404.63 errors in app service

Question

404.63 errors in app service

Mario Rentería 1

Hi, I have a problem with my app service. Almost 20 percent of my traffic are bad requests (404.63).

I checked the diagnostics and in the "HTTP 4xx Errors" section shows me the following message

The below table shows you the count of all HTTP 4xx errors that happened for your app. The errors are categorized as Front End or Worker based on the instance that returned the error. Front End in Azure App Service is a layer seven-load balancer, acting as a proxy, distributing incoming HTTP requests between different applications and their respective Workers. Web Workers are the backbone of the App Service scale unit and they run your application code.

What does the 404.63 error mean?
How can I fix it?

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Answer 1

ajkuma 28,036 Microsoft Employee Moderator

@Mario Rentería , To isolate the issue, kindly try these steps:

1.Please review to see if it’s due to dynamic IP security rule: elements denyByConcurrentRequests and denyByRequestRate.
-Azure App Service access restrictions

2.App Service Always On also generate periodic (every 5min) probes to app instances and causing failed requests with error 404, and this can be found in Application Insights.

If this is the case -It can be fixed by rewriting the Always on path. After a cold start of your application, AlwaysOn will send a request to the ROOT of your application “/”.
Whatever file is delivered when a request is made to / is the one which will be warmed up, which will fail because the root doesn’t exist.

Kindly see this blog for more details - https://devblogs.microsoft.com/premier-developer/404-response-code-caused-by-app-services-alwayson-feature/

If the issue still persists, kindly let me know I’ll follow-up with you privately to fetch more details about your WebApp and subscription/for performing deeper investigation.

ajkuma 28,036 Reputation points Microsoft Employee Moderator

2021-12-28T12:52:49.967+00:00

@Mario Rentería , Additionally -

Since when are you facing this issue? Do you recall making any changes? Is your App running on a single instance or multiple instances?
Has this always been the case (after the deployment) or just recently started experience this behavior?

Also, please review Azure Service Health and Azure Resource Health to fetch additional info.

Azure Service health provides a personalized view of the health of the Azure services and regions you're using. This is the best place to look for service impacting communications about outages, planned maintenance activities, and other health advisories.

Azure Resource Health helps you diagnose and get support for service problems that affect your Azure resources. It reports on the current and past health of your resources.
https://learn.microsoft.com/azure/service-health/overview
Mario Rentería 1 Reputation point

2021-12-28T17:00:26.003+00:00

@Anonymous
We have tried those steps

1.- There are not any access restrictions, only the default that allows any source.

2.- There is just one 404 error from the root application path "/". The 404 errors are part of the custom health check path we defined, in fact, on every request we do there is a probabilty that returns 404.

3.- We also reviewed Azure Service health and Azure Resourse health but it shows us that everything is ok.

At some point we moved the app service from different resource group, I think this cause this problem.
We have two instances for this webapp.
We are facing this issue for almost one month.
ajkuma 28,036 Reputation points Microsoft Employee Moderator

2021-12-29T11:45:04.187+00:00

@Mario Rentería , Thanks for sharing the requested details. Apologies you're still seeing this issue.
As mentioned, to fetch some more details on your App and subscription, I'll follow-up with you privately.

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404.63 errors in app service

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