X-Clacks-Overhead
loreStands for: X-Clacks-Overhead: GNU Terry Pratchett
The HTTP header thousands of sites send in Terry Pratchett's memory: X-Clacks-Overhead: GNU Terry Pratchett.
In Pratchett's Going Postal the clacks are semaphore towers that relay messages across the Discworld, and a message's overhead can carry control codes: G, send it on to the next tower; N, do not log it; U, at the end of the line, turn it around and send it back. A name sent with GNU circulates forever, because, as the book has it, a man is not dead while his name is still spoken. When Pratchett died on 12 March 2015, readers on r/discworld proposed putting his name in the one overhead the internet has, and within days Apache and Nginx snippets, a WordPress plugin and a Chrome extension that lights up when the header is present were in circulation. The header is still sent by a great many servers, and this one is among them.
The clacks of the Discworld novels are an optical telegraph: towers with shuttered lamps, each repeating what the previous one showed, so that a message crosses a continent in hours. In Going Postal (2004) the system has grown its own culture, including the overhead, the housekeeping codes sent along with the traffic. Three of them matter to the memorial. G tells a tower to pass the message on. N tells it not to log the message. U tells the tower at the end of the line to turn the message around and send it back the way it came. A name sent with all three never stops moving through the towers and is never written down, which is how the clacks operators keep a dead colleague's name alive: the book's line is that a man is not dead while his name is still spoken. Terry Pratchett died on 12 March 2015. Within days, readers on the Discworld subreddit asked what the internet's overhead would be, and settled on an HTTP response header, which is exactly a piece of housekeeping sent along with the traffic and seen only by the machines and the people who look. The name X-Clacks-Overhead follows the X- convention for private headers of the time; the value is GNU Terry Pratchett. Configuration snippets for Apache (a Header set line) and Nginx (add_header) appeared on 15 March 2015, then a WordPress plugin, then browser extensions that show a small marker when a site sends it. Reddit itself added it. The L-Space wiki, the Discworld fan encyclopaedia, lists later adopters, among them Minecraft, which carries GNU Terry Pratchett as splash text number 338 from Java Edition 1.8.5. The header does nothing. Browsers ignore it; caches pass it through; it adds a few dozen bytes to every response. That is the point: a tribute that costs the reader nothing and is found only by someone who went looking, which is the shape of the best things in this corner of the web, and the reason it is the model for the other hidden headers in this glossary.
Also known as: GNU Terry Pratchett, clacks overhead, GNU
Sources
- The L-Space Web wiki, 'GNU Terry Pratchett': the clacks overhead codes from Going Postal (G, send the message on to the next tower; N, do not log the message; U, at the end of the line, return the message), the line 'a man is not dead while his name is still spoken', the r/discworld origin of hiding GNU text in site headers, and the adopters, down to Minecraft splash text 338 in Java Edition 1.8.5
- Boing Boing, 15 March 2015, 'Sending Terry Pratchett home with HTTP headers': Apache and Nginx configurations that transmit the X-Clacks-Overhead header reading GNU Terry Pratchett, so that his name lives on in the overhead
- IT Pro, 17 March 2015, 'How Reddit ensured Sir Terry Pratchett's name lives on': redditors and other administrators adding the header to the servers they control, with a Chrome extension and a WordPress plugin