Passive DNS enrichment scanner

[danielmichaels]

Part of #61 · Tier 5 · scanner · Effort: L · lower priority — needs a vendor

Implement the wishlist PassiveDNSSourceScanner: enrich a domain with historical DNS data from a passive-DNS source.

Purpose

• Subdomain/host discovery from historical resolutions.
• Detect abandoned/dangling records that no longer resolve but were once published (takeover leads).
• Infrastructure-change signal (IP/provider churn over time).

⚠️ Cost / dependency flag

Requires a passive-DNS vendor (e.g. a commercial pDNS API) — API key + per-query cost. Defer until a source is chosen. Cache results; treat as enrichment, not a per-scan default.

Wiring

• choose source; add config (API key, enable flag, off by default)
• internal/scanner/scan_passivedns.go + ScanPassiveDNSArgs/Worker; register
• feed discovered hosts through processDiscoveredHost
• TDD tests with a fake pDNS client

Decisions for owner

• Which passive-DNS provider (drives the whole design).
• Cadence (almost certainly not every scan).