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 206.51.235.200
and the result would be:
; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.10.rc1.el6_3.6 <<>> -x 206.51.235.200
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 17735
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0
;; QUESTION SECTION:
;200.235.51.206.in-addr.arpa. IN PTR
;; ANSWER SECTION:
200.235.51.206.in-addr.arpa. 0 IN PTR enterprisevpssolutions.com.
;; AUTHORITY SECTION:
235.51.206.in-addr.arpa. 68000 IN NS rdns2.hivelocity.net.
235.51.206.in-addr.arpa. 68000 IN NS rdns1.hivelocity.net.
;; Query time: 1 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Jan 15 18:51:42 2013
;; MSG SIZE rcvd: 139
or
host 206.51.235.200
and the result would be:
200.235.51.206.in-addr.arpa domain name pointer enterprisevpssolutions.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..
Detailed explanation of the reverse DNS lookup can be found in the following article:
http://en.wikipedia.org/wiki/Reverse_DNS_lookup
Currently we provide reverse DNS setup using PTR records on our Virtual Dedicated servers, where all the server system resources are solely reserved for your account(s).
