Amazon Web Services VPC Terraform Module

This Terraform module creates a configurable general purpose Amazon Web Services VPC. The module offers an opinionated but flexible network topography geared towards general purpose situations with separate public and private subnets. Each VPC can be configured to support one to four availability zones. Private subnet NAT can be configured via NAT Gateways. A single Internet Gateway is created to provide public routing for public subnets. The module does not configure a bastion or VPN instance for private subnet instance access.

This module has been tested with Terraform version 0.7.9.

Example VPC Layout: 3 AZ's

Usage

• Include the module in your main.tf:

module "vpc" {
  source = "git::ssh://git@github.com/reactiveops/terraform-vpc.git?ref=2.0.2"

  aws_access_key = "${var.aws_access_key}"
  aws_secret_key = "${var.aws_secret_key}"
  aws_region = "${var.aws_region}"

  az_count =  "${var.az_count}"
  aws_azs = "${var.aws_azs}"

  vpc_cidr_base = "${var.vpc_cidr_base}"

}

• Create the required variables either in main.tf or a separate variables.tf file:

variable "aws_access_key" {}
variable "aws_secret_key" {}
variable "aws_region" {}

variable "aws_azs" {}
variable "az_count" {}

variable "vpc_cidr_base" {}

• Assign variable values, for example in a terraform.tfvars file:

aws_azs = "us-west-2a, us-west-2b, us-west-2c, us-west-2d"
az_count = 3
vpc_cidr_base = "10.0"

This repo contains a few example *.tfvars.examples files showing example variable configurations.

Configuration Options

VPC IP Addresses

Generated VPC's will have a /16 CIDR block providing up to 65,536 IP addresses. Choose the IP range you want by setting the vpc_cidr_base variable to the first two numbers of the desired IP range. All subnets will use IP CIDR's built on this pattern.

vpc_cidr_base = "10.1"

The following subnets will be created in each AZ:

• Public
  • Resources requiring public IP addresses such as VPN/bastion instances and Elastic Load Balancers.
• Private working
  • Internal non-production resources such as web servers and database instances.
• Private production
  • Internal production resources such as web servers and database instances.
• Private admin
  • Internal shared administrative resources such as build server instances.

Each subnet will be a /21 block providing up to 2,048 IP addresses per subnet and AZ.

AZ Count

Your VPC can span between one and four AZ's. You can select the specific AZ's that should be used.

aws_azs = "us-west-2a, us-west-2b, us-west-2c, us-west-2d"
az_count = 4

NAT Gateways

multi_az_nat_gateway

Ideally, in a multi-AZ setup, there is at least one NAT Gateway residing in each availability zone. This allows the outbound traffic from private subnets in each AZ to function independently, and allow for some resilience in-case of an AZ outage.

single_nat_gateway

In some cases, it may be necessary to use a single NAT Gateway, in a single AZ, to pass all outbound traffic from the VPC. This is usually a result of more than one private subnet needing to share a single route table. Concessions must be made in this situation since a route table can only contain a single default route. This configuration introduces a single point of failure (SPOF) in to a multi-AZ environment and should be used only when necessary.

Default values assume a multi-AZ NAT Gateway configuration:

variable "multi_az_nat_gateway" {
  description = "place a NAT gateway in each AZ"
  default = 1
}

variable "single_nat_gateway" {
  description = "use a single NAT gateway to serve outbound traffic for all AZs"
  default = 0
}

To use a single NAT gateway, set multi_az_nat_gateway = 0 and single_nat_gateway = 1 in terraform.tfvars

Tagging

The subnets created can include custom tags by setting variables of the form SUBNETNAME_subnet_tags.

Subnet	Variable
admin	admin_subnet_tags
public	public_subnet_tags
private_prod	private_prod_subnet_tags
private_working	private_working_subnet_tags

The routing tables can include custom tags by setting variables of the form TABLENAME_route_table_tags.

Route Table	Variable
public	public_route_table_tags
private	private_route_table_tags

The internet gateway can be tagged with the variable internet_gateway_tags

Testing

This repo contains a few .tfvars.example files in the root illustrating different module usage configuration patterns. Each .tfvars.example file has a corresponding tfplan output file under test/fixtures representing the expected output. The project Makefile includes targets for installing a specific version of Terraform and comparing results of a terraform plan against expected output files.

Setup

Running make test requires an actual AWS account for plan generation. The AWS account used requires read-only access to VPC/EC2 resources. No changes are applied. Credentials can be set via environment variables.

export TF_VAR_aws_access_key=XXXXXXXXXXXXXXXXX
export TF_VAR_aws_secret_key=XXXXXXXXXXXXXXXXXXXXXXXXXXXX

Executing tests

> make test

Makefile defaults expect execution on OS X. To execute on Linux:

> make test TF_PLATFORM=Linux