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’ll be happy to help.