What purpose is of "&rnd=" parameter in http requests?

Question

Why do some web-applications use the http-get parameter rnd? What is the purpose of it? What problems are solved by using this parameter?

Didn't observed that, but this could help in getting rid of cache. — Riz, Oct 19 '11 at 12:42

score 7 · Accepted Answer · answered Oct 19 '11 at 12:51

This could be to make sure the page/image/whatever isn't taken from the user's cache. If the link is different every time then the browser will get it from the server rather than from the cache, ensuring it's the latest version.

It could also be to track people's progress through the site. Best explained with a little story:

A user visits example.com. All the links are given the same random number (let's say 4).
The user opens a link in a new window/tab, and the link is page2.php?rnd=4. All the links in this page are given the random number 7.
The user can click the link to page3.php from the original tab or the new one, and the analytics software on the server can tell which one by whether it has rnd=4 or rnd=7.

All we can do is suggest possibilities though. There's no one standard reason to put rnd= in a URL, and we can't know the website designer's motives without seeing the server software.

score 6 · Answer 2 · answered Oct 19 '11 at 12:45

Internet Explorer and other browsers will read an image URL, download the image, and store it in a cache.

If your application is going to be updating the image regular, and so you want your users to not see a cached image, the URL needs to be unique each time.

Therefore, adding a random string ensures this will be unique and downloaded into the cache each time.

score 4 · Answer 3 · answered Oct 19 '11 at 12:43

4

It's almost always for cache-busting.

answered Oct 19 '11 at 12:43

Dave Newton

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score 2 · Answer 4 · edited May 23 '17 at 11:58

As has been suggested by others. This kind of behaviour is usually used to avoid caching issues when you are calling a page that returns dynamic content data.

For example, say you have a page that gets some current user information such as "mysite.com/CurrentUserData". Now on the first call to this page, the user data will be returned as expected, but depending on the timing and caching settings, the second call may return the same data - even though the expected data may have been updated.

The main reason for caching is of course to optimise the speed of frequent request. But in the instance where this is not wanted, adding a random value as a query string parameter is known to be a widely used solution.

There are however other ways to get around this issue. For example if you were doing an Ajax request with javascript/JQuery. You could set the cache to false in your call...

$.ajax({url: 'page.html', cache: false});

you could also change it for all page calls on document load with...

$.ajaxSetup({cache: false}});

If you were to do an MVC application, you can even disable the caching on the control action methods with an attribute like so...

[OutputCache(NoStore = true, Duration = 0, VaryByParam = "*")]
public ActionResult NonCacheableData()
{
    return View();
}

(thanks to a quick copy and paste from here)

I dare say there are also settings in IIS you could apply to get the same affect - though I have not been that far with this yet.

What purpose is of "&rnd=" parameter in http requests?

4 Answers4