IIS is automatically call to http://localhost after application pool get recycled or stop and start

Simon Dao 0

Hi,
We have some .Net applications running on Windows servers and backed by IIS.
We noticed that when the application pool got recycled or stop&start.
There will be a call to http://localhost/ with the clientIP 127.0.0.1
This is annoying us because we didn't have anything on / so it returned 404 and it reported to our monitoring tool(Azure Application Insight)

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Is there any config to prevent this behavior(disable auto call to http://localhost/)?
This is our IIS version
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JasonPan - MSFT 5,981 Reputation points Microsoft Vendor

2024-05-03T09:21:51.08+00:00
Hi @Simon Dao,

After investigating the issue, I can confirm the issue is related to Application Insight SDK, not related to IIS. I have tried in my local and not found this behavior in network trace.

And I also found a lot of threads about how to filter localhost telemetry using TelemetryProcessor. Here is the working one, you can refer it.

public class LocalHostTelemetryFilter : ITelemetryProcessor { private ITelemetryProcessor next; public LocalHostTelemetryFilter(ITelemetryProcessor next) { this.next = next; } public void Process(ITelemetry item) { var requestTelemetry = item as RequestTelemetry; if (requestTelemetry != null && requestTelemetry.Url.Host.Equals("localhost", StringComparer.OrdinalIgnoreCase)) { return; } else { this.next.Process(item); } } }

And then register it in ApplicationInsights.config:

<TelemetryProcessors> <Add Type="LocalhostFilterSample.LocalHostTelemetryFilter, LocalHostFilterSample"/> </TelemetryProcessors>

For more details, you can check the official document https://learn.microsoft.com/en-us/azure/azure-monitor/app/api-filtering-sampling?tabs=javascriptwebsdkloaderscript#create-a-telemetry-processor

Best Regards

Jason
Simon Dao 0 Reputation points

2024-05-03T10:09:00.8733333+00:00

Hi @JasonPan - MSFT

Thanks for your response.
To be clear we didn't have any intended request to localhost.
And the app we run on PROD and NON-PROD env.
NON-PROD is running on AKS.
PROD running on on-prem.

With the same codebase and SDK version.
NON-PROD instance does not have that problem.
only PROD instances have that problem.

So if it was SDK issue. we should have the error in all instances, right?
But we only get that on on-prem IIS. So I think SDK is not the case here
JasonPan - MSFT 5,981 Reputation points Microsoft Vendor

2024-05-03T10:28:32.4833333+00:00

Hi @Simon Dao,

Thanks for your sharing, I will investigate the issue further.

Best Regards

Jason
Simon Dao 0 Reputation points

2024-05-06T07:51:41.8666667+00:00

Thank @JasonPan - MSFT ,
I'm waiting for your investigation result
JasonPan - MSFT 5,981 Reputation points Microsoft Vendor

2024-05-06T12:15:12.3033333+00:00

Hi @Simon Dao,

I tried to reproduce the issue but failed. Could you share me the detailed steps ? If we can reproduce the issue in brand new project, then we can continue in the right direction.

Best Regards

Jason
Simon Dao 0 Reputation points

2024-05-08T02:59:52.0833333+00:00

Hi @JasonPan - MSFT ,

I tried to reproduce it with a sample test app but didn't see the same behavior.
So I started package capture on the VM that have the problem.
I capture it on loopback interface. From the tracer file, I can not see anything related to 404on localhost But on Application Insight, It still reporting 404 on request to http://localhost with client ip 127.0.0.1 which is very strange.
At this stage, I run out of idea what to check next. Perhap you could have some suggestion for us to continue analyzing this

Thank you in advance,
Simon
JasonPan - MSFT 5,981 Reputation points Microsoft Vendor

2024-05-08T06:29:27.8766667+00:00

Hi @Simon Dao,

Now the brand new sample can't reproduce the issue in your side. So we can confirm the issue related to your project, I mean maybe it's code issue.

My suggestion is you have to debug it via disabling the middlewares in your asp.net core application for test. Then we can find the issue related what middleware in your project.

Best Regards

Jason

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