Add tags to EKS-related subnets to support discovery for ELB

https://aws.amazon.com/premiumsupport/knowledge-center/eks-vpc-subnet-discovery/

Author: Vladimir Andrijevikj

aws/eks/main.tf

@@ -1,5 +1,11 @@
 locals {
-  tags = merge({ Name = var.name }, var.tags)
+  elb_discovery_tag = var.uses_nat_gateway ? "kubernetes.io/role/internal-elb" : "kubernetes.io/role/elb"
+  tags              = merge({ Name = var.name }, var.tags)
+
+  subnet_tags = merge(local.tags, {
+    (local.elb_discovery_tag) = true,
+    "kubernetes.io/cluster/${var.name}" = "shared"
+  })
 }
 
 module "eks-vpc" {
@@ -35,17 +41,10 @@ module "eks-subnets" {
   source = "../vpc/subnets"
 
   exclude_names = var.subnet_module.exclude_names
-  netnum_offset = var.subnet_module.netnum_offset
-
   internet_gateway_id = module.eks-vpc.internet_gateway_id
-  nat_gateway_id = var.uses_nat_gateway ? module.eks-vpc-nat-gateway[0].nat_gateway_id : 0
-
-  tags = merge(
-    local.tags,
-    {
-      "kubernetes.io/cluster/${var.name}" = "shared"
-    },
-  )
+  nat_gateway_id      = var.uses_nat_gateway ? module.eks-vpc-nat-gateway[0].nat_gateway_id : 0
+  netnum_offset = var.subnet_module.netnum_offset
+  tags = local.subnet_tags
   vpc_id = module.eks-vpc.vpc_id
 }
 

aws/nat_gateway/main.tf

@@ -1,3 +1,7 @@
+locals {
+  subnet_tags = merge(var.tags, { "kubernetes.io/role/elb" = true })
+}
+
 # Create subnets for use by the LoadBalancer for ingress
 # And use the first of these subnets for the NAT Gateway
 
@@ -18,14 +22,14 @@ resource "aws_subnet" "mod" {
     var.subnet_cidr_netnum_offset + count.index + 1,
   )
   map_public_ip_on_launch = true
-  tags                    = var.tags
+  tags                    = local.subnet_tags
   vpc_id                  = var.vpc_id
 }
 
 # ElasticIP address for use with the NAT Gateway
 resource "aws_eip" "nat-gw-eip" {
-  vpc   = true
-  tags  = var.tags
+  vpc  = true
+  tags = var.tags
 }
 
 # NAT Gateway in the first subnet