Add support to configure custom routes to TGWs

Adding a new variable custom_route_to_tgw which is map of objects.
The map key is used to identify required subnets.
Each object contains route configuration.
Right now I'm creating a new route in the private and public subnets.
Not sure if this can be useful also for the transit_gateway and / or core_network subnets.
It is fair to mention that there is one limitation of this variable - one cannot create more routes per a subnet.
Not sure how to achieve it without too much changes.

README.md

@@ -340,11 +340,13 @@ Please see our [developer documentation](https://github.com/aws-ia/terraform-aws
 | [aws_route.ipv6_private_to_tgw](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
 | [aws_route.ipv6_public_to_cwan](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
 | [aws_route.ipv6_public_to_tgw](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
+| [aws_route.private_subnets_custom_route_to_tgw](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
 | [aws_route.private_to_cwan](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
 | [aws_route.private_to_egress_only](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
 | [aws_route.private_to_nat](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
 | [aws_route.private_to_tgw](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
 | [aws_route.public_ipv6_to_igw](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
+| [aws_route.public_subnets_custom_route_to_tgw](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
 | [aws_route.public_to_cwan](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
 | [aws_route.public_to_igw](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
 | [aws_route.public_to_tgw](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/route) | resource |
@@ -377,6 +379,7 @@ Please see our [developer documentation](https://github.com/aws-ia/terraform-aws
 | <a name="input_core_network"></a> [core\_network](#input\_core\_network) | AWS Cloud WAN's core network information - to create a VPC attachment. Required when `cloud_wan` subnet is defined. Two attributes are required: the `id` and `arn` of the resource. | <pre>object({<br>    id  = string<br>    arn = string<br>  })</pre> | <pre>{<br>  "arn": null,<br>  "id": null<br>}</pre> | no |
 | <a name="input_core_network_ipv6_routes"></a> [core\_network\_ipv6\_routes](#input\_core\_network\_ipv6\_routes) | Configuration of IPv6 route(s) to AWS Cloud WAN's core network.<br>For each `public` and/or `private` subnets named in the `subnets` variable, optionally create routes from the subnet to the core network.<br>You can specify either a CIDR range or a prefix-list-id that you want routed to the core network.<br>Example:<pre>core_network_ivp6_routes = {<br>  public  = "::/0"<br>  private = "pl-123"<br>}</pre> | `any` | `{}` | no |
 | <a name="input_core_network_routes"></a> [core\_network\_routes](#input\_core\_network\_routes) | Configuration of route(s) to AWS Cloud WAN's core network.<br>For each `public` and/or `private` subnets named in the `subnets` variable, optionally create routes from the subnet to the core network.<br>You can specify either a CIDR range or a prefix-list-id that you want routed to the core network.<br>Example:<pre>core_network_routes = {<br>  public  = "10.0.0.0/8"<br>  private = "pl-123"<br>}</pre> | `any` | `{}` | no |
+| <a name="input_custom_route_to_tgw"></a> [custom\_route\_to\_tgw](#input\_custom\_route\_to\_tgw) | Add a custom route to a transit gateway in a subnet.<br>  The subnet is determined based on a map key that matches the key<br>  provided in the var.subnets variable. This allows you to define<br>  the specific subnet where the custom route should be applied.<br><br>Example:<pre>subnets = {<br>  # Dual-stack subnet<br>  public = {<br>    netmask                   = 24<br>    assign_ipv6_cidr          = true<br>    nat_gateway_configuration = "single_az"<br>  }<br>  # IPv4 only subnet<br>  private = {<br>    netmask                  = 24<br>    connect_to_public_natgw  = true<br>  }<br>}<br>custom_route_to_tgw = {<br>  private = {<br>    destination_cidr_block = "10.0.0.0/16"<br>    transit_gateway_id     = "tgw-0282179847129898sd"<br>  }<br>}</pre> | <pre>map(object({<br>    destination_cidr_block     = optional(string)<br>    destination_prefix_list_id = optional(string)<br>    transit_gateway_id         = string<br>  }))</pre> | `{}` | no |
 | <a name="input_tags"></a> [tags](#input\_tags) | Tags to apply to all resources. | `map(string)` | `{}` | no |
 | <a name="input_transit_gateway_id"></a> [transit\_gateway\_id](#input\_transit\_gateway\_id) | Transit gateway id to attach the VPC to. Required when `transit_gateway` subnet is defined. | `string` | `null` | no |
 | <a name="input_transit_gateway_ipv6_routes"></a> [transit\_gateway\_ipv6\_routes](#input\_transit\_gateway\_ipv6\_routes) | Configuration of IPv6 route(s) to transit gateway.<br>For each `public` and/or `private` subnets named in the `subnets` variable,<br>Optionally create routes from the subnet to transit gateway. Specify the CIDR range or a prefix-list-id that you want routed to the transit gateway.<br>Example:<pre>transit_gateway_ipv6_routes = {<br>  public  = "::/0"<br>  private = "pl-123"<br>}</pre> | `any` | `{}` | no |

