This task combats link rot by using the Internet Archive Wayback Machine to provide archive copies of now dead links in references and citations or marking them with {{dead link}} if a suitable archived copy is unavailable.
The bot currently only processes citation templates that have |url= and |accessdate= set. The recognized citations are: {{Citation}}, {{Cite news}}, {{Cite web}}, {{Cite journal}}, {{Cite book}}, {{Cite mailing list}}, {{Cite video}}, {{Vcite web}}, {{Vcite book}}, {{Vcite news}}, and {{Vcite journal}}. The bot will attempt to retrieve the archived copy from Wayback and add |archiveurl= and |archivedate= to the citation (the bot will respect whitespace formatting). The bot will also add <!-- Added by H3llBot --> comment, so it is possible to track bot added archvies. Failing that, it will mark dead links with {{dead link|bot=H3llBot}} or set |deadurl=yes if it was a preemptively archived citation with |deadurl=no.
The retrieved Wayback archive's date is either (1) the closest archived copy before the citation's |accessdate= up to 3 month range or (2) the first archived copy after the access date up to 1 month range (used to be ±6 months). The date format is derived either from {{Use dmy dates}} or {{Use mdy dates}} templates or the citation's |accessdate= or |date= field.
Dead links are URLs whose HTTP status responses are 404 or 301. Other error codes or failed connections are ignored. The 404 check is carried out twice within 3 days (used to be 1 day) to make sure the link is really dead and not just down for maintenance. GET (as opposed to HEAD) requests are used and redirects followed as some servers redirect to both 404 and 200 pages.