Firefox loses Request headers after redirect

Firefox does not remember some request headers after a redirect. This can cause problems with AJAX requests that use the X-Requested-With: XmlHttpRequest header.

I noticed a little problem with Firefox. In my scenario I have added the SEO optimising IIS Url Rewriting module pattern that redirects URL with upper case in to a lower case version.

For all browsers the AJAX request comes in with the X-Requested-With: XmlHttpRequest request header, a 301 status code is returned with the lower cased location, that new location then is requested with the X-Requested-With: XmlHttpRequest header in tact.

Firefox, however, does not honour this header in its redirect. This causes problems with my ASP.NET MVC code that checks for the request being via AJAX using IsAjaxRequest(). This method checks if X-Requested-With: XmlHttpRequest is in the headers (or in other exciting places).

Work around

I was going to try and do something clever like detect the browser and this header and then not redirect under some circumstances because (brace yourself SEO consultants) functionality is more important than SEO. I decided to instead make sure that one troublesome link was lowercase and so avoided the redirect.

Just hope this answers your questions if you come across this oddity in Firefox.

ASP.NET MVC IsAjaxRequest() not recognising request from jQuery $.ajax()

If you are finding that an AJAX request from, say, a piece of jQuery AJAX code such as $.ajax() $.get() or $.post() or you own custom AJAX code (you legend) is not registering as an AJAX request when using the IsAjaxRequest() then the problem is probably to do with X-Requested-With.

I used .NET Reflector to have a look at how IsAjaxRequest() works and it is something like this:

return ((request["X-Requested-With"] == "XMLHttpRequest") || ((request.Headers != null) && (request.Headers["X-Requested-With"] == "XMLHttpRequest")));

So that means you need to have a header of "X-Requested-With" set to "XMLHttpRequest" which is usually the case when you are using AJAX libraries like ASP.NET's or jQuery's. You can see if this is the case using Fiddler.

However, interestingly you can also set this as either part of your form POST or even a GET querystring! Such as www.example.com?x-requested-with=XMLHttpRequest (case sensitive). This will also make IsAjaxRequest() return true.