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A comment on Ask HN: How did internet discover my subdomain?. Parent comment

Re:Ask HN: How did internet discover my subdomain? by okasaki on Friday March 07, 2025 @06:44AM (#43289396) 

    $ host 209.216.230.207
    207.230.216.209.in-addr.arpa domain name pointer news.ycombinator.com.
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Re:Ask HN: How did internet discover my subdomain? by dspillett on Friday March 07, 2025 @07:53AM (#43289769)
That is when there is an explicit PTR record, for instance one of my assigned addresses can be named that way due to:

    74.231.187.81.in-addr.arpa. 3600 IN PTR ns2.nogoodnamesareleft.com.
in the zone file for that IPv4, but unless they've explicitly configured, or are using a hosting service that does it without asking, this it won't be what is happening.

It isn't practical to do a reverse lookup from “normal” name-to-address records like

    ns2.nogoodnamesareleft.com. IN A 81.187.231.74
(it is possible to build a partial reverse mapping by collecting a huge number of DNS query results, but not really practical unless you are someone like Google or Cloudflare running a popular resolution service)
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Re:Ask HN: How did internet discover my subdomain? by mr_mitm on Friday March 07, 2025 @07:15AM (#43289544)
Not sure what you are trying to tell me. This isn't guaranteed to work. If you define a reverse lookup record for your domain, then that counts as published in my book.
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Re:Ask HN: How did internet discover my subdomain? by drpossum on Friday March 07, 2025 @09:10AM (#43290288)
This is correct.
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Re:Ask HN: How did internet discover my subdomain? by DonHopkins on Friday March 07, 2025 @10:18AM (#43290839)
I love how the ARPANET still lives on through reverse DNS PTRs.

https://www.youtube.com/watch?v=V78GUSOS-EM

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