Firewall Rule Complexity Estimator

ANA›Life Services Authority›National Calculator Authority›Firewall Rule Complexity Estimator

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Firewall Rule Complexity Estimator

Estimates the overall complexity score of a firewall policy based on rule count, object diversity, rule types, and advanced features like negation and logging. Higher scores indicate harder-to-maintain, audit, and troubleshoot rulesets.

Total Number of Rules

Unique Source Objects (IPs, subnets, groups)

Unique Destination Objects (IPs, subnets, groups)

Unique Service / Port Objects

Number of Deny / Drop Rules

Number of NAT Rules

Number of Rules Using Negation (NOT source/destination)

Number of Rules with Logging Enabled

Number of Any-Any or Overly Permissive Rules

Number of Security Zones / Interfaces

Calculate Complexity Score

Fill in the fields above and click Calculate.

function firCalc() { var resultDiv = document.getElementById("fir_result");

// --- Parse inputs --- var totalRules = parseFloat(document.getElementById("fir_total_rules").value); var uniqueSrc = parseFloat(document.getElementById("fir_unique_src_objects").value); var uniqueDst = parseFloat(document.getElementById("fir_unique_dst_objects").value); var uniqueSvc = parseFloat(document.getElementById("fir_unique_services").value); var denyRules = parseFloat(document.getElementById("fir_deny_rules").value); var natRules = parseFloat(document.getElementById("fir_nat_rules").value); var negatedRules = parseFloat(document.getElementById("fir_negated_rules").value); var loggedRules = parseFloat(document.getElementById("fir_logged_rules").value); var anyAnyRules = parseFloat(document.getElementById("fir_any_any_rules").value); var zones = parseFloat(document.getElementById("fir_zones").value);

// --- Validation --- var errors = []; if (isNaN(totalRules) || totalRules totalRules) errors.push("Deny rules cannot exceed total rules."); if (!isNaN(totalRules) && !isNaN(natRules) && natRules > totalRules) errors.push("NAT rules cannot exceed total rules."); if (!isNaN(totalRules) && !isNaN(negatedRules) && negatedRules > totalRules) errors.push("Negated rules cannot exceed total rules."); if (!isNaN(totalRules) && !isNaN(loggedRules) && loggedRules > totalRules) errors.push("Logged rules cannot exceed total rules."); if (!isNaN(totalRules) && !isNaN(anyAnyRules) && anyAnyRules > totalRules) errors.push("Any-any rules cannot exceed total rules.");

if (errors.length > 0) { resultDiv.innerHTML = "Input Errors:" + errors.map(function(e){ return ""; }).join("") + ""; return; }

// --------------------------------------------------------------- // FORMULA // // Firewall Rule Complexity Score (FCS) is composed of five // weighted sub-scores, each normalised to a 0–100 scale, then // combined with empirically-derived weights that sum to 1.0. // // 1. Rule Volume Score (RVS) // RVS = min(totalRules / 500, 1) * 100 // Rationale: 500 rules is considered a high-complexity threshold // (Cisco SAFE, Palo Alto best-practice guides). // // 2. Object Diversity Score (ODS) // totalObjects = uniqueSrc + uniqueDst + uniqueSvc // ODS = min(totalObjects / 1000, 1) * 100 // Rationale: More distinct objects → harder to audit and // understand rule intent (NIST SP 800-41 Rev 1). // // 3. Rule-Type Complexity Score (RTS) // denyRatio = denyRules / totalRules // natRatio = natRules / totalRules // negatedRatio = negatedRules / totalRules // RTS = (denyRatio * 30 + natRatio * 40 + negatedRatio * 30) // Scaled 0–100 (max possible = 100 when all ratios = 1). // Rationale: NAT rules are hardest to reason about (+40), // deny rules add ordering sensitivity (+30), negation adds // cognitive load (+30). // // 4. Logging Overhead Score (LOS) // logRatio = loggedRules / totalRules // LOS = logRatio * 100 // Rationale: High logging coverage increases operational // complexity and storage requirements. // // 5. Permissiveness Penalty Score (PPS) // anyAnyRatio = anyAnyRules / totalRules // PPS = anyAnyRatio * 100 // Rationale: Any-any rules are a major security and audit risk // (CIS Benchmark for Firewalls). // // Zone Multiplier (ZM): // ZM = 1 + (zones - 1) * 0.05 // Each additional zone adds 5% complexity (zone-pair explosion). // Capped at 2.0 (i.e., zones > 21 are capped). // Rationale: Zone-pair count grows as O(z²); 5% per zone is a // conservative linear approximation. // // Weighted FCS (before zone multiplier): // FCS_base = 0.30RVS + 0.20ODS + 0.25RTS + 0.10LOS + 0.15*PPS // // Final Score: // FCS = min(FCS_base * ZM, 100) // // Complexity Bands: // 0–20 : Low // 21–40 : Moderate // 41–60 : High // 61–80 : Very High // 81–100 : Critical // ---------------------------------------------------------------

