terraform-aws-dynamic-subnets

Terraform module to provision public and private subnets in an existing VPC

IMPORTANT: This module provisions NAT instance with public IP. Note: this module is intended for use with an existing VPC and existing Internet Gateway. To create a new VPC, use terraform-aws-vpc module.

This project is part of our comprehensive "SweetOps" approach towards DevOps.

It's 100% Open Source and licensed under the APACHE2.

We literally have hundreds of terraform modules that are Open Source and well-maintained. Check them out!

Security & Compliance

Security scanning is graciously provided by Bridgecrew. Bridgecrew is the leading fully hosted, cloud-native solution providing continuous Terraform security and compliance.

Benchmark	Description
	Infrastructure Security Compliance
	Center for Internet Security, KUBERNETES Compliance
	Center for Internet Security, AWS Compliance
	Center for Internet Security, AZURE Compliance
	Payment Card Industry Data Security Standards Compliance
	National Institute of Standards and Technology Compliance
	Information Security Management System, ISO/IEC 27001 Compliance
	Service Organization Control 2 Compliance
	Center for Internet Security, GCP Compliance
	Health Insurance Portability and Accountability Compliance

Usage

IMPORTANT: We do not pin modules to versions in our examples because of the difficulty of keeping the versions in the documentation in sync with the latest released versions. We highly recommend that in your code you pin the version to the exact version you are using so that your infrastructure remains stable, and update versions in a systematic way so that they do not catch you by surprise.

Also, because of a bug in the Terraform registry (hashicorp/terraform#21417), the registry shows many of our inputs as required when in fact they are optional. The table below correctly indicates which inputs are required.

module "subnets" {
  source = "cloudposse/dynamic-subnets/aws"
  # Cloud Posse recommends pinning every module to a specific version
  # version = "x.x.x"
  namespace           = "eg"
  stage               = "prod"
  name                = "app"
  vpc_id              = "vpc-XXXXXXXX"
  igw_id              = "igw-XXXXXXXX"
  cidr_block          = "10.0.0.0/16"
  availability_zones  = ["us-east-1a", "us-east-1b"]
}

module "subnets_with_existing_ips" {
  source = "cloudposse/dynamic-subnets/aws"
  # Cloud Posse recommends pinning every module to a specific version
  # version = "x.x.x"
  namespace           = "eg"
  stage               = "prod"
  name                = "app"
  vpc_id              = "vpc-XXXXXXXX"
  igw_id              = "igw-XXXXXXXX"
  cidr_block          = "10.0.0.0/16"
  availability_zones  = ["us-east-1a", "us-east-1b"]
  nat_gateway_enabled = true
  nat_elastic_ips     = ["1.2.3.4", "1.2.3.5"]
}

Learn about using providers with terraform modules.

Subnet calculation logic

terraform-aws-dynamic-subnets creates a set of subnets based on ${var.cidr_block} input and number of Availability Zones in the region.

For subnet set calculation, the module uses Terraform interpolation

cidrsubnet.

${
  cidrsubnet(
  signum(length(var.cidr_block)) == 1 ?
  var.cidr_block : data.aws_vpc.default.cidr_block,
  ceil(log(length(data.aws_availability_zones.available.names) * 2, 2)),
  count.index)
}

1. Use ${var.cidr_block} input (if specified) or use a VPC CIDR block data.aws_vpc.default.cidr_block (e.g. 10.0.0.0/16)

2. Get number of available AZ in the region (e.g. length(data.aws_availability_zones.available.names))

3. Calculate newbits. newbits number specifies how many subnets be the CIDR block (input or VPC) will be divided into. newbits is the number of binary digits.

   Example:

   newbits = 1 - 2 subnets are available (1 binary digit allows to count up to 2)

   newbits = 2 - 4 subnets are available (2 binary digits allows to count up to 4)

   newbits = 3 - 8 subnets are available (3 binary digits allows to count up to 8)

   etc.

   1. We know, that we have 6 AZs in a us-east-1 region (see step 2).

   2. We need to create 1 public subnet and 1 private subnet in each AZ, thus we need to create 12 subnets in total (6 AZs * (1 public + 1 private)).

   3. We need 4 binary digits for that ( 2⁴ = 16 ). In order to calculate the number of binary digits we should use logarithm function. We should use base 2 logarithm because decimal numbers can be calculated as powers of binary number. See Wiki for more details

      Example:

      For 12 subnets we need 3.58 binary digits (log₂12)

      For 16 subnets we need 4 binary digits (log₂16)

      For 7 subnets we need 2.81 binary digits (log₂7)

      etc.

   4. We can't use fractional values to calculate the number of binary digits. We can't round it down because smaller number of binary digits is insufficient to represent the required subnets. We round it up. See ceil.

      Example:

      For 12 subnets we need 4 binary digits (ceil(log₂12))

      For 16 subnets we need 4 binary digits (ceil(log₂16))

      For 7 subnets we need 3 binary digits (ceil(log₂7))

      etc.

   5. Assign private subnets according to AZ number (we're using count.index for that).

   6. Assign public subnets according to AZ number but with a shift according to the number of AZs in the region (see step 2)

Makefile Targets

Available targets:

  help                                Help screen
  help/all                            Display help for all targets
  help/short                          This help short screen
  lint                                Lint terraform code

Requirements

Providers

Modules

Resources

Inputs

{
  "additional_tag_map": {},
  "attributes": [],
  "delimiter": null,
  "enabled": true,
  "environment": null,
  "id_length_limit": null,
  "label_key_case": null,
  "label_order": [],
  "label_value_case": null,
  "name": null,
  "namespace": null,
  "regex_replace_chars": null,
  "stage": null,
  "tags": {}
}

Outputs

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Related Projects

Check out these related projects.

• terraform-aws-vpc - Terraform Module that defines a VPC with public/private subnets across multiple AZs with Internet Gateways
• terraform-aws-vpc-peering - Terraform module to create a peering connection between two VPCs
• terraform-aws-kops-vpc-peering - Terraform module to create a peering connection between a backing services VPC and a VPC created by Kops
• terraform-aws-named-subnets - Terraform module for named subnets provisioning.

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Copyright

Copyright © 2017-2021 Cloud Posse, LLC

License

See LICENSE for full details.

Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  https://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied.  See the License for the
specific language governing permissions and limitations
under the License.

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Contributors

Erik Osterman	Andriy Knysh	Sergey Vasilyev	Vladimir	Konstantin B	dcowan-vestmark	Ivan Pinatti	Oscar Sullivan	Joe Niland