Reverse DNS Lookup (rDNS)
A Domain Name System (DNS) query that resolves an IP address back to its associated hostname, used as a foundational step in crawler authenticity verification.
AI Summary: Reverse DNS (rDNS) is a DNS query that resolves an IP address to a domain hostname. Combined with a forward DNS check (FCrDNS), it provides cryptographic certainty when verifying whether a crawler claiming to be Googlebot or OAI-SearchBot is genuine or spoofed.
Technical Definition
Reverse DNS (rDNS) is the process of mapping an IP address back to a fully qualified domain name (FQDN) using specialized PTR (Pointer) records housed under the .in-addr.arpa (IPv4) or .ip6.arpa (IPv6) zones.
In web security, Forward-Confirmed Reverse DNS (FCrDNS) is the industry gold standard for verifying whether an incoming HTTP request actually originated from the vendor declared in the User-Agent header.
Two-Step Verification Protocol
# Step 1: Perform reverse lookup on incoming client IP (e.g., 66.249.66.1)
dig -x 66.249.66.1 +short
# Output: crawl-66-249-66-1.googlebot.com.
# Step 2: Perform forward lookup on returned hostname
dig crawl-66-249-66-1.googlebot.com +short
# Output: 66.249.66.1
If the forward lookup matches the original connecting IP, the bot identity is cryptographically validated. If it fails, the request is an imposter and should be rejected.
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