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Leech/HotLink Protection


Category: Web technologies

Are you tired of people using your server as a storage space for their pictures on different websites? Use this option to block them from "leeching" your files.

The problem of people "leeching" files has become quite a serious one for some webmasters so it has been developed a solution for them. Servers can be configured to prevent "hotlinks" by using complicated config files .. or even through web interfaces.

CPanel is being used on many web servers (some servers are also using CPanel-like interfaces with similar options though) - which has a very nifty option of turning HotLink protection for your server/domain, setting up exclusions and what page to load in case an user tried to link directly to a file.

Site Management Tools - CPanel

But how does this system really works? Well, it simply checks for the referrer field sent by the browser. If the user came from a page on your server (or on the exclusion list), it is considered "safe" and allowed to view the specified resource, if it is not, the "HotLink" mechanism is triggered.

A simple customized system for hot-linking can be made using a server-side scripting language such as PHP or ASP. In PHP for example, you can create a page which to load a file specified by a parameter and comparing the $_SERVER['HTTP_REFERER'] field with a pattern specified by yourself.

Make sure you do a proper check of the parameter sent! It can be become a SERIOUS security issue otherwise. A simple solution for this would be to have all images/files in a single directory and files contain only letters and numbers, then check the filename sent as parameter using Regular Expressions:

if (eregi("^([a-z0-9]+\.([a-z]{1,3})$", $file))
    // load the page

Keep in mind that some of your visitors might use referrer blocking programs which will trigger indeed HotLink even if they are visiting the website normally. This is a quite common situation if server is configured with anti-leeching only set for images/video files and might save you from some headaches if your visitors are complaining images are not showing up.

For more informations about this situation check out this article: http://bubble.ro/How_to_block_referrer_detection.html

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Posted by: Indy on February 28, 2006 at 19:48.

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