Wikipedia:LINKROT

The effort required to prevent link rot is significantly less than the effort required to repair or mitigate a rotten link. Therefore, prevention of link rot strengthens the encyclopedia. This guide provides strategies for preventing link rot before it happens. These include the use of web archiving services and the judicious use of citation templates.

Editors are encouraged to add an archive link as a part of each citation, or at least submit the referenced URL for archiving, at the same time that a citation is created or updated.

However, link rot cannot always be prevented, so this guide also explains how to mitigate link rot by finding previously archived links and other sources. These strategies should be implemented in accordance with , which describes the steps to take when a link cannot be repaired.

Except for URLs in the External links section that have not been used to support any article content, do not delete cited information solely because the URL to the source does not work any longer. Recovery and repair options and tools are available. Verifiability does not require that all information be supported by a working link, nor does it require the source to be published online.

As you edit, if an article has bare URLs in its citations, fix them or at least tag the References section with {{}} as a reminder to complete citation details as above, and to categorize the article as needing cleanup.

A second way to prevent link rot is to use a web archiving service. The two most popular services are the Wayback Machine, which crawls and archives many web pages as well as having a form to suggest a URL to be archived, and WebCite, which provides on-demand web archiving. These services collect and preserve web pages for future use even if the original web page is moved, changed, deleted, or placed behind a pay wall. Web archiving is especially important when citing web pages that are unstable or prone to changes, like time sensitive news articles or pages hosted by financially distressed organizations. Once you have the URL for the archived version of the web page, use the archiveurl= and archivedate= parameters in the citation template that you are using. The template will automatically incorporate the archived link into reference.

...
Wikipedia