Here's a concrete example of how using PTR records gets things mixed up.
keyserver.stack.nl offers keyserver service on port 443.
It has an A record at 131.155.141.70.
But the ptr is to mud.stack.nl:
70.141.155.131.in-addr.arpa. 69674 IN PTR mud.stack.nl.
and the https SNI and HTTP Host: directives provide an entirely different
website depending on whether you access it with:
https://mud.stack.nl/
or
https://keyserver.stack.nl/
If you access it as https://hkps.pool.sks-keyservers.net/, you get the
"keyserver" view. But if you access it by the name in the PTR record
("mud.stack.nl") then you get the mud view (and a 404 on any /pks URLs)
Even more troubling is that dirmngr successfully connects to mud.stack.nl and
does the query, even though it is configured to only talk to
hkps.pool.sks-keyservers.net
This suggests that anyone able to spoof a PTR record to me can get my dirmngr to
send my potentially-sensitive keyserver queries to an entirely different webserver.