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Terraform module for creating a simple private Terraform registry in AWS with DynamoDB
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README.md

Terraform Private Registry for AWS

This Terraform module establishes a private registry for Terraform, allowing you to publish your own modules in a location you control independent of Terraform's public registry at registry.terraform.io.

Terraform module addresses can include an optional hostname part which allows them to be downloaded from services other than the public registry:

module "awesomeapp" {
  source = "tf.example.com/awesomecorp/awesomeapp/aws"
}

The module in this repository provides the API endpoints necessary to provide such a module source hostname. Terraform's documented registry HTTP API is implemented via Amazon API Gateway relaying requests to a DynamoDB table that contains a simple index of modules. The module packages themselves can be stored at any non-registry module source address supported by Terraform, including in an S3 bucket with standard AWS authentication.

This module requires Terraform 0.12 or newer.

If using this module in production, be sure to select a specific version via a version constraint argument in your module block to avoid surprising changes during upgrades. This module uses semantic versioning, and so new minor releases may introduce additional features that may lead to additional cost.

For the moment this module remains EXPERIMENTAL. Once we've heard good feedback about how it behaves in real-world situations it will be blessed with a non-experimental version number (1.0 or greater).

Simple Usage

When called with no arguments, this module will create a registry API with no access control backed by a DynamoDB table called TerraformRegistry-modules:

module "tf_registry" {
  source = "apparentlymart/tf-registry/aws"
}

output "rest_api_id" {
  value = module.tf_registry.rest_api_id
}

output "services" {
  value = module.tf_registry.services
}

Since the registry is just an index for module packages stored elsewhere, it may be acceptable in some environments to allow unauthenticated access to the registry API while protecting access to the packages themselves, and so the above may be sufficient to get started.

After the initial creation of the registry, the DynamoDB table will be empty. Use any normal strategy for population of the registry, such as the DynamoDB management console, the AWS CLI, or a custom program using an AWS SDK. For the sake of example here, we'll create an alias for the HashiCorp Consul module in the public registry using the AWS CLI:

aws dynamodb put-item \
  --table-name TerraformRegistry-modules \
  --item '{
      "Id": {"S":"hashicorp/consul/aws"},
      "Version": {"S":"0.4.4"},
      "Source": {"S":"https://api.github.com/repos/hashicorp/terraform-aws-consul/tarball/v0.4.4//*?archive=tar.gz"}
  }'

The {"S":...} objects here are the DynamoDB convention for indicating a string value. The Id and Version attributes together form the primary key for the table, and Source specifies a module source address where the module package can be downloaded. In this case, we indicate a .tar.gz archive of a tag from a repository on GitHub.

The terraform apply log should include a value for the services output indicated in the configuration above, which will look something like this:

Outputs:

rest_api_id = b9h60hion6
services = {
  "modules.v1" = "https://b9h60hion6.execute-api.us-west-2.amazonaws.com/live/modules.v1/"
}

This map value is a service discovery document for Terraform's service discovery protocol. For normal use it would be necessary to publish a JSON version of this document at /.well-known/terraform.json on an HTTPS server running at the hostname that will be used to install modules, but for initial testing we can use an override configuration in the Terraform CLI config file (not your infrastructure configuration in .tf files):

host "tf.example.com" {
  services = {
    "modules.v1" = "https://b9h60hion6.execute-api.us-west-2.amazonaws.com/live/modules.v1/"
  }
}

With this block in place, Terraform will use this hard-coded map instead of trying to request a discovery document over the network. From another separate Terraform configuration we should then be able to request the Consul module via this private registry:

module "consul" {
  source = "tf.example.com/hashicorp/consul/aws"

  # ...
}

The module installer in terraform init should then be able to download the module by first requesting its package location from our private registry.

The remaining sections of this README will cover some other options and configuration details.

Customizing AWS Object Names

By default, this module creates various objects across a number of different AWS services using names starting with TerraformRegistry. You can customize this prefix by setting the name_prefix argument:

module "tf_registry" {
  source = "apparentlymart/tf-registry/aws"

  name_prefix = "AnotherTerraformRegistry"
}

Changing this name after the module is initially created requires re-creating all remote objects, including the underlying DynamoDB table. That means any data in that table would be lost and must be restored from backup.

Publishing Module Packages in Amazon S3

If all of the users or compute instances where you run the Terraform CLI have access to suitable AWS credentials, it may be convenient to publish your private modules as archives in a protected Amazon S3 bucket.

To do this, create a zip file containing the module source code with the main (top-level) module at the root of the file. Place this file in an S3 bucket with suitable permissions. The naming scheme for objects in this bucket is up to you, but we'd suggest using a systematic scheme like namespace/module/provider/module_provider_version.zip, giving (for example) hashicorp/consul/aws/consul_aws_v0.4.4.zip.

When you record these in the registry's DynamoDB table, use the s3:: prefix followed by an S3 URL to instruct Terraform to use the S3 authentication protocol when credentials are available:

aws dynamodb put-item \
  --table-name TerraformRegistry-modules \
  --item '{
    "Id": {"S":"hashicorp/consul/aws"},
    "Version": {"S":"0.4.4"},
    "Source": {"S":"s3::https://s3-us-west-2.amazonaws.com/hashicorp/consul/aws/consul_aws_v0.4.4.zip"}
  }'

For more information on how Terraform retrieves modules from S3 buckets, and in particular where it will look to obtain AWS credentials for the request, see the S3 Bucket source documentation.

