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Reverse DNS Lookup

Given an IP address, return the PTR record. Useful when confirming an outbound mail server's rDNS matches its EHLO banner (missing or mismatched PTR is a classic spam-filter trip), tracing who owns a suspicious IP, or validating an auto-assigned PaaS hostname.

Accepts IPv4 and IPv6. Private (RFC 1918), loopback, and link-local ranges are rejected. They only resolve on the local network.

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