You can customize entire IP addresses or ranges to block particular bots through server-side restrictions on IPs. Most of the major engines crawl from a limited number of IP ranges, making it possible to identify them and restrict access. This technique is, ironically, popular with webmasters who mistakenly assume that search engine spiders are spammers attempting to steal their content, and thus block the IP ranges to restrict access and save bandwidth. Use caution when blocking bots, and make sure you’re not restricting access to a spider that could bring benefits, either from search traffic or from link attribution.
Blocking and cloaking by user agent:
At the server level, it is possible to detect user agents and restrict their access to pages or websites based on their declaration of identity. As an example, if a website detected a rogue bot, you might double-check its identity before allowing access. The search engines all use a similar protocol to verify their user agents via the Web: a reverse DNS lookup followed by a corresponding forward DNS IP lookup. An example for Google would look like this:
host 22.214.171.124 126.96.36.199.in-addr.arpa domain name pointer crawl-66-249-66-1.googlebot.com.
> host crawl-66-249-66-1.googlebot.com crawl-66-249-66-1.googlebot.com has address 188.8.131.52
A reverse DNS lookup by itself may be insufficient, because a spoofer could set up reverse DNS to point to xyz.googlebot.com or any other address.
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