I have a containerized ASP.NET MVC application running on Docker, and I'm encountering an issue when attempting to send an HTTP request from the application to an external API. Interestingly, when I run the Docker image locally, this request executes successfully. However, upon deploying the application to ECS (Elastic Container Service), I'm confronted with a timeout error. Notably, the ECS Service is operating behind a load balancer.
Adding to the context, the ECS service is deployed to a target group on port 80, while the load balancer listener is configured on port 443. Remarkably, the application deploys without any hitches, and I can readily access its web page without issues. Nonetheless, the challenge continues to persist specifically with this particular HTTP request.
In my quest to resolve this matter, I've meticulously scrutinized the configuration settings for both the load balancer's security groups and the ECS Task's security groups. I've been thorough in confirming that ports 80 and 443 are correctly configured for both inbound and outbound traffic. Additionally, I made an effort to further troubleshoot the problem by deploying the ECS Service in a public subnet. Moreover, I attempted to send requests to external HTTP addresses on port 80 as part of the diagnostic process. Unfortunately, despite these additional measures, the issue remains unresolved.
Furthermore, it's worth noting that I also experimented with the IAM (Identity and Access Management) permissions of the ECS Task. As a temporary measure, I granted the task Admin permissions, but regrettably, this adjustment did not yield a resolution to the problem either.
Inbound requests to your application go to the load balancer, and then from the load balancer to the ECS server. Outbound requests from your application do not involve the load balancer at all, so focusing on the load balancer here is a dead end.
Also, IAM permissions are not involved in the outbound network request from your ECS service. Those permissions are used when connecting to other AWS services, not when making external API calls to non-AWS services.
First, you need to make sure that the security group assigned to the ECS service allows outbound traffic to any address. If that doesn't fix it, then you have not configured your ECS tasks to have Internet access. You can give your ECS tasks Internet access in one of two ways:
Add a NAT Gateway to your public VPC subnet(s). Configure private subnets in your VPC that have a route to the NAT Gateway. Deploy your ECS tasks to only those private subnets with a route to the NAT Gateway. The outbound requests from the ECS service will now go through the NAT Gateway and out to the Internet.
Make sure the ECS tasks are deployed only to public VPC subnets (subnets with a route to the VPC Internet Gateway). In the ECS Service network configuration, enable "Assign Public IP".
Note that if you are deploying to EC2 instances (not Fargate) and you are using the awsvpc network mode for your ECS tasks, you have to use the NAT Gateway method, as you can't assign a public IP to your tasks in that scenario.