terraform-aws-apigateway-custom-domain

This is intended to simplify the process of providing access to AWS API Gateway Private REST API's. It draws heavily upon the blog post Enabling Private APIs with Custom Domain Names (AWS API Gateway) and the repository serverless-samples/apigw-private-custom-domain-name that draws heavily from that same blog post.

In addition, it provides an extra VPC Endpoint Service in the account so that the CNAME does not point directly to the load balancers which have IP addresses that are subject to change.

An implementer may also provide vpc_endpoint_remote that is the DNS of a VPC Endpoint created in another connectivity account. That would cause the DNS alias to point to that interface rather than the local interface created by this module.

Usage

Alias Direct to NLB

In its most simple form:

module "this" {
  source = "theherk/apigateway-custom-domain/aws"

  api_id           = "jqr4abcdef"
  certificate_arn  = "arn:aws:acm:eu-north-1:123412341234:certificate/3f7ba345-30e0-481e-b513-f869b8834d42"
  domain_name      = "api.your.internal.domain"
  name             = "h4s-test"
  subnet_ids       = module.network.private_subnets[*].id
  vpc_endpoint_api = module.network.vpc_endpoint.api.id
  vpc_id           = module.network.vpc_id
  zone_id          = module.network.zones.internal.id
}

This is the most inexpensive and simple option provided by this module . It is also the most simple. Here, the alias record points directly to the NLB. If you don't need to route traffic through a connectivity account and you don't need to add the IP's to an allow list, this is probably fine.

ref: Sequence Diagram

Alias to VPC Endpoint in API Account

If one needs to add the request IP's to an allow list in a firewall, you shouldn't rely on the permanence of NLB's IP addresses. They change; sometimes not for a long time, but they change. In these cases, we put another VPC Endpoint Service in front of the NLB, then direct the DNS record toward a VPC Endpoint Interface that connects to that service.

module "this" {
  source = "theherk/apigateway-custom-domain/aws"

  api_id             = "jqr4abcdef"
  certificate_arn    = "arn:aws:acm:eu-north-1:123412341234:certificate/3f7ba345-30e0-481e-b513-f869b8834d42"
  domain_name        = "api.your.internal.domain"
  name               = "h4s-test"
  subnet_ids         = module.network.private_subnets[*].id
  vpc_endpoint_api   = module.network.vpc_endpoint.api.id
  vpc_endpoint_local = true
  vpc_id             = module.network.vpc_id
  zone_id            = module.network.zones.internal.id
}

ref: Sequence Diagram

Alias to VPC Endpoint in Connectivity Account

In situations where your private network connections ingress from another account, assuming your have a TGW route covering both VPC's, you can first use vpc_endpoint_local. That will create the infrastructure above, but gives as an output vpc_endpoint_service. You can then use that to create a VPC Endpoint Interface in your connectivity account that targets the created vpc_endpoint_service. Giving this service back as vpc_endpoint_service_remote, will remove the local VPC Endpoint Interface and point the DNS record to the VPC Endpoint Interface in the connectivity account. Yielding the following sequence.

After having done the above with vpc_endpoint_local = true, create your VPC Endpoint Interface toward the output vpc_endpoint_service. Then modify the configuration to:

module "this" {
  source = "theherk/apigateway-custom-domain/aws"

  api_id              = "jqr4abcdef"
  certificate_arn     = "arn:aws:acm:eu-north-1:123412341234:certificate/3f7ba345-30e0-481e-b513-f869b8834d42"
  domain_name         = "api.your.internal.domain"
  name                = "h4s-test"
  subnet_ids          = module.network.private_subnets[*].id
  vpc_endpoint_api    = module.network.vpc_endpoint.api.id
  vpc_endpoint_remote = "url-of-endpoint-created-in-connectivity-account"
  vpc_id              = module.network.vpc_id
  zone_id             = module.network.zones.internal.id
}

ref: Sequence Diagram

Cost

Originally, this was designed with the ability to use existing load balancers and ALB path routing, but it was much more complex. So although if you look at the documents referenced and blog posts about this topic you'll find ALB is supported, for simplicity this module takes a much more opinionated approach. For each API custom domain, a new NLB is created and associated VPC Endpoints. This does incur some cost. This will likely be in the tens of dollars per month, but you should definitely check out the pricing pages for your region.

