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Request Limit Length Limits for IIS’s requestFiltering Module

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Today I updated my CodePaste.net site to MVC 3 and pushed an update to the site. The update of MVC went pretty smooth as well as most of the update process to the live site. Short of missing a web.config change in the /views folder that caused blank pages on the server, the process was relatively painless.

However, one issue that kicked my ass for about an hour – and not foe the first time – was a problem with my OpenId authentication using DotNetOpenAuth. I tested the site operation fairly extensively locally and everything worked no problem, but on the server the OpenId returns resulted in a 404 response from IIS for a nice friendly OpenId return URL like this:


A 404 of course isn’t terribly helpful – normally a 404 is a resource not found error, but the resource is definitely there. So how the heck do you figure out what’s wrong?

If you’re just interested in the solution, here’s the short version: IIS by default allows only for a 1024 byte query string, which is obviously exceeded by the above. The setting is controlled by the RequestFiltering module in IIS 6 and later which can be configured in ApplicationHost.config (in \%windir\system32\inetsvr\config). To set the value configure the requestLimits key like so:

        <requestLimits maxQueryString="2048">

This fixed me right up and made the requests work.

How do you find out about problems like this? Ah yes the troubles of an administrator? Read on and I’ll take you through a quick review of how I tracked this down.

Finding the Problem

The issue with the error returned is that IIS returns a 404 Resource not found error and doesn’t provide much information about it.

If you’re lucky enough to be able to run your site from the localhost IIS is actually very helpful and gives you the right information immediately in a nicely detailed error page.

The bottom of the page actually describes exactly what needs to be fixed. One problem with this easy way to find an error: You HAVE TO run localhost. On my server which has about 10 domains running localhost doesn’t point at the particular site I had problems with so I didn’t get the luxury of this nice error page.

Using Failed Request Tracing to retrieve Error Info

The first place I go with IIS errors is to turn on Failed Request Tracing in IIS to get more error information. If you have access to the server to make a configuration change you can enable Failed Request Tracing like this:

Find the Failed Request Tracing Rules in the IIS Service Manager.



Select the option and then Edit Site Tracing to enable tracing. Then add a rule for * (all content) and specify status codes from 100-999 to capture all errors. if you know exactly what error you’re looking for it might help to specify it exactly to keep the number of errors down.

Then run your request and let it fail. IIS will throw error log files into a folder like this C:\inetpub\logs\FailedReqLogFiles\W3SVC5 where the last 5 is the instance ID of the site.


These files are XML but they include an XSL stylesheet that provides some decent formatting. In this case it pointed me straight at the offending module:


Ok, it’s the RequestFilteringModule. Request Filtering is built into IIS 6-7 and configured in ApplicationHost.config. This module defines a few basic rules about what paths and extensions are allowed in requests and among other things how long a query string is allowed to be. Most of these settings are pretty sensible but the query string value can easily become a problem especially if you’re dealing with OpenId since these return URLs are quite extensive.

Debugging failed requests is never fun, but IIS 6 and forward at least provides us the tools that can help us point in the right direction. The error message the FRT report isn’t as nice as the IIS error message but it at least points at the offending module which gave me the clue I needed to look at request restrictions in ApplicationHost.config. This would still be a stretch if you’re not intimately familiar, but I think with some Google searches it would be easy to track this down with a few tries…

Hope this was useful to some of you. Useful to me to put this out as a reminder – I’ve run into this issue before myself and totally forgot. Next time I got it, right?

Posted in ASP.NET  Security  

The Voices of Reason


October 29, 2010

# re: Request Limit Length Limits for IIS&rsquo;s requestFiltering Module

Having a "How did I find the problem" part is pretty neat. Good post.

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