Domain Intelligence — Passive OSINT

Passive domain reconnaissance using only Python stdlib. Zero dependencies. Zero API keys. Works on Linux, macOS, and Windows.

Helper script

This skill includes scripts/domain_intel.py — a complete CLI tool for all domain intelligence operations.

# Subdomain discovery via Certificate Transparency logs
python SKILL_DIR/scripts/domain_intel.py subdomains example.com

# SSL certificate inspection (expiry, cipher, SANs, issuer)
python SKILL_DIR/scripts/domain_intel.py ssl example.com

# WHOIS lookup (registrar, dates, name servers — 100+ TLDs)
python SKILL_DIR/scripts/domain_intel.py whois example.com

# DNS records (A, AAAA, MX, NS, TXT, CNAME)
python SKILL_DIR/scripts/domain_intel.py dns example.com

# Domain availability check (passive: DNS + WHOIS + SSL signals)
python SKILL_DIR/scripts/domain_intel.py available coolstartup.io

# Bulk analysis — multiple domains, multiple checks in parallel
python SKILL_DIR/scripts/domain_intel.py bulk example.com github.com google.com
python SKILL_DIR/scripts/domain_intel.py bulk example.com github.com --checks ssl,dns

SKILL_DIR is the directory containing this SKILL.md file. All output is structured JSON.

Available commands

Command	What it does	Data source
subdomains	Find subdomains from certificate logs	crt.sh (HTTPS)
ssl	Inspect TLS certificate details	Direct TCP:443 to target
whois	Registration info, registrar, dates	WHOIS servers (TCP:43)
dns	A, AAAA, MX, NS, TXT, CNAME records	System DNS + Google DoH
available	Check if domain is registered	DNS + WHOIS + SSL signals
bulk	Run multiple checks on multiple domains	All of the above

When to use this vs built-in tools

• Use this skill for infrastructure questions: subdomains, SSL certs, WHOIS, DNS records, availability
• Use web_search for general research about what a domain/company does
• Use web_extract to get the actual content of a webpage
• Use terminal with curl -I for a simple "is this URL reachable" check

Task	Better tool	Why
"What does example.com do?"	web_extract	Gets page content, not DNS/WHOIS data
"Find info about a company"	web_search	General research, not domain-specific
"Is this website safe?"	web_search	Reputation checks need web context
"Check if a URL is reachable"	terminal with curl -I	Simple HTTP check
"Find subdomains of X"	This skill	Only passive source for this
"When does the SSL cert expire?"	This skill	Built-in tools can't inspect TLS
"Who registered this domain?"	This skill	WHOIS data not in web search
"Is coolstartup.io available?"	This skill	Passive availability via DNS+WHOIS+SSL

Platform compatibility

Pure Python stdlib (socket, ssl, urllib, json, concurrent.futures). Works identically on Linux, macOS, and Windows with no dependencies.

• crt.sh queries use HTTPS (port 443) — works behind most firewalls
• WHOIS queries use TCP port 43 — may be blocked on restrictive networks
• DNS queries use Google DoH (HTTPS) for MX/NS/TXT — firewall-friendly
• SSL checks connect to the target on port 443 — the only "active" operation

Data sources

All queries are passive — no port scanning, no vulnerability testing:

• crt.sh — Certificate Transparency logs (subdomain discovery, HTTPS only)
• WHOIS servers — Direct TCP to 100+ authoritative TLD registrars
• Google DNS-over-HTTPS — MX, NS, TXT, CNAME resolution (firewall-friendly)
• System DNS — A/AAAA record resolution
• SSL check is the only "active" operation (TCP connection to target:443)

Notes

• WHOIS queries use TCP port 43 — may be blocked on restrictive networks
• Some WHOIS servers redact registrant info (GDPR) — mention this to the user
• crt.sh can be slow for very popular domains (thousands of certs) — set reasonable expectations
• The availability check is heuristic-based (3 passive signals) — not authoritative like a registrar API

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Contributed by @FurkanL0