Clacks checker

What the header is

In Going Postal, Terry Pratchett's semaphore operators keep the names of their dead moving through the towers with a three-letter code. G means send the message on. N means don't log it. U means turn it round at the end of the line. A message marked GNU never arrives anywhere, so it never stops.

After he died in March 2015, people started doing the same on the web, with a response header called X-Clacks-Overhead. Browsers don't show it. Servers pass it along anyway, quietly, with every page.^*

Add it to your own site

One line, wherever your server sets headers. Pick yours:

[[headers]]
  for = "/*"
  [headers.values]
    X-Clacks-Overhead = "GNU Terry Pratchett"

/*
  X-Clacks-Overhead: GNU Terry Pratchett

Header set X-Clacks-Overhead "GNU Terry Pratchett"

add_header X-Clacks-Overhead "GNU Terry Pratchett" always;

app.use((req, res, next) => {
  res.set("X-Clacks-Overhead", "GNU Terry Pratchett");
  next();
});

To carry more than one name, separate them with commas.

Take it with you

The browser extension puts a small tower in your toolbar. Visit a site that carries a name in its overhead and the shutters light up; click the tower to see who is being kept on the line. It reads headers inside your browser and sends nothing anywhere.^†

Privacy

This page: the address you type goes to lolb.link's server, which asks that website for its headers (never the page itself) and tells you what it found. The answer is cached for ten minutes so repeat checks don't bother the site again. Nothing else is kept, apart from Netlify's standard request logs.

The extension: it asks your browser to show it the response headers of pages you open, so it can spot X-Clacks-Overhead. That is why the browser warns that it can read data on all websites. It reads nothing else, remembers the result for each tab only while the tab is open, never sends anything to lolb.link or anyone else, and has no analytics. The code is short and readable inside the zip.