The module registry protocol itself cannot support AWS-style authentication, so you must either allow unauthenticated requests to the registry endpoints (which will disclose only metadata about the modules) or configure the registry to use a Lambda authorizer, as described in a later section.

Re-deploying the API after Changes

Most customizations in the following sections cause changes to the API Gateway configuration. Due to the design of API Gateway, such changes must be explicitly re-deployed after Terraform has finished applying them.

To do this, look for the rest_api_id output value in the terraform apply output and insert as the --rest-api-id value in the following AWS CLI command line:

aws apigateway create-deployment \
    --rest-api-id b9h60hion6 \
    --stage-name live

This will be necessary after any terraform apply whose plan includes changes to resources with types starting with aws_api_gateway_.

Publishing the Discovery Document

If you already have an HTTPS server running on a suitable hostname then you can make your private registry accessible via that hostname by publishing a JSON version of the discovery map at /.well-known/terraform.json on that server:

{
  "modules.v1": "https://b9h60hion6.execute-api.us-west-2.amazonaws.com/live/modules.v1/"
}

For example, to make the above example hostname tf.example.com work without local overrides, the discovery document would need to be published at https://tf.example.com/.well-known/terraform.json.

Automatic Discovery Document

If you'd rather keep the whole registry deployment self-contained, this tf-registry module can optionally publish itself at a hostname of your choice and host its own JSON discovery document like the above.

For this to work you will first need to create and verify an AWS Certificate Manager certificate in the same region where this module is being deployed. Such a certificate can be provisioned automatically in Terraform if your domain is hosted in Route53.

With the certificate created, the optional friendly_hostname argument for this module calls for the hostname mapping to be configured:

module "tf_registry" {
  source = "apparentlymart/tf-registry/aws"

  friendly_hostname = {
    host                = aws_acm_certificate.cert.domain_name
    acm_certificate_arn = aws_acm_certificate_validation.cert.certificate_arn
  }
}

When friendly_hostname is set, the module will additionally configure API Gateway to serve the registry API and an automatically-generated discovery document at the given hostname and with the given certificate.

To complete this configuration, you'll need to create an entry in your DNS zone to point requests at the registry API. If you're using Route53 then you can create an alias record using aws_route53_record:

resource "aws_route53_record" "tf" {
  zone_id = var.your_zone_id

  name = aws_acm_certificate.cert.domain_name
  type = "A"
  alias {
    name    = module.tf_registry.dns_alias.hostname
    zone_id = module.tf_registry.dns_alias.route53_zone_id
  }
}

After giving time for the changes to propagate, you should be able to request the discovery document at your hostname using curl, such as the following example continuing to use tf.example.com:

$ curl https://tf.example.com/.well-known/terraform.json
{
  "modules.v1":"/modules.v1/"
}

Once this works, Terraform should be able to find modules via that hostname. If you added a host block to the Terraform CLI configuration during the "Simple Usage" steps above, remember to remove it to allow Terraform to do normal discovery over the network.

Access Control

Terraform CLI supports bearer-token authentication credentials when making API requests. Credentials are configured on a per-hostname basis and apply to all services at that hostname. An authentication token for a particular hostname can be configured using a credentials block in the CLI configuration.

This module has no built-in support for authentication, but you can add token authentication by writing an AWS Lambda-based authorizer function that checks submitted API tokens in any way that makes sense for your environment.

Once you have written an authorizer function, you can enable it for your registry using the optional lambda_authorizer argument:

module "tf_registry" {
  source = "apparentlymart/tf-registry/aws"

  lambda_authorizer = {
    type          = "TOKEN"
    function_name = aws_lambda_function.auth.function_name
  }
}

The type attribute can be set to either TOKEN or REQUEST, depending on which calling convention the authorizer function is expecting. If TOKEN is selected, the function recieves the content of the Authorization HTTP header, where Terraform CLI places any configured bearer token.

When writing your authorizer function, remember that the Authorization header value has a Bearer prefix to indicate that Terraform is using bearer token authentication. Your function must check for this and then strip it off before checking whether the rest of the header value is a valid token.

The details of writing an authorizer function are beyond the scope of this readme. For more information, see Introducing custom authorizers in Amazon API Gateway from the AWS Compute Blog.

Advanced Scenarios Using the Submodules

The top-level module in this package is intended as a good set of defaults for a simple registry deployment. If you need more control in your environment, you may prefer to use directly the sub-modules that the top-level module is constructed from, which can be composed together in different ways to make different tradeoffs:

  • modules-store manages the DynamoDB table that stores the module registry index.
  • modules.v1 implements the version 1 HTTP API that Terraform CLI expects, using API Gateway against a given DynamoDB table which is assumed to be one created by the modules-store module.
  • disco adds a /.well-known/terraform.json discovery document to the root of any given API Gateway REST API.

None of these sub-modules create an API Gateway REST API themselves, so you can write your own module that creates and configures a REST API as meets your needs and then use these sub-modules to populate it with the modules API functionality and, if desired, a discovery document.

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