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rDNS explained in detail

Have you ever heard of rDNS? It may not be as well-known as its opposite, forward DNS, but in today’s digital world, rDNS plays a critical role in preventing spam and network troubleshooting. In this article, we’ll explain rDNS in more detail and discuss its importance and various use cases. Let’s get started!

What is rDNS?

You probably already know what a forward DNS is. It links the associated hostname/domain to the IP address. Now think from the opposite direction. rDNS, also known as Reverse DNS is doing the mirror action, using the IP address to find the hostname/domain name. You might be surprised that this is actually needed, but it has significant application. It is very useful for e-mail verifications B2B and troubleshooting.

rDNS

Imagine this situation, you don’t have a reverse DNS set up, but you are sending a very important e-mail to another company. They have a mail server with Anti-Spam protection. It will check if you have rDNS and if you are missing it, your message will go directly to the spam bin. This protection reduces the phishing significantly by putting straight into the spam, all e-mails that have IPs that don’t correspond to IPs of the domains they say they come from.

All devices, connected to the internet have their IP addresses. This makes it easy to do a reverse DNS lookup and see who is it on the other side. For this purpose, you will need a PTR record.

Check out Reverse DNS service by ClouDNS!

Why is rDNS important?

There are several reasons why rDNS is crucial. Some of them are the following: 

  • Email Authentication: rDNS helps authenticate email servers and prevent spam. A large number of email servers will only accept incoming emails from hosts with a valid PTR record. That way, they avoid spam and phishing emails from spoofed IP addresses.
  • Network Troubleshooting: rDNS can be used to help troubleshoot network issues. If a network administrator is trying to diagnose a problem with a specific IP address, they can use rDNS to determine which hostname is associated with it. This can help them identify the device or service causing the problem.
  • Protection: Some security systems use rDNS to identify and block malicious traffic. For example, some intrusion detection and prevention systems will use rDNS to recognize and stop traffic coming from known malicious IP addresses.
  • Access Control: In some cases, rDNS can be used to control access to a network or specific services. For example, some firewalls and VPNs use rDNS to allow or deny access to particular hosts or services based on their hostname.

How does rDNS work?

The main goal of rDNS is to map an IP address to a hostname. In order to achieve that, it works by performing a reverse lookup of an IP address to find the associated domain name.

The process of rDNS starts with a query to a DNS server, asking for the hostname associated with a specific IP address. The DNS server then looks up the available IP address in its records, and if it finds a match, it returns the associated hostname.

Reversing the forward DNS process and resolving an IP address to a domain name requires a Reverse DNS zone and a PTR record. So, let’s explain a little bit more about them!

PTR record

The PTR record is also called a Pointer record, and its primary purpose is to link the IP address to its corresponding hostname/domain name. It is important to note that you should have an A or AAAA record for each PTR record you create. The explanation behind this practice is pretty simple. An A record or AAAA record maps a hostname/domain name to its IP address, and PTR goes exactly in the opposite direction. That is why it is essential to ensure you configure your DNS records accurately without any mistakes. Otherwise, there is a possibility for your emails to land directly into the spam folder of your recipients.

Reverse DNS zone

Using rDNS requires creating a special type of DNS zone called a Reverse DNS zone. That is the only place where PTR records are able to exist and function correctly. Reverse DNS can work both with IPv4 addresses and IPv6 addresses, yet they should be written in reverse. That is because there is a specific root domain in-addr.arpa that uses the IP addresses in reverse order. So, for example, the IP address 111.123.101.1 becomes 1.101.123.111.in-addr.arpa.

How to start using rDNS?

The first step is to create a Reverse DNS zone; you can do that from the Control Panel, add a new zone and click on the Master Reverse DNS zone.

Watch the video to find out how to add Master Reverse DNS zone

This zone is directly related to the size of your IP network. For example in IPv4/24 network, you will have 255 IP addresses. In the network 192.168.1.0/24, all the available IPs will be from 192.168.1.1 to 192.168.1.255, and the Reverse DNS zone will have this format 1.168.192.in-addr.arpa

Now in this Reverse zone, we can add PTR records that match each IP from the network. You just add a new record; this will be for the first IP 192.168.1.1

Type: PTR

Host: 1

Points to: hostname1.example.com

The PTR records will look like this in the Control Panel:

Host:Type:Points to:TTL
1.1.168.192.in-addr.arpaPTRhostname1.example.com1 Hour

Then, make sure that there is a matching A record. Each PTR must have one.

Find more information about PTR records and how to create them!

The last step is to change the name servers of your Reverse zone at your IP provider or ask them to do it. A configured NS records at the IP provider must point to the name servers listed in your Control Panel.

rDNS lookup – How to do it?

Now that we have explained the basics of rDNS, it’s time to show you how to perform an rDNS lookup. Besides, the process is simple, and you don’t need to have a background in IT to complete it. 

  • Windows

If you are a Windows user, you can perform rDNS lookup with the NSlookup command. First, open the Command Prompt by pressing the Windows key + R, then typing “cmd” and pressing Enter. Type the following command:

$ nslookup 1.2.3.4

  • macOS and Linux

If you are a macOS or Linux user, you can complete the rDNS lookup with the Dig command or the Host command. Open the Terminal application and write the following:

$ dig -x 1.2.3.4

or 

$ host -t PTR 1.2.3.4

*Please, make sure to replace “1.2.3.4” with the IP address (IPv4 or IPv6) you wish to check.

It’s important to note that in some cases, the rDNS lookup may not return any results if the IP address does not have a PTR record associated with it. Besides, in other cases, the response may show “NXDOMAIN” or “NXRRset”. Also, some firewalls or security systems may block reverse DNS queries.

Conclusion

rDNS is essential for email authentication, network security, and troubleshooting network issues. It performs a reverse lookup of an IP address to find the associated hostname. To achieve that, creating a Reverse DNS zone and adding PTR records is required. Thanks to this amazing technology, the Internet is a more secure place by identifying the authenticity of the source of the email, and it is beneficial for network troubleshooting.

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Tags: , , , , , , Last modified: January 26, 2023
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