Posted by Aaron on September 19, 2013
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During beta testing we upgraded a production site to 2013 beta 2. This site, however, was running on IIS 6. Until this upgrade we had not tested IIS 6, though for 2013 we specifically chosen to support ASP.NET 4.0, rather than 4.5, so it would still be compatible. Surprisingly it worked perfectly with no Web.config or IIS changes (other than changing the app pool to .NET 4).
However, we did notice one issue. On the first request to the site homepage (new session), the URL shown in the browser had this tacked on to the end of it: /eurl.axd/a9e530c3ac1a9b48a61ce9a633523a51/. After some research we found that ASP.NET uses this URL under the hood to support extension-less URLs in IIS 6. You can read more about this here: http://www.asp.net/whitepapers/aspnet4/breaking-changes#0.1__Toc256770153.
What had us stumped was that the site we upgraded did not have any virtual directories or applications under it that are running in ASP.NET 2.0 like the Microsoft article discusses. After some further digging we found that ISAPI Rewrite, which was installed on this server, was grabbing the request earlier in the pipeline when the URL still had the eurl.axd piece attached. We were able to modify the ISAPI Rewrite rule as noted below to strip this piece of the URL out of the redirect.
Original Rule:
RewriteRule (.*) http\://www.yourdomain.com$1 [NC,R=301]
New Rule:
RewriteRule (.*)(eurl.axd/.*)$ http\://www.yourdomain.com$1 [NC,R=301]