About PTRarchive.com

This site is designed to archive DNS PTR records -- otherwise known as reverse DNS entries.

Every IP used on the Internet is expected to have a PTR record that points to a hostname identifying it. For servers, this could be something like 'www.example.com'. For clients, it could be something like 'client04-88b.example.net'.

Reverse DNS entries are used for many purposes. One of the most common is to help identify visitors to websites. They can also be used to help block spam (legitimate E-mail from your bank isn't going to come from an IP address with a reverse DNS entry that declares it to be a client). They can also be helpful in tracking down bad guys on the Internet.

Starting in 2008, several attempts were made at collecting all reverse DNS entries, and in 2013 Project Sonar started regularly collecting reverse DNS data. Our goal is to preserve this data.

About the Data

The reverse DNS data comes from a number of sources. The bulk of the data comes from Project Sonar, which collects data weekly (it was twice a month). Other data comes from the Internet Census 2012 and private sources.

The quality of the data varies. The first issue is how complete the data is -- were all possible valid IPv4 addresses checked? How were errors and timeouts handled (e.g. were multiple authoritative DNS servers checked if needed)? Some data sets include multiple PTR records, other do not. Some preserve case ("PTRarchive.com" versus "ptrarchive.com"), others do not. Some record who the reverse DNS is delegated to if no PTR record exists, others do not.

As of this writing, we do not return multiple reverse DNS entries for an IP address, but expect this to change.

Our data is stored in a unique format that was designed with the right balance of high compression while offering quick access to the data.