Resolves IP addresses to hostnames through PTR records over DNS-over-HTTPS (Cloudflare, Google or Quad9): reverse names are built for IPv4 (in-addr.arpa) and IPv6 (ip6.arpa, with :: expansion), multiple PTR answers are listed, and forward-confirmed reverse DNS (FCrDNS) checks that each hostname resolves back to the address — the test mail servers and abuse desks apply. NXDOMAIN, empty answers and resolver errors are reported per address. Raw JSON is the secondary output. Addresses are sent to the chosen resolver; nothing is stored.