How to Route Multiple ECS Services Behind a Single ALB Domain Using Different Target Ports

Yes, you can definitely route traffic to multiple ECS services running on different target ports using a single Application Load Balancer (ALB) with its default DNS name.

For your port-based routing scenario, you have a couple of approaches:

1. Multiple Listeners on the ALB: You can configure your ALB with multiple listeners, each listening on a different port. For example:

• Listener on port 80 routes to ECS service A
• Listener on port 8080 routes to ECS service B

This way, users would access your services via:

• my-alb-123456.ap-southeast-1.elb.amazonaws.com:80 for service A
• my-alb-123456.ap-southeast-1.elb.amazonaws.com:8080 for service B

2. Path-based Routing: Even without custom domains, you can use path-based routing rules with a single listener:

• my-alb-123456.ap-southeast-1.elb.amazonaws.com/service-a/* routes to service A
• my-alb-123456.ap-southeast-1.elb.amazonaws.com/service-b/* routes to service B

For ECS services, the ALB supports dynamic host port mapping, which is particularly useful. For example, if your task's container definition specifies port 80 for a container port and port 0 for the host port, ECS will dynamically choose a port from the ephemeral port range (like 32768-61000). The container is then registered with the ALB as an instance ID and port combination, allowing multiple tasks from a single service on the same container instance.

Best practices for your PoC setup:

• For services requiring access to multiple load-balanced ports, configure separate listeners for each port
• Ensure your load balancer subnet configuration includes all Availability Zones where your container instances reside
• Optimize load balancer health check parameters to speed up deployments
• If using the awsvpc network mode for your tasks, set the target type to ip instead of instance
• Use a unique target group for each service to avoid issues during service deployments

This approach works well for a PoC and can be extended later with custom domains if needed.
Sources
Use an Application Load Balancer for Amazon ECS - Amazon Elastic Container Service
Registering multiple target groups with an Amazon ECS service - Amazon Elastic Container Service
Using multiple Elastic Load Balancing listeners - AWS Elastic Beanstalk
How to prepare architecture for 1 public elastic IP and multiple ec2 instances with routing based on port | AWS re:Post