ELB target returns http 500 but load balancer returns 200

Question

I have entries in my ELB access logs showing that the target returned an HTTP 500, however the load balancer return code is 200. What conditions would cause this to happen?

Here is an example log entry:

> 2022-07-23T00:15:45.006137Z xxxxxxxxxxx XX.XXX.XXX.XX:12173 XXX.XX.X.XX:8080 0.000028 0.002797 0.000014 200 500 149 40 "POST http://xxx-xxxxxxx.xxxxxxxx.xxx:80/xxx-xxx HTTP/1.1" "-" - -

Answer

Hello,

Check if the below is applicable in your case:

----

**Target.ResponseCodeMismatch**

Description: The health checks didn't return an expected HTTP code.

Resolution:

* Success codes are the HTTP codes to use when checking for a successful response from a target. You can specify values or ranges of values between 200 and 499. `The default value is 200`. Check your load balancer health check configuration to verify which success codes that it's expecting to receive. Then, inspect your web server access logs to see if the expected success codes are being returned. [Modify the success code value](https://docs.aws.amazon.com/elasticloadbalancing/latest/application/target-group-health-checks.html#modify-health-check-settings) if necessary.
* Verify that the ping path is valid. The ping path is the destination on the targets for health checks. Be sure to specify a valid URI (/path?query). The default is /. [Modify the ping path value](https://docs.aws.amazon.com/elasticloadbalancing/latest/application/target-group-health-checks.html#modify-health-check-settings) if necessary.

Reference: https://aws.amazon.com/premiumsupport/knowledge-center/elb-fix-failing-health-checks-alb/

ELB target returns http 500 but load balancer returns 200

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