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Subfinder

Tool manual · Kali & common usage

OSINT · Beginner · OSINT lane

BeginnerRecon
Authorized usage only. Capture and inspect traffic only on systems and networks you own or have written permission to analyze.

What is Subfinder?

Subfinder passively discovers subdomains from public sources (APIs/certificates/search).

Fast subdomain inventory before deeper probing — authorized domains only.

Official website: Subfinder · Documentation

When do you use it?

  • You need Passive subdomain discovery from public sources.
  • You are following a CYNARKS lab or authorized engagement that calls for Subfinder.
  • You already tried simpler checks and need this tool’s deeper capability.

Who uses it?

  • OSINT analysts
  • Investigators (lawful use)
  • Red teamers (authorized recon)

How do you use it? (typical flow)

  1. Install Subfinder using the steps below (prefer Kali package or official installer).
  2. Confirm you have authorization for the target (lab VM, own system, or written scope).
  3. Run --help / man (or the Help section) to see current flags for your version.
  4. Start with a safe example from Basic commands against localhost or an authorized lab target.
  5. Record what you ran, what you observed, and any follow-up (filter, report, remediation).

Help & manual (like man / --help)

Every serious tool has a built-in way to look up flags. Learn these first so you are not stuck memorizing every option.

  1. 1. Help

    subfinder -h

    Providers and output options.

Kali Linux

Often installed via go install or apt; popular companion to amass/httpx.

1. Installation

OSInstallation
Kali LinuxOften installed via go install or apt; popular companion to amass/httpx.
Linuxgo install github.com/projectdiscovery/subfinder/v2/cmd/subfinder@latest

2. Basic commands

  1. 1. Passive subdomain discovery

    subfinder -d example.com

    Passive subdomain discovery

  2. 2. Show help

    subfinder --help

    List flags for your installed version (same habit as man/--help).

  3. 3. Short help

    subfinder -h

    Alternate help flag when supported.

  4. 4. Show all options and usage

    --help

    Show all options and usage

Options / flags

OptionPurpose
--helpShow all options and usage

5. Useful features

  • Provider API keys
  • Output formats

6. Practical examples

  1. Example 1: Passive subdomain discovery

    subfinder -d example.com

    Passive subdomain discovery

  2. Example 2: Show help

    subfinder --help

    List flags for your installed version (same habit as man/--help).

  3. Example 3: Short help

    subfinder -h

    Alternate help flag when supported.

  4. Example 4: Show all options and usage

    --help

    Show all options and usage

7. Important notes

  • Subfinder is used legitimately by security teams for authorized testing, defense validation and education.
  • Authorized targets only — labs, systems you own, or written permission.
  • Prefer reading --help / man / official docs for your installed version; flags change over time.

Related course: ethical hacking penetration testing · soc analyst blue team · Commands: /osint