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Forward-confirmed reverse DNS

published · September 12, 2026

Forward-confirmed reverse DNS looks up an address's PTR name, then resolves that name back; a name that comes back to the same address proves who owns it.

Forward-confirmed reverse DNS is a two-step check on an IP address: look up the PTR record that names the address (crawl-66-249-66-1.googlebot.com, say), then resolve that name forward to its A or AAAA records and see whether the original address is among them. Anyone who controls a block can publish any PTR name they like, so the reverse lookup alone proves nothing; the forward confirmation is what turns a name into evidence, because only the owner of the name's domain can make it resolve back.

What Spaw answers

The check runs only when a lookup asks for it with hostname: true, because it is a live DNS query with a two-second budget. hostname is the PTR name, hostname_verified whether it forward-confirmed, and a verified name under an operator's documented domain — googlebot.com, google.com, search.msn.com, applebot.apple.com, yandex., crawl.baidu. — proves the crawler and sets is_crawler even where no published range does. hostname_kind reads the naming convention (dynamic for pool and DHCP names, static for fixed and hosting names) and hostname_domain is the registrable domain, so the operator behind a name is one field.

What it does not mean

An unverified name is just text the block's owner chose; it is never used to prove anything. And a verified name proves the operator, not the intent: a real Googlebot address is Google's, whatever it happens to be fetching.

Related

markdown version: /glossary/forward-confirmed-reverse-dns.md

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