Are you experiencing problems with your e-mails? Perhaps they are going to the spam folder or missing. If this is your case, it’s possible that the origin of the problem is that you haven’t configured reverse DNS (rDNS).

What is reverse DNS (rDNS)?

The rDNS or reverse DNS is the DNS lookup of an IP address, with the goal to find the domain name behind it. Yes, it’s exactly the opposite of the regular DNS lookup, in which a domain name points to the IP address where it’s hosted.

The rDNS service usually is supplied by managed DNS providers. Basically, when you get it, you have the chance to create a reverse Master zone. By adding PTR records to this zone, you can verify the correct match between IP addresses and their domain name. PTR records become the evidence you need to prove the IP address really corresponds to the domain name and, therefore, avoid frauds.

The rDNS functions with IPv4 (A) and IPv6 (AAAA). 

What is a reverse Master zone?

A reverse Master zone is a Primary DNS zone on an authoritative DNS server you need for resolving IP addresses to domains. Its creation is a must because it’s the environment PTR records require for functioning correctly.

When a rDNS lookup is triggered, it will travel through the DNS hierarchy like a regular DNS lookup. The difference is rDNS lookups require a specific root domain: in-addr.arpa. Within this domain, subdomains are produced with the numbers of an IP address in reverse. There’s a reason for this: the IP address information is more specific from the left to the right direction.

What is a PTR record?

The PTR or pointer record contains an IP address linked to the domain name. It’s the antithesis of an A record. This last supplies the corresponding IP address of a domain name. And actually, being opposites, they are teammates. You need an A or AAAA record for each PTR you add.

Let’s explain it through an example. When an e-mail is sent, the recipient’s mail server will verify the PTR record to confirm that the IP address corresponds to the domain it says to come from. This is an rDNS lookup. To ensure this match and avoid risks, there will be a second check, but the attention will go to the A or AAAA record.

PTR records are helpful for servers to confirm senders’ identity, for the recipients to trust that the sender’s IP address is not a forged one.

Suppose you wonder what happens when an e-mail server doesn’t have rDNS enabled. Well, they still try to protect security in a basic way. They discard the message or mark it as dangerous. The choice depends on their configuration.

Why do we need rDNS?

To prevent spam. Unwanted messages can be simply rubbish or dangerous threats. They can be looking for advertising or spreading malware, phishing, etc. rDNS can really reduce spam by verifying the IP addresses. If they are legit, rDNS will find it out!

To get easily readable logs. Logging systems register just IP addresses, those long sequences of numbers you perfectly know. Well, with rDNS those numbers can become domain names easier to be read on the logs.

To fix e-mail delivery problems. If your e-mails are going to the spam folder or missing, you can stop these problems by enabling rDNS. 

Conclusion.

The rDNS is an efficient solution you should add to enhance your security. So check your configuration and set it up today!