Contributing

To work on this repository, you need to install the pre-commit hooks, and dependencies from pre-commit-terraform.

make pre-commit

That should be the easy way, but if you use another package manager than apt, brew, or yum or want to configure these differently on your system, you can do so by following the guidance here. For instance, you can set this up to use docker for running checks rather than installing directly to your filesystem.

After doing this, several checks will be run when attempting commits.

---

note: The following is generated by terraform docs.

Requirements

Name	Version
terraform	>= 0.15
aws	>= 3.64

Providers

Name	Version
aws	5.41.0

Modules

No modules.

Resources

Name	Type
aws_api_gateway_base_path_mapping.this	resource
aws_api_gateway_domain_name.this	resource
aws_lb.this	resource
aws_lb_listener.this	resource
aws_lb_target_group.this	resource
aws_lb_target_group_attachment.this	resource
aws_route53_record.this	resource
aws_security_group.api_endpoint	resource
aws_vpc_endpoint.api	resource
aws_vpc_endpoint.local	resource
aws_vpc_endpoint_service.this	resource
aws_network_interface.this	data source
aws_region.current	data source
aws_subnet.this	data source
aws_vpc_endpoint.api	data source

Inputs

Name	Description	Type	Default	Required
allowed_principals	Principals allowed to discover the VPC Endpoint Service. By default, principals outside the VPC will not be able to create interfaces to the Endpoint Service.	list(string)	null	no
api_id	API to which the requests are destined.	string	n/a	yes
api_stage	Name of the stage used for the base path mapping.	string	n/a	yes
certificate_arn	ACM certificate to attach to the listener.	string	n/a	yes
domain_name	Domain name to access api. This will point to the remote VPC Endpoint if remote_vpc_endpoint is given, or to a generated VPC Endpoint otherwise.	string	n/a	yes
domain_names_alternate	Alternate domain names to access the api. These alternate names are for subject alternative names in the given certificate.	list(string)	[]	no
lb_enable_deletion_protection	Boolean indicating if deletion protection should be enabled for the created load balancer. If lb is given, this is ignored.	bool	null	no
lb_log_bucket	S3 bucket into which the created load balancer will store access logs. If lb is given, this is ignored. Even if lb is not given, this is not required.	string	null	no
name	A name used to identify associated resources throughout, like endpoints and load balancer.	string	n/a	yes
routing_policy	Routing policy applied to the alias A record. This can be useful if you intend to failover to an alternate API. It is not required, and when not given, a simple routing policy will be used.	object({ set_identifier = string cidr = optional(object({ collection_id = string location_name = string })) failover = optional(object({ type = string })) geolocation = optional(object({ continent = string country = string subdivision = optional(string) })) geoproximity = optional(object({ aws_region = optional(string) bias = optional(string) local_zone_group = optional(string) coordinates = optional(object({ latitude = string longitude = string })) })) latency = optional(object({ region = string })) weighted = optional(object({ weight = number })) })	null	no
subnet_ids	Subnets for the target group.	list(string)	n/a	yes
vpc_endpoint_api	VPC Endpoint for execute-api. If not given, one will be created. Given as an object, to avoid unknown until apply-time errors for variable data.	object({ id = string })	null	no
vpc_endpoint_local	Boolean indicating is a VPC Endpoint Interface should be created in the account with the API. If false and no vpc_endpoint_remote given, the DNS record will point directly to the NLB. This will be slightly less expensive, but it works. However, if you have any intention of using an IP allow list to allow traffic through a firewall, this may not be a good option since the IP addresses of NLB's can change unexpectedly. Ignored if vpc_endpoint_remote given.	bool	false	no
vpc_endpoint_remote	VPC Endpoint in another connectivity account for calling the VPC Endpoint Service created in the API's account. If not provided, a second VPC Endpoint will be created in the API's account to point toward the VPC Endpoint Service which reaches the NLB. When added, the endpoint created in this account to reach the service will be removed.	string	null	no
vpc_id	ID of the VPC.	string	n/a	yes
zone_id	Zone into which the custom domain should be added.	string	n/a	yes

Outputs

Name	Description
vpc_endpoint_service	VPC Endpoint Service toward load balancer. This can be used to generate a VPC Endpoint Interface in another connectivity account.

