A Timeout error occurs when EC2 and ECS using the same security group communicate

Question

I'm having trouble communicating with EC2 and ECS.

When I send a request to ECS from EC2 that has the same security group as ECS, I get a Timeout error. However, the request can be received from ECS from outside.

Security group A allows all its own traffic.

EC2 Instance

• private subnet A
• security group A

ECS Load Balancer(internet facing)

• public subnet A
• security group A (for internal communication)
• security group B (for external communication)

ECS Container

• private subnets A and B
• security group A

Answer 1

When you send traffic to a public facing load balancer it exits the VPC and comes back in. When the traffic exits the VPC it is no longer associated to a security group. Thus the security group no longer sees the traffic as coming from "itself" and it blocks the request.