XClacksOverhead.org

In Terry Pratchett's science-fantasy Discworld series, "The Clacks" is a network infrastructure of Semaphore Towers, that operate in a similar fashion to telegraph - named "Clacks" because of the clicking sound the system makes as signals send.

In Sir Terry's novel "Going Postal", the story explains that the inventor of the Clacks - a man named Robert Dearheart, lost his only son in a suspicious workplace accident, and in order to keep the memory of his son alive, he transmitted his son's name as a special operational signal through the Clacks to forever preserve his memory:

The nature of the 'GNU' code would keep the name of his son forever transmitting through The Clacks network so long as the network still existed.

As a way to preserve the memory of Sir Terry Pratchett, the users of the SubReddit for the Discworld series came up with the idea behind the X-Clacks-Overhead HTTP Header. It allows web authors to silently commemorate someone through the use of a non-invasive header that can be transmitted from server to server, or server to client without operational interference.

You would only know the header is present if you analysed the transmission headers of your content requests on web sites serving the header.

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