# README

Manage AWS Prefix Lists

This is a tool for automatically keeping one or more AWS prefix lists in sync with ip-ranges.json. This allows you to create security group rules that allow traffic from a specific AWS service or region without having to specify those ranges manually.

Architecture

The central piece of this is a Lambda function that manages the prefix lists.

The Lambda function is invoked whenever:

(1a) AWS updates the ip-ranges.json document, triggering a (1b) notification to an AWS-managed SNS topic that we subscribe to. This causes an (2a) AWS Event Bridge rule to invoke the Lambda function.
(2b) A time-based AWS Event Bridge rule (e.g., daily, every 4 hours, etc.) fires and invokes the Lambda function.

When the Lambda function runs, it:

(3) Reads ip-ranges.json,
(4) Creates or updates a set of prefix lists in your account
(5) Optionally updates AWS Systems Manager parameters with the prefix lists, and
(6) Optionally notifies an AWS Simple Notification Service (SNS) topic when changes are made.

Examples

Allowing security group access for CloudFront would look like the following:

CloudFormation

CloudFormation requires some way of setting the prefix list ids. We assume that the prefix list ids have been saved to AWS Systems Manager Parameter Store under the names /ManagedPrefixLists/CloudFront/IPv4 and /ManagedPrefixLists/CloudFront/IPv6.

VpcId:
  Type: AWS::EC2::VPC::Id
  Description: The VPC to create the security groups in.
CloudFrontPrefixListsIpv4:
  Type: AWS::SSM::Parameter::Value<List<String>>
  Description: The AWS SSM parameter holding the prefix list IDs for CloudFront's IPv4 ranges.
  Default: /ManagedPrefixLists/CloudFront/IPv4
CloudFrontPrefixListsIpv4:
  Type: AWS::SSM::Parameter::Value<List<String>>
  Description: The AWS SSM parameter holding the prefix list IDs for CloudFront's IPv6 ranges.
  Default: /ManagedPrefixLists/CloudFront/IPv6
Resources:
  # Ideally you should have a CloudFormation macro here that expands into the required number of security groups instead of
  # hard-coding the number directly. This is an advanced CloudFormation setup.
  AllowCloudFront0:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupName: CloudFront-HTTPS (Group 0)
      GroupDescription: Allow CloudFront to access HTTPS ports
      VpcId: ...
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 443
          ToPort: 443
          SourcePrefixListId: !Select [0, !Ref CloudFrontPrefixListsIpv4]
        - IpProtocol: tcp
          FromPort: 443
          ToPort: 443
          SourcePrefixListId: !Select [0, !Ref CloudFrontPrefixListsIpv6]
  AllowCloudFront1:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupName: CloudFront-HTTPS (Group 1)
      GroupDescription: Allow CloudFront to access HTTPS ports
      VpcId: ...
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 443
          ToPort: 443
          SourcePrefixListId: !Select [1, !Ref CloudFrontPrefixListsIpv4]

Terraform

As of this writing, Terraform's aws_prefix_list data source cannot query customer-managed prefix lists properly. Here, we assume that the prefix list ids have been saved to AWS Systems Manager Parameter Store under the names /ManagedPrefixLists/CloudFront/IPv4 and /ManagedPrefixLists/CloudFront/IPv6.

variable "region" {
    type = string
    description = "The AWS region to deploy to."
}

variable "aws_vpc_id" {
    type = string
    description = "The VPC to create the security groups in."
}

variable "prefix_list_ssm_parameter_ipv4" {
    type = string
    description = "The AWS SSM parameter holding the prefix list IDs for CloudFront's IPv4 ranges."
    default = "/ManagedPrefixLists/CloudFront/IPv4"
}

variable "prefix_list_ssm_parameter_ipv4" {
    type = string
    description = "The AWS SSM parameter holding the prefix list IDs for CloudFront's IPv6 ranges."
    default = "/ManagedPrefixLists/CloudFront/IPv6"
}

provider "aws" {
    region = var.region
}

data "aws_ssm_parameter" "prefix_list_ipv4" {
    name = var.prefix_list_ssm_parameter.ipv4
}

data "aws_ssm_parameter" "prefix_list_ipv6" {
    name = var.prefix_list_ssm_parameter.ipv6
}

resource "aws_security_group" "cloudfront" {
    # Create as many groups as needed according to the number of prefix lists seen.
    count = max(length(data.aws_ssm_parameter.prefix_list.ipv4.value, data.aws_ssm_parameter.prefix_list.ipv6.value))
    name = "CloudFrfront-HTTPS (Group ${count.index})"
    description = "Allow CloudFront to access HTTPS ports"
    vpc_id = var.aws_vpc_id
}

resource "aws_security_group_rule" "cloudfront_ipv4" {
    count = length(data.aws_ssm_parameter.prefix_list.ipv4.value)
    security_group_id = aws_security_group.cloudfront[count.index].id
    type = "ingress"
    protocol = "tcp"
    from_port = 443
    to_port = 443
    prefix_list_ids = [data.aws.ssm_parameter.prefix_list_ipv4.value[count.index]]
}

resource "aws_security_group_rule" "cloudfront_ipv6" {
    count = length(data.aws_ssm_parameter.prefix_list.ipv6.value)
    security_group_id = aws_security_group.cloudfront[count.index].id
    type = "ingress"
    protocol = "tcp"
    from_port = 443
    to_port = 443
    prefix_list_ids = [data.aws.ssm_parameter.prefix_list_ipv6.value[count.index]]
}

module "managed_prefix_list_core" {
    source  = "ionosphere-io/managed-prefix-list-core/aws"
    version = "v0.2.0"
    region = var.region
}

module "cloudfront_prefix_list" {
    source  = "ionosphere-io/managed-prefix-list/aws"
    version = "v0.2.0"
    region = var.region
    lambda_function_arn = module.managed_prefix_list_core.lambda_function_arn

    filter_service = "CLOUDFRONT"
}

Caution!

Security groups have a default limit of 60 IP rules per group -- this cannot be increased without consequences.

Each customer-managed prefix list has a "maximum number of entries" setting. If you assign a customer-managed prefix list to a security group, the value of this setting counts against your limit for that security group. The actual number of CIDR blocks in the prefix list is irrelevant here.