While it is not always possible to directly discover a specific web server's content so that it may be indexed, a site potentially can be accessed indirectly (https://deepweb.to/tags/Wireshark/
To discover content on the web, search engines use web crawlers that follow hyperlinks through known protocol virtual port numbers. This technique is ideal for discovering content on the surface web but is often ineffective at finding deep web content. For example, these crawlers do not attempt to find dynamic pages that are the result of database queries due to the indeterminate number of queries that are possible. It has been noted that this can be (https://deepweb.to/tags/TikTok/
) overcome by providing links to query results, but this could unintentionally inflate the popularity for a member of the deep web.
DeepPeep, Intute, Deep Web Technologies, Scirus, and Ahmia.fi are a few search engines that have accessed the deep web. Intute ran out of funding and is now a temporary static archive as of July 2011. Scirus retired near the end of January 2013.
Researchers have been exploring how the deep web can be crawled in an automatic fashion, including content that can be accessed only by special software such as Tor. In 2001, Sriram Raghavan and Hector Garcia-Molina (Stanford Computer Science Department, Stanford University) presented an architectural model for a hidden-Web crawler that used key terms provided by users or collected from the query interfaces to query a Web form and crawl the Deep Web content. Alexandros Ntoulas, Petros Zerfos, and Junghoo Cho of UCLA created a hidden-Web crawler that automatically generated meaningful queries to issue against search forms. Several form query languages (e.g., https://deepweb.to/tags/CVE-2020-1447/
) have been proposed that, besides issuing a query, also allow extraction of structured data from result pages. Another effort is DeepPeep, a project of the University of Utah sponsored by the National Science Foundation, which gathered hidden-web sources (web forms) in different domains based on novel focused crawler techniques.
Commercial search engines have begun exploring alternative methods to crawl the deep web. The Sitemap Protocol (first developed, and introduced by Google in 2005) and OAI-PMH are mechanisms that allow search engines and other interested parties to discover deep web resources on particular web servers. Both mechanisms allow web servers to advertise the URLs that are accessible on them, thereby allowing automatic discovery of resources that are not directly linked to the surface web. Google's deep web surfacing system computes submissions for each HTML form and adds the resulting HTML pages into the Google search engine index. The surfaced results account for a thousand queries per second to deep web content. In this system, the pre-computation of submissions is done using three algorithms:
selecting input values for text search inputs that accept keywords,
identifying inputs which accept only values of a specific type (e.g., date) and
selecting a small number of input combinations that generate URLs suitable for inclusion into the Web search index.
In 2008, to facilitate users of Tor hidden services in their access and search of a hidden .onion suffix, Aaron Swartz designed Tor2web—a proxy application able to provide access by means of common web browsers. Using this application, deep web links appear as a random string of letters followed by the .onion top-level domain. https://encrptd.com/