Architecture patterns for consuming private APIs cross-account

This blog written by Thomas Moore, Senior Solutions Architect and Josh Hart, Senior Solutions Architect.

Amazon API Gateway allows developers to create private REST APIs that are only accessible from a virtual private cloud (VPC). Traffic to the private API uses secure connections and does not leave the AWS network, meaning AWS isolates it from the public internet. This makes private API Gateway endpoints a good fit for publishing internal APIs, such as those used by backend microservice communication.

In microservice architectures, where multiple teams build and manage components, different AWS accounts often consume private API endpoints.

This blog post shows how a service can consume a private API Gateway endpoint that is published in another AWS account securely over AWS PrivateLink.

Consuming API Gateway private endpoint cross-account via AWS PrivateLink.

This blog covers consuming API Gateway endpoints cross-account. For exposing cross-account resources behind an API Gateway, read this existing blog post.

Overview

To access API Gateway private endpoints, you must create an interface VPC endpoint (named execute-api) inside your VPC. This creates an AWS PrivateLink connection between your AWS account VPC and the API Gateway service VPC. The PrivateLink connection allows traffic to flow over private IP address space without traversing the internet.

PrivateLink allows access to private API Gateway endpoints in different AWS accounts, without VPC peering, VPN connections, or AWS Transit Gateway. A single execute-api endpoint is used to connect to any API Gateway, regardless of which AWS account the destination API Gateway is in. Resource policies control which VPC endpoints have access to the API Gateway private endpoint. This makes the cross-account architecture simpler, with no complex routing or inter-vpc connectivity.

The following diagram shows how interface VPC endpoints in a consumer account create a PrivateLink connection back to the API Gateway service account VPC. The resource policy applied to the private API determines which VPC endpoint can access the API. For this reason, it is critical to ensure that the resource policy is correct to prevent unintentional access from other AWS account VPC endpoints.

Access to private API Gateway endpoints requires an AWS PrivateLink connection to an AWS service account VPC.

In this example, the resource policy denies all connections to the private API endpoint unless the aws:SourceVpce condition matches vpce-1a2b3c4d in account A. This means that connections from other execute-api VPC endpoints are denied. To allow access from account B, add vpce-9z8y7x6w to the resource policy. Refer to the documentation to learn about other condition keys you can use in API Gateway resource policies.

For more detail on how VPC links work, read Understanding VPC links in Amazon API Gateway private integrations.

The following sections cover three architecture patterns to consume API Gateway private endpoints cross-account:

Regional API Gateway to private API Gateway
Lambda function calling API Gateway in another account
Container microservice calling API Gateway in another account using mTLS

Regional API Gateway to private API Gateway cross-account

When building microservices in different AWS accounts, private API Gateway endpoints are often used to allow service-to-service communication. Sometimes a portion of these endpoints must be exposed publicly for end user consumption. One pattern for this is to have a central public API Gateway, which acts as the front-door to multiple private API Gateway endpoints. This allows for central governance of authentication, logging and monitoring.

The following diagram shows how to achieve this using a VPC link. VPC links enable you to connect API Gateway integrations to private resources inside a VPC. The API Gateway VPC interface endpoint is the VPC resource that you want to connect to, as this is routing traffic to the private API Gateway endpoints in different AWS accounts.

API Gateway Regional endpoint consuming API Gateway private endpoints cross-account

VPC link requires the use of a Network Load Balancer (NLB). The target group of the NLB points to the private IP addresses of the VPC endpoint, normally one for each Availability Zone. The target group health check must validate the API Gateway service is online. You can use the API Gateway reserved /ping path for this, which returns an HTTP status code of 200 when the service is healthy.

You can deploy this pattern in your own account using the example CDK code found on GitHub.

Lambda function calling private API Gateway cross-account

Another popular requirement is for AWS Lambda functions to invoke private API Gateway endpoints cross-account. This enables service-to-service communication in microservice architectures.

The following diagram shows how to achieve this using VPC connections for Lambda, which allows access to private resources inside your VPC. This allows Lambda to access the API Gateway VPC endpoint and, therefore, the private API Gateway endpoints in another account.

Consuming API Gateway private endpoints from Lambda cross-account

Unlike the previous example, there is no NLB or VPC link required. The resource policy on the private API Gateway must allow access from the VPC endpoint in the account where the consuming Lambda function is.

As the Lambda function has a VPC attachment, it will use DNS resolution from inside the VPC. This means that if you selected the Enable Private DNS Name option when creating the interface VPC endpoint for API Gateway the https://{restapi-id}.execute-api.{region}.amazonaws.com endpoint will automatically resolve to private IP addresses. Note that this DNS configuration can block access from Regional and edge-optimized API endpoints from inside the VPC. For more information, refer to the knowledge center article.

You can deploy this pattern in your own account using the sample CDK code found on GitHub.

Calling private API Gateway cross-account with mutual TLS (mTLS)

Customers that operate in regulated industries, such as open banking, must often implement mutual TLS (mTLS) for securely accessing their APIs. It is also great for Internet of Things (IoT) applications to authenticate devices using digital certificates.

Mutual TLS (mTLS) verifies both the client and server via certificates with TLS

Regional API Gateway has native support for mTLS but, currently, private API Gateway does not support mTLS, so you must terminate mTLS before the API Gateway. One pattern is to implement a proxy service in the producer account that resolves the mTLS handshake, terminates mTLS, and proxies the request to the private API Gateway over regular HTTPS.

The following diagram shows how to use a combination of PrivateLink, an NGINX-based proxy, and private API Gateway to implement mTLS and consume the private API across accounts.

Consuming API Gateway private endpoints cross-account with mTLS

In this architecture diagram, Amazon ECS Fargate is used to host the container task running the NGINX proxy server. This proxy validates the certificate passed by the connecting client before passing the connection to API Gateway via the execute-proxy VPC endpoint. The following sample NGINX configuration shows how the mTLS proxy service works by using ssl_verify_client and ssl_client_certificate settings to verify the connecting client’s certificate, and proxy_pass to forward the request onto API Gateway.

server {
    listen 443 ssl;

    ssl_certificate     /etc/ssl/server.crt;
    ssl_certificate_key /etc/ssl/server.key;
    ssl_protocols       TLSv1.2;
    ssl_prefer_server_ciphers on;
    ssl_ciphers ECDH+AESGCM:ECDH+AES256:ECDH+AES128:DH+3DES:!ADH:!AECDH:!MD5;

    ssl_client_certificate /etc/ssl/client.crt;
    ssl_verify_client      on;

    location / {
        proxy_pass https://{api-gateway-endpoint-api};
    }
}

The connecting client must supply the client certificate when connecting to the API via the VPC endpoint service:

curl --key client.key --cert client.crt --cacert server.crt https://{vpc-endpoint-service-url}

Use VPC security group rules on both the VPC endpoint and the NGINX proxy to prevent clients bypassing the mTLS endpoint and connecting directly to the API Gateway endpoint.

There is an example NGINX config and Dockerfile to configure this solution in the GitHub repository.

Conclusion

This post explores three solutions to consume private API Gateway across AWS accounts. A key component of all the solutions is the VPC interface endpoint. Using VPC Endpoints & PrivateLink, you can consume resources securely and even your own microservices across AWS accounts. For more details, read Enabling New SaaS Strategies with AWS PrivateLink. Visit the GitHub repository to get started implementing one of these solutions today.

For more serverless learning resources, visit Serverless Land.

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