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.

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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)
}

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

Name	Version
terraform	>= 0.13.0
aws	>= 2.0
local	>= 1.2
null	>= 2.0

Providers

Name	Version
aws	>= 2.0

Modules

Name	Source	Version
nat_instance_label	cloudposse/label/null	0.24.1
nat_label	cloudposse/label/null	0.24.1
private_label	cloudposse/label/null	0.24.1
public_label	cloudposse/label/null	0.24.1
this	cloudposse/label/null	0.24.1
utils	cloudposse/utils/aws	0.4.0

Resources

Name	Type
aws_eip.default	resource
aws_eip.nat_instance	resource
aws_eip_association.nat_instance	resource
aws_instance.nat_instance	resource
aws_nat_gateway.default	resource
aws_network_acl.private	resource
aws_network_acl.public	resource
aws_route.default	resource
aws_route.nat_instance	resource
aws_route.public	resource
aws_route_table.private	resource
aws_route_table.public	resource
aws_route_table_association.private	resource
aws_route_table_association.public	resource
aws_route_table_association.public_default	resource
aws_security_group.nat_instance	resource
aws_security_group_rule.nat_instance_egress	resource
aws_security_group_rule.nat_instance_ingress	resource
aws_subnet.private	resource
aws_subnet.public	resource
aws_ami.nat_instance	data source
aws_availability_zones.available	data source
aws_eip.nat_ips	data source
aws_vpc.default	data source

Inputs

Name	Description	Type	Default	Required
additional_tag_map	Additional tags for appending to tags_as_list_of_maps. Not added to `tags`.	`map(string)`	`{}`	no
attributes	Additional attributes (e.g. `1`)	`list(string)`	`[]`	no
availability_zone_attribute_style	The style of Availability Zone code to use in tags and names. One of `full`, `short`, or `fixed`.	`string`	`"short"`	no
availability_zones	List of Availability Zones where subnets will be created	`list(string)`	n/a	yes
aws_route_create_timeout	Time to wait for AWS route creation specifed as a Go Duration, e.g. `2m`	`string`	`"2m"`	no
aws_route_delete_timeout	Time to wait for AWS route deletion specifed as a Go Duration, e.g. `5m`	`string`	`"5m"`	no
cidr_block	Base CIDR block which will be divided into subnet CIDR blocks (e.g. `10.0.0.0/16`)	`string`	n/a	yes
context	Single object for setting entire context at once. See description of individual variables for details. Leave string and numeric variables as `null` to use default value. Individual variable settings (non-null) override settings in context object, except for attributes, tags, and additional_tag_map, which are merged.	`any`	{ "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": {} }	no
delimiter	Delimiter to be used between `namespace`, `environment`, `stage`, `name` and `attributes`. Defaults to `-` (hyphen). Set to `""` to use no delimiter at all.	`string`	`null`	no
enabled	Set to false to prevent the module from creating any resources	`bool`	`null`	no
environment	Environment, e.g. 'uw2', 'us-west-2', OR 'prod', 'staging', 'dev', 'UAT'	`string`	`null`	no
id_length_limit	Limit `id` to this many characters (minimum 6). Set to `0` for unlimited length. Set to `null` for default, which is `0`. Does not affect `id_full`.	`number`	`null`	no
igw_id	Internet Gateway ID the public route table will point to (e.g. `igw-9c26a123`)	`string`	n/a	yes
label_key_case	The letter case of label keys (`tag` names) (i.e. `name`, `namespace`, `environment`, `stage`, `attributes`) to use in `tags`. Possible values: `lower`, `title`, `upper`. Default value: `title`.	`string`	`null`	no
label_order	The naming order of the id output and Name tag. Defaults to ["namespace", "environment", "stage", "name", "attributes"]. You can omit any of the 5 elements, but at least one must be present.	`list(string)`	`null`	no
label_value_case	The letter case of output label values (also used in `tags` and `id`). Possible values: `lower`, `title`, `upper` and `none` (no transformation). Default value: `lower`.	`string`	`null`	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	`number`	`0`	no
metadata_http_endpoint_enabled	Whether the metadata service is available	`bool`	`true`	no
metadata_http_put_response_hop_limit	The desired HTTP PUT response hop limit (between 1 and 64) for instance metadata requests.	`number`	`1`	no
metadata_http_tokens_required	Whether or not the metadata service requires session tokens, also referred to as Instance Metadata Service Version 2.	`bool`	`true`	no
name	Solution name, e.g. 'app' or 'jenkins'	`string`	`null`	no
namespace	Namespace, which could be your organization name or abbreviation, e.g. 'eg' or 'cp'	`string`	`null`	no
nat_elastic_ips	Existing Elastic IPs to attach to the NAT Gateway(s) or Instance(s) instead of creating new ones.	`list(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
private_subnets_additional_tags	Additional tags to be added to private subnets	`map(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
public_subnets_additional_tags	Additional tags to be added to public subnets	`map(string)`	`{}`	no
regex_replace_chars	Regex to replace chars with empty string in `namespace`, `environment`, `stage` and `name`. If not set, `"/[^a-zA-Z0-9-]/"` is used to remove all characters other than hyphens, letters and digits.	`string`	`null`	no
root_block_device_encrypted	Whether to encrypt the root block device	`bool`	`true`	no
stage	Stage, e.g. 'prod', 'staging', 'dev', OR 'source', 'build', 'test', 'deploy', 'release'	`string`	`null`	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 (e.g. `map('BusinessUnit','XYZ')`	`map(string)`	`{}`	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`	n/a	yes

Outputs

Name	Description
availability_zones	List of Availability Zones where subnets were created
nat_gateway_ids	IDs of the NAT Gateways created
nat_gateway_public_ips	EIP of the NAT Gateway
nat_instance_ids	IDs of the NAT Instances created
nat_ips	IP Addresses in use for NAT
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

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
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