Telling webmasters to use DNS to verify on a case-by-case basis seems like the best way to go. I think the recommended technique would be to do a reverse DNS lookup, verify that the name is in the googlebot.com domain, and then do a corresponding forward DNS->IP lookup using that googlebot.com name; eg:> host 184.108.40.206.66.249.66.in-addr.arpa domain name pointer crawl-66-249-66-1.googlebot.com.> host crawl-66-249-66-1.googlebot.comcrawl-66-249-66-1.googlebot.com has address 220.127.116.11I don't think just doing a reverse DNS lookup is sufficient, because a spoofer could set up reverse DNS to point to crawl-a-b-c-d.googlebot.com.
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