Allow passing in EIPs for the NAT Gateways

main.tf

@@ -1,3 +1,7 @@
+terraform {
+  required_version = ">= 0.10.13" # introduction of Local Values configuration language feature
+}
+
 ######
 # VPC
 ######
@@ -152,16 +156,28 @@ resource "aws_elasticache_subnet_group" "elasticache" {
 ##############
 # NAT Gateway
 ##############
+# Workaround for interpolation not being able to "short-circuit" the evaluation of the conditional branch that doesn't end up being used
+# Source: https://github.com/hashicorp/terraform/issues/11566#issuecomment-289417805
+#
+# The logical expression would be
+#
+#    nat_gateway_ips = var.reuse_nat_ips ? var.external_nat_ip_ids : aws_eip.nat.*.id
+#
+# but then when count of aws_eip.nat.*.id is zero, this would throw a resource not found error on aws_eip.nat.*.id.
+locals {
+  nat_gateway_ips = "${split(",", (var.reuse_nat_ips ? join(",", var.external_nat_ip_ids) : join(",", aws_eip.nat.*.id)))}"
+}
+
 resource "aws_eip" "nat" {
-  count = "${var.enable_nat_gateway ? (var.single_nat_gateway ? 1 : length(var.azs)) : 0}"
+  count = "${(var.enable_nat_gateway && !var.reuse_nat_ips) ? (var.single_nat_gateway ? 1 : length(var.azs)) : 0}"
 
   vpc = true
 }
 
 resource "aws_nat_gateway" "this" {
   count = "${var.enable_nat_gateway ? (var.single_nat_gateway ? 1 : length(var.azs)) : 0}"
 
-  allocation_id = "${element(aws_eip.nat.*.id, (var.single_nat_gateway ? 0 : count.index))}"
+  allocation_id = "${element(local.nat_gateway_ips, (var.single_nat_gateway ? 0 : count.index))}"
   subnet_id     = "${element(aws_subnet.public.*.id, (var.single_nat_gateway ? 0 : count.index))}"
 
   tags = "${merge(var.tags, map("Name", format("%s-%s", var.name, element(var.azs, (var.single_nat_gateway ? 0 : count.index)))))}"

variables.tf

@@ -65,6 +65,17 @@ variable "single_nat_gateway" {
   default     = false
 }
 
+variable "reuse_nat_ips" {
+  description = "Should be true if you don't want EIPs to be created for your NAT Gateways and will instead pass them in via the 'external_nat_ip_ids' variable"
+  default     = false
+}
+
+variable "external_nat_ip_ids" {
+  description = "List of EIP IDs to be assigned to the NAT Gateways (used in combination with reuse_nat_ips)"
+  type        = "list"
+  default     = []
+}
+
 variable "enable_dynamodb_endpoint" {
   description = "Should be true if you want to provision a DynamoDB endpoint to the VPC"
   default     = false