, . Google, Yahoo, MSN/Live/Bing/whatever-it-is-today, Ask .., DNS .
1) ( , googlebot)
Mozilla/5.0 (; Googlebot/2.1; + http://www.google.com/bot.html
2) DNS IP- , .
rdns 66.249.71.202 - crawl-66-249-71-202.googlebot.com( , googlebot.com)
3) On this, you can fake your own step 2, so now check the dns of the A record for the result returned in step 2, and make sure that you have the original requesting IP.
For example, the dns for the above is crawl-66-249-71-202.googlebot.com. A 66.249.71.202
66.249.71.202 was the requesting IP address, so this is a valid googlebot.
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