X-Clacks-Overhead

After Terry Pratchett's death in 2015, the web community adopted the X-Clacks-Overhead unofficial response header as a memorial. The header carries no technical function and references the Discworld clacks semaphore network.

Usage

The header references the "clacks" semaphore network from Terry Pratchett's Discworld novels. In the fiction, the clacks is a series of towers transmitting coded messages across great distances. The code "GNU" is a set of operator instructions: G means "send the message on," N means "do not log the message," and U means "turn the message around at the end of the line." Together, GNU keeps a person's name cycling through the network indefinitely, a way of remembering the dead.

After Pratchett's death on 12 March 2015, web developers adopted X-Clacks-Overhead as a memorial. Server plugins and modules exist for Apache (mod_clacks), Nginx, WordPress, Express, and other platforms. The header appears on sites ranging from news outlets to e-commerce stores. Adding the header is a one-line configuration change in most environments and has zero impact on response handling or client behavior.

Values

The standard value is GNU Terry Pratchett. Some servers extend the value with additional names as a broader memorial.

Example

The most common form sends the original tribute value. The three-letter code GNU followed by the name is the format established by the community after 2015.

X-Clacks-Overhead: GNU Terry Pratchett

Some servers include additional names alongside Pratchett, honoring other figures in science and literature. Each name is separated by a comma.

X-Clacks-Overhead: GNU Terry Pratchett, Douglas Adams, Stephen Hawking

A full response from a web server with the memorial header alongside standard HTTP headers.

HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
Server: nginx
X-Clacks-Overhead: GNU Terry Pratchett

See also

• Server
• HTTP headers