boma/docs/superpowers/plans/2026-06-06-firewall-strategy.md

# Firewall Strategy (ADR-020) Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** Land the firewall *strategy* as ADR-020 and fold it into the living docs — no firewall code is built here (the host-nftables and OPNsense-as-code builds are separate follow-up specs).

**Architecture:** This is a documentation-only change. It creates `docs/decisions/020-firewall.md` from the approved design spec, then updates CLAUDE.md (Further reading + the firewall guardrail), `docs/TODO.md` (mark 3.5 decided), and `docs/CAPABILITIES.md` (point the firewall note at ADR-020). There is no executable code, so verification is consistency greps + `make lint`.

**Tech Stack:** Markdown docs only. `make lint` (yamllint + ansible-lint + check-tags) must stay green; none of these tools lint Markdown content, but the run confirms nothing else broke.

---

## File structure

| File | Responsibility | Action |
|------|----------------|--------|
| `docs/decisions/020-firewall.md` | The firewall strategy ADR (two-layer model, shared catalog, deferred builds) | Create |
| `CLAUDE.md` | Add ADR-020 to *Further reading*; harden the firewall guardrail bullet to reference the catalog/ADR-020 | Modify |
| `docs/TODO.md` | Mark item 3.5 DECIDED (ADR-020) | Modify |
| `docs/CAPABILITIES.md` | Point the existing firewall parenthetical at ADR-020 + the two-layer model | Modify |

Notes for the implementer:
- The design spec this ADR is based on is `docs/superpowers/specs/2026-06-06-firewall-strategy-design.md` — read it if you need the full rationale, but the ADR text below is complete and self-contained.
- Existing ADRs live in `docs/decisions/` numbered 001–019; this is 020. Match their concise, decision-focused tone (ADR-019 is a good recent reference).
- Before any `git commit`, the pre-commit hook runs and decrypts `vault.yml`, so the vault agent must be unlocked: run `rbw unlocked` (exit 0 = good). If locked, ask the user to `rbw unlock` and wait. None of these tasks touch vault files.
- Run `make lint` via the repo venv wiring (the Makefile handles paths).

---

### Task 1: Write ADR-020

**Files:**
- Create: `docs/decisions/020-firewall.md`

- [ ] **Step 1: Create the ADR**

Create `docs/decisions/020-firewall.md` with exactly this content:

````markdown
# ADR-020 — Firewall strategy: two-layer model with a shared service catalog

## Status

Accepted (2026-06-06). Resolves TODO 3.5 ("Decide the firewall strategy — which
firewall, ruleset, per-host vs central").

**Strategy ADR.** It pins the architecture and each layer's responsibilities; the
detailed builds are separate follow-up efforts (see *Scope*).

## Context

boma needs a firewall strategy that is predictable, declarative, and defends the stated
threat model — opportunistic external, lateral movement / blast radius, operator/agent
error (ADR-002). The pieces were already committed across other ADRs (`nftables`
default-deny on hosts — ADR-002; OPNsense at the perimeter — ADR-007; Docker with
`iptables: false` — ADR-004), but nothing tied them together: which layer owns what,
where firewall intent is declared, and how the layers stay consistent. Without that,
ports drift open ad-hoc and "per-host vs central" stays unanswered.

## Decision

### Two layers, distinct jobs

**OPNsense — perimeter + inter-VLAN.** Owns the WAN edge and all policy *between zones*:
`lan`/`iot`/`guest` → `srv`, `mgmt` access, and the per-VLAN egress rules (ADR-007). It
is **structurally blind to intra-`srv` traffic** — services share the switched `srv`
subnet (VLAN 20), which never reaches the gateway.

**Host nftables — host-local + east-west within `srv`** (in the `base` role, every VM):

- **Default-deny inbound**; allow loopback + established/related.
- **East-west allowlist**: a service host accepts a connection only from declared
  sources (e.g. the reverse proxy, a named peer) — the lateral-movement control OPNsense
  cannot provide.
- **Permissive egress**: allow outbound + established/related; per-VLAN egress
  restriction stays at OPNsense (ADR-007). Host-level egress allowlisting is
  high-friction (every DNS/NTP/update/registry/webhook must be enumerated) for limited
  added benefit once the VLAN already bounds where a host can go.
- **Docker**: daemon runs with `"iptables": false`; nftables owns all filtering,
  including container traffic (ADR-004).
- **Guaranteed management plane**: loopback, established/related, and `wt0` (NetBird,
  ADR-016) for SSH + Ansible are always allowed, independent of the catalog, applied
  atomically — a malformed or empty catalog can never lock out management. (ADR-016: SSH
  is allowed only on `wt0`.)

So "per-host vs central" is answered: **both**, with clear ownership.

### Single source of truth — a shared service catalog

A central, declarative **service catalog** in `group_vars/` is the one source of truth
for firewall intent (aligning with ADR-002's "port definitions live in `group_vars/`",
and keeping connectivity *topology* in inventory rather than in any one self-contained
service role — ADR-004). Each entry describes a service's **ingress**:

