacenter region
vnet_subnet_id = module.network.vnet_subnets[0]
depends_on = [azurerm_resource_group.lab_autodeploy]
}
module "network" {
source = "Azure/network/azurerm"
resource_group_name = azurerm_resource_group.lab_autodeploy.name
subnet_prefixes = ["10.4.0.0/24"]
subnet_names = ["subnet1"]
depends_on = [azurerm_resource_group.lab_autodeploy]
}
output "windows_vm_public_name" {
value = module.windowsservers.public_ip_dns_name
}

Answer 1

This isn't an issue specific to Terraform - in Azure you cannot change a subnet that has things attached to it. The fact that the VM is powered off makes no difference.
To get around this without destroying the VM, you could move the NIC to a different subnet (create a temporary subnet if necessary), perform the address space change and then move the NIC back.