You can think of the PTR record as an opposite of the A record. While the A record points a domain name to an IP address, the PTR record resolves the IP address to a domain/hostname.
PTR records are used for the reverse DNS (Domain Name System) lookup. Using the IP address you can get the associated domain/hostname. An A record should exist for every PTR record.
The usage of a reverse DNS setup for a mail server is a good solution. Some external mail exchange servers make reverse DNS lookups before accepting messages originating from your mail server.
You can check whether there is a PTR record set for a defined IP address. The syntax of the commands on a Linux OS are:
dig -x IP
or
host IP
For example, you can run the following commands in your Linux terminal:
dig -x 209.85.102.36
and the result would be:
; <<>> DiG 9.5.1-P2.1 <<>> -x 209.85.102.36
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 41537
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2
;; QUESTION SECTION:
;36.102.85.209.in-addr.arpa. IN PTR
;; ANSWER SECTION:
36.102.85.209.in-addr.arpa. 7200 IN PTR serv01.siteground.com.
;; AUTHORITY SECTION:
102.85.209.in-addr.arpa. 7200 IN NS ns1.ev1servers.net.
102.85.209.in-addr.arpa. 7200 IN NS ns2.ev1servers.net.
or
host 209.85.102.36
and the result would be:
36.102.85.209.in-addr.arpa domain name pointer serv01.siteground.com.
The corresponding command under the Windows OS command prompt is:
nslookup IP
The reverse DNS lookup for IP version 4 uses a special domain - in-addr.arpa., while the lookup towards IPs version 6 uses ip6.arpa..
Currently we provide reverse DNS setup using PTR records on our Dedicated servers, where all the server system resources are solely reserved for your account(s).
The rDNS service is provided for free and it can be requested by posting a ticket in the Domain/DNS issues category. There can be only one PTR record per IP address. Multiple PTR records can not be set for one IP.