```yaml
photoprism:
  ingress:
    - { from: reverse_proxy, port: 2342, proto: tcp }
reverse_proxy:
  ingress:
    - { from: lan, port: 443, proto: tcp }
```

`from` is **symbolic**, resolved at render time: a host/group → IP(s) from inventory; a
role (`reverse_proxy`) → the host(s) filling it; a VLAN/zone (`lan`) → the subnet from
the ADR-007 table. This keeps the catalog readable and resilient to IP changes.

### Each layer renders only its own slice

| Ingress rule | Host nftables | OPNsense |
|---|---|---|
| `from: reverse_proxy` (a `srv` peer) | allow proxy IP → port | — (intra-`srv`, invisible) |
| `from: lan` (cross-VLAN) | allow `lan` subnet → port | allow `lan` → host:port |

The dominant pattern falls out naturally: most services are **proxied** — their only
ingress is `from: reverse_proxy`, and users reach them through the reverse proxy, which
alone carries `from: lan, port: 443` (matches "services sit behind the reverse proxy
with authentication", ADR-002).

This was chosen over a single connectivity-model-generates-both (too much machinery,
tight coupling of two very different rule domains) and over fully independent per-layer
declarations (real drift risk).

### OPNsense automation — owned here, mechanism deferred

OPNsense is Ansible-managed (CLAUDE.md: "OPNsense is entirely Ansible; no Terraform
OPNsense provider"). It renders the cross-VLAN slice of the catalog plus the static
ADR-007 facts. The **how** — config-XML templating vs the OPNsense API vs a plugin — is
deferred to the OPNsense-as-code follow-up spec. Recorded as an explicit open
sub-decision.

## Guardrails

- **The catalog is authoritative.** If a port is not in the catalog, it does not exist —
  hardening the existing rule "never open a firewall port ad-hoc on a host" (ADR-002).
- **The `firewall` tag** (ADR-019) marks firewall tasks; `--tags firewall` re-renders
  rules.
- **Drift detection (aspiration).** A deterministic check — in the spirit of
  `scripts/check-tags.py` — comparing each host's live `nft` ruleset / listening ports
  against the catalog and flagging anything undeclared. Ties to TODO 8.5
  (`/security-review`). Not necessarily built first.

## Consequences

- Lateral movement within `srv` is constrained — the gap OPNsense structurally can't
  close.
- One declarative catalog → no ad-hoc ports and no cross-layer drift on shared facts
  (ports, IPs, sources).
- Cost: the catalog + render-per-layer machinery must be built and maintained; east-west
  allowlisting adds per-service ingress declarations (mitigated by proxied-by-default,
  which keeps most entries to a single line).

## Scope

**Decided here:** the two-layer model and responsibilities; host nftables = default-deny
inbound + east-west allowlist + permissive egress + guaranteed management plane + Docker
`iptables:false`; the shared `group_vars` catalog as single source of truth with
symbolic sources; each layer renders its own slice; the no-ad-hoc-ports guardrail.

**Deferred to follow-up specs (each its own brainstorm → plan):**

1. **Host nftables implementation** in `base` — catalog schema, nftables template,
   Docker `iptables:false` integration, fail-safe ordering, Molecule tests. The natural
   next spec.
2. **OPNsense-as-code** — tooling mechanism + cross-VLAN rule rendering.
3. **Drift-detection check** — if/when built.

## Related

ADR-002 (security baseline: nftables default-deny, fail2ban, blast radius),
ADR-004 (Docker model: `iptables:false`), ADR-007 (network topology, VLANs, OPNsense,
per-VLAN egress), ADR-016 (NetBird mesh: SSH on `wt0` only), ADR-019 (`firewall` tag).
````

- [ ] **Step 2: Verify the file is well-formed**

Run:
```bash
test -f docs/decisions/020-firewall.md && grep -c "^## " docs/decisions/020-firewall.md
```
Expected: exit 0 and a printed count of `7` (the H2 sections: Status, Context, Decision, Guardrails, Consequences, Scope, Related — H3 subsections under Decision are not counted by `^## `).

