terraform-aws-dynamic-subnets

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

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.

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Usage

IMPORTANT: The master branch is used in source just as an example. In your code, do not pin to master because there may be breaking changes between releases. Instead pin to the release tag (e.g. ?ref=tags/x.y.z) of one of our latest releases.

module "subnets" {
  source              = "git::https://github.com/cloudposse/terraform-aws-dynamic-subnets.git?ref=master"
  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"]
}

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

Inputs

Name	Description	Type	Default	Required
additional_tag_map	Additional tags for appending to each tag map	map(string)	<map>	no
attributes	Any extra attributes for naming these resources	list(string)	<list>	no
availability_zones	List of Availability Zones where subnets will be created	list(string)	-	yes
cidr_block	Base CIDR block which will be divided into subnet CIDR blocks (e.g. 10.0.0.0/16)	string	-	yes
context	Default context to use for passing state between label invocations	object	<map>	no
delimiter	Delimiter to be used between namespace, stage, name and attributes	string	-	no
environment	The environment name if not using stage	string	``	no
igw_id	Internet Gateway ID the public route table will point to (e.g. igw-9c26a123)	string	-	yes
label_order	The naming order of the ID output and Name tag	list(string)	<list>	no
map_public_ip_on_launch	Instances launched into a public subnet should be assigned a public IP address	bool	true	no
max_subnet_count	Sets the maximum amount of subnets to deploy. 0 will deploy a subnet for every provided availablility zone (in availability_zones variable) within the region	string	0	no
name	Solution name, e.g. 'app' or 'jenkins'	string	``	no
namespace	Namespace, which could be your organization name or abbreviation, e.g. 'eg' or 'cp'	string	``	no
nat_gateway_enabled	Flag to enable/disable NAT Gateways to allow servers in the private subnets to access the Internet	bool	true	no
nat_instance_enabled	Flag to enable/disable NAT Instances to allow servers in the private subnets to access the Internet	bool	false	no
nat_instance_type	NAT Instance type	string	t3.micro	no
private_network_acl_id	Network ACL ID that will be added to private subnets. If empty, a new ACL will be created	string	``	no
public_network_acl_id	Network ACL ID that will be added to public subnets. If empty, a new ACL will be created	string	``	no
regex_replace_chars	Regex to replace chars with empty string in namespace, environment, stage and name. By default only hyphens, letters and digits are allowed, all other chars are removed	string	/[^a-zA-Z0-9-]/	no
stage	Stage, e.g. 'prod', 'staging', 'dev', or 'test'	string	``	no
subnet_type_tag_key	Key for subnet type tag to provide information about the type of subnets, e.g. cpco.io/subnet/type=private or cpco.io/subnet/type=public	string	cpco.io/subnet/type	no
subnet_type_tag_value_format	This is using the format interpolation symbols to allow the value of the subnet_type_tag_key to be modified.	string	%s	no
tags	Additional tags to apply to all resources that use this label module	map(string)	<map>	no
vpc_default_route_table_id	Default route table for public subnets. If not set, will be created. (e.g. rtb-f4f0ce12)	string	``	no
vpc_id	VPC ID where subnets will be created (e.g. vpc-aceb2723)	string	-	yes

Outputs

Name	Description
availability_zones	List of Availability Zones where subnets were created
nat_gateway_ids	IDs of the NAT Gateways created
nat_instance_ids	IDs of the NAT Instances created
private_route_table_ids	IDs of the created private route tables
private_subnet_cidrs	CIDR blocks of the created private subnets
private_subnet_ids	IDs of the created private subnets
public_route_table_ids	IDs of the created public route tables
public_subnet_cidrs	CIDR blocks of the created public subnets
public_subnet_ids	IDs of the created public subnets

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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 © 2017-2019 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
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Contributors

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