How do I troubleshoot a disconnected Amazon ECS agent?

Last updated: 2022-03-21

My container instances for Amazon Elastic Container Service (Amazon ECS) are disconnected.

Short description

Your Amazon ECS container agent might disconnect and reconnect several times an hour as part of the normal operation. These change events are normal and aren't a cause for concern. Connection events that last for only a few minutes might not indicate issues with container agent or your container instance. However, if the container agent remains in the disconnected state for a longer time, then the container instance can't operate as part of your Amazon ECS cluster. This issue might be caused due to the following reasons:

  • Networking issues prevent communication between the instance and Amazon ECS.
  • The container agent doesn't have the required AWS Identity and Access Management (IAM) permissions to communicate with Amazon ECS endpoints.
  • There are problems with the host or Docker daemon inside the container instance.
  • There is resource contention in the underlying host.

Resolution

Note: The following resolution applies to Amazon ECS-optimized Amazon Linux 2 AMIs. For a resolution that applies to Amazon ECS-optimized Amazon Linux 1 AMIs, see Why are my Amazon ECS container instances with Amazon Linux 1 AMIs disconnected?

You can connect to your Amazon EC2 instances using SSH keys. If you don't have the SSH keys generated, you can connect to your instance using Session Manager. By default, AWS Systems Manager Agent is installed on Amazon Linux 2 AMIs and Amazon Linux 2 ECS-optimized base AMI.

Verify that the container agent is running on the container instance

To verify the status and connectivity of the Amazon ECS container agent, run either of the following commands on your container instance:

$ sudo systemctl status ecs
            
$ sudo docker ps -f name=ecs-agent

The output specifies active (running) and looks similar to the following:

ecs.service - Amazon Elastic Container Service - container agent
   Loaded: loaded (/usr/lib/systemd/system/ecs.service; enabled; vendor preset: disabled)
   Active: active (running) since Tue 2022-02-15 15:51:09 UTC; 37min ago
     Docs: https://aws.amazon.com/documentation/ecs/
  Process: 30039 ExecStopPost=/usr/libexec/amazon-ecs-init post-stop (code=exited, status=0/SUCCESS)
  Process: 29987 ExecStop=/usr/libexec/amazon-ecs-init stop (code=exited, status=0/SUCCESS)
  Process: 30077 ExecStartPre=/usr/libexec/amazon-ecs-init pre-start (code=exited, status=0/SUCCESS)
 Main PID: 30123 (amazon-ecs-init)
    Tasks: 5
   Memory: 3.7M
   CGroup: /system.slice/ecs.service
           └─30123 /usr/libexec/amazon-ecs-init start
CONTAINER ID   IMAGE                                            COMMAND    CREATED      STATUS                PORTS     NAMES
eb1dc8d4ab3b   amazon/amazon-ecs-agent:latest   "/agent"        3 days ago   Up 3 days (healthy)                        ecs-agent

If the issue is caused due to a disconnected agent, then restart the ECS agent by running the following command:

$ sudo systemctl restart ecs

Note: You don't see any output after running these commands.

To verify that the agent is running, run the following command:

sudo systemctl status ecs

Verify that the Docker service is running on the container instance

To verify that the Docker service is running on the affected container instance, run the following command:

sudo systemctl status docker

The output specifies active (running) and looks similar to the following:

docker.service - Docker Application Container Engine
   Loaded: loaded (/usr/lib/systemd/system/docker.service; disabled; vendor preset: disabled)
   Active: active (running) since Fri 2022-02-11 17:42:32 UTC; 3 days ago
     Docs: https://docs.docker.com
  Process: 4307 ExecStartPre=/usr/libexec/docker/docker-setup-runtimes.sh (code=exited, status=0/SUCCESS)
  Process: 4296 ExecStartPre=/bin/mkdir -p /run/docker (code=exited, status=0/SUCCESS)
 Main PID: 4315 (dockerd)
    Tasks: 24
   Memory: 360.5M
   CGroup: /system.slice/docker.service
           ├─4315 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock --default-ulimit nofile=32768:65536
           ├─6010 /usr/bin/docker-proxy -proto tcp -host-ip 0.0.0.0 -host-port 80 -container-ip 172.17.0.2 -container-port 80
           └─6016 /usr/bin/docker-proxy -proto tcp -host-ip :: -host-port 80 -container-ip 172.17.0.2 -container-port 80

If the Docker service is inactive, then run the following command to restart the Docker service:

sudo systemctl restart docker

Note: The command doesn't return any output.

