AWS VPC Terraform module

Upstream version 6.7.3
2 controls from CISA Cyber Essentials requirements

Terraform Module Source

cisacyberessentials.compliance.tf/terraform-aws-modules/vpc/aws
ComplianceInputsOutputsResourcesRulesDiff (0)README

AWS VPC Terraform module

Terraform module which creates VPC resources on AWS.

SWUbanner

Usage

module "vpc" {
  source = "terraform-aws-modules/vpc/aws"

  name = "my-vpc"
  cidr = "10.0.0.0/16"

  azs             = ["eu-west-1a", "eu-west-1b", "eu-west-1c"]
  private_subnets = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"]
  public_subnets  = ["10.0.101.0/24", "10.0.102.0/24", "10.0.103.0/24"]

  enable_nat_gateway = true
  enable_vpn_gateway = true

  tags = {
    Terraform = "true"
    Environment = "dev"
  }
}

[!WARNING] v6.x of the module still supports creating a VPC Flow Log within the root (VPC) module. However, this is deprecated behavior and will be removed in v7.0.0. Please use the standalone flow log module instead.

External NAT Gateway IPs

By default this module will provision new Elastic IPs for the VPC's NAT Gateways. This means that when creating a new VPC, new IPs are allocated, and when that VPC is destroyed those IPs are released. Sometimes it is handy to keep the same IPs even after the VPC is destroyed and re-created. To that end, it is possible to assign existing IPs to the NAT Gateways. This prevents the destruction of the VPC from releasing those IPs, while making it possible that a re-created VPC uses the same IPs.

To achieve this, allocate the IPs outside the VPC module declaration.

resource "aws_eip" "nat" {
  count = 3

  vpc = true
}

Then, pass the allocated IPs as a parameter to this module.

module "vpc" {
  source = "terraform-aws-modules/vpc/aws"

  # The rest of arguments are omitted for brevity

  enable_nat_gateway  = true
  single_nat_gateway  = false
  reuse_nat_ips       = true                    # <= Skip creation of EIPs for the NAT Gateways
  external_nat_ip_ids = "${aws_eip.nat.*.id}"   # <= IPs specified here as input to the module
}

Note that in the example we allocate 3 IPs because we will be provisioning 3 NAT Gateways (due to single_nat_gateway = false and having 3 subnets). If, on the other hand, single_nat_gateway = true, then aws_eip.nat would only need to allocate 1 IP. Passing the IPs into the module is done by setting two variables reuse_nat_ips = true and external_nat_ip_ids = "${aws_eip.nat.*.id}".

NAT Gateway Scenarios

This module supports three scenarios for creating NAT gateways. Each will be explained in further detail in the corresponding sections.

  • One NAT Gateway per subnet (default behavior)
    • enable_nat_gateway = true
    • single_nat_gateway = false
    • one_nat_gateway_per_az = false
  • Single NAT Gateway
    • enable_nat_gateway = true
    • single_nat_gateway = true
    • one_nat_gateway_per_az = false
  • One NAT Gateway per availability zone
    • enable_nat_gateway = true
    • single_nat_gateway = false
    • one_nat_gateway_per_az = true

If both single_nat_gateway and one_nat_gateway_per_az are set to true, then single_nat_gateway takes precedence.

One NAT Gateway per subnet (default)

By default, the module will determine the number of NAT Gateways to create based on the max() of the private subnet lists (database_subnets, elasticache_subnets, private_subnets, and redshift_subnets). The module does not take into account the number of intra_subnets, since the latter are designed to have no Internet access via NAT Gateway. For example, if your configuration looks like the following:

database_subnets    = ["10.0.21.0/24", "10.0.22.0/24"]
elasticache_subnets = ["10.0.31.0/24", "10.0.32.0/24"]
private_subnets     = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24", "10.0.4.0/24", "10.0.5.0/24"]
redshift_subnets    = ["10.0.41.0/24", "10.0.42.0/24"]
intra_subnets       = ["10.0.51.0/24", "10.0.52.0/24", "10.0.53.0/24"]

Then 5 NAT Gateways will be created since 5 private subnet CIDR blocks were specified.

Single NAT Gateway

If single_nat_gateway = true, then all private subnets will route their Internet traffic through this single NAT gateway. The NAT gateway will be placed in the first public subnet in your public_subnets block.

One NAT Gateway per availability zone

If one_nat_gateway_per_az = true and single_nat_gateway = false, then the module will place one NAT gateway in each availability zone you specify in var.azs. There are some requirements around using this feature flag:

  • The variable var.azs must be specified.
  • The number of public subnet CIDR blocks specified in public_subnets must be greater than or equal to the number of availability zones specified in var.azs. This is to ensure that each NAT Gateway has a dedicated public subnet to deploy to.

"private" versus "intra" subnets

By default, if NAT Gateways are enabled, private subnets will be configured with routes for Internet traffic that point at the NAT Gateways configured by use of the above options.

If you need private subnets that should have no Internet routing (in the sense of RFC1918 Category 1 subnets), intra_subnets should be specified. An example use case is configuration of AWS Lambda functions within a VPC, where AWS Lambda functions only need to pass traffic to internal resources or VPC endpoints for AWS services.

Since AWS Lambda functions allocate Elastic Network Interfaces in proportion to the traffic received (read more), it can be useful to allocate a large private subnet for such allocations, while keeping the traffic they generate entirely internal to the VPC.

