Reverse DNS (rDNS)

Reverse DNS (rDNS) is a technique used to map an IP address to a domain name using a Pointer (PTR) record. It involves querying the DNS system in reverse, starting with the IP address, to retrieve the associated domain name(e.g. -> localhost). Reverse DNS is commonly used for verifying server identities, preventing email spam, and troubleshooting network issues.

What is a reverse DNS record used for?

A reverse DNS record, also known as a PTR (Pointer) record, is used for reverse DNS lookups. It associates an IP address with a domain name, enabling identification of the domain name associated with a given IP address. Reverse DNS records are primarily used for verification of server identities, preventing email spam, troubleshooting network issues, and establishing trust in various online services.

Why should an email server have a valid Reverse DNS record?

An email server should have a valid Reverse DNS (rDNS) record for several important reasons:

In summary, having a valid Reverse DNS record for an email server is essential for maintaining email deliverability, preventing spam, establishing server credibility, and aiding in sender identification. It helps ensure that legitimate emails reach their intended recipients and builds trust in the email communication ecosystem.

Where can I set a reverse DNS record for an IP address?

Setting a reverse DNS (rDNS) record for an IP address typically involves working with the entity that manages the IP address block or the DNS configuration for that IP address. The specific process and options for setting a rDNS record may vary depending on your situation. Here are some common scenarios:

It’s important to note that the exact steps and procedures may vary depending on your specific situation and the organization responsible for managing the DNS configuration of your IP address.

How can I check a reverse DNS record?

You can check a reverse DNS entry by querying an IP address with an according parameter of your favorite DNS tool.

To have a valid setup let’s take a look at a typical email provider like GMX and query their mail out DNS record for an IP address:

$ dig a

;			IN	A

;; ANSWER SECTION:		78401	IN	A		78401	IN	A		78401	IN	A		78401	IN	A		78401	IN	A		78401	IN	A

As the next step use one of the IP addresses to make a reverse DNS lookup:

$ dig -x



As most IP addresses have a valid reverse DNS record, you can try one by your own to get a better understanding.