About

A PTR record is a DNS record with the type PTR that:

• gives back a domain name from a ip address
• is the result of a reverse lookup

Usage: Authentication

A PTR record is important because it's a part of request authentication on the Internet.

ie when a server gets a request from an IP, it performs:

• first an reverse lookup to determine its host name
• then a forward lookup to validate the host name

If they don't match, the request is classified as bad.

For instance:

• an email server should have its it configured
• this is the way to validate google bot

Example of Gmail SMTP error message:

The IP address sending this message does 550-5.7.25 not have a PTR record setup, 
or the corresponding forward DNS entry 550-5.7.25 does not point to the sending IP. 
As a policy, Gmail does not accept 550-5.7.25 messages from IPs with missing PTR records. 
Please visit 550-5.7.25

Who can set the PTR record?

Only the owner of the IP can set a reverse record, not the owner of the domain. Therefore if you want to set a PTR record:

• for a server, you need to do it at your server provider
• for your internet connection, you need to ask it to your telecom provider

You Server provider should give you an interface to create it for a host

Example ovh on the cloud planel:

Zone

The reverse DNS database of the Internet is rooted (ie the PTR records are stored) in the .arpa top-level domain.