X-Clacks-Overhead
The HTTP X-Clacks-Overhead response header is an unofficial HTTP header added as a tribute to author Terry Pratchett. The header carries no technical function and serves purely as a cultural memorial.
Note
The "X-" naming convention for HTTP headers, "X" referring to "experimental", has been deprecated and needs to be transitioned to the formal naming convention for HTTP headers.
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
Takeaway
The X-Clacks-Overhead header is a community-driven memorial to Terry Pratchett, embedding his name in HTTP traffic as the Discworld clacks network kept names alive in fiction.