Check the log message for unregistered instance events.

• If the instance shows no container instance ID or the log reports unregistered events, the ECS agent likely didn’t register with the cluster.

Make sure the instance has correct user data.

• Go to the cluster in the console and select Actions > Edit configuration > Compute.

• Add this script, replacing the placeholder with your cluster name:

  #!/bin/bash
  echo ECS_CLUSTER="your-cluster-name" >> /etc/ecs/ecs.config

Confirm that the AMI includes a working ECS agent.

• The ECS‑optimized AMI includes the ECS agent and required configuration.

• Using a non‑ECS‑optimized AMI may prevent the agent from registering.

Verify required security group rules.

• Port 22 (SSH): Optional, only for administrative access.

• Port 2375 (TCP): Used by the ECS agent for communication with ECS.

• Port 2376 (TCP): Used for secure, encrypted agent communication.

Check the IAM instance profile.

• The instance must use an IAM role with permissions required for ECS agent registration.
   

Verify IP and networking settings.