Knowledge Base

How to configure reverse DNS

This article describes what reverse DNS is, when it is used, and how to configure it for your account.

What is reverse DNS?

Reverse DNS lookups (also known as reverse DNS resolution or rDNS), resolve IP addresses to domain names. This is the opposite of regular DNS resolution (also known as forward DNS lookups), which takes a domain name as input and return the associated IP address. Forward DNS lookups use A records to determine a domain name's IP address, while reverse DNS lookups use PTR records to determine an IP address' associated domain name.

As an anti-spam measure, external e-mail servers frequently require sending domains to have PTR records. If a sending domain does not have a PTR record, the e-mail server rejects the message, usually with an error message in the SMTP headers indicating that reverse DNS is not configured.

Testing reverse DNS

To determine if there is a PTR record associated with a particular IP address, you can log in to your account using SSH and then type the following command. Replace xxx.xxx.xxx.xxx with the IP address that you want to test:

dig -x xxx.xxx.xxx.xxx

For example, if you type the following command:

dig -x 75.98.175.166

You receive the following output:

;; QUESTION SECTION:
;166.175.98.75.in-addr.arpa. IN PTR

;; ANSWER SECTION:
166.175.98.75.in-addr.arpa. 42989 IN PTR 75.98.175.166.static.a2webhosting.com.

This output shows that IP address 75.98.175.166 is associated with the domain name 75.98.175.166.static.a2webhosting.com.

Configuring reverse DNS

If you have a VPS or dedicated hosting account, we can set up reverse DNS for you. To do this, please open a support ticket on the Customer Portal at https://my.a2hosting.com. In the ticket, please specify the domain you want to configure for reverse DNS, as well as your account's IP address.

Shared and reseller servers already have reverse DNS configured. If you find that this is not the case for your account, please open a support ticket and let us know. Make sure you include your domain name and IP address in the ticket.

More Information

For more information about reverse DNS, please visit http://en.wikipedia.org/wiki/Reverse_DNS_lookup.