Reverse DNS is the process of resolving an IP address back to a hostname. It is performed using PTR (pointer) records stored in a special region of DNS called in-addr.arpa for IPv4 and ip6.arpa for IPv6. Unlike most DNS records, PTR records are controlled by whoever owns the IP address block — not the domain owner.
To look up the hostname for an IPv4 address, DNS reverses the octets and appends .in-addr.arpa.. For example, to resolve 192.0.2.1, a resolver queries for the PTR record at 1.2.0.192.in-addr.arpa.. The PTR record at that name points to a hostname.
1.2.0.192.in-addr.arpa. 3600 IN PTR mail.example.com.
IPv6 reverse DNS works the same way but uses the full expanded address with each nibble reversed, appended to .ip6.arpa.. Because IPv6 addresses are much longer, the resulting query name is correspondingly long.
This is the key distinction that surprises many engineers: PTR records for an IP address can only be set by whoever controls the IP address block — your hosting provider, cloud provider, or ISP. If you host a server at AWS or Google Cloud, you configure reverse DNS through that provider's console or API, not through your domain registrar's DNS panel.
The reverse DNS zone for a given address block is delegated from the regional internet registry (such as ARIN, RIPE, or APNIC) down to the entity that received the IP allocation. That entity may further delegate control of sub-ranges to their customers. When you rent IP addresses from a provider, they typically offer a way for you to set PTR records for those addresses, but the mechanism varies by provider.
Receiving mail servers routinely check reverse DNS on the IP address that connected to them. The check serves as a basic hygiene signal: a mail server with no PTR record, or one whose PTR does not resolve back to the connecting IP, is commonly associated with residential connections and compromised hosts used for spam.
Many receiving servers will reject or heavily penalize mail from IPs without a valid PTR record. The stricter check — called forward-confirmed reverse DNS (FCrDNS) — also requires that the hostname returned by the PTR record, when looked up via a forward A or AAAA query, resolves back to the original IP. This two-way match is harder to fake and is used by some providers as a reputation signal.
For outbound mail servers, setting a meaningful PTR record (such as mail.yourdomain.com) is considered standard practice and is required by some receiving domains and anti-spam services.
mail.example.com but an A lookup of mail.example.com returns a different IP, FCrDNS checks fail. Keep PTR and forward records synchronized.Run a reverse DNS lookup for any IP address
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