X-Clacks-Overhead is a non-standardised HTTP header based upon the fictional work of the late, great, Sir Terry Pratchett.
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:
GNU John Dearheart
G: Send the message onto the next Clacks Tower.
N: Do not log the message.
U: At the end of the line, return the message.
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.
"A man is not dead while his name is still spoken."
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.
At the time of writing, Mozilla.org (makers of the Firefox web browser), the makers of Debian (a popular Linux Operation System), and Xml.com (a major repository of standards information) are examples of some of the backbones of the Internet who transmit the Signal "GNU Terry Pratchett". The Internet news website The Register also transmits headers to commemorate both Sir Terry Pratchett and one of their staffers; Lester Haines.
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