Ask HN: Where to buy Reverse DNS lookup database from?

You can look it up yourself, but watch out for CDN's and other third-party services and be aware that not every IP address has a reverse DNS entry. On MacOS:

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Note that the IP address bytes are reversed in the following command to lookup 18.9.25.15

host -a 15.25.9.18.in-addr.arpa

;; ANSWER SECTION:

15.25.9.18.in-addr.arpa. 1800 IN PTR dmz-mailsec-scanner-4.mit.edu.

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host -a dmz-mailsec-scanner-4.mit.edu

;; ANSWER SECTION:

dmz-mailsec-scanner-4.mit.edu. 1800 IN A 18.9.25.15

dmz-mailsec-scanner-4.mit.edu. 1800 IN HINFO "VMWARE/VM" "LINUX"