Simple definition
PTR links an IP address to a host name in the DNS Inverse.
Technical definition
Recordings PTR are published in the in-addr.arpa areas for IPv4 and ip6.arpa for IPv6.
What is it used for?
identify the name associated with an IP and improve the reputation of some email servers.
Practical example
a server SMTP public often has a PTR consistent with his name HELO.
Common issues
- PTR absent
- PTR inconsistent with HELO
- reverse zone managed by the operator
Key takeaway: Always adapt the diagnosis to the product version and the architecture actually deployed.