Robots Exclusion Protocol for Yahoo!, Google and Microsoft
Common REP Directives
The following are all the major REP features currently implemented by Google, Microsoft and Yahoo!. Each of these directives can be specified to be applicable for all crawlers or for specific crawlers by targeting them to specific user-agents, which is how any crawler identifies itself. Each of us also supports Reverse DNS based authentication of our crawler, and you can use this validate the identity of any crawlers claiming a particular user-agent.
1. Robot.txt Directives
|Disallow||Tells a crawler not to crawl your site or parts of your site -- your site's robots.txt still needs to be crawled to find this directive, but the disallowed pages will not be crawled.||'No crawl' pages from a site. This directive in the default syntax prevents specific path(s) of a site from crawling.|
|Allow||Tells a crawler the specific pages on your site you want indexed so you can use this in combination with Disallow. If both Disallow and Allow clauses apply to a URL, the most specific rule - the longest rule - applies.||This is useful in particular in conjunction with Disallow clauses, where a large section of a site is disallowed, except a small section within it.|
|$ Wildcard Support||Tells a crawler to match everything from the end of a URL -- large number of directories without specifying specific pages.||'No Crawl' files with specific patterns, for eg., files with certain filetypes that always have a certain extension, say pdf; etc.|
|Sitemap Location||Tells a crawler where it can find your sitemaps.||Point to other locations where feeds exist to point the crawlers to the site's content.|
2. HTML META Directives
These directives can either be placed in the HTML of a page or in the HTTP header for non-HTML content like PDF, video, etc. using an X-Robots-Tag. The X-Robots-Tag mechanism allows these directives to be available for all types of documents -- HTML or otherwise. If both forms of the tag, HTML META and X-Robots-Tag in the header are present, the most restrictive one applies.
|NOINDEX META Tag||Tells a crawler not to index a given page.||Don't index the page. This allows pages that are crawled to be kept out of the index.|
|NOFOLLOW META Tag||Tells a crawler not to follow a link to other content on a given page.||Prevent publicly writeable areas from being abused by spammers looking for link credit. By NOFOLLOW, you let the robot know that you are discounting all outgoing links from this page.|
|NOSNIPPET META Tag||Tells a crawler not to display snippets in the search results for a given page.||Present no abstract for the page on search results.|
|NOARCHIVE META Tag||Tells a search engine not to show a "cached" link for a given page.||Do not make a copy of the page available to users from the search engine cache.|
|NOODP META Tag||Tells a crawler not to use a title and snippet from the Open Directory Project for a given page.||Do not use the ODP (Open Directory Project) title and abstract for this page in Search.|
source : ysearchblog