random cidr for GCP vpc

Author: clstokes

terraform/google/google-compute-instance/main.tf

@@ -38,19 +38,6 @@ module "vpc" {
   region     = local.region
 
   name = local.name
-
-  subnets = [
-    {
-      subnet_name   = "subnet-${local.region}-10-0-121"
-      subnet_ip     = "10.0.121.0/24"
-      subnet_region = local.region
-    },
-    {
-      subnet_name   = "subnet-${local.region}-10-0-122"
-      subnet_ip     = "10.0.122.0/24"
-      subnet_region = local.region
-    }
-  ]
 }
 
 resource "tailscale_tailnet_key" "main" {

terraform/google/google-compute-instance/outputs.tf

@@ -2,6 +2,10 @@ output "instance_id" {
   value = module.tailscale_instance.instance_id
 }
 
+output "subnets_ips" {
+  value = module.vpc.subnets_ips
+}
+
 output "user_data_md5" {
   description = "MD5 hash of the VM user_data script - for detecting changes"
   value       = module.tailscale_instance.user_data_md5

terraform/google/internal-modules/google-vpc/main.tf

@@ -1,3 +1,28 @@
+locals {
+  cidr = length(var.subnets) == 0 ? [cidrsubnet("10.0.0.0/16", 6, random_integer.vpc_cidr[0].result)] : [] # /22
+  #   subnets = length(var.subnets) == 0 ? [cidrsubnet(local.cidr[0], 2, 0), cidrsubnet(local.cidr[0], 2, 1)] : var.subnets # /24 inside the /22
+  subnets = length(var.subnets) == 0 ? [
+    {
+      subnet_name   = "subnet-0"
+      subnet_ip     = cidrsubnet(local.cidr[0], 2, 0)
+      subnet_region = var.region
+    },
+    {
+      subnet_name   = "subnet-1"
+      subnet_ip     = cidrsubnet(local.cidr[0], 2, 1)
+      subnet_region = var.region
+    }
+  ] : var.subnets
+}
+
+# Pick a random /22 within 10.0.0.0/16
+resource "random_integer" "vpc_cidr" {
+  count = length(var.subnets) == 0 ? 1 : 0
+
+  min = 0
+  max = 63 # 2^(22-16)-1 = 64 slices in a /16
+}
+
 module "vpc" {
   # https://registry.terraform.io/modules/terraform-google-modules/network/google/latest
   source  = "terraform-google-modules/network/google"
@@ -6,7 +31,7 @@ module "vpc" {
   project_id   = var.project_id
   network_name = var.name
 
-  subnets = var.subnets
+  subnets = local.subnets
 }
 
 module "cloud_router" {

terraform/google/internal-modules/google-vpc/variables.tf

@@ -25,4 +25,5 @@ variable "subnets" {
       subnet_region = string
     }
   ))
+  default = []
 }