Conditional creation

Prior to Terraform 0.13, you were unable to specify count in a module block. If you wish to toggle the creation of the module's resources in an older (pre 0.13) version of Terraform, you can use the create_vpc argument.

# This VPC will not be created
module "vpc" {
  source = "terraform-aws-modules/vpc/aws"

  create_vpc = false
  # ... omitted
}

Public access to RDS instances

Sometimes it is handy to have public access to RDS instances (it is not recommended for production) by specifying these arguments:

  create_database_subnet_group           = true
  create_database_subnet_route_table     = true
  create_database_internet_gateway_route = true

  enable_dns_hostnames = true
  enable_dns_support   = true

Network Access Control Lists (ACL or NACL)

This module can manage network ACL and rules. Once VPC is created, AWS creates the default network ACL, which can be controlled using this module (manage_default_network_acl = true).

Also, each type of subnet may have its own network ACL with custom rules per subnet. Eg, set public_dedicated_network_acl = true to use dedicated network ACL for the public subnets; set values of public_inbound_acl_rules and public_outbound_acl_rules to specify all the NACL rules you need to have on public subnets (see variables.tf for default values and structures).

By default, all subnets are associated with the default network ACL.

Public access to Redshift cluster

Sometimes it is handy to have public access to Redshift clusters (for example if you need to access it by Kinesis - VPC endpoint for Kinesis is not yet supported by Redshift) by specifying these arguments:

  enable_public_redshift = true  # <= By default Redshift subnets will be associated with the private route table

Transit Gateway (TGW) integration

It is possible to integrate this VPC module with terraform-aws-transit-gateway module which handles the creation of TGW resources and VPC attachments. See complete example there.

VPC CIDR from AWS IP Address Manager (IPAM)

It is possible to have your VPC CIDR assigned from an AWS IPAM Pool. However, In order to build subnets within this module Terraform must know subnet CIDRs to properly plan the amount of resources to build. Since CIDR is derived by IPAM by calling CreateVpc this is not possible within a module unless cidr is known ahead of time. You can get around this by "previewing" the CIDR and then using that as the subnet values.

Note: Due to race conditions with terraform plan, it is not possible to use ipv4_netmask_length or a pools allocation_default_netmask_length within this module. You must explicitly set the CIDRs for a pool to use.

# Find the pool RAM shared to your account
# Info on RAM sharing pools: https://docs.aws.amazon.com/vpc/latest/ipam/share-pool-ipam.html
data "aws_vpc_ipam_pool" "ipv4_example" {
  filter {
    name   = "description"
    values = ["*mypool*"]
  }

  filter {
    name   = "address-family"
    values = ["ipv4"]
  }
}

# Preview next CIDR from pool
data "aws_vpc_ipam_preview_next_cidr" "previewed_cidr" {
  ipam_pool_id   = data.aws_vpc_ipam_pool.ipv4_example.id
  netmask_length = 24
}

data "aws_region" "current" {}

# Calculate subnet cidrs from previewed IPAM CIDR
locals {
  partition       = cidrsubnets(data.aws_vpc_ipam_preview_next_cidr.previewed_cidr.cidr, 2, 2)
  private_subnets = cidrsubnets(local.partition[0], 2, 2)
  public_subnets  = cidrsubnets(local.partition[1], 2, 2)
  azs             = formatlist("${data.aws_region.current.name}%s", ["a", "b"])
}

module "vpc_cidr_from_ipam" {
  source            = "terraform-aws-modules/vpc/aws"
  name              = "vpc-cidr-from-ipam"
  ipv4_ipam_pool_id = data.aws_vpc_ipam_pool.ipv4_example.id
  azs               = local.azs
  cidr              = data.aws_vpc_ipam_preview_next_cidr.previewed_cidr.cidr
  private_subnets   = local.private_subnets
  public_subnets    = local.public_subnets
}

Known limitations (Terraform/OpenTofu, not this module)

A few requests come up again and again and cannot be implemented by this module, or by any module: Terraform requires lifecycle arguments to be literal values inside the resource block. hashicorp/terraform#18367 has been open since 2018, #22544 since 2019, and opentofu/opentofu#1329 is the same request for OpenTofu.

  • Terraform keeps removing the tags that Kubernetes and load balancer controllers add - Native options: move tag ownership into Terraform with provider default_tags, or fork this module and add ignore_changes = [tags, tags_all].

Compliance.tf serves this module with these rules applied at download time, on top of whatever your organization already has enabled there. Inputs and outputs do not change; the source line does. Drop the version argument and pin the release you use by adding &version= and that release number to the URL. To get started, register a free compliance.tf account and configure an access token:

source = "https://registry.compliance.tf/terraform-aws-modules/vpc/aws?add_rules=lifecycle_ignore_tags"

The full workaround for each item above, and the exact diff each rule makes, are in the compliance.tf docs for this module. To preview a diff without an account, open this module in the Rules Playground.

Disclosure: written by this module's maintainer, who also builds compliance.tf.

Examples

Contributing

Report issues/questions/feature requests on in the issues section.

Full contributing guidelines are covered here.

Authors

Module is maintained by Anton Babenko with help from these awesome contributors.

License

Apache 2 Licensed. See LICENSE for full details.

Additional information for users from Russia and Belarus