The Domain Name System, or DNS, correlates domain names with IP addresses. A DNS pointer record (PTR) provides the domain name associated with an IP address. A DNS PTR record is exactly the opposite of the ‘A’ record, which provides the IP address associated with a domain name.

DNS PTR records are used in reverse DNS lookups. When a user attempts to reach a domain name in their browser, a DNS lookup occurs, matching the domain name to the IP address. A reverse DNS lookup is the opposite of this process: it is a query that starts with the IP address and looks up the domain name.

How are DNS PTR records stored?

In IPv4:

While DNS A records are stored under the given domain name, DNS PTR records are stored under the IP address reversed, and with “.in-addr.arpa” added. For example, the PTR record for the IP address 192.0.2.255 would be stored under “255.2.0.192.in-addr.arpa”.

“in-addr.arpa” has to be added because PTR records are stored within the .arpa top-level domain in the DNS. .arpa is a domain used mostly for managing network infrastructure, and it was the first top-level domain name defined for the Internet.

(The name “ARPA” dates back to the earliest days of the Internet: it takes its name from the Advanced Research Projects Agency (ARPA), which created ARPANET, an important precursor to the Internet.) in-addr.arpa is the namespace within .arpa for reverse DNS lookups in IPv4.

In IPv6:

IPv6 addresses are constructed differently from IPv4 addresses, and IPv6 PTR records exist in a different namespace within .arpa. IPv6 PTR records are stored under the IPv6 address, reversed and converted into four-bit sections (as opposed to 8-bit sections, as in IPv4), plus “.ip6.arpa”.

What are some of the main uses of PTR records?

PTR records are used in reverse DNS lookups; common uses for reverse DNS include:

Anti-spam: Some email anti-spam filters use reverse DNS to check the domain names of email addresses and see if the associated IP addresses are likely to be used by legitimate email servers.

Troubleshooting email delivery issues: Because anti-spam filters perform these checks, email delivery problems can result from a misconfigured or missing PTR record. If a domain has no PTR record, or if the PTR record contains the wrong domain, email services may block all emails from that domain.

Logging: System logs typically record only IP addresses; a reverse DNS lookup can convert these into domain names for logs that are more human-readable.