Like most large websites, Wikipedia suffers from the phenomenon known as link rot, where external links become dead, as the linked web pages or complete websites disappear, change their content, or move without HTML redirection. This presents a significant threat to Wikipedia's reliability policy and its source citation guideline.
In general, do not delete cited information solely because the URL to the source does not work any longer. Tools, procedures, and processes are available as outlined in this document.
Links added by editors to the English Wikipedia mainspace are automatically saved to Wayback Machine within about 24 hours (nb. in practice not every link is getting saved for various reasons). This is done with a program called which Internet Archive runs and maintains; other language wiki sites are included. It monitors , extracts new external URLs and adds a snapshot to the Wayback. This system became active sometime after 2015, though previous efforts were also made. Also, sometime after 2012, archive.today (aka archive.is) attempted to archive all external links then existing on Wikipedia at that time. This was incomplete but a significant number of links were added to archive.today during this period making it a major archival source filling in gaps of coverage. Archive.today is still making some automated archives as of 2020, though the extent of coverage and frequency is unknown.
As of 2015, there is a Wikipedia bot and tool called WP:IABOT that automates fixing link rot. It runs continuously checking all articles on Wikipedia if a link is dead, adding archives to Wayback Machine (if not yet there), and replacing dead links in the wikitext with an archived version. This bot runs automatically but it can also be directed by end users through its web interface. It is available when viewing any page's history, located near the top of the page on the line of "External Tools", with the "Fix dead links" option.
As of 2015, the periodic bot WP:WAYBACKMEDIC checks for link rot in the archive links themselves. Archive databases are dynamic and changing, archives go missing, move, new ones added etc.. this bot maintains existing archive links on English Wikipedia. It also does archiving on request at WP:URLREQ, it is a flexible tool that can do many custom jobs.
Most citation templates have a
|quote= parameter that can be used to store text quotes of the source material. This can be used to store a limited amount of text from the source within the citation template. This is especially useful for sources that cannot be archived with web archiving services. It can also provide insurance against failure of the chosen web archiving service. Storing the entire text of the source is not appropriate under fair use policies, so choose only the most important portions of the text that most support the assertions in the Wikipedia article. Where applicable, public domain materials can be copied to Wikisource.
If the dead link includes enough information (article title, names, etc.) it is often possible to use it to find the Web page at a different location, either on the same site or elsewhere.
Often web pages simply moved within the same site. A site index or site-specific search feature is a useful place to locate the moved page. If these tools are not available, many Internet search engines allow a search on a specified site.
Failing this, searching the Internet for the page can find alternatives.
Check for archived versions at one of the many web archive services. The "Big 3" archive services are web.archive.org, webcitation.org and archive.is. These account for over 90% of all archives on Wikipedia, with web.archive.org being over 80% of all archive links. Other archive services are listed at WP:WEBARCHIVES.
The allows one to search multiple archiving services with a single search. The Memento database is cached, meaning results are returned quickly, but the cache also becomes out of date. Therefore, it should not be relied on as the final word – very often it may report no archives are available, when they actually are. You may still need to do the work of checking individual archive sites, but Mementos can be a quick first check.
If multiple archive dates are available, use the one that is most likely to be the contents of the page seen by the editor who entered the reference on the
|access-date=. If that parameter is not specified, a can be performed to determine when the link was added to the article.
View the archive to verify that it contains valid page information. Usually dates closer to the time the link was placed in the Wikipedia page, or earlier, are more likely to show valid information.
At times, all attempts to repair the link will be unsuccessful. In that event, consider finding an alternate source so that the loss of the original does not harm the verifiability of the article. Alternate sources about broad topics are usually easily located. A simple search engine query might locate an appropriate alternative, but be extremely careful to avoid citing mirrors and forks of Wikipedia itself, which would violate Wikipedia:Verifiability.
Sometimes, finding an appropriate source is not possible, or would require more extensive research techniques, such as a visit to a library or the use of a subscription-based database. If that is the case, consider consulting with Wikipedia editors at Wikipedia:WikiProject Resource Exchange, the Wikipedia:Village pump, or Wikipedia:Help desk. Also, consider contacting experts or other interested editors at a relevant WikiProject.
Sometimes a link is dead because the website moved the URL (e.g. moved to ). If you discover an URL change like this, please submit a request at WP:BOTREQ for a url move. A bot will make the change.
Non-Wikimedia sites are also susceptible to link rot. Following a page move or page deletion, links to Wikipedia pages from other websites may break. In most page moves, a redirect will remain at the old page—this won't cause a problem. But if a page is completely deleted or usurped (i.e. replaced with other content) then link rot will have been caused on any external websites that link to it.
Replacement of page content with a disambiguation page may still cause link rot, but is less harmful because a disambiguation page is essentially a type of soft redirect that will lead the reader to the required content. If a page is usurped with content for another subject that shares its name, a hatnote may be placed at the top that directs readers to the original content on its new page—this again is a type of soft redirect, but less obvious. In these cases, readers arriving from an external rotten link should be able to find what they're looking for, but the situation is best avoided as they would have to get there via an additional page, potentially giving a poor impression of both Wikipedia and the linking website.
Because the Wikipedia software does not store
Referer information, it will be impossible to tell how many external web pages will be affected by a move or deletion, but the risk of link rot will probably be greatest on older and higher profile pages. In truth, there is not a lot that can be done; maintenance of non-Wikimedia websites is not within the scope of being a Wikimedian, nor in most cases within our capability (although if they can be fixed, it would be helpful to do so). However, it may be good practice to think about the potential impact on other sites when deleting or moving Wikipedia pages, especially if no redirect or hatnote will remain. If a move or deletion is expected to cause significant damage, then this might be a factor to consider in WP:RM, WP:AFD and WP:RFD discussions, although other factors may carry more weight.