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Understanding Reverse DNS and PTR Records

Understand the role of Reverse DNS and PTR records in networking.

Introduction

This article explains what reverse DNS and PTR records are, why they are important, and how you can set them up for your rented IP prefix(es). Proper configuration of these records is essential for improving email delivery and adding credibility to your outbound emails.

What is a Reverse DNS Lookup Zone?

A reverse DNS lookup zone is a special DNS zone that maps an IP address (IPv4 or IPv6) to a hostname using a PTR (Pointer) record. This process is essentially the opposite of a standard DNS lookup, which maps a hostname to an IP address.

  • For IPv4: The reverse DNS zone is represented in the format in-addr.arpa.
    For example, if your IP address is 80.240.113.62, the reverse DNS query will look like 62.113.240.80.in-addr.arpa.

  • For IPv6: The reverse DNS zone uses the ip6.arpa format. An IPv6 address such as 2a03:90c0:501:2801::62 would be represented as 2.1.0.8.2.1.0.5.0.c.0.9.0.3.0.a.2.ip6.arpa.


What are PTR Records?

PTR records, or Pointer records, are DNS records that map an IP address to a hostname. They serve as the reverse of an A record, which maps a hostname to an IP address.

Uses of PTR Records:

  1. Anti-Spam Measures in Email Delivery: PTR records are used by mail servers to verify that the IP address sending an email matches the expected hostname. This can help reduce spam and fraudulent emails.

  2. Convenience in Tracing: PTR records allow IP addresses to be mapped to human-readable domain names, making it easier to trace and manage dedicated subnets.

How to Set Up Reverse DNS and PTR Records

To set up a reverse zone, you need to create a reverse DNS zone and add PTR records for forward resolution.

Step 1 - Create a reverse DNS zone

  1. Within your account, click Add site.

  2. For your site name, use the reverse IP address:
    ​

    • For IPv4 /24 prefixes, the pattern is:

      • IP prefix: <octet_1>.<octet_2>.<octet_3>.0/24

      • Reverse zone address: <octet_3>.<octet_2>.<octet_1>.in-addr.arpa

    • For IPv4 /16 prefixes, the pattern is:

      • IP prefix: <octet_1>.<octet_2>.0.0/16

      • Reverse zone address: <octet_2>.<octet_1>.in-addr.arpa

    • For IPv6, consider the following examples:

      • IPv6 prefix: 2001:DB8::0/32

      • Reverse zone: 8.b.d.0.1.0.0.2.ip6.arpa

      • IPv6 prefix: 2001:DB8::0/48

      • Reverse zone: 0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa

  3. Skip the rest of the onboarding process.


Step 2 - Add PTR records

  1. Go to DNS > Records.

  2. For each IP within the prefix, add a PTR record using the least significant octet(s) as the subdomain.

  3. Add the two Cloudflare nameservers provided for the zone at your Regional Internet Registry (RIR).

After this process, your reverse zone will be activated and you can perform reverse DNS lookups.

Helpful tools

While setting up reverse zones, the following third-party tools may be useful:

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