ColdFusion Muse

Custom URLs using the missing File handler

Mark Kruger April 29, 2005 3:42 PM Coldfusion Tips and Techniques Comments (5)

(reprinted from a previous blog)

If you have ever worked on a community site, or a site with salesman profiles or profiles of individuals, you may have said to yourself, "Self, it sure would be nice if each of these folks could have their own website". Doing it would require you to make a unique host entry for each user or create separate folders for each user and of course, that's not really easily managed. Or is it? Perhaps there's another way. Actually IIS and Cold Fusion give you a way to accomplish this pretty easily. 2 ways in fact.

> The first method is through the use of the dreaded "missing file template". When IIS can't find a file to serve it has 2 choices. 1 - it can send a "missing file status code" back to the browser in the response header. The actual code is 404, which is why "404 error" has become a euphemism for "missing file". The second choice is to actually serve a file. What file you ask? Why.... whatever file is specified in the "custom errors" for that site. By default, in fact, IIS comes equipped with a host of custom error templates. These are simple HTML files that are served in leu of the actual status code. They say extremely informative things like "404 error - file not found". Honestly, you'd think as much cash as Microsoft has they would hire someone other than Dilbert's boss to write their error messages.

Ok, here's the deal. IIS error messages need not be written by Microsoft. In fact you can write them yourself or specify any file you like to be served. You can hire bolivian contractors to write them, warn people about global warning instead of throwing an error, or even (dare I say it) display something actually helpful - like a link to your support page for further help. In fact (and here's where our solution comes in), you don't actually have to use a "file" at all - you can use a URL from your site. That's work CF comes in.

Try this:

  1. Open your site properties and choose "custom errors"
  2. Click on the 404 custom error and choose "edit"
  3. Change the message type to "URL"
  4. Enter the relative path to a .cfm script on your site.
Remember, you don't enter a full URL here - the file must reside on your site and be accessible from a relative path. Why? Well, IIS is going to process the this value as if it were the file part of an HTTP request. Remember what an HTTP header looks like?
GET /mycode/myPage.cfm      HTTP/1.1
   Content-Type: text/html
   User-Agent: Mozilla/4.0
   Content-Length: 488
The Host address is separate from the "GET" part of the request. If you put in a full URL IIS would actually have to retrieve content using HTTP using a completely separate request. Instead, IIS wants to simply replace the erroneous part of the header information with the "known good" file mapping and continue on with the request as if it had been for the missing file template in the first place. That means that a request that hits the missing file template is about as fast as if the file specified had existed in the first place.

So, if IIS merely replaces the non-existent "/mycode/myPage.cfm" with a mapping from the custom error handler setting from the 404 error and continues on, does that mean I can use a .cfm page for my 404 handler? Yes indeed - it means exactly that! If you have specified "404Handler.cfm" and set the message type to URL in the custom error page. The request continues as before and the it is handed off to the Coldfusion server for processing with the original path sent forward as a the cgi.query_string variable. CF server does its thing, parsing through the script looking for commands and cf tags and sends the results back to IIS which serves it up to the user.

That being the case it's pretty easy to imagine how to use this to your advantage if you wanted to create a custom URL for a community site, or custom directories for a real estate site (to use another example). In the file handler simply parse out the string that represents the missing directory and use it as a variable to determine what actually should be served up to the user. For example, if you wanted to use the zip code for house searches on a real estate site you could have the user enter "". Since there is no directory titled 68154, this request would be forwarded to the missing file handler - we'll call it "haveYouSeenThisFile.cfm" with a querystring that contains ";404/68156" as the last item. All you have to do is <zipcode = Listlast(cgi.query_string,'/')>" and you have the zip code being sought. Display or redirect the user to the page with houses from that zip code and you are done.

DNS wildcard

The second method is actually cooler - but it can confuse people. First, set a wild card entry in your DNS server that points to your web server (or have your DNS provider do it. This entry is a rule that says, "If you receive a request for DNS resolution and there is no specific match for that string - forward it to the web server." In other words, if your DNS has an entry for "" and a user requests that IP address then that specific IP address is given in response. If however a request is received for and there is no entry for Bill (why would there be) then that request is forwarded to the web server.

To use our zip code example, a user could enter "" in their browser and that request would be forwarded to the web server. On the web server, the default document (a .cfm file - let's say "index.cfm") examines the value of cgi.Http_Host (usually looking at Listfirst(cgi.Http_host,"."). If that item is not "www" and not "myhomes" then it takes it to be a zip code and directs the user accordingly - simple right?

Well actually this approach works well if you are sending out links to people via email or you have links on a page or web site or in a document. But if you have a need to actually speak or explain the URL to someone it works less well. Most novice users simply don't grasp that a web site doesn't need a "www" in the URL. So you end up with or other variations. You can account for this in your parsing code of course, but you have added complexity to the process instead of making it easier (which is the point of this whole thing after all - right?).

Either approach can result in a nicely customized URL for many uses and a very minimal code base accounting for it. Neither requires extra "hard to track and see" things like mod_rewrite or ISAPI filters. And neither approach incurs a heavy performance penalty.

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  • Mal's Gravatar
    Posted By
    Mal | 2/28/06 3:06 AM
    Thanks, gives me some ideas to work on.
    Sample code would have been handy ;)
  • James Allen's Gravatar
    Posted By
    James Allen | 9/12/07 5:41 PM
    Hey there,

    I used the 404 error handler technique on a site I developed last year (the world cup one in my website link), which worked pretty well to allow me to eradicate any trace of .cfm files. However, the big problem I found with this method was the ability to POST to non-existent URL's - e.g <form action="/contactus"...

    This would always fail with another IIS error code (I can't remember the exact code - 503 perhaps). I believe it was possible to fix this by redirecting the error code handler to the same one as used by the 404, but the ISP I used wouldn't allow this - only for 404 etc.

    Just one thing to consider when using this approach. There are workarounds to allow form posting but they aren't as elegant as being able to use the custom URL's exclusively.

    Although I'd be interested in other people's ideas on this. I have a big 6 month contract coming up to develop a *huge* community site and I really want to use clear SEO friendly links throughout.. I wasn't sure if the 404 approach was the best way to do it as I thought it had abit of a performance hit - though it's good to read in this article that it doesn't..
  • Tucson Web Design's Gravatar
    Posted By
    Tucson Web Design | 10/26/10 3:59 PM
    I have created a solution for posting form to custom urls. My solution does involve using a cfm page as a controller. Within that page i take the form info and build the custom url then I use a dynamic cflocation tag to redirect to the proper seo friendly url.
  • Alas's Gravatar
    Posted By
    Alas | 7/22/11 6:27 PM
    Saved me loads of work, thanks.
  • Omega replica's Gravatar
    Posted By
    Omega replica | 3/30/12 12:15 AM
    Good job thanks