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Discovering subdomains of a domain is an essential part of hacking reconnaissance, and thanks to following online tools, which make life easier.

Having an unsecured subdomain can lead to a serious risk to your business, and lately, there have been some security incidents where the hacker used subdomains tricks.

The most recent one was Vine, where the entire code was available to download from a vulnerably exposed subdomain.

If you are a website owner or security researcher, you can use the following tools to find the subdomains of any domain.

WhoisXML API Subdomain Lookup

WhoisXML API Subdomain Lookup tools allow users to discover a domain name’s subdomains easily. The subdomains product line is fueled by a comprehensive repository that includes 2.3+ billion subdomain records with 1+ million subdomains added daily. 

The tools allow researching any target domain name and reveal the list of all subdomains found for the domain with timestamps of the first time the record was seen and the last update for a specific record. 

The product line includes a/an:

  • API with output queries in XML and JSON formats for easy integration
  • Data feed with files available in unified and consistent CSV format, updated both on a daily and weekly basis. Download the CSV sample to test the data in your environment
  • GUI lookup tool that creates reports with shareable links

Check this product sheet to learn how WhoisXML API’s subdomain data can match specific data requirements. 

Criminal IP

Criminal IP is an up-and-coming security OSINT search engine with a revolutionary IP-based search system and tracking technology. Domain Search is a Criminal IP feature that scans target domains in real time and provides exhaustive information on that domain with a final 5-level risk score, detection for probability of phishing, mapped IP, real IP, tech stack, redirection, DNS record, certificates, etc. 

This service has the added benefit of the scan detecting previously unknown subdomains of a certain domain and listing it to users for easy viewing. Once a certain URL is put to scan, chrome is launched to conduct scanning and AI-driven analysis, diagnosing whether URLs are malicious and should be blocked accordingly. This service provides direct URL search, to-the-point keyword search, and easy-to-use filters to aid users in finding what they need.

DNS Dumpster

DNSDumpster is a domain research tool to find host-related information. It’s the project.

Not just subdomain, but it gives you information about DNS server, MX record, TXT record, and excellent mapping of your domain.


An online tool to find subdomains using Anubis, Amass, DNScan, Sublist3r, Lepus, Censys, etc.

I tried NMMAPPER for one of the domains, and the results were accurate. Go ahead and give it a try for your research works.


Sublist3r is a python tool to find subdomains using a search engine. Currently, it supports Google, Yahoo, Bing, Baidu, Ask, Netcraft, Virustotal, ThreatCrowd, DNSdumpster, and PassiveDNS.

Sublist3r is supported only on Python 2.7 version and has few dependencies in a library.

You can use this tool on Windows, CentOS, Rehat, Ubuntu, Debian, or any other UNIX-based OS. The following example is from CentOS/Linux.

  • Login to your Linux server
  • Download latest Sublist3r
wget .

Extract the downloaded file

  • It will create a new folder called “Sublist3r-master”

As I mentioned earlier, it has the following dependencies, and you can install it using a yum command.

yum install python-requests python-argparse

Now you are set to discover the subdomain by using the following command.

./ -d

As you can see, it did discover my subdomains.


Netcraft has a large number of domain databases, and you don’t want to miss this in finding public subdomain information.

The search result will contain all the domains and subdomains with first seen, netblock, and OS information.

If you need more information about the website, click on-site report, and you will be given tons of information about technologies, ranking, etc.


Detectify can scan subdomains against hundreds of pre-defined words, but you can’t do this to a domain you don’t own.

However, if you have authorized a user, you can enable subdomain discovery in the overview under settings.


SubBrute is one of the most popular and accurate subdomain enumeration tools. It’s a community-driven project, and it uses the open resolver as a proxy, so SubBrute doesn’t send traffic to the domain’s name servers.

It’s not an online tool, and you need to install this on your computer. You can use Windows or UNIX-based OS, and installation is very easy. The following demonstration is based on CentOS/Linux.

  • Log in to your CentOS/Linux
  • Download the latest SubBrute
wget .
  • Unzip the downloaded zip file

It will create a new folder called “subbrute-master”. Go inside the folder and execute the with the domain.


It will take a few seconds and result in you with any subdomain found.


Knock is another python-based subdomain discovery tool tested with Python 2.7.6 version. It finds the subdomain of a target domain using a wordlist.

  • You can download and install this on a Linux-based OS.
wget .
  • Extract the downloaded zip file with unzip command
  • it will extract and create a new folder, “knock-knock3.
  • Go inside this folder and install with the following command
python install

Once installed, you can scan for subdomains by following


DNSRecon on Kali Linux

Kali Linux is an excellent platform for a security researcher, and you can use DNSRecon on Kali without installing anything.

It checks all NS records for zone transfers, overall DNS records, wildcard resolution, PTR record, etc.

To use DNSRecon, execute the following, and you are all done.

dnsrecon –d


Pentest-tools search for subdomain using multiple methods like DNS zone transfer, DNS enumeration based on wordlist, and public search engine.

You can save the output in PDF format.


If you want to resolve domain names in bulk, MassDNS is the tool for you. This tool can resolve over 350,000 domain names per second! It uses publicly available resolvers, and it is suited for people who want to resolve millions or even billions of domain names.

One issue you may face while using this tool is that it may increase the load on public resolvers and lead to your IP address being flagged for abuse. Therefore, this tool must be used with caution.


Amass has been created to help information security professionals perform network mapping of attack surfaces and perform external asset discovery.

The tool is entirely free to use, and its clientele includes the leading IT company Accenture.


By using the above tools, I hope you should be able to discover subdomains of the target domain for your security research. You may also want to try an online port scanner.

If you are interested in learning ethical hacking, check out this course.

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  • Chandan Kumar
    Chandan Kumar is a seasoned technology enthusiast and entrepreneur passionate about empowering businesses and individuals globally. As the founder of Geekflare, a leading technology publication, Chandan has spearheaded the development…

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