NOTE: I have updated the Status Code returned by image.cfm to be 204 as per Justin's comments (see below). According to the w3c, 204 stands for No Content (which can/should be used in conjunction with the zero-length header).
The idea here is that you contact the server by setting an image's source equal to the ColdFusion page that you are trying to hit. Then, the server grabs all the URL params you send as the image SRC request and returns an image such as a 1x1 GIF image. However, what happens if you don't return an image at all? Why go through the overhead? Not returning an image has always made me nervous. Not sure why. It's not like the page is gonna explode or anything?
Notice here that source of the image is the images.cfm ColdFusion template. This would theoretically allow us to pass variables to the server via the IMG source URL.
Then, I put together a small ColdFusion page that returns nothing. And, not only does it return nothing, it tells the browser, using the ColdFusion CFHeader tag, to expect zero bytes of content. This should get the browser to stop downloading the requested file immediately (what we want).
<!--- Kill extra output. ---> <cfsilent> <!--- Tell the browser that the image was found but that it has no content. Status 204 means no content. ---> <cfheader statuscode="204" statustext="No Content" /> <!--- Tell the browser that the image has no content. ---> <cfheader name="content-length" value="0" /> <!--- Set the content type. ---> <cfcontent type="text/gif" reset="true" /> </cfsilent>
Running the code above, we get the following output:
Zero-length Image Test
Hi Ben, you could also pass back a status code of 204 (http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.5) which tells the client that the request was successful but that it has no data to return... I've been using this technique ever since Brandon Harper had wrote about in the CFDJ a few years back (http://www.sys-con.com/story/?storyid=46789&de=1)
Awesome tip! I rare use anything other that 200, 500, 404, 301 and 302. Good to know there is one like that (that stands for no data). I will modify the post to reflect this (after I test it).