data.tf

@@ -27,6 +27,16 @@ locals {
 
   # constructed list of <private_subnet_key>/az
   private_per_az = flatten([for az in local.azs : [for subnet in local.private_subnet_names : "${subnet}/${az}"]])
+  # List of private subnets names where will be added a route to a TGW.
+  # The tricky part here is that we have to identify the subnet names where a route
+  # should be added because the subnet name has suffix /az in the name.
+  custom_route_to_tgw_subnet_names = flatten([
+    for subnet_name, _ in var.custom_route_to_tgw :
+    [
+      for subnet_name_az in local.private_per_az :
+      subnet_name_az if startswith(subnet_name_az, "${subnet_name}/")
+    ]
+  ])
   # list of private subnet keys with connect_to_public_natgw = true
   private_subnets_nat_routed = [for type in local.private_subnet_names : type if try(var.subnets[type].connect_to_public_natgw == true, false)]
   # private subnets with cidrs per az if connect_to_public_natgw = true ...  "privatetwo/us-east-1a"

main.tf

@@ -305,6 +305,26 @@ resource "aws_route" "private_to_tgw" {
   ]
 }
 
+resource "aws_route" "private_subnets_custom_route_to_tgw" {
+  for_each = toset(local.custom_route_to_tgw_subnet_names)
+
+  destination_cidr_block     = var.custom_route_to_tgw[split("/", each.key)[0]].destination_cidr_block
+  destination_prefix_list_id = var.custom_route_to_tgw[split("/", each.key)[0]].destination_prefix_list_id
+
+  route_table_id     = aws_route_table.private[each.key].id
+  transit_gateway_id = var.custom_route_to_tgw[split("/", each.key)[0]].transit_gateway_id
+}
+
+resource "aws_route" "public_subnets_custom_route_to_tgw" {
+  for_each = lookup(var.custom_route_to_tgw, "public", null) != null ? toset(local.azs) : toset([])
+
+  destination_cidr_block     = var.custom_route_to_tgw["public"].destination_cidr_block
+  destination_prefix_list_id = var.custom_route_to_tgw["public"].destination_prefix_list_id
+
+  route_table_id     = aws_route_table.public[each.key].id
+  transit_gateway_id = var.custom_route_to_tgw["public"].transit_gateway_id
+}
+
 # Route: IPv6 routes from private subnets to the Transit Gateway (if configured in var.transit_gateway_ipv6_routes)
 resource "aws_route" "ipv6_private_to_tgw" {
   for_each = toset(local.ipv6_private_subnet_key_names_tgw_routed)

variables.tf

@@ -250,6 +250,44 @@ EOF
   }
 }
 
+variable "custom_route_to_tgw" {
+  description = <<-EOF
+    Add a custom route to a transit gateway in a subnet.
+    The subnet is determined based on a map key that matches the key
+    provided in the var.subnets variable. This allows you to define
+    the specific subnet where the custom route should be applied.
+
+  Example:
+  ```
+  subnets = {
+    # Dual-stack subnet
+    public = {
+      netmask                   = 24
+      assign_ipv6_cidr          = true
+      nat_gateway_configuration = "single_az"
+    }
+    # IPv4 only subnet
+    private = {
+      netmask                  = 24
+      connect_to_public_natgw  = true
+    }
+  }
+  custom_route_to_tgw = {
+    private = {
+      destination_cidr_block = "10.0.0.0/16"
+      transit_gateway_id     = "tgw-0282179847129898sd"
+    }
+  }
+  ```
+  EOF
+  type = map(object({
+    destination_cidr_block     = optional(string)
+    destination_prefix_list_id = optional(string)
+    transit_gateway_id         = string
+  }))
+  default = {}
+}
+
 variable "tags" {
   description = "Tags to apply to all resources."
   type        = map(string)