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PTR Records (Reverse DNS Lookups)


PTR records, also commonly known as reverse DNS lookup or reverse DNS resolution, maps an IP address to a host/domain name. This is the opposite of resolving a domain name to an IP address which is commonly known as the A record in IPv4 and the AAAA record in IPv6.

Published Standard: RFC 2317

PTR records, why are they important?


This record is used by spam filters. Spammers often use fake domain names to send out email and generally won't have a correct PTR record set up in DNS. This is one of the main requirements of many spam filters and if you don't have a PTR Record set up many of your emails will be blocked and never reach the intended recipient.

What is a Generic PTR record?


Generic PTR are records that look like they have a repeating, psuedo random string, or a alpha-numeric sequence. Something similar to 123-123-123-123.your.isp.com. Your PTR record should be unique and usually take on the form of "mail.domain.com". Many spam filters run your PTR record through a series of regex's to determine if it matches a generic string. You can also read some of the different Postmaster Guideines. You can test to see if you have a Generic PTR record, by sending an email to mailtest@unlocktheinbox.com, which will auto-respond with the results. Keep in mind "Mailtest" will perform the test on all your MX records and IP's in your email header, but only your out-going mail servers or LSIP (Last sending IP address) needs to have a rDNS PTR record. But it's good pratice to set one up for all of them.
 
Here's a very small snippet of code from SpamCannibal.org Generic PTR Record Test, so show how matching is done. The real test is a few thousands lines of regex's
  # test for these regular expression (case insensitive)
  # d.d.d.d
         '\d{1,3}[-.]\d{1,3}[-.]\d{1,3}[-.]\d{1,3}',
	 '\d+[a-z_\-\.]\d+[a-z_\-\.]\d+[a-z_\-\.]\d+|\d{12}',
  # 180.Red-80-34-112.staticIP.rima-tde.net
         '\d+\.red-\d+-\d+-\d+\.staticip\.rima-tde\.net',
  # 216.subnet125-161-2.speedy.telkom.net.id
         '\d+\.subnet\d+-\d+-\d+\.speedy\.telkom\.net\.id',
  # 122.sub-75-199-30.myvzw.com
         '\d+\.sub-\d+-\d+-\d+\.myvzw\.com',
  # ip-90.net-89-3-110.rev.numericable.fr
         'ip-\d+\.net-\d+-\d+-\d+\.rev\.numericable\.fr'

PTR record, How do I set it up?


If you need a PTR Record set up for your mail server you'll need to contact your ISP (Internet Service Provider) as this is something they'll set up for you. If your mail server's domain name were mail.yourdomain.com, then you would ask your ISP to set up a reverse PTR Record for your mail server and give them the IP address of the server. You can check to see if your mail server has a PTR Record set up by using our lookup tool: PTR Record Lookup Tool

What's all this talk about LSIP?


LSIP stands for "Last Sending IP Address", it's the last IP used to send the email to its final destination. It's very important that this IP has a rDNS record and this IP is also the IP used in SPF and Sender ID checking.

An Example of Reverse DNS in Action


Reverse DNS Chart


This company receives an average of 1348 messages per hour. On average 33 of those messages per hour are rejected because there isn't a correct PTR Record set up. The rejection line is in blue across the bottom.

Source: Unlock The Inbox

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