add reverse DNS support (#20)

outputs.tf

@@ -52,3 +52,13 @@ output "instance_ids_spot" {
   description = "List of instance IDs for spot instances"
   value       = { for name, instance in module.spot_requests : name => instance.instance_id }
 }
+
+output "reverse_dns_zone_name" {
+  description = "Reverse DNS zone name"
+  value       = aws_route53_zone.reverse.name
+}
+
+output "reverse_dns_zone_id" {
+  description = "Reverse DNS zone ID"
+  value       = aws_route53_zone.reverse.zone_id
+}

route53.tf

@@ -0,0 +1,61 @@
+# Reverse DNS zone for subnet
+locals {
+  # 1) Split CIDR into [address, mask]
+  cidr_parts = split("/", var.environment_cidr)
+
+  # 2) Convert mask to number
+  mask_bits = tonumber(local.cidr_parts[1])
+
+  # 3) Compute the network address (host bits = 0)
+  #    "cidrhost(..., 0)" returns the base network address of that CIDR.
+  network_address = cidrhost(var.environment_cidr, 0)
+
+  # 4) Split the network address (e.g. "10.0.1.0") into ["10", "0", "1", "0"]
+  address_octets = split(".", local.network_address)
+
+  # 5) Figure out how many octets are part of the “network” portion
+  #    (Assuming mask_bits is multiple of 8, e.g. /8, /16, /24)
+  #
+  #    /8  => first 1 octet
+  #    /16 => first 2 octets
+  #    /24 => first 3 octets
+  #    ...
+  relevant_octets = slice(local.address_octets, 0, ceil(local.mask_bits / 8))
+
+  # 6) Reverse the order (e.g. ["10", "0", "1"] => ["1", "0", "10"])
+  reversed_octets = reverse(local.relevant_octets)
+
+  # 7) Join reversed octets with '.' and add .in-addr.arpa
+  reversed_zone_string = join(".", local.reversed_octets)
+  reverse_dns_zone     = format("%s.in-addr.arpa", local.reversed_zone_string)
+}
+
+resource "aws_route53_zone" "reverse" {
+  name = local.reverse_dns_zone
+
+  vpc {
+    vpc_id = var.vpc_id
+  }
+
+  tags = merge(var.default_tags, { Environment = var.tag_environment })
+}
+
+# PTR records for on-demand instances
+resource "aws_route53_record" "ondemand_ptr" {
+  for_each = var.on_demand_requests
+  zone_id  = aws_route53_zone.reverse.zone_id
+  name     = "${join(".", reverse(split(".", module.on_demand_requests[each.key].private_ip)))}.in-addr.arpa"
+  type     = "PTR"
+  ttl      = "300"
+  records  = ["${module.on_demand_requests[each.key].hostname}."]
+}
+
+# PTR records for spot instances
+resource "aws_route53_record" "spot_ptr" {
+  for_each = var.spot_requests
+  zone_id  = aws_route53_zone.reverse.zone_id
+  name     = "${join(".", reverse(split(".", module.spot_requests[each.key].private_ip)))}.in-addr.arpa"
+  type     = "PTR"
+  ttl      = "300"
+  records  = ["${module.spot_requests[each.key].hostname}."]
+}