What is a PTR (Reverse DNS) records and why they are needed?

PTR records serve as as a reverse IP-address display in the host name.

Example: PTR record (or Reverse DNS) translates the domain name ns.cloudhost.lv in IP-address 91.220.43.5. And it also allows the address 91.220.43.5 return to ns.cloudhost.lv. Some hosters indicate PTR record for their pages for example:

$ nslookup 91.220.43.6
Server:		8.8.8.8
Address:	8.8.8.8#53

Non-authoritative answer:
6.43.220.91.in-addr.arpa	name = web.cloudhosting.lv.

$ nslookup 91.220.43.6 ns.cloudhost.lv
Server:		ns.cloudhost.lv
Address:	91.220.43.5#53

6.43.220.91.in-addr.arpa	name = web.cloudhosting.lv.

Basically these records is used as a spam filter. Spammers often forge domain names to send e-mail, which generally will not have a proper PTR record created from the DNS. The presence of reverse Reverse DNS - one of the basic requirements of many spam filters. His absence could lead to blocking of emails. Also, when a web analysis and the analysis of Internet traffic using a reverse search Reverse DNS, to find out which service providers in your visitors.

For PTR records do not have specific technical requirements. However, with an increase in unwanted electronic messages (spam), they are necessary for some security schemes.

Exactly therefore Most of Providers For PTR records applies the following rules:

For each IP-address, there is one PTR record.
For a server with multiple IP-addresses will be registered multiple PTR records, one for each IP-address.
For any mail server must be running MX-record and an A-record - this rule is one of the most important.
For each PTR record should be an A record, but that's not necessarily the contrary.

In other words, we must able to take IP-address, look it PTR-record, find an A record for that host and return the IP-address.

$ host -a cloudhosting.lv
Trying "cloudhosting.lv"
;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;cloudhosting.lv.		IN	ANY

;; ANSWER SECTION:
cloudhosting.lv.	14397	IN	A	91.220.43.6
cloudhosting.lv.	21597	IN	NS	ns2.cloudhost.lv.
cloudhosting.lv.	21597	IN	NS	ns.cloudhost.lv.
cloudhosting.lv.	14397	IN	MX	5 mailhost.energy.lv.
cloudhosting.lv.	14397	IN	TXT	"v=spf1 a mx ip4:91.220.43.4 ?all"

Received 221 bytes from 8.8.8.8#53 in 18 ms

$ nslookup -type=soa cloudhosting.lv
Server:		8.8.8.8
Address:	8.8.8.8#53

Non-authoritative answer:
cloudhosting.lv
	origin = ns.cloudhost.lv
	mail addr = dns.energy.lv.lv
	serial = 2013090901
	refresh = 86400
	retry = 7200
	expire = 604800
	minimum = 86400

$ nslookup -type=any cloudhosting.lv
Server:		8.8.8.8
Address:	8.8.8.8#53

Non-authoritative answer:
cloudhosting.lv
	origin = ns.cloudhost.lv
	mail addr = dns.energy.lv.lv
	serial = 2013090901
	refresh = 86400
	retry = 7200
	expire = 604800
	minimum = 86400
Name:	cloudhosting.lv
Address: 91.220.43.6
cloudhosting.lv	nameserver = ns2.cloudhost.lv.
cloudhosting.lv	nameserver = ns.cloudhost.lv.
cloudhosting.lv	mail exchanger = 5 mailhost.energy.lv.
cloudhosting.lv	text = "v=spf1 a mx ip4:91.220.43.4 ?all"