I am aware that apex records (records at the zone "root") can have PTR records, but frequently I see that zone root records do not have a matching PTR, or they are pointing to a different place than the associated host record.
dig umich.edu @188.8.131.52; dig -x 184.108.40.206 (truncated for brevity) ;; ANSWER SECTION: umich.edu. 709 IN A 220.127.116.11 (truncated for brevity) ;; ANSWER SECTION: 251.243.211.141.in-addr.arpa. 1800 IN PTR www.umich.edu. (truncated for brevity)
Reasons not to have a PTR:
- A reason not to put in a PTR is if you are not authoritative for the address zones anyway. E.g. I am making an A record point to a cloud provider, and I cannot use a CNAME because it is a zone root.
- Another reason I could understand is if you are using anycast for your DNS services, and may not get the same answer (geographic) place-to-place. Google.com seems to do this.
But is there a reason one should "never" or "always" do this aside from an exception like the above? I would think there should always be a correct PTR by default.