How do I log HTTP 525 errors in IIS?

Question

I have Windows Server 2016 Datacenter (1607) running on AWS EC2 serving .NET Framework 4.7.2 application on IIS (10.0.14393.0). We use Cloudflare as a proxy (DDoS protection). Cloudflare tells us that IIS intermittently returns HTTP 525 when negotiating SSL.

Cloudflare insist that the problem is at the server and not with them.

AWS did not have outages and I have other services on the server that were running correctly at the time. The certificates are with LetsEncrypt and are valid at the time (the service that renews and updates runs at a different time to the errors occuring). Server is patched up to date.

There are no IIS logs at time of the errors. How do I log 525 errors either in IIS or via Windows Event Viewer?

Many thanks in advance.

Answer 1

Check the logs in C:\Windows\System32\LogFiles\HTTPERR

Turn on failed request tracing and monitor for 525 status codes.

https://learn.microsoft.com/en-us/iis/troubleshoot/using-failed-request-tracing/using-failed-request-tracing-rules-to-troubleshoot-application-request-routing-arr

https://learn.microsoft.com/en-us/iis/troubleshoot/using-failed-request-tracing/troubleshoot-with-failed-request-tracing