Subfinder: Fast, Passive Subdomain Enumeration for Bug Bounty and Pentesting

Discover Subdomains the Smart Way with Subfinder

Whether you’re into bug bounty hunting, penetration testing, or just love exploring internet surface area, Subfinder by ProjectDiscovery is a must-have tool in your cybersecurity toolkit. This open-source tool specializes in passive subdomain enumeration, making it ideal for stealthy and efficient reconnaissance.

Purpose and Use Cases

Subfinder is designed to find valid subdomains of target domains using passive online sources. This means it doesn’t send direct queries to the target infrastructure, making it stealthy and low-risk for detection. It’s perfect for:

• Bug bounty hunters identifying attack surfaces
• Penetration testers performing reconnaissance
• Security analysts mapping domain assets
• Red teamers staying under the radar

Installation and Setup

Installing Subfinder is straightforward. Make sure you have Go 1.21 or later installed, then run:

go install -v github.com/projectdiscovery/subfinder/v2/cmd/subfinder@latest

After installation, you can run Subfinder directly. However, to maximize its power, some passive data sources require API keys. Learn more about setting up provider configurations here: Post-Install Configuration.

Core Features

• Blazing fast performance with optimized modules
• Curated passive sources like crt.sh and GitHub for rich subdomain data
• Multiple output formats: JSON, text files, standard output
• Wildcard and DNS resolution support for filtering noise
• STDIN/STDOUT compatibility for smooth automation and scripting
• Recursive subdomain support for deeper discovery

Example Commands

Run Subfinder on a single domain:

subfinder -d example.com

Scan a list of domains:

subfinder -dL domains.txt

Use all sources (slow but comprehensive):

subfinder -d example.com -all

Exclude noisy or unreliable sources:

subfinder -d example.com -es alienvault,zoomeyeapi

Output results to a file:

subfinder -d example.com -o results.txt

Security Considerations

Since Subfinder performs only passive reconnaissance, it’s inherently safe and doesn’t alert targets. However, be cautious when integrating it with active tools or APIs that may log access or trigger alerts.

Technical Terms Explained

• Passive Enumeration: Gathering data from third-party sources without direct interaction with the target system.
• Wildcard Domains: DNS records that match multiple subdomains; filtering these reduces false positives.
• Resolvers: DNS servers used to resolve domain names into IP addresses, used in validation steps.
• STDIN/STDOUT: Standard input/output – useful for chaining Subfinder with other tools in shell pipelines.

Library Use for Developers

Subfinder can also be integrated into Go applications as a library. Minimal examples of SDK usage are available in the Subfinder GitHub examples directory.

Join the Community

Connect with like-minded hackers and researchers on the ProjectDiscovery Discord to share tips, get help, and stay updated.

Conclusion

Subfinder is a lightweight, high-speed subdomain enumerator that fits seamlessly into any recon workflow. Built for passive recon, it respects API limits, stays stealthy, and delivers results that matter. If you’re serious about asset discovery and mapping attack surfaces, Subfinder should be one of your go-to tools.

Learn more and download it here: Subfinder on GitHub