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.

bash
# Subdomain discovery via Certificate Transparency logs
python3 SKILL_DIR/scripts/domain_intel.py subdomains example.com

# SSL certificate inspection (expiry, cipher, SANs, issuer)
python3 SKILL_DIR/scripts/domain_intel.py ssl example.com

# WHOIS lookup (registrar, dates, name servers — 100+ TLDs)
python3 SKILL_DIR/scripts/domain_intel.py whois example.com

# DNS records (A, AAAA, MX, NS, TXT, CNAME)
python3 SKILL_DIR/scripts/domain_intel.py dns example.com

# Domain availability check (passive: DNS + WHOIS + SSL signals)
python3 SKILL_DIR/scripts/domain_intel.py available coolstartup.io

# Bulk analysis — multiple domains, multiple checks in parallel
python3 SKILL_DIR/scripts/domain_intel.py bulk example.com github.com google.com
python3 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

Contributed by @FurkanL0