To verify that the Docker service has restarted, run the following command:

sudo systemctl status docker

Review log files for the container agent and Docker

If your container instance is still disconnected, then review the log files on the container host for the container agent and Docker.

Check the following log files for keywords, such as "error", "warn", or "agent transition state":

  • View the Amazon ECS container agent's latest logs at /var/log/ecs/ecs-agent.log Note: You can view the rotated log by filtering to /var/log/ecs/ecs-agent-log.timestamp
  • View the Amazon ECS init log at /var/log/ecs/ecs-init.log
  • View the userdata execution logs at /var/log/cloud-init.log
  • View the Docker Daemon logs with the command sudo journalctl -u docker

Note: You can choose to use the Amazon ECS logs collector to collect general operating system logs, Docker logs, and container agent logs for Amazon ECS.

Verify that the IAM instance profile has the necessary permissions

If the container agent is still disconnected, verify that the IAM instance profile associated with the container instance has the necessary IAM permissions.

1.    Connect to the instance using SSH or Session Manager.

2.    To view the instance metadata on the instance profile associated with the instance, run the following command:

curl http://169.254.169.254/latest/meta-data/iam/info

The output looks similar to the following:

{
  "Code" : "Success",
  "LastUpdated" : "2022-02-16T22:42:17Z",
  "InstanceProfileArn" : "arn:aws:iam::1122334455:instance-profile/ecsInstanceRole",
  "InstanceProfileId" : "AIPA4VIZXOFF55F72XIZN"
}

3.    Verify that the IAM role contains the correct permissions for your container instances.

4.    To verify specific credential errors with the container agent, run a command similar to the following to check the container agent log for a list of ECS logs:

Be sure to replace YYYY-MM-DD-** with the relevant timestamp.

cat /var/log/ecs/ecs-agent.log.YYYY-MM-DD-**

Note: The container agent log is rotated every hour. The suffix automatically changes to reflect the current date and time. Update the command to include the date range and log ID for when the issue occurred.

Verify that your container instance has enough resources to run the ECS agent

If your tasks have a high memory/CPU utilization, then your container instance might not have enough resources to run the ECS agent.

The Amazon ECS container agent uses the Docker ReadMemInfo() function to query the amount of memory that's available for the operating system.

Run the following command on your container instance to view the total memory that's recognized by the operating system:

free -b

Example output for an t2.large instance running the Amazon ECS-optimized Amazon Linux AMI:

                          total        used         free                   shared     buff/cache    available
Mem:                    8361193472   298577920     7325388800              405504      737226752    7844274176
Swap:                     0              0           0

You can choose to reserve some memory for the Amazon ECS container agent and other critical system processes on your container instances, so that your task's containers don't contend for the same memory. For more information, see Container instance memory management.

Verify that the environment variable ECS_CLUSTER has the correct cluster name

If the Amazon ECS container agent configuration parameter ECS_CLUSTER has the incorrect cluster name, then the container instance can't join the cluster. Check the contents of the /etc/ecs/ecs.config file to verify this parameter.

cat /etc/ecs/ecs.config

Verify that the ECS agent can communicate to ECS endpoints

Be sure that the network access control lists and security group used by the container instance allow outbound connections on port 443 (HTTPS) to connect with ECS endpoints.

Run either of the following commands on your container instance to check the outbound connections to ECS endpoints (ACS/TCS):

sudo yum install telnet -y
$ telnet ecs.region.amazonaws.com 443

-or-

$ curl https://ecs.region.amazonaws.com

The following are some of the best practices to keep in mind: