How to debug 502 errors on ELB

To debug 502 Bad Gateway errors on your Elastic Load Balancer (ELB), you should consider the following steps:

1. Check the response from your ECS tasks: A 502 Bad Gateway error typically indicates that the load balancer was unable to parse the response sent from a registered instance (in your case, the ECS tasks). Verify that the responses being sent from your ECS tasks conform to HTTP specifications.

2. Monitor CloudWatch metrics: Pay attention to the HTTPCode_ELB_5XX and HTTPCode_Backend_5XX metrics. These can provide insights into the frequency and timing of the 502 errors.

3. Analyze access and connection logs: Although you mentioned not finding references to 502 errors in these logs, double-check to ensure you're not missing any relevant information. Look for patterns in requests that might be triggering the errors.

4. Check target group health: Ensure that your ECS tasks are healthy and properly registered with the target group. Unhealthy instances or misconfigured health checks can sometimes lead to 502 errors.

5. Verify network configuration: Make sure that security groups, network ACLs, and VPC settings are correctly configured to allow traffic between the ALB and your ECS tasks.

6. Examine application logs: Although you mentioned no error logs from the tasks, review your application logs in detail for any potential issues that might not be immediately apparent.

7. Test direct connections: Try sending requests directly to your ECS tasks, bypassing the ALB, to see if you can reproduce the error. This can help isolate whether the issue is with the ALB or the application.

8. Check for capacity issues: Ensure that your ECS cluster has sufficient capacity to handle the incoming requests. Overloaded instances can sometimes result in 502 errors.

9. Verify ALB configuration: Double-check your ALB settings, including listener rules, target group settings, and health check configurations.

If after these steps you're still unable to identify the root cause, you may want to contact AWS Support for further assistance. They can help investigate potential issues within the ELB service itself or provide more advanced troubleshooting guidance.
Sources
Troubleshoot a Classic Load Balancer: HTTP errors - Elastic Load Balancing
Troubleshoot a Classic Load Balancer: Response code metrics - Elastic Load Balancing
Troubleshoot your Classic Load Balancer - Elastic Load Balancing