TL;DR
DNS PTR records enable reverse lookups — translating IP addresses into domain names. Security professionals use them to validate email servers, investigate traffic origins, and verify communication partners. Storage format varies between IPv4 (using .in-addr.arpa) and IPv6 (using .ip6.arpa).
How reverse DNS lookups work
A pointer (PTR) record resolves an Internet Protocol (IP) address (made up of numbers and decimals) into a domain name (such as okta.com).
The act of translating an IP address into a domain name is known as a reverse lookup in the Domain Name System (DNS).
Let us explain how this process works.
What does a DNS PTR record do?
Every computer has a unique IP address, per the DNS that powers the internet. You may never know your IP address, but no other computer can reach you without it.
Connected devices use IP addresses exclusively. But if you know the IP address, you may want to know the formal domain name too.
Enter the DNS PTR tool.
Use the DNS PTR tool to perform a reverse lookup and translate the numbers into a web address you can both read and understand.
Why do you need PTR records?
As we mentioned, the average computer user has no need for a DNS PTR record. But there are many reasons for security experts to use the tool.
You might use PTR records for:
Validation. Some email servers won't accept notes from servers that don't have PTR records set up properly. Attending to this step is a little like proving that you have pure intentions.
Safety. Just as you need PTR records to validate your outgoing email, you should expect your communication partners to do the same.
Investigation. Some systems store only IP addresses, not domain names. You might need PTR records to understand where traffic originates.
| IP Version | PTR Record Storage Format |
|---|---|
| Internet Protocol version 4 (IPv4) | Stored under the IP address with '.in-addr.arpa' appended |
| Internet Protocol version 6 (IPv6) | Stored under the address converted into four-bit sections with '.ip6.arpa' appended |
What does a DNS PTR record look like?
An example could help you understand just how these systems work. Try a tool like this.
The DNS is critical to effective internet communication. But understanding the terminology takes time and a bit of practice. Check out our blog post for more DNS details.
Frequently asked questions
What is the difference between a DNS PTR record and a regular DNS record?
A standard DNS record translates a domain name into an IP address, while a PTR record does the reverse — it resolves a numeric IP address back into a human-readable domain name. This reverse process is called a reverse lookup.
Who actually needs to use DNS PTR records?
PTR records are primarily used by security professionals and system administrators, not everyday computer users. Common use cases include email server validation, traffic origin investigation, and verifying the identity of communication partners.
Why would an email server reject messages without a PTR record?
Some email servers require that sending servers have properly configured PTR records as a trust signal. Without one, the sending server cannot prove its legitimacy, and the email may be rejected or flagged as suspicious.
How are PTR records stored differently for IPv4 and IPv6?
For Internet Protocol version 4 (IPv4), PTR records are stored under the IP address with '.in-addr.arpa' appended. For Internet Protocol version 6 (IPv6), the address is first converted into four-bit sections and then '.ip6.arpa' is appended.
Can a PTR record help identify where internet traffic is coming from?
Yes. Because some systems log only IP addresses rather than domain names, a PTR record reverse lookup can help administrators identify the origin of network traffic by translating those IP addresses into recognizable domain names.
Where can I perform a DNS PTR reverse lookup?
A DNS PTR reverse lookup can be performed using free online tools such as DNS query utilities, where you enter an IP address to retrieve its associated domain name and see PTR records in action.
References
- What Is an IP Address and What Can It Reveal About You? (May 2015). Insider.
- How to Set Up Reverse DNS and PTR Records. (June 2013). Computerworld.
- Reverse DNS Lookup and PTR Query. DNS Insiders.