Sequence Diagrams

Alias Direct to NLB Sequence

sequenceDiagram
  box Consuming Account
  actor caller
  end
  box API Account
  participant dns as DNS A Record
  participant nlb as NLB TLS Listener
  participant tg as Target Group
  participant vpce as VPC Endpoint for API Gateway
  participant apidomain as API Custom Domain
  participant bpm as Base Path Mapping
  participant apigw as Private API Gateway
  end
  caller->>dns: resolve api-xyz.internal.example.com
  dns-->>caller: api-xys-nlb.elb.eu-north-1.amazonaws.com
  caller->>dns: resolve api-xys-nlb.elb.eu-north-1.amazonaws.com
  dns-->>caller: IP's
  caller->>+nlb: terminate TLS
  nlb->>tg: targetting execute-api Endpoint network interfaces
  tg->>+vpce: new TLS connection
  vpce->>apidomain: looking for API for api-xyz.internal.example.com
  apidomain->>bpm: what stage?
  bpm->>apigw: request toward API stage x
  apigw-->>vpce: response
  vpce-->>-nlb: response
  nlb-->>-caller: response

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Alias to VPC Endpoint in API Account Sequence

sequenceDiagram
  box Consuming Account
  actor caller
  end
  box API Account
  participant dns as DNS A Record
  participant vpcel as VPC Endpoint in API Account
  participant vpces as VPC Endpoint Service toward NLB
  participant nlb as NLB TLS Listener
  participant tg as Target Group
  participant vpce as VPC Endpoint for API Gateway
  participant apidomain as API Custom Domain
  participant bpm as Base Path Mapping
  participant apigw as Private API Gateway
  end
  caller->>dns: resolve api-xyz.internal.example.com
  dns-->>caller: vpce-x-y.vpce-svc-z.eu-north-1.vpce.amazonaws.com
  caller->>dns: resolve vpce-x-y.vpce-svc-z.eu-north-1.vpce.amazonaws.com
  dns-->>caller: IP's
  caller->>vpcel: request
  vpcel->>vpces: take me to your master
  vpces->>+nlb: terminate TLS
  nlb->>tg: targetting execute-api Endpoint network interfaces
  tg->>+vpce: new TLS connection
  vpce->>apidomain: looking for API for api-xyz.internal.example.com
  apidomain->>bpm: what stage?
  bpm->>apigw: request toward API stage x
  apigw-->>vpce: response
  vpce-->>-nlb: response
  nlb-->>-vpcel: response
  vpcel-->>caller: response

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Alias to VPC Endpoint in Connectivity Account Sequence

sequenceDiagram
  box Consuming Account
  actor caller
  end
  box Connectivity Account
  participant vpcer as VPC Endpoint in Connectivity Account
  end
  box API Account
  participant dns as DNS A Record
  participant vpces as VPC Endpoint Service toward NLB
  participant nlb as NLB TLS Listener
  participant tg as Target Group
  participant vpce as VPC Endpoint for API Gateway
  participant apidomain as API Custom Domain
  participant bpm as Base Path Mapping
  participant apigw as Private API Gateway
  end
  caller->>dns: resolve api-xyz.internal.example.com
  dns-->>caller: vpce-a-b.vpce-svc-c.eu-north-1.vpce.amazonaws.com
  caller->>dns: resolve vpce-a-b.vpce-svc-c.eu-north-1.vpce.amazonaws.com
  dns-->>caller: IP's
  caller->>vpcer: request via connectivity
  vpcer->>vpces: take me to your master via TGW
  vpces->>+nlb: terminate TLS
  nlb->>tg: targetting execute-api Endpoint network interfaces
  tg->>+vpce: new TLS connection
  vpce->>apidomain: looking for API for api-xyz.internal.example.com
  apidomain->>bpm: what stage?
  bpm->>apigw: request toward API stage x
  apigw-->>vpce: response
  vpce-->>-nlb: response
  nlb-->>-vpcer: response
  vpcer-->>caller: response

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