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This is an older version of Eleventy. Go to the newest Eleventy docs (current path: /docs/quicktips/not-found/) or the full release history.
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Quick Tip #006—Adding a 404 Not Found Page to your Static Site

A 404: Not Found page is a pretty standard thing on the web. It’s particularly useful when someone shares a broken or mistyped link. You don’t need dynamic middleware to do this, it’s pretty easy to add to a statically generated site. There are a few options, depending on your web host:

But first let’s make our 404.html template:

---
title: Oops! Not Found
permalink: 404.html
---
<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>{{ title }}</title>
  </head>
  <body>
    <h1>{{ title }}</h1>
    <p>This is where you should tell the user how to find their content. Maybe on the <a href="/">home page?</a></p>
  </body>
</html>

Eleventy will output this template to 404.html.

If you’re using GitHub Pages or Netlify, there is no step two! A 404.html file in your output directory is all you need.

Netlify even has lovely multi-language 404 page support too using Redirects.

.htaccess Jump to heading

For other hosts, if you use .htaccess for configuration you can use ErrorDocument. Make sure your root directory matches here!

ErrorDocument 404 /404.html

With --serve Jump to heading

INFO:
If you’re using the new Eleventy Dev Server Added in v2.0.0, the 404.html convention is now provided to you for free! You can skip this section!

If you’d like to backport this functionality to Browsersync, keep reading. (Note also that you can use Browsersync with Eleventy 2.0!)

If you're using Browsersync with Eleventy (with eleventy --serve), you can configure it to do the 404 routing by passing a callback in your config. Read more on the BrowserSyncConfig option, the Browsersync callbacks option, and how to provide a 404 using a Browsersync callback.

Filename .eleventy.js
const fs = require("fs");
const NOT_FOUND_PATH = "_site/404.html";

module.exports = function(eleventyConfig) {
  eleventyConfig.setBrowserSyncConfig({
    callbacks: {
      ready: function(err, bs) {

        bs.addMiddleware("*", (req, res) => {
          if (!fs.existsSync(NOT_FOUND_PATH)) {
            throw new Error(`Expected a \`${NOT_FOUND_PATH}\` file but could not find one. Did you create a 404.html template?`);
          }

          const content_404 = fs.readFileSync(NOT_FOUND_PATH);
          // Add 404 http status code in request header.
          res.writeHead(404, { "Content-Type": "text/html; charset=UTF-8" });
          // Provides the 404 content without redirect.
          res.write(content_404);
          res.end();
        });
      }
    }
  });
};

Thank you Cassey Lottman.