policies to potential supporters. In Europe, the NotFound project, created by multiple European organizations including Missing Children Europe and Child Focus, encourages site operators to add a snippet of code to serve customized 404 error pages which provide data about missing children. While many websites send additional information in a 404 error message—such as a link to the homepage of a website or a search box—some also endeavor to find the correct web page the user wanted. Extensions are available for some content management systems (CMSs) to do this. Tracking 404 errors A number of tools exist that crawl through a website to find pages that return 404 status codes. These tools can be helpful in finding links that exist within a particular website. The limitation of these tools is that they only find links within one particular website, and ignore 404s resulting from links on other websites. As a result, these tools miss out on 83% of the 404s on websites. One way around this is to find 404 errors by analyzing external links. One of the most effective ways to discover 404 errors is by using Google Search Console, Google Analytics or crawling software. Another common method is tracking traffic to 404 pages using log file analysis. This can be useful to understand more about what 404s users reached on the site. Another method of tracking traffic to 404 pages is using JavaScript-based traffic tracking tools. See also Blue screen of death Funky caching Link rot List of HTTP status codes References External links A More Useful 404 404 Not Found of the Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content specification, at the Internet Engineering Task Force ErrorDocument Directive – instructions on custom error pages for the Apache 2.0 web server 404: Not Found – an award-winning song about the error code Computer errors Hypertext Transfer Protocol status codes Internet terminology