AWS: How to Fix Unhealthy Instance Group Targets

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If your target group shows that EC2 instances are unhealthy, even though they are running fine, there are a few common reasons to look into and solutions to try.

First, check your health check settings. Make sure the health check path is correct and that it returns a 200 status code, which is normal for successful responses. Sometimes, your application may respond with a different status code, so you might need to adjust the settings to accept other codes as healthy.

Next, review your network setup. Ensure that your Virtual Private Cloud (VPC) network is configured properly, with correct routing between the load balancer and your EC2 instances. Also, verify that your security groups allow traffic on the app’s port, usually 80 or 443, from the load balancer to your instances. Both inbound and outbound rules should be correctly set to permit this communication.

Then, look at your application configuration. Confirm that your app listens on the right port and on all network interfaces (not just localhost). Make sure it responds to the specific health check URL and that it has fully started and is running without errors.

If the health status remains unhealthy, dig into the details. Check the reason code for the unhealthy indicator in your target group, and review logs on your EC2 instances to see if there are any errors related to health check requests. You might also try temporarily adjusting your security groups to allow more broad access for testing purposes, which can help identify if blocked traffic is the issue.

Even if your instances are marked as unhealthy, the load balancer might still route traffic to them in some fail-open scenarios. However, relying on this isn’t recommended for production environments.

To fix these problems, start by double-checking your security settings, ensuring your application is responding correctly to health checks, and confirming network connectivity between the load balancer and your EC2 instances. Proper configuration and testing will help keep your setup healthy and functioning smoothly.