Inline Linking

Inline linking (also known as hotlinking, leeching, piggy-backing, direct linking, offsite image grabs, bandwidth theft) is the use of a linked object, often an image, from one site by a web page belonging to a second site. The second site is said to have an inline link to the site where the object is located.

 

Inline linking and HTTP

 

The technology behind the World Wide Web, the Hypertext Transfer Protocol (HTTP), does not make any distinction of types of links—all links are functionally equal. Resources may be located on any server at any location.

When a web site is visited, the browser first downloads the textual content in the form of an HTML document. The downloaded HTML document may call for other HTML files, images, scripts and/or stylesheet files to be processed. These files may contain <img> tags which supply the URLs which allow images to display on the page. The HTML code generally does not specify a server, meaning that the web browser should use the same server as the parent code (<img src="picture.jpg" />). It also permits absolute URLs that refer to images hosted on other servers (<img src="http://www.example.com/picture.jpg" />).

When a browser downloads an HTML page containing such an image, the browser will contact the remote server to request the image content.

 

Common uses of linked content

 

The ability to display content from one site within another is part of the original design of the Web's hypertext medium. Common uses include:

• Web architects may deliberately segregate the images of a site on one server or a group of servers. Hosting images on separate servers allows the site to divide the bandwidth requirements between servers. As an example, the high-volume site Slashdot stores its "front page" at slashdot.org; individual stories on servers such as games.slashdot.org or it.slashdot.org; and serves images for each host from images.slashdot.org.
• An article on one site may refer to copyrighted images or content on another site, avoiding rights and ownership issues that copying the original files might raise, although this practice is generally not accepted.
• Many web pages include banner ads. Banner ads are images hosted by a company that acts as middleman between the advertisers and the web sites on which the ads appear. The <img> tag may specify a URL to a CGI script on the ad server, including a string uniquely identifying the site producing the traffic, and possibly other information about the person viewing the ad, previously collected and associated with a cookie. The CGI script determines which image to send in response to the request.
• Some websites hotlink from a faster server to increase client loading speed.
• Hit counters or Web counters show how many times a page has been loaded. Several companies provide hit counters that are maintained off site and displayed with an inline link.

 

Controversial uses of inline linking

 

The blurring of boundaries between sites can lead to other problems when the site violates users' expectations. Other times, inline linking can be done for malicious purposes.

• Content sites where the object is stored and from which it is retrieved may not like the new placement.
• Inline linking to an image stored on another site increases the bandwidth use of that site even though the site is not being viewed as intended. The complaint may be the loss of ad revenue or changing the perceived meaning through an unapproved context.
• Cross-site scripting and phishing attacks may include inline links to a legitimate site to gain the confidence of a victim.
• Pay-per-content services may attempt to restrict access to their content through complex scripting and inline linking techniques.
• Inline objects can be used to perform drive-by attacks on the client, exploiting faults in the code that interprets the objects. When an object is stored on an external server, the referring site, for example Wikipedia, has no control over if and when an originally beneficial object's content is replaced by malicious content.
• The requests for inline objects usually contain the referrer information. This leaks information about the browsed pages to the servers hosting the objects.

Prevention

Client side

Most web browsers will blindly follow the URL for inline links, even though it is a frequent security complaint. Embedded images may be used as a web bug to track users or to relay information to a third party. Many ad filtering browser tools will restrict this behavior to varying degrees.

 

Server side

 

Some servers are programmed to use the HTTP referrer to detect hot-linking and return a condemnatory message, commonly in the same format, in place of the expected image or media clip. Most servers can be configured to partially protect hosted media from inline linking, usually by not serving the media or by serving a different file.

Nearly all of these measures will transmit an alternate content. Most types of electronic media can be redirected this way, including video files, music files, and animations (such as Flash).

Other solutions usually combine the standard mod_rewrite module with some custom complex server side scripting (i.e. to allow hotlinking for a limited amount of time).