I have made a custom handler called MyIndexHandler and I want it to handle *.index requests, now as I want this handler to be executed by any website, I installed my handler with following two steps in IIS manager.

1. Add MyIndexHandler.dll in GAC
2. Add Managed Module for *.index, the drop down in IIS displays my index handler correctly so it means it did find it correctly from the GAC.
3. I added Script map for aspnet_isapi.dll for *.index (this is required for classic mode)

Now in any of my website if I try xyz.index, my handler does not get called and it returns "Failed to execute URL", now this only happens when IIS is unable to find the mapped handler, but I already have defined mapped handler at IIS level and when i try to add entry in web.config manually for this handler, it tells me you can not define multiple handler, it means that IIS finds my handler, mapping but for some reason it does not execute it.