What Is Reverse DNS? PTR Records Explained

Reverse DNS, often shortened to rDNS, looks up a hostname associated with an IP address. It uses a DNS record called a PTR record, short for pointer record. This reverses the direction of a familiar address lookup: instead of starting with a name and finding an address, you start with an address and look for a name.

Reverse DNS is useful for mail-server checks, readable logs, and network troubleshooting. It is a separate configuration from the DNS records that send visitors to a website.

Forward DNS vs reverse DNS

Suppose a mail server uses the hostname mail.example.net and the IPv4 address 192.0.2.10. Its intended records could look like this:

These are illustrative records, not live lookup results. The IPv4 address is from a range reserved for documentation by RFC 5737.

Creating the A record does not create the PTR record. Changing the PTR record does not redirect website visitors to a different server. Each record answers its own kind of question.

How does a PTR record work?

For IPv4, reverse DNS uses a special domain ending in in-addr.arpa. The address’s four numbers are placed in reverse order before that suffix.

For our example:

IP address:          192.0.2.10
Reverse lookup name: 10.2.0.192.in-addr.arpa
PTR answer:          mail.example.net

A DNS resolver looks up the reverse name and returns the PTR answer if one is available. The IPv4 reverse namespace and PTR record format are defined in RFC 1035.

In a provider’s control panel, you may only need to enter the IP address and desired hostname. The provider can handle the reverse-zone format for you.

Who can change reverse DNS?

Reverse DNS authority follows the address space. It is not automatically controlled by the company hosting your ordinary domain’s DNS.

For a provider-assigned server address, the hosting provider might offer a PTR setting or handle changes through support. For a block you control, the registry or upstream provider may delegate the reverse zone to nameservers operated by you or your DNS provider.

ARIN’s reverse DNS guide explains how resource holders manage reverse delegations and how some provider-assigned resources can have shared authority. The exact process depends on how the address space is registered and delegated.

This explains a common setup problem: adding a PTR record inside an ordinary example.net DNS zone does not make it the authoritative answer for the public address’s reverse lookup. The record needs to be served through the correct reverse zone.

Before planning a change, establish who controls that zone and whether your account has permission to edit it. If the address is leased, include reverse DNS responsibilities in the agreement.

Why does reverse DNS matter for email?

Receiving mail services can use reverse DNS as part of their checks on a connecting mail server. For example, Gmail’s sender guidelines require a sending server’s public IP to have a PTR hostname, with that hostname resolving back to the same IP through an A or AAAA record.

This relationship is often called forward-confirmed reverse DNS. In our example, the reverse lookup returns mail.example.net, and the forward lookup returns 192.0.2.10.

It is one part of a complete mail setup. A correct PTR does not replace SPF, DKIM, or DMARC, and does not guarantee inbox placement. Sending behavior and IP address reputation also matter.

How do you check reverse DNS?

Start with the address used by the connection you are investigating. For outbound mail, this means the public source address seen by the receiving server. A mail relay or NAT gateway can make that different from the address shown on the application server.

Then work through these checks:

1. Look up the PTR record. Use a DNS lookup tool that supports reverse lookups or PTR queries. Record both the answer and the IP you checked.
2. Look up the returned hostname. Check its A record for IPv4 or AAAA record for IPv6. For a forward-confirmed setup, it should resolve back to the relevant address.
3. Confirm who serves the reverse zone. If the answer is wrong, find the provider or delegated DNS operator able to change it.
4. Recheck after the change. An authoritative answer may be updated while a recursive resolver still holds an older cached response.

Record the result with a time and resolver name when troubleshooting. This helps distinguish different cached answers from an actual configuration difference.

Does reverse DNS work with IPv6?

Yes. IPv6 reverse lookups use ip6.arpa, with the fully expanded address’s hexadecimal digits reversed and separated by dots. Forward IPv6 lookups use AAAA records. These conventions are defined in RFC 3596.

The purpose is the same: find a hostname associated with an address. If a service sends over both IPv4 and IPv6, check the reverse DNS for each actual source address. A correct IPv4 PTR does not configure the IPv6 one.

What changes when you move a BYOIP block?

Keeping the same addresses can simplify a provider move, but it does not remove the need to review DNS responsibilities.

Determine whether the current reverse DNS service will continue, whether delegation needs changing, and who can update the records after the move. Also confirm that any hostname returned by a PTR still describes the intended service and has appropriate forward records.

Treat routing and reverse DNS as separate checklist items. BGP announcements affect reachability; PTR records affect reverse lookup answers. Our BYOIP guide explains how address continuity fits into a broader migration.

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