Web Spiders, often called Robots, are WWW search engines that "crawl" across the Internet and index pages on Web servers. A Web Spider will then catalog that information and make it available to the Internet for searching. This makes it easier for users to find specific information on the Internet by allowing "one-stop shopping" through the Spider's WWW site. Most Robots also prioritize documents that are on the Internet, allowing search results to be "scored" or arranged in order of most likely matches on a search.

A Robots.txt file is a special text file that is always located in your Web server's root directory. This file contains restrictions for Web Spiders, telling them where they have permission to search. It should be noted that Web Robots are not required to respect Robots.txt files, but most well-written Web Spiders follow the rules you define.

A Robot identifies itself when it browses your site, which is known as the "User-agent" and appears in the logs for IIS. Generally, the flow of events when a Web Spider crawls your site is similar to the following:

1.	The Robot asks for your /robots.txt file and looks for a "User- agent:" line that refers to it specifically. • If it finds an entry for itself, such as "User-agent: WebRobot," then it follows the rules that pertain to it. • If is does not find an entry for itself, it looks for a global set of rules, such as "User-agent: *," and obeys those rules. • If the Robot has an entry for itself and a global set of rules is also present, the Robot's personal rules will supersede the global rules.
2.	Rules for a user-agent are set up as "Disallow:" statements that tell a robot where it cannot search. A disallow statement is applied to any address that may have been requested by the Robot. For example: • "Disallow: /test" causes a Web Spider to ignore /test/index.htm, and so on. • "Disallow: /" causes a Web Spider to ignore the whole site; sometimes this is desirable. • "Disallow: " allows a Web Spider to crawl the whole site.
3.	Lines that begin with the pound symbol (#) denote comments, which can be useful when creating long sets of rules.Examples • This example disallows all Web Spiders for the entire site: # Make changes for all web spiders User-agent: * Disallow: / • The following example disallows a Robot named "WebSpider" from the virtual paths "/marketing" and "/sales": # Tell "WebSpider" where it can't go User-agent: WebSpider Disallow: /marketing Disallow: /sales # Allow all other robots to browse everywhere User-agent: * Disallow: • This example allows only a Web Spider named "SpiderOne" into a site, while denying all other Spiders: # Allow "SpiderOne" in the site User-agent: SpiderOne Disallow: # Deny all other spiders User-agent: * Disallow: / • This last example disallows FrontPage-related paths in the root of your Web site: # Ignore FrontPage files User-agent: * Disallow: /_borders Disallow: /_derived Disallow: /_fpclass Disallow: /_overlay Disallow: /_private Disallow: /_themes Disallow: /_vti_bin Disallow: /_vti_cnf Disallow: /_vti_log Disallow: /_vti_map Disallow: /_vti_pvt Disallow: /_vti_txt

For more information on writing Robots.txt files, see the following Web sites: