Check the “Server” HTTP header for the source of an error returned from IIS

When an error response is returned from an HTTP request submitted to an IIS web server, the error response might actually be coming from http.sys (the “Hypertext Transfer Protocol Stack”), which processes incoming HTTP requests before they are passed along to IIS.

You can determine the source of the error by looking at the “Server” HTTP header in the returned HTTP response.  An error coming from http.sys will have the header:

Server:Microsoft-HTTPAPI/2.0

An error coming from IIS will instead have a header like:

Server:Microsoft-IIS/XX.X

Here’s some handy C# code to dump all headers from a given WebResponse to the console:

for (int i = 0; i < response.Headers.Count; i++) 
{ 
    string header = response.Headers.GetKey(i); 
    string[] values = response.Headers.GetValues(i); 
    Console.WriteLine(header + ": " + String.Join(", ", values)); 
}

Bonus tip: For a bit more information on errors generated by http.sys, check out the log files in this folder on the web server PC:

%windir%\System32\LogFiles\HTTPERR

References:

• http://stackoverflow.com/a/24179362/12484 (Thanks, snautz!)
• http://stackoverflow.com/a/32022511/12484 (Thanks, Peter Hahndorf!)
• http://stackoverflow.com/a/12284697/12484 (Thanks, ony!)