TermOctober 10, 2026
Forward-confirmed reverse DNS
- Category
- Web
- Also known as
- FCrDNS
- Double reverse DNS lookup
- Related terms
- User agent
- Crawler verification
01
One-line definition
Looking up an IP's hostname, then resolving that hostname back, to prove the IP really belongs to the domain it claims.
02
Plain-English explanation
Anyone can set a reverse DNS name on their own IP, but they can't make someone else's domain resolve back to it. So a request from Googlebot's IP should reverse-resolve to a googlebot.com or google.com hostname, and that hostname should resolve back to the same IP.
03
Real example from my work
Alerts on my site said Applebot had read /.env.example. The IPs reverse-resolved to rented Google Cloud machines. Real Applebot is on Apple's network, and every request had returned 404.
04
Why it matters
It's how you tell a real crawler from a scanner using its name.