Robots Exclusion Standard
The robots exclusion standard or robots.txt protocol is a convention to prevent cooperating web spiders and other web robots from accessing all or part of a website. The information specifying the parts that should not be accessed is specified in a file called robots.txt in the top-level directory of the website.
The robots.txt patterns are matched by simple substring comparisons, so care should be taken to make sure that patterns matching directories have the final '/' character appended: otherwise all files with names starting with that substring will match, rather than just those in the directory intended.
This example allows all robots to visit all files because the wildcard "*" specifies all robots.
User-agent: * Disallow:
This example keeps all robots out:
User-agent: * Disallow: /
The next is an example that tells all crawlers not to enter into four directories of a website:
User-agent: * Disallow: /cgi-bin/ Disallow: /images/ Disallow: /tmp/ Disallow: /private/
Example that tells a specific crawler not to enter one specific directory:
User-agent: BadBot Disallow: /private/
Example demonstrating how comments can be used:
# Comments appear after the "#" symbol at the start of a line, or after a directive User-agent: * # match all bots Disallow: / # keep them out
In order to prevent access to all pages by robots,
should be used.
robots.txt is older and more widely accepted, but there are other methods (which can be used together with robots.txt) that allow greater control, like disabling indexing of images only or disabling archiving of page contents.
<meta name="robots" content="noindex,nofollow" />
HEAD section of an HTML document tells search engines such as Google, Yahoo!, or MSN to exclude the page from its index and not to follow any links on this page for further possible indexing.
Directives within a page
The <NOINDEX> tag is a non-standard HTML tag whose intent is to indicate portions of a page that should not be indexed, such as common navigation or footer. Using it without a namespace will make XHTML pages invalid.
Google uses comments for the same purpose: <!--googleoff: index--> ... <!--googleon: index-->
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