Question
Where do I update rDNS for my IP?
Answer
rDNS will need to be updated at the authoritative nameservers for the IP range in questions. Typically this will be the hosting provider or internet service provider. Using the dig command is one way to find out where the rDNS for your IP is being served from. Below is an example command to check this. From this output, we can see that the rDNS for the 216.58.193.0/24 IP range is hosted at the Google nameservers, specifically at IPs 216.239.38.10, 216.239.34.10, 216.239.32.10, and 216.239.36.10.
root@server [~] # dig +nssearch 193.58.216.in-addr.arpa
SOA ns1.google.com. dns-admin.google.com. 327928838 900 900 1800 60 from server 216.239.38.10 in 27 ms.
SOA ns1.google.com. dns-admin.google.com. 327978237 900 900 1800 60 from server 216.239.34.10 in 28 ms.
SOA ns1.google.com. dns-admin.google.com. 327978237 900 900 1800 60 from server 216.239.32.10 in 29 ms.
SOA ns1.google.com. dns-admin.google.com. 327978237 900 900 1800 60 from server 216.239.36.10 in 29 ms.
To look up the authoritative source of rDNS for IP 1.2.3.4, you would use the following command. Note that the octets of the IP are reversed, and the 4th octet is not used.
dig +nssearch 3.2.1.in-addr.arpa
If you own your own block of IPs and want to use your WHM/cPanel server as the authoritative source of rDNS for your IPs, we have the following guide to get you started.
How to Configure Reverse DNS in WHM (cPanel Docs)
Resources
Please see "How to setup PTR (rDNS) records on your cPanel & WHM server" for instructions for specific providers.