Url encoded # (%23) causing 404 in ASP.NET application


I have a deep-linking Silverlight RIA trying to consume a Twitter OAuth callback. The URL of the callback "page" in the RIA is:


Twitter calls back to this URL so long as the # sign is URL encoded; so the callback url I supply to Twitter is:


RiaTestPage.aspx does of course exist, but when Twitter calls back to this URL, I'm getting a 404 (from the VS 2010 ASP.NET Development server)


Server Error in '/' Application

The resource cannot be found.

Description: HTTP 404. The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable. Please review the following URL and make sure that it is spelled correctly.

Requested URL: /RiaTestPage.aspx#callback


Although the # sign has been correctly decoded in the error message above, the 404 seems to be the result of the encoded # sign. If I manually change the callback url that caused the 404,


to this:


The callback page in the RIA loads normally. Why am I receiving a 404 in this situation?


You get a 404 error because the #callback part is not part of the URL. It's a bookmark that is used by the browser, and it's never sent in the request to the server. If you encode the hash, it becomes part of the file name instead, and there is no file named RiaTestPage.aspx#callback.


Cause of the problem explained by Guffa already. Several solutions exist though:

<h2>1. Redirection service:</h2>

Use a redirection service (like bit.ly, or even your own server) for the URL you publish. It will provide a standard link that Twitter will allow and send it to the correct page on your site. <strong>These services can also make the URL much shorter, which is a plus for Twitter posts</strong>.

<h2>2. Custom 404:</h2>

Implement a custom 404 page on your site and when specific URLs, like your example come in, correct the encoding and redirect to your correct page.

<h2>3. <em>Simply add a ? to your link</em>!:</h2>

The ? character marks the end of the file specification and the start of parameters. Parameters are not actually required before a bookmark so this URL should work for a Twitter link:


will call


which is the same as:



In IIS7 you can use the URL rewrite module or manually edit the web.config file to add this rewrite rule which turns %23 into #. It doesn't look intuitive because you put the # in both the regex find and replace boxes, but it works. This redirects incoming URLs like www.domain.com/page%23anchorname to www.domain.com/page#anchorname, and avoids the 404 error.

<rewrite> <rules> <rule name="unencode hashmark"> <match url="^([^\#]*)#(.*)$" /> <action type="Redirect" url="{R:1}#{R:2}" /> </rule> </rules> </rewrite>


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