random cidr for Azure vnet

Author: clstokes

terraform/azure/azure-linux-vm/main.tf

@@ -29,7 +29,7 @@ locals {
   vpc_id                    = module.vpc.vnet_id
   subnet_id                 = module.vpc.public_subnet_id
   network_security_group_id = azurerm_network_security_group.tailscale_ingress.id
-  instance_type             = "Standard_DS1_v2"
+  instance_type             = "Standard_D2as_v6"
   admin_public_key_path     = var.admin_public_key_path
 }
 
@@ -47,12 +47,6 @@ module "vpc" {
   location            = local.location
   resource_group_name = local.resource_group_name
 
-  cidrs = ["10.0.0.0/22"]
-  subnet_cidrs = [
-    "10.0.0.0/24",
-    "10.0.1.0/24",
-    "10.0.2.0/24",
-  ]
   subnet_name_public               = "public"
   subnet_name_private              = "private"
   subnet_name_private_dns_resolver = "dns-inbound"

terraform/azure/azure-linux-vm/outputs.tf

@@ -2,6 +2,10 @@ output "vpc_id" {
   value = module.vpc.vnet_id
 }
 
+output "vpc_cidrs" {
+  value = module.vpc.vnet_address_space
+}
+
 output "nat_public_ips" {
   value = module.vpc.nat_public_ips
 }

terraform/azure/internal-modules/azure-network/main.tf

@@ -1,3 +1,16 @@
+locals {
+  cidrs        = length(var.cidrs) == 0 ? [cidrsubnet("10.0.0.0/16", 6, random_integer.vpc_cidr[0].result)] : var.cidrs                                                    # /22
+  subnet_cidrs = length(var.subnet_cidrs) == 0 ? [cidrsubnet(local.cidrs[0], 2, 0), cidrsubnet(local.cidrs[0], 2, 1), cidrsubnet(local.cidrs[0], 2, 2)] : var.subnet_cidrs # /24 inside the /22
+}
+
+# Pick a random /22 within 10.0.0.0/16
+resource "random_integer" "vpc_cidr" {
+  count = length(var.cidrs) == 0 ? 1 : 0
+
+  min = 0
+  max = 63 # 2^(22-16)-1 = 64 slices in a /16
+}
+
 module "vpc" {
   # https://registry.terraform.io/modules/Azure/network/azurerm/latest
   source  = "Azure/network/azurerm"
@@ -9,8 +22,8 @@ module "vpc" {
   vnet_name = var.name
   tags      = var.tags
 
-  address_spaces  = var.cidrs
-  subnet_prefixes = var.subnet_cidrs
+  address_spaces  = local.cidrs
+  subnet_prefixes = local.subnet_cidrs
   subnet_names = [
     var.subnet_name_public,
     var.subnet_name_private,

terraform/azure/internal-modules/azure-network/variables.tf

@@ -24,10 +24,12 @@ variable "tags" {
 variable "cidrs" {
   description = "IPv4 CIDR block for the VPC"
   type        = list(string)
+  default     = []
 }
 variable "subnet_cidrs" {
   description = "List of CIDR blocks"
   type        = list(string)
+  default     = []
 }
 variable "subnet_name_public" {
   description = "Name of the `public` subnet"