The naive approach of reversing the address, converting to nibbles
and appending a suffix won't scale.
For IPv6 if you did the reverse of /48, /52, /56, /60 and /64
prefixes, which matches delegation patterns along with NXDOMAIN
synthesis, you would still be fine. You stop the search on NXDOMAIN
or data with perhaps a new value which says to continue searching
for white listed records. One could even start with /32 if one is
worried about spammers pretending to be ISPs.
I don't necessarily disagree, but now you've just upgraded the DNS
(NXDOMAIN synthesis is far from universal) and layered a probing protocol
on top of it. We can have a theological argument about whether that
counts as "using the DNS".
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