Open Resolver Risk Calculator

Calculator Inputs

Example Data Table

Use the table as guidance. Real risk depends on controls and observed abuse.

Formula Used

This tool computes three subscores on a 0–10 scale: Exposure, Amplification, and Hygiene.

The final risk score is a normalized weighted sum:

• E increases with internet exposure, open port 53, recursion, and no ACL.
• A increases with amplification factor, larger UDP payload, and no RRL.
• H increases with stale/unknown patching, no monitoring, no DNSSEC.

Weights auto-normalize to sum to 1. This keeps scoring consistent.

How to Use This Calculator

1. Enter resolver posture details like recursion, ACL, and internet exposure.
2. Provide an estimated amplification factor and UDP payload size.
3. Adjust weights to match your threat model and environment.
4. Press Calculate Risk to see score, level, and guidance.
5. Download CSV or PDF for evidence, reviews, or remediation tracking.

FAQs

1) What is an open resolver?

A DNS resolver that answers recursive queries from any source. Attackers can abuse it for reflection, amplification, and traffic laundering.

2) Why is recursion risky on internet-facing resolvers?

Recursion allows arbitrary clients to trigger downstream lookups. This expands abuse options and can amplify DDoS impact, cache poisoning attempts, and resource exhaustion.

3) What is amplification factor in this calculator?

It is the ratio of response bytes to request bytes. Higher ratios increase reflection damage. The tool treats large factors as higher amplification subscore.

4) How do ACLs reduce risk?

ACLs restrict who can query or recurse. Limiting to internal networks and trusted forwarders prevents the resolver from being a public DDoS amplifier.

5) Does Response Rate Limiting always help?

It reduces abusive bursts, but needs tuning. Incorrect settings can block legitimate traffic. Combine RRL with filtering, segmentation, and response size controls.

6) Why include DNSSEC validation and monitoring?

DNSSEC validation improves integrity for signed zones. Monitoring catches spikes, unusual queries, and abuse early. Together they reduce silent compromise and operational surprises.

7) Is this score a compliance standard?

No. It is a practical risk estimate using weighted signals. Use it to prioritize remediation, then validate with scanning, traffic analysis, and incident response requirements.