- [ ] **Step 3: Commit**

```bash
git add docs/decisions/020-firewall.md
git commit -m "docs(adr): ADR-020 firewall strategy (two-layer + shared catalog)"
```

---

### Task 2: Wire ADR-020 into CLAUDE.md

**Files:**
- Modify: `CLAUDE.md` (Further reading table; firewall guardrail bullet)

- [ ] **Step 1: Add ADR-020 to the Further reading table**

In `CLAUDE.md`, find this row (around line 225):

```markdown
| Tagging & run-targeting | `docs/decisions/019-tagging.md` |
```

Add this row immediately after it:

```markdown
| Firewall strategy      | `docs/decisions/020-firewall.md`      |
```

(Exact column padding need not match perfectly — just produce a valid Markdown table row consistent with the surrounding rows.)

- [ ] **Step 2: Harden the firewall guardrail bullet**

In `CLAUDE.md`, find this bullet (around line 172, under "What Claude must not do without explicit instruction"):

```markdown
- Open a firewall port anywhere but the `group_vars` firewall definitions — never ad-hoc on a host (ADR-002)
```

Replace it with:

```markdown
- Open a firewall port anywhere but the `group_vars` service catalog — never ad-hoc on a host. If it's not in the catalog, it doesn't exist (ADR-002, ADR-020)
```

- [ ] **Step 3: Verify both edits**

Run:
```bash
grep -n "020-firewall" CLAUDE.md && grep -n "service catalog" CLAUDE.md
```
Expected: the Further reading row matches `020-firewall`, and the guardrail bullet now contains "service catalog".

- [ ] **Step 4: Commit**

```bash
git add CLAUDE.md
git commit -m "docs: link ADR-020; harden firewall guardrail to the service catalog"
```

---

### Task 3: Mark TODO 3.5 decided

**Files:**
- Modify: `docs/TODO.md` (item 3.5)

- [ ] **Step 1: Strike through and annotate item 3.5**

In `docs/TODO.md`, find this line (around line 26):

```markdown
   5. Decide the firewall strategy (which firewall, ruleset, per-host vs central).
```

Replace it with:

```markdown
   5. ~~Decide the firewall strategy (which firewall, ruleset, per-host vs central).~~
      DECIDED (ADR-020): two layers — OPNsense (perimeter + inter-VLAN) + host nftables
      (default-deny inbound + east-west allowlist, permissive egress). Single source of
      truth: a `group_vars` service catalog with symbolic sources; each layer renders
      its own slice. Builds deferred to follow-up specs (host nftables in `base`, then
      OPNsense-as-code).
```

- [ ] **Step 2: Verify**

Run: `grep -n "DECIDED (ADR-020)" docs/TODO.md`
Expected: one match on the item 3.5 annotation.

- [ ] **Step 3: Commit**

```bash
git add docs/TODO.md
git commit -m "docs(todo): mark 3.5 firewall strategy decided (ADR-020)"
```

---

### Task 4: Update CAPABILITIES.md firewall note

**Files:**
- Modify: `docs/CAPABILITIES.md` (the firewall parenthetical in §1 Edge & networking, around line 32)

- [ ] **Step 1: Point the firewall note at ADR-020**

In `docs/CAPABILITIES.md`, find this line (around line 32, just under the §1 table):

```markdown
_(DHCP, firewall, mDNS reflection live on OPNsense — Ansible-managed, not containers.)_
```

Replace it with:

```markdown
_(DHCP, firewall, mDNS reflection live on OPNsense — Ansible-managed, not containers.)_

_Firewalling is two-layer (ADR-020): OPNsense at the perimeter + inter-VLAN, plus
per-host `nftables` (default-deny inbound + east-west allowlist) rendered by the `base`
role from a shared `group_vars` service catalog. Both layers are still to be built._
```

- [ ] **Step 2: Verify and run the full lint suite**

Run:
```bash
grep -n "ADR-020" docs/CAPABILITIES.md && make lint
```
Expected: the new ADR-020 note is found, and `make lint` passes (yamllint clean, ansible-lint clean, `check-tags: OK`).

- [ ] **Step 3: Commit**

```bash
git add docs/CAPABILITIES.md
git commit -m "docs(capabilities): note two-layer firewall model (ADR-020)"
```

---

## Final verification

- [ ] Confirm cross-references resolve:
  ```bash
  ls docs/decisions/020-firewall.md && grep -rl "ADR-020\|020-firewall" CLAUDE.md docs/TODO.md docs/CAPABILITIES.md
  ```
  Expected: the ADR file exists and all three living docs reference it.
- [ ] `make lint` passes end to end.
- [ ] `git log --oneline -4` shows the four task commits.
- [ ] Sanity: the ADR's *Scope* section names the two deferred build specs (host nftables in `base`, OPNsense-as-code) so the next brainstorm has an obvious starting point.