Reverse WHOIS

Can reverse WHOIS API help my company?

I am not sure if many companies have asked themselves if reverse WHOIS can help them. But I do know this: oftentimes business owners would ask me for a certain piece of information. More often than not, my response would be: “You need reverse WHOIS” for that! Who is the largest web-hosting provider in Australia? Who is the biggest domain registrar? How many registered .club domain names are there? How many domain names does Joe own?

What is a reverse WHOIS API?

Everybody knows about Live WHOIS API, but what is a reverse WHOIS API? Is it like WHOIS, only backward? The term “reverse” doesn’t begin to explain the benefits of this great tool. Which is a shame because it can be a very, very powerful tool in uncovering crucial data. So let’s dig in.

A WHOIS database gives you all sorts of data behind a domain name: date of registration; date of expiry, owner name, owner email, and other owner details like address, town, phone number, and some technical details like Nameserver. When you make a request to a WHOIS API, you need to put a domain name in that request (such as The response then consists of all the WHOIS data previously mentioned.

With a reverse WHOIS, you can potentially enter any information from a data perspective:

  • email,
  • phone number,
  • address,
  • city,
  • nameserver
  • or any piece of text in a WHOIS record

and get a list of domain names associated with that data.
So for example, if is listed as the owner of the domain name, in a reverse WHOIS API request, you could get a list of all the domain names that are also associated with that particular email address.

Reverse WHOIS API use cases

Here are a few use cases that will hopefully help you relate to this amazing service.

  1. Let’s say you are a sales executive for a large web hosting company or a website builder, and you want to build a marketing campaign around website owners that host their websites with a particular provider (any provider). You could put that provider’s nameservers in the request, and get a list of domain names that are using that particular nameserver. Keep in mind that as of November 2017, there are over 300 million registered domain names. So you could end up with a very long list of domain names.
  2. On the other hand, if you are a trademark lawyer, copyright attorney or brand protection agency, and you found a website that’s squatting on your name or using some of your branding, you will use reverse WHOIS in a different way. Once you find the infringing website, you might want to check its email (if it’s not a private registration), or you might want to check technical information like nameservers or IP address. You could also cross-reference that information with any other domain names, and check for same keywords in the domain name.
  3. You could also use reverse WHOIS API for research purposes. For example, you might be researching startups or companies or websites, and you would like to know how many of them have their domain names registered in the city of San Francisco. You could request San Francisco as the city, and then cross-reference that list of domain names with your list of domain names.
Reverse WHOIS

Reverse WHOIS

Reverse WHOIS or reverse WHOIS API?

Obviously, there’s a difference between a reverse WHOIS service and a reverse WHOIS API. Hopefully I don’t have to explain here what an API or a SAAS is, so we will sum it up by saying that in order to get reverse WHOIS data within an API you have to be a developer (know how to code), compared to with a software in the cloud, you don’t have to be one. So in a SAAS environment, you can just copy-paste any data (email, city, nameserver) and you can get the data back. The advantage is clear: no coding is necessary! You just paste or upload your list, and you get the data back. However, if you are a developer, and you can use an API, then you are going to lack some freedom and opportunities for building a tool or a system yourself.

Does WhoAPI have a reverse WHOIS API?

Unfortunately, at the moment WhoAPI does not have its own reverse WHOIS API or a reverse WHOIS service. However, we are evaluating the possibility of building one. So, if you are interested, please reach out to us, and you will have the opportunity of becoming our very first beta-tester with one month of free usage.

Potential issues with reverse WHOIS

In order for a reverse WHOIS lookup service to function in the first place, there has to be an entire database. Over 300 million domain names is a very large database. Updating this database on a daily basis comes with its own challenges. That’s why some companies cache the data. Caching the data could mean that you get obsolete information when you are making your request.

There you have it. I hope that it’s a bit clearer now what reverse WHOIS can do for your company. And that next time you have some domain data requirements, reverse WHOIS will be there to help you.


Goran Duskic has been the Founder and CEO of WhoAPI Inc. since 2011, a company that specializes in developing APIs, including the well-known Whois API. He started his career in internet entrepreneurship in 2006 and has co-founded several online businesses, including a web hosting company that he later sold. Goran's work primarily involves creating practical API solutions to meet technological needs.

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