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Reverse Proxy (ARR) and Script Resource Issues


When executing this page directly from the application server everything works as expected.

Going through the reverse proxy server, results are in a 'pending' call to a script resource when the application is accessed. The request eventually times out. Then, JavaScript errors may occur (such as 'Sys.WebForms.PageRequestManager' is null or not an object, 'Telerik' is undefined).

Basically, the Telerik controls do not work behind a reverse proxy (ARR).


The problem is most likely due to the fact that the proxy server is caching the HTTP handlers we use to get the needed WebResources and this makes them invalid. For this reason the controls are unable to load their scripts and resources and do not work.

The Telerik UI for ASP.NET AJAX controls rely heavily on WebResource requests to fetch the necessary scripts and styles they require to operate. This is the standard (default) way to fetch resources in the MS AJAX library and if it does not work, it is likely the the framework itself will not operate. Thus, Telerik UI for ASP.NET AJAX does not officially support (reverse) proxy scenarios.

You need to ensure that all requests return successfully.


To fix it you can try the following:

  • Exclude the Telerik-specific handlers from the proxy caching
  • Use a RadScriptManager instead of the asp:ScriptManager as then the requests for the Telerik controls scripts will go through the Telerik handler and not the standard ScriptResource.axd handler. This may help in case the standard handler has some special settings built-in into IIS.
  • Other possible issues with the URL:
  • Enable the CDN scripts as explained in the following help articles: Script CDN Overview and StyleSheet CDN.
  • If the issue is with file uploads only, you can create a custom file upload handler that will not get broken by the proxy.
  • More tips for load balancing and HTTPS scenario are available in the following forum thread: Ajax Control with reverse proxy (ARR) on IIS.
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