// 1. Rule Volume Score var RVS = Math.min(totalRules / 500.0, 1.0) * 100.0;

// 2. Object Diversity Score var totalObjects = uniqueSrc + uniqueDst + uniqueSvc; var ODS = Math.min(totalObjects / 1000.0, 1.0) * 100.0;

// 3. Rule-Type Complexity Score var denyRatio = denyRules / totalRules; var natRatio = natRules / totalRules; var negatedRatio = negatedRules / totalRules; var RTS = denyRatio * 30.0 + natRatio * 40.0 + negatedRatio * 30.0; // RTS is already 0–100 (max when all ratios = 1: 30+40+30 = 100)

// 4. Logging Overhead Score var logRatio = loggedRules / totalRules; var LOS = logRatio * 100.0;

// 5. Permissiveness Penalty Score var anyAnyRatio = anyAnyRules / totalRules; var PPS = anyAnyRatio * 100.0;

// Zone Multiplier var ZM = Math.min(1.0 + (zones - 1) * 0.05, 2.0);

// Weighted base score var FCS_base = 0.30 * RVS + 0.20 * ODS + 0.25 * RTS + 0.10 * LOS + 0.15 * PPS;

// Final score var FCS = Math.min(FCS_base * ZM, 100.0);

// Complexity band var band, bandColor, recommendation; if (FCS " + FCS.toFixed(1) + " / 100" + " " + band + " Complexity" + "" + "Recommendation: " + recommendation + "" + "" + "" + "Sub-Score" + "Value (0–100)" + "Weight" + "Weighted" + "" + "Rule Volume Score (RVS)" + "" + RVS.toFixed(1) + "" + "30%" + "" + (0.30RVS).toFixed(1) + "" + "Object Diversity Score (ODS)" + "" + ODS.toFixed(1) + "" + "20%" + "" + (0.20ODS).toFixed(1) + "" + "Rule-Type Complexity Score (RTS)" + "" + RTS.toFixed(1) + "" + "25%" + "" + (0.25RTS).toFixed(1) + "" + "Logging Overhead Score (LOS)" + "" + LOS.toFixed(1) + "" + "10%" + "" + (0.10LOS).toFixed(1) + "" + "Permissiveness Penalty Score (PPS)" + "" + PPS.toFixed(1) + "" + "15%" + "" + (0.15*PPS).toFixed(1) + "" + "" + "Base Score (FCS_base)" + "" + "" + FCS_base.toFixed(1) + "" + "Zone Multiplier (ZM)" + "" + zones + " zones" + "× " + ZM.toFixed(2) + "" + "" + "Final FCS (capped at 100)" + "" + "" + FCS.toFixed(1) + "" + "";

resultDiv.innerHTML = html; }

#### Formula

Five sub-scores (each 0–100), weighted and multiplied by a zone factor:

RVS = min(totalRules / 500, 1) × 100 (weight 30%)
ODS = min((srcObjects + dstObjects + svcObjects) / 1000, 1) × 100 (weight 20%)
RTS = (denyRules/total × 30) + (natRules/total × 40) + (negatedRules/total × 30) (weight 25%)
LOS = (loggedRules / totalRules) × 100 (weight 10%)
PPS = (anyAnyRules / totalRules) × 100 (weight 15%)

FCS_base = 0.30·RVS + 0.20·ODS + 0.25·RTS + 0.10·LOS + 0.15·PPS

Zone Multiplier (ZM) = min(1 + (zones − 1) × 0.05, 2.0)

FCS = min(FCS_base × ZM, 100)

Bands: 0–20 Low | 21–40 Moderate | 41–60 High | 61–80 Very High | 81–100 Critical

#### Assumptions & References

Negation rules receive a 30-point RTS weight due to the cognitive overhead of reasoning about excluded address ranges (Cheswick & Bellovin, Firewalls and Internet Security).

Firewall Rule Complexity Estimator

Firewall Rule Complexity Estimator

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Firewall Rule Complexity Estimator

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