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What reverse DNS is and what it is for

Updated · 3 min read

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The DNS everyone knows turns names into numbers: you type example.com and the computer finds the IP. Reverse DNS goes the other way: it starts from an IP and finds the name associated with it. It sounds like a technical detail, but for anyone sending email it can be the difference between the inbox and a rejection.

How it works

The name associated with an IP is stored in a record called PTR (pointer). It does not live in your domain's DNS zone, but in a special zone controlled by whoever owns the address block. For the IP 200.160.2.3, the lookup is made on the name 3.2.160.200.in-addr.arpa, with the numbers reversed.

In practice, this means that the PTR is set by the IP provider: the VPS or dedicated server company, or your internet service provider. You cannot create a PTR just by editing your domain's DNS panel.

Forward confirmation (FCrDNS)

A PTR alone does not prove much, because any IP owner can write whatever name they want. That is why mail servers run a second check, called FCrDNS (forward-confirmed reverse DNS):

  1. They look up the IP's PTR and get a name, for example mail.example.com.
  2. They look up that name's A record.
  3. They check that it points back to the same IP.

If the loop closes, the server is "confirmed". If it does not, the message picks up spam points or is rejected outright.

What it is for

  • Email delivery: this is the most important use. Gmail, Outlook and most servers check the sender's reverse DNS. Without a PTR, or with a generic residential PTR, the message is likely to be rejected.
  • Logs and troubleshooting: names are easier to recognize than numbers in access logs and in a traceroute. Many ISP routers have names that indicate the city, such as mia for Miami or gru and sp for São Paulo.
  • Identification: it helps you tell at a glance who owns an IP that shows up on your server. Combine it with the IP lookup to see the provider and ASN.

How to set up your server's PTR

  1. Pick the name, usually the mail server's, such as mail.yourdomain.com.
  2. In your domain's DNS, create an A record for that name pointing to the server's IP.
  3. In your VPS or dedicated server provider's panel, look for Reverse DNS, rDNS or PTR and enter the same name. If there is no such option, open a support ticket.
  4. Wait a few minutes and check it with TestePing's reverse DNS tool. The result should show the name and "reverse DNS confirmed".

Use a single name per IP and keep the same name in your mail server's greeting (HELO/EHLO). That consistency helps your reputation.

What if I don't send email?

For a server that only hosts websites, the PTR is optional. It does not affect how the site works or its SEO. For residential connections, the ISP already sets a generic name, and there is nothing to configure.