rohan singh

software engineer. bicyclist & rock climber. craft beer addict.

Getting ASP.NET to Play Nice With Opera & Wget

A few weeks ago we noticed an issue with some crawlers and search engines being unable to crawl Around the same time, Karl Dubost from Opera discovered that visitors using Opera were getting a parse error. In fact, a number of high-profile ASP.NET sites seem to be impacted by the latter issue.

If you’re just looking for the quick fix, skip ahead to the bottom of this post.

To sum up the issue, our server was responding with a Content-Type header of application/xhtml+xml to both Opera and wget. In Opera, this triggers an XML parser, which would fail since our content is actually text/html.

Of course, nowhere in our code were explicitly setting the Content-Type header to anything other than text/html, so the behavior was puzzling. Additionally, the server was responding with an application/xhtml+xml type even when the Accept header from the browser specified */*. This made no sense at all, since if the browser was willing to accept anything, we should be sending the content in its default text/html.

ASP.NET Browser Detection

As it turns out, ASP.NET has a somewhat questionable feature that allows you detect browser types and capabilities, largely based on the browser’s request headers. This is mediated through a browser definition file (*.browser), which is just a bunch of XML that matches up request header patterns to browser types and know capabilities. This file lives in your ASP.NET applicaiton’s App_Browsers folder.

The known capabilities for the current user agent are all available through the HttpContext.Request.Browser object.

As I said, all very questionable. The idea of having a giant database of browsers and what they are like just rubs me wrong and strikes me as unmaintainable. In fact, we only tried out the whole browser definition file as part of a proof-of-concept for some mobile pages. The feature never quite worked correctly, so we abandoned it and thought that was the end of that.

The insidious preferredRenderingMime

Of course, that wasn’t the end of that. For each browser definition in a browser definition file, you can define a preferredRenderingMime value for a browser. For example:

  <capability name="preferredRenderingMime" value="application/xhtml+xml" />

Most of the larger *.browser file compilations floating around the Web have Opera set to preferring a MIME type of application/xhtml+xml. A lot of files will default to a preferred type of application/xhtml+xml for all browsers that pass an Accept: / request header.

And even though we use ASP.NET MVC and no longer are using any of the browser detection stuff at all, the System.Web.UI.Page class has this fun code that runs when the page is processed:

private void SetIntrinsics(HttpContext context, bool allowAsync)

    HttpCapabilitiesBase browser = this._request.Browser;
    this._response.ContentType = browser.PreferredRenderingMime;


Brilliant, right? Since Opera prefers XHTML, this code does you the favor of automatically setting your response content type to application/xhtml+xml. Of course, since your actual content is still HTML, this causes an XML parse error and all your Opera visitors are hosed.

This might make sense if you are doing the classic ASP.NET Web Forms thing with server controls that adapt their rendering based on the browser’s preferred MIME. But even if you are using ASP.NET MVC, your .aspx views are still essentially pages, and this old code will still run.

Broken wget

Unfortunately, this SetIntrinsics code has another nasty side effect. If your application has already sent out response headers or content, it will just throw an exception since Response.ContentType can’t be set after response headers have already been sent.

In the case of, this meant that issue wasn’t just that Opera visitors experienced a parse error, but that an exception would be thrown for any browser for which ASP.NET tried to switch the Response.ContentType. This would result in absolutely no content being served, resulting in a blank page for Opera visitors.

Furthermore, a large percentage of crawlers and search engines use wget to grab pages. wget sends an Accept header of */*, and runs into the same no-content issue. A fine mess all around.

The fix

The simplest fix, of course, is to get rid of any *.browser files you may be using in your application. I understand redirecting to a mobile version of your site for mobile browsers or the like, but basing any major functionality on guesses about the user’s browser is a great path to future pain.

If, however, you want to keep your browser definition files around, consider removing any uses of the preferrendRenderingMime capability. Here’s a regex that should be able to find those instances for you:


Just do a find & replace on that and you should be good to go. This is what we ended up doing for and, which I’m happy to say now work perfectly for Opera and wget users alike!