Add DNS dangling scanner with examples, requirements, and readme

[Pnkcaht]

What I Did

• Added a proactive DNS dangling scanner for Netlify under sig-security-tooling/scan-dangling-dns/.
• Implemented dns_scan.py with:
  • Safe HTTP requests with retries and timeouts
  • Signature-based detection of non-existent Netlify sites
  • JSON structured output for CI/Prow integration
  • Detailed logging and fail-safe design
• Added example DNS records in examples/dns_records.yaml.
• Added requirements.txt for dependency management (PyYAML and requests).
• Added a fully descriptive README with Table of Contents, usage, YAML format, and contribution notes.
• Verified scanner detects dangling Netlify records from the example YAML.

Related Issue

• Fixes: Scanning and private triage of dangling DNS records #172

Test: DNS Dangling Scanner

Objective:
Verify that dns_scan.py correctly detects dangling Netlify DNS records from a sample YAML file.

Command executed:

python3 sig-security-tooling/scan-dangling-dns/dns_scan.py sig-security-tooling/scan-dangling-dns/examples/dns_records.yaml

Explanation:

• The scanner reads examples/dns_records.yaml and iterates over all DNS records.

• For each record pointing to a Netlify target, it performs a passive HTTP GET to check if the site exists.

• Sites that respond with known Netlify “not found” signatures are flagged as dangling.

• The scanner outputs INFO logs for each check and ERROR logs for detected dangling records.

• A JSON structured list of dangling records is printed for CI/Prow integration.

• Exit codes:

  • 0 → No dangling records

  • 2 → Dangling records detected

Result:

2026-02-06 19:54:49,231 - INFO - Checking Netlify target: docs.k8s.io -> kubernetes-docs.netlify.app
2026-02-06 19:54:52,486 - ERROR - Dangling Netlify DNS detected: docs.k8s.io -> kubernetes-docs.netlify.app
2026-02-06 19:54:52,486 - INFO - Checking Netlify target: dangling.k8s.io -> definitely-not-existing-k8s.netlify.app
2026-02-06 19:54:52,669 - ERROR - Dangling Netlify DNS detected: dangling.k8s.io -> definitely-not-existing-k8s.netlify.app
2026-02-06 19:54:52,670 - INFO - Checking Netlify target: old-preview.k8s.io -> old-k8s-preview-123.netlify.app
2026-02-06 19:54:53,070 - ERROR - Dangling Netlify DNS detected: old-preview.k8s.io -> old-k8s-preview-123.netlify.app
2026-02-06 19:54:53,070 - INFO - Checking Netlify target: schema-test.k8s.io -> missing-schema-test.netlify.app
2026-02-06 19:54:53,258 - ERROR - Dangling Netlify DNS detected: schema-test.k8s.io -> https://missing-schema-test.netlify.app
2026-02-06 19:54:53,258 - INFO - Checking Netlify target: cname-style.k8s.io -> alias-to-something.netlify.app
2026-02-06 19:54:53,441 - ERROR - Dangling Netlify DNS detected: cname-style.k8s.io -> alias-to-something.netlify.app
2026-02-06 19:54:53,441 - ERROR - Dangling DNS records detected:
[
  {
    "hostname": "docs.k8s.io",
    "target": "kubernetes-docs.netlify.app",
    "provider": "netlify"
  },
  {
    "hostname": "dangling.k8s.io",
    "target": "definitely-not-existing-k8s.netlify.app",
    "provider": "netlify"
  },
  {
    "hostname": "old-preview.k8s.io",
    "target": "old-k8s-preview-123.netlify.app",
    "provider": "netlify"
  },
  {
    "hostname": "schema-test.k8s.io",
    "target": "https://missing-schema-test.netlify.app",
    "provider": "netlify"
  },
  {
    "hostname": "cname-style.k8s.io",
    "target": "alias-to-something.netlify.app",
    "provider": "netlify"
  }
]

Conclusion:
The scanner successfully detected all dangling Netlify DNS records in the example YAML.
Logs are detailed, and JSON output is ready for CI/Prow automation.

Diagram

flowchart TD
    A[Load DNS records from YAML] --> B{For each record}
    B --> C{Is target a Netlify site?}
    C -- No --> B
    C -- Yes --> D[Normalize hostname]
    D --> E[Perform HTTP GET with retries]
    E --> F{Check for dangling signatures}
    F -- Found --> G[Log ERROR & add to dangling list]
    F -- Not found --> H[Log INFO - site exists]
    G --> B
    H --> B
    B --> I[Print JSON of dangling records]
    I --> J[Exit with code 0 or 2]

Loading

Remaining future step:

Investigate flagged records and remove or reassign DNS entries to prevent subdomain takeover

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Pnkcaht
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[k8s-ci-robot]

Hi @Pnkcaht. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[PushkarJ]

/ok-to-test
/cc tabbysable