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Published: Saturday, December 30, 2000

Creating "Quick-Links" Using a Custom 404 Page

By Johan Alveborg

The other day one of the marketing guys came around and asked if we couldnít fix up some easy-to-type URLís for some pages on the site. The site uses frames and the page requested should of course show up in the main content frame in the same way as if the user had chosen it from the menu. Until now, we had solved this with a parameter to the frame-definition page, such as SomePage.asp?Page=42, and then checking the Page-param in a Select Case statement for the correct page to display.

- continued -

Now, the marketing departement had dreamed up about 35 new direct-links they wanted implemented, and they were of the opinion that http://www.foo.com/somepage?page=42 didnít really look very good in print ads, and I had to agree :-)

The two most obvious ways to solve this were:

    1.) Add 35 new virtual roots to our 25 IIS boxes, or
    2.) Add 35 new directories in the webroot, each with a default page that does a Response.Redirect to SomePage.asp with the correct parameter.

On of our hired hands started off along the lines of option #2, but I started thinking there hade to be a better way, and fortunately there is! What happens if you request http://www.foo.com/bar and the directory doesnít exist? You get a "404 Page not found"-message. Hrmmm, let's check... yes you can configure what page IIS serves when the requested file or directory doesnít exist, but it doesnít seem to run the ASP-code put in the page. What eluded me at first was that in the IIS MMC you can set what type of page IIS should use for the error messages, the default is "File" which just shoots the page to the user without any processing. Changing the type to "URL" does the trick and any ASP code in the page is now processed. IIS sends the URL the user requested in the query string so itís available to our ASP code to check.

For More Information on Custom 404 Error Pages
To learn more about creating custom 404 error pages be sure to read Creating a Custom 404 Error Page. It is a very good and thorough article explaining how to setup such a custom 404 error page! Another very worthwhile read on the subject is Nathan Pond's Tracking 404 Errors.

So, the rest is pretty straightforward: In the ASP page IIS serves when it canít find a requested folder or file, check what folder the user requests and if itís one that you have anticipated, you redirect the user to the correct page Ė in our case the frame definition page with a parameter indicating what page to display in the main content frame.

Begin by creating a 404.asp page and configure IIS to serve that page for the 404 error code (donít forget to set "Type" to "URL"; for more information on setting up IIS properly and creating such a custom 404 error page, be sure to read Creating a Custom 404 Error Page!)

In 404.asp, snarf out the directory the user has tried to access using the following code:

SomeDir = Request.ServerVariables("Query_String")
SomeDir = UCase(Mid(SomeDir, InStrRev(SomeDir, "/") + 1, Len(SomeDir)))
'SomeDir now contains whatever the user typed after the last
'slash in the URL, if he typed http://www.foo.com/books SomeDir 
'will contain "BOOKS".

and then use a Select Case to check and redirect as needed:

Select Case SomeDir
  Case "BOOKS"
  Case "FOO"
  Case Else
    Response.Write("The page you requested doesnít exist...")
End Select

You could of course tidy things up by using an array of names/numbers (in the format BOOKS;25) stored in an application variable and loop through it. This would allow for an easy web-based updating mechanism for adding new pages without having to change the 404.asp page. To accomplish this, in Global.asa (or an admin page) set the array up as:

Dim RedirPages(2)
RedirPages(1) = "BOOKS;25"
RedirPages(2) = "FOO;42"
Application("404RedirectionPagesArray") = RedirPages

And in the 404.asp page:

RedirPages = Application("404RedirectionPagesArray")
'Put it in a local var to save typing later on

For I = 0 To UBound(RedirPages) Ė 1 
  If (Split(RedirPages(I), ";")(0) = SomeDir) Then
    Response.Redirect("/SomePage.asp?Page=" & _
                      Split(RedirPages(I), ";")(1))
  End If

Don't use the dictionary object (which otherwise would have been the perfect method) since it's not thread safe. The LookupTable object looks good but I havenít tried it yet.

One of the coolest things about this hack is that it's very easy to handle user requests for misspelled URL's: In the above example, just add BOKS and BOOK to the array and you'll at least have increased the chances that the user will get to the page that they intended!

Happy Programming!

  • By Johan Alveborg

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