What Is A Reverse DNS Lookup?

Date First Published: 24th April 2022

Topic: Computer Networking

Subtopic: Internet Protocols

Article Type: Computer Terms & Definitions

Difficulty Level: 9/10

Learn more about what a reverse DNS lookup is in this article.

As suggested in the name, a reverse DNS lookup, also known as an rDNS, is the opposite of a forwards lookup. It works by first looking up the IP address and then returning the domain name mapped to that IP address. A forwards DNS lookup works in the opposite way. In order for a reverse DNS lookup to properly work, it is necessary for a PTR record to be configured. These records store IP addresses with their segments reversed and ‘.in-addr.arpa’ is added to that. For example, a domain name with the IP address ‘173.245.58.63’ would have its information stored as ‘173.245.58.63. in-addr.arpa’.

Note:

PTR records are stored with ‘.ip6.arpa’ in IPv6, the more recent and advanced version of an IP address.

Purpose Of Reverse DNS Lookups

Reverse DNS lookups are not crucial to the normal functioning of the internet and are not used universally. In accordance with the standards of the IETF, it is suggested that every domain name should be capable of performing a reverse DNS lookup.

Reverse DNS lookups are mostly used by email servers for the purposes of spam filtering and validating incoming messages. This is performed by checking the IP address of the sender’s mail server in order to find out whether the mail server is legitimate. If the reverse DNS lookup resolver matches the IP address to the domain name of the email address, the mail server is legitimate. Due to this, it is very important that PTR records are configured on mail servers.

How To Perform A Reverse DNS Lookup?

1. Open the Start Menu. It is the Windows button in the bottom-left corner of the screen. The start menu can additionally be displayed by a simple keyboard shortcut. The keyboard shortcut is the button on the keyboard that has the Windows logo on it.
2. Find and open the folder 'Windows System' and right-click on 'Command Prompt' and select 'Run as administrator'.
3. In Command Prompt, type 'nslookup' with the IP address that you want to look up. For example, if you wanted to look up the IP address '123.123.123.123', you would type 'nslookup 123.123.123.123' without the quotes.

4. Tap 'Enter' and the reverse DNS lookup should be performed. Some reverse DNS lookups will timeout, as shown below.