Why Reverse DNS Lookups?

Reverse DNS records are an essential setting for those running a mail server since many recipient servers reject, or mark as spam, all email that originates from an “unauthenticated” server.

This basically means that after the sending IP address is checked, if the Reverse DNS does not match the sending domain, then it is classed as “unauthenticated”.

We put ”unauthenticated” in quotes because having a Reverse DNS record attached to your domain does not automatically guarantee acceptance of email originating from your domain by the recipient's email server. It's just that non-matching or generic reverse DNS lookup (RDNS) settings are often rejected out of hand.  Having a Reverse DNS record for your domain will prevent email originating from your domain from getting immediately rejected.

RDNS can also be very useful  when tracking down network issues and was the original driving force of RDNS. When pinging a website or IP address, one part of the output is the server's RDNS record.

How Does it Work?

When you enter a domain name into your browser, the DNS system will find the IP address of the server the domain is associated with.

A reverse DNS lookup does the opposite. It establishes what domain is associated with the IP address. This is a useful setting to configure for anyone but essential for customers running a mail server on their Cloud Server.

You can easily set up reverse DNS  through the control panel. Just perform these steps:

1. Log in to your Rackspace account.
2. Click the Hosting menu item on the left.
3. After that drops down, click Cloud Servers.
4. This presents you with an overview page of your Cloud Servers. Click the link for your Cloud Server.
5. Click the DNS tab at the top. This is where you  modify your DNS records.

6.  Scroll down to the section  called Reverse DNS Management. You will see your IP address for that server listed. Click the hostname under the DNS Record column.

7.  A window appears and asks you for the hostname that you would like to set it to. After you enter your hostname, press Update. This change is immediate.

8.  Simply entering the hostname of your server will not be enough.  Your hostname needs to be a Fully Qualified Host Name (FQHN) that is set for a domain that you control.  You will see the red square next to the DNS Record field change to green when an FQHN has been entered.