What is Reverse DNS and why is it important
Written by Eric Benson
Updated at November 2nd, 2022
What is reverse DNS and why is it important?
If you’ve been involved in email marketing or website management in the past, you’ve probably heard of DNS and DNS records. Reverse DNS lookups are a related process that can help with email deliverability, email delivery troubleshooting, and error logging. It’s a form of email authentication that helps mail servers confirm that your sending domain (the @yourdomain.com part of your email address) matches the IP address associated with your domain.
What is reverse DNS?
A reverse DNS lookup, also known as reverse DNS or rDNS, is a way for a server to determine the domain name associated with a particular IP address. This is essentially the opposite of a normal DNS lookup, in which an IP address is identified from a domain name.
Reverse DNS uses pointer records (also known as PTR records) to identify a domain name from an associated IP address.
When you send an email the receiving mail server will check your server's IP address, and will then “ask” your DNS server what the associated domain name is by checking the PTR record. Without a correct PTR record to allow reverse DNS lookups, your emails may not reach your recipients.
How does reverse DNS work?
Reverse DNS lookups are primarily used by email services and for troubleshooting email deliverability, and work by double checking a sending domain name with the associated IP address.
Here’s a brief overview of the basics and why it’s important:
First, an email will be sent, and a mail server will identify the associated domain name for both the sender and receiver of the email.
The mail server will use a reverse DNS lookup to ensure the IP address of the sender matches the domain name’s IP address. This is a way for servers to ensure that the sending domain isn’t being used by spammers.
A PTR record contains the data verifying the IP address matches the domain name, and tells the mail server that the email is legitimate.
The email is delivered.
This process allows mail servers to identify spammers and prevent their emails from being delivered. It can also cause issues for legitimate senders if their server does not have a PTR record set up. Without a PTR record, a reverse DNS lookup cannot be resolved. Many mail servers will reject messages sent from a server that doesn’t support reverse lookups.
Why is reverse DNS important?
Reverse DNS is important for a few reasons. First, it allows mail servers to identify spammers who are using hijacked machines or spoofing domain names and prevent their emails from being delivered. This significantly cuts down on the number of spam emails that get through.
Some error-logging software also uses reverse DNS lookups to add domain names to logs instead of raw IP addresses, making the logs easier to read and identify issues in. This can also be used by troubleshooting tools.
Legitimate senders should also be aware of reverse DNS because of the spam restrictions some mail servers place on them. If your server doesn’t support reverse DNS, or you haven’t set it up for your server, this can cause email delivery issues. Be sure to set up PTR records, and ensure you’re using the correct domain inside of them.
Without a correct PTR record to allow reverse DNS lookups, your emails may not reach your recipients.