Traffic Flow In and Out of ECS

Question

I'm designing a secure ECS workload that runs in **private subnets** (no NAT Gateway), and I want to ensure reliable connectivity for both **incoming traffic** from the internet and **outgoing requests** to AWS APIs like Secrets Manager, STS, and ECS control plane.

Here’s what I have in place:

* A **public Application Load Balancer (ALB)** exposed via **Route 53**
* ECS tasks hosted in **private subnets**, fronted by the ALB
* **Interface VPC endpoints** deployed for:

* `com.amazonaws.eu-west-1.secretsmanager`
  * `com.amazonaws.eu-west-1.sts`
  * `com.amazonaws.eu-west-1.ecs`
* Security groups allow:

* ELB to communicate with ECS over HTTP/S
  * ECS tasks to reach VPC endpoints
* **RDS is also in private subnets**, accessed by ECS tasks

I **plan to disable the NAT Gateway on weekends** to reduce cost, assuming that:

* All outbound traffic to AWS APIs is covered via VPC endpoints
* ECS tasks do not need to reach public APIs during that time

📌 **My questions are:**

1. Is my understanding of the traffic flow correct?
   (Internet → Route 53 → ALB → ECS → RDS or AWS service via endpoint)

2. Are there any caveats or hidden dependencies where ECS tasks might still require the NAT Gateway?

3. Is this setup **resilient** enough to operate without NAT for periods of time?

4. Am I missing any AWS interface endpoints that ECS, Lambda (in VPC), or Fargate might implicitly depend on?

Would appreciate any input, especially from those who've run NAT-less ECS or Lambda setups in production.

Thanks!

Riku_Kobayashi · Accepted Answer

Hello.

Do container images for ECS Fargate come from a private ECR?  
In that case, you need to configure the following VPC endpoint.  
https://docs.aws.amazon.com/AmazonECS/latest/developerguide/vpc-endpoints.html

* com.amazonaws.region.ecr.dkr  
* com.amazonaws.region.ecr.api  
* com.amazonaws.region.s3 (Gateway VPC Endpoint)  
* com.amazonaws.region.logs

> Is my understanding of the traffic flow correct? (Internet → Route 53 → ALB → ECS → RDS or AWS service via endpoint)

Yes, the traffic flow is correct.

> Are there any caveats or hidden dependencies where ECS tasks might still require the NAT Gateway?

If an application hosted on ECS Fargate runs an external public API, a timeout error will occur if there is no NAT Gateway.

> Is this setup resilient enough to operate without NAT for periods of time?

I have built ECS Fargate environments using VPC endpoints several times, and as long as it was configured as a multi-AZ and redundant, I have never encountered any major issues.

re:Post Agent · Answer

Your understanding of the traffic flow is generally correct. The flow follows: Internet → Route 53 → ALB → ECS → RDS or AWS service via endpoint.

For your ECS workload in private subnets with interface VPC endpoints, you've covered the main services needed, but there are some important considerations:

1. For ECS, you should have additional endpoints beyond just the main ECS endpoint. You need:
           - com.amazonaws.region.ecs-agent
           - com.amazonaws.region.ecs-telemetry
           - com.amazonaws.region.ecr.api
           - com.amazonaws.region.ecr.dkr
           - com.amazonaws.region.logs (for CloudWatch Logs)

2. Regarding disabling the NAT Gateway on weekends:
           - This can work if your containers don't need to access any public internet resources
           - However, be aware that ECS may need to pull container images from ECR or Docker Hub
           - If using Docker Hub or other external image repositories, you'll still need NAT Gateway access
           - Container health checks or dependency checks might fail if they rely on external endpoints

3. Resilience considerations:
           - Your setup can operate without NAT if all required services have interface endpoints
           - Ensure your security groups are properly configured to allow traffic between your ECS tasks and all VPC endpoints
           - Consider that AWS services occasionally update their dependencies, which might introduce new requirements

4. For Lambda in VPC:
           - You'll need additional endpoints for Lambda execution
           - com.amazonaws.region.lambda
           - com.amazonaws.region.logs

Using AWS PrivateLink with interface VPC endpoints keeps your traffic within the AWS network, which is more secure than routing through the public internet. This approach restricts all network traffic between your VPC and AWS services to the Amazon network, eliminating the need for internet gateways or NAT devices during normal operation.

If you're using Secrets Manager or Systems Manager Parameter Store for sensitive data with ECS, ensure you have VPC endpoints for these services as well. Additionally, enforce TLS 1.2 when using AWS SDKs to encrypt data in transit between your application and AWS services.
      
**Sources**  
[AWS SDK for .NET credential loading - Replatform .NET Applications with Windows Containers](https://docs.aws.amazon.com/whitepapers/latest/replatform-dotnet-apps-with-windows-containers/aws-sdk-for-.net-credential-loading.html)  
[SEC05-BP02 Control traffic flow within your network layers - AWS Well-Architected Framework](https://docs.aws.amazon.com/wellarchitected/latest/framework/sec_network_protection_layered.html)  
[SEC05-BP02 Control traffic flow within your network layers - Security Pillar](https://docs.aws.amazon.com/wellarchitected/latest/security-pillar/sec_network_protection_layered.html)

Traffic Flow In and Out of ECS

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