A trackback is one of three types of linkbacks, methods for Web authors to request notification when somebody links to one of their documents. This enables authors to keep track of who is linking, and so referring, to their articles. Some weblog software programs, such as Serendipity, WordPress, Movable Type, Typo, Telligent Community, Kentico CMS and Drupal (via contrib modules), support automatic pingbacks where all the links in a published article can be pinged when the article is published. The term is used colloquially for any kind of linkback.
The TrackBack specification was created by Six Apart, which first implemented it in its Movable Type blogging software in August 2002. The TrackBack has since been implemented in most other blogging tools. Six Apart started a working group in February 2006 to improve the Trackback protocol with the goal to eventually have it approved as an Internet standard by the IETF. One notable blogging service that does not support trackback is Blogger. Instead, Blogger provides “backlinks”, which allow users to employ Google’s search infrastructure to show links between blog entries.
A trackback is an acknowledgment. This acknowledgment is sent via a network signal (ping) from the originating site to the receiving site. The receptor often publishes a link back to the originator indicating its worthiness. Trackback requires both sites to be trackback-enabled in order to establish this communication. Trackback does not require the originating site to be physically linked to the receiving site.
Trackbacks are used primarily to facilitate communication between blogs; if a blogger writes a new entry commenting on, or referring to, an entry found at another blog, and both blogging tools support the TrackBack protocol, then the commenting blogger can notify the other blog with a “TrackBack ping“; the receiving blog will typically display summaries of, and links to, all the commenting entries below the original entry. This allows for conversations spanning several blogs that readers can easily follow.
Blogging software that supports the TrackBack protocol displays a “TrackBack URL” with every entry. This URL is used by the commenting blogger, whose software will send XML-formatted information about the new entry to this URL. Some blogging tools are able to discover these TrackBack URLs automatically, others require the commenting blogger to enter them manually.
Some individuals or companies have abused the TrackBack feature to insert spam links on some blogs. This is similar to comment spam but avoids some of the safeguards designed to stop the latter practice. As a result, TrackBack spam filters similar to those implemented against comment spam now exist in many weblog publishing systems. Many blogs have stopped using trackbacks because dealing with spam became too much of a burden.