Firewall Config Diff

Firewall Config Diff

Overview

Use this skill to compare two firewall or NGFW configurations and report how they differ, by pivoting both through the parsing-* intermediate JSON schema. Each side is normalized to the same vendor-neutral sections — address_objects, address_groups, service_objects, service_groups, security_policies, nat_rules, zones, interfaces, static_routes / virtual_routers / routing, vpn_tunnels, ha_config, admin_users, and system — and then compared section by section. Because the comparison happens on the schema pivot rather than the raw text, same-vendor and cross-vendor diffs are one mechanism: the only cross-vendor addition is a normalization step for constructs that two vendors model differently.

The comparison is always semantic, never textual. Items are paired by what they mean — an address object by its value, a policy by its match-and-action tuple — so a rename, a reorder, or a different vendor's syntax does not register as a difference. A plain diff of two config files would flag every cosmetic and ordering change and miss the fact that two differently-named objects describe the same subnet; it would add nothing here and actively mislead. This skill compares meaning; it does not emit a target config (that is firewall-config-conversion) and does not judge the rulebase against best practice (that is firewall-best-practices-audit).

The output is always two parts: a per-section report listing what was added, removed, and changed, followed by a single parity verdict (EQUIVALENT or DIFFERENCES FOUND). Where a feature on one side has no equivalent on the other — common in cross-vendor comparisons — it is flagged not-comparable and excluded from the diff rather than reported as a false difference. The report never claims more than the schema supports and always lists what could not be compared.

Scope and routing

Compare two normalized parser outputs. Use a text-diff tool for literal line changes, firewall-config-conversion to emit target syntax, and firewall-best-practices-audit to assess one rulebase.

Input Handling

There are two inputs, A and B. Route each side independently on what you were given:

• Parsed intermediate schema (the vendor-neutral JSON produced by any parsing-* skill; the schema definition lives in the parsing-srx-configs skill) — diff directly. Read each side's source vendor from metadata.source_vendor (the canonical schema field); for robustness also accept a legacy metadata.vendor key if a non-conformant parse provides one — read whichever is present, and label the two sides A and B by their vendors.
• Raw config — identify the vendor from the syntax, run the matching parsing-* skill (parsing-cisco-configs, parsing-fortinet-configs, parsing-palo-configs, parsing-srx-configs) to produce the intermediate schema, then diff the result. Never re-implement parsing in this skill.

A and B may be the same vendor (drift, HA-pair, pre/post-change) or different vendors (migration parity, round-trip). The two sides may arrive in different forms — one parsed schema and one raw config — in which case parse the raw side first so both are schema before any comparison begins.

Semantic Identity

Items pair across A and B by MEANING, not order. Identity is value/tuple-based by default;
a stable name is a valid anchor only in the same-vendor case (see the convention below):
- address objects: by value (e.g. 10.0.1.10/32), not object name.
- service objects: by protocol + port(s). applications: by canonical app name.
- address/service groups: by their expanded member set.
- security policies: by the tuple (src_addresses, dst_addresses, service/app, action, src_zones, dst_zones), addresses compared by value.
- nat_rules: by (match addresses/zones + translation).
- interfaces: by name/unit + address. static routes: by prefix + next-hop. OSPF/BGP: by area/AS + neighbor.
- system / admin_users / dhcp_config / ha_config / screen_config / schedules / security_services: by their natural key (name or the field identity).
A matched pair is `unchanged` (all attributes equal) or `changed` (same identity, ≥1 differing attribute — e.g. logging, profile, description). Unmatched in A only = `removed`; in B only = `added`.
Security-policy ORDER differences are reported separately (order affects shadowing) and do NOT count as add/remove.

Name-anchoring convention (resolves how value changes are reported):
- CROSS-vendor → identity is strictly value/tuple-based (names differ across platforms), so a
  value change is a `removed` + `added` pair.
- SAME-vendor drift → when object/policy NAMES are stable you MAY anchor pairs by name and
  report the value/attribute delta as `changed` (applies to address objects, service objects,
  address/service groups, AND security policies). A policy whose referenced object value
  shifts stays paired by name; the shift is noted on the `changed` pair, not as add+remove.

Diff Workflow

1. Normalize both sides to schema. Confirm A and B are each the intermediate schema (parse any raw side first), and record each side's source vendor.
2. Pair items per section. For every schema section, pair items between A and B by their semantic identity from the rules above — by value, member set, or match-and-action tuple. This is the cross-vendor default; for same-vendor drift a stable name is a valid anchor (see the Name-anchoring convention). Never pair by position.
3. Classify each pairing. Mark each pair unchanged (all attributes equal) or changed (same identity, differing attributes); items unmatched in A are removed, in B are added. Note security_policies rule-order differences separately, since order affects shadowing — a reorder is not an add/remove.
4. Normalize cross-vendor constructs. When A and B are different vendors, normalize the constructs that the two vendors model differently using references/equivalence-rules.md before pairing, and flag any feature with no equivalent on the other side as not-comparable — never let a non-isomorphic feature surface as a false diff.
5. Emit the report and verdict. Produce the per-section report and the single parity verdict in the templates below, and list everything that could not be compared.

Output & Verdict

Section:    unchanged   added   removed   changed [  | order:  reordered]
  + [B] 
  - [A] 
  ~ [A→B] : 
Not comparable: 
Parity verdict: )>  (A=, B=)

The per-section report comes first; the single Parity verdict: line is always last (the worked examples follow this order). ` in the verdict equals the total added + removed + changed across all sections (comparable items only — not-comparable` entries are listed but never counted).

Framings: drift (A=old, B=new → what changed) and parity (A vs B → equivalent?) — same mechanism. Never claim more than the schema supports; always list what could not be compared.

Reference Material (load on demand)

• references/equivalence-rules.md — the cross-vendor normalization rules: how constructs that two vendors model differently (zones vs security-levels, App-ID vs port-based services, address/service representations, NAT models) are reduced to a common form before pairing, and which features are inherently not-comparable.
• references/example-diff.md — a worked diff against parsing-* fixtures, including a same-vendor drift case and a cross-vendor round-trip case with its parity verdict.

Common Pitfalls

• Doing a textual or line diff — this skill compares by MEANING over the schema; never diff raw text.
• Treating a rename or a reorder as add+remove — match by semantic identity, and report order separately.
• Emitting false diffs on cross-vendor non-isomorphic features — flag them not-comparable instead.
• Re-implementing parsing instead of delegating to the matching parsing-* skill.
• Comparing object NAMES instead of VALUES across vendors — two configs can use different names for the same subnet (same-vendor drift may anchor by stable name — see Name-anchoring convention).
• Forgetting that NAT can change the effective meaning of traffic when reasoning about policy equivalence.
• Path-referencing another skill's files in the round-trip example — keep this skill self-contained with inline snippets.

Verification Checklist

• [ ] Confirm both inputs are intermediate schema (parse raw first); note each side's vendor.
• [ ] Items paired by semantic identity (value/tuple), not name or order.
• [ ] Each section reports unchanged/added/removed/changed; rule-order changes reported separately.
• [ ] Cross-vendor non-isomorphic features flagged not-comparable, not as false diffs.
• [ ] Output has a parity verdict and lists everything that could not be compared.
• [ ] No textual line-diff; no secrets surfaced in the report.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: fastrevmd-lab
• Source: fastrevmd-lab/fwskillsshare
• License: Apache-2.0
• Homepage: https://mechub.org

Install and usage instructions live in the source repository linked above.