PTR Record Reference

This article serves as a reference for the formal structure, format, and key technical details of a PTR (Pointer) record.

PTR record format

A PTR record is a type of DNS record that facilitates a reverse DNS lookup. It has a type code of 12 and is defined in RFC 1035. Unlike most DNS records that live in a domain’s forward zone, PTR records are stored in special reverse lookup zones (in-addr.arpa for IPv4 and ip6.arpa for IPv6).

The name of a PTR record is not a domain name but a specially formatted IP address.

• IPv4: The address is reversed, and .in-addr.arpa is appended (e.g., 192.0.2.5 becomes 5.2.0.192.in-addr.arpa).
• IPv6: Each hexadecimal digit is reversed, separated by dots, and .ip6.arpa is appended.

The core data of a PTR record, its RDATA (Resource Data) section, contains a single value:

The canonical representation of a PTR record is:

<reversed_ip_address> IN PTR <domain-name>

PTR record behavior and examples

PTR records are critical for several internet operations, primarily for security, trust, and logging.

Email anti-spam verification: Many mail servers perform a reverse DNS lookup on an incoming email’s IP address. If the IP address does not resolve to a hostname via a PTR record, or if the resolved hostname does not match the forward DNS record, the email may be flagged as spam or rejected.

Forward-confirmed reverse DNS (FCrDNS): This is a security practice where a PTR record is configured to point to a domain name, and the A/AAAA record for that domain name points back to the original IP address. This two-way verification is often used to establish trust, especially for mail servers.

Logging and analytics: Server logs often record connecting IP addresses. A reverse DNS lookup allows administrators to display human-readable hostnames in these logs instead of just raw IP addresses, making analysis and troubleshooting easier.

Example in a zone file:
If a server at IP address 192.0.2.5 has the hostname mail.example.com, its PTR record might appear in a DNS zone file as:

5.2.0.192.in-addr.arpa. IN PTR mail.example.com.

For an IPv6 address like 2001:db8::1, its PTR record would resolve:

1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. IN PTR ipv6host.example.com.

PTR record management

You cannot create or manage PTR records for your domain’s IP address within a standard DNS hosting service like DNSimple. PTR records are managed by the owner of the IP address block, which is typically your:

• Internet Service Provider (ISP)
• Cloud provider (e.g., AWS, GCP, Azure)
• Hosting company

Have more questions?

If you have additional questions or need any assistance with your DNS records, just contact support, and we will be happy to help. You can also read more about DNS hosting at DNSimple.