diff --git a/docs/superpowers/specs/2026-06-06-firewall-strategy-design.md b/docs/superpowers/specs/2026-06-06-firewall-strategy-design.md new file mode 100644 index 0000000..f3f6002 --- /dev/null +++ b/docs/superpowers/specs/2026-06-06-firewall-strategy-design.md @@ -0,0 +1,164 @@ +# Design — Firewall strategy (two-layer model + shared catalog) + +- **Date:** 2026-06-06 +- **Status:** Approved design — pending implementation plan +- **Resolves:** TODO 3.5 ("Decide the firewall strategy — which firewall, ruleset, + per-host vs central") +- **Becomes:** ADR-020 (this design is the basis for that ADR) +- **Scope note:** This is the **strategy** ADR. It pins the architecture and + responsibilities; the detailed builds (host nftables in `base`, OPNsense-as-code) are + separate follow-up specs (see *Scope*). + +--- + +## Problem + +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 ADRs already commit to pieces of this — `nftables` +default-deny on hosts (ADR-002), OPNsense at the perimeter (ADR-007), Docker with +`iptables: false` (ADR-004) — but no document ties them together: *which layer owns +what, where firewall intent is declared, and how the two layers stay consistent.* +Without that, ports drift open ad-hoc and "per-host vs central" stays unanswered. + +The roles that would hold the host firewall (`base`, `docker_host`) are empty, and there +is no OPNsense automation yet — so this is greenfield strategy work. + +## The two-layer model + +Two firewall layers, each with a distinct job; the host layer adds deliberate +defense-in-depth for the one thing the perimeter structurally cannot see. + +### OPNsense — perimeter + inter-VLAN + +Owns everything *between zones* and at the edge: + +- WAN edge (the internet boundary). +- Inter-VLAN policy: `lan`/`iot`/`guest` → `srv`, `mgmt` access, the documented + per-VLAN egress rules (ADR-007). +- **Structurally blind to intra-`srv` traffic**: services share the `srv` subnet + (VLAN 20), which is switched and never reaches the OPNsense gateway. + +### Host nftables — host-local + east-west within `srv` (in `base`) + +Runs on every Debian 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). This is the lateral-movement control + OPNsense cannot provide — the blast-radius goal in ADR-002. +- **Permissive egress**: allow outbound + established/related. Per-VLAN egress + restriction stays at OPNsense (where it already lives, ADR-007). Rationale: host-level + egress allowlisting is high-friction (every DNS/NTP/update/registry/webhook call must + be enumerated) for limited additional benefit given OPNsense already bounds where each + VLAN can go. +- **Docker integration**: Docker daemon runs with `"iptables": false`; nftables owns all + filtering, including container traffic (ADR-004). +- **Guaranteed management plane**: loopback, established/related, and `wt0` (the NetBird + overlay, ADR-016) for SSH + Ansible are *always* allowed, independent of the catalog, + and the ruleset is applied atomically — so a malformed or empty catalog can never lock + out management. (ADR-016: SSH is allowed only on `wt0`, not the LAN.) + +## The shared service catalog (single source of truth) + +A central, declarative **service catalog** in `group_vars/` is the one source of truth +for firewall intent. This aligns with ADR-002's existing rule that "port definitions +live in `group_vars/` so rules stay in sync with deployed services," and keeps +connectivity *topology* (inherently cross-cutting) in inventory rather than in any one +self-contained service role (ADR-004). + +Each entry describes a service's **ingress** as a list of allow rules: + +```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 or group** → IP(s) from inventory; +- a **role** (e.g. `reverse_proxy`) → the host(s) filling it; +- a **VLAN/zone** (e.g. `lan`) → the subnet from the ADR-007 table. + +Symbolic sources keep the catalog readable and resilient to IP changes. + +### Each layer renders only its own slice + +The same catalog feeds both layers; each filters for the rules it owns: + +| 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`; users reach them *through* the reverse proxy, which +alone carries `from: lan, port: 443`. This matches "services sit behind the reverse +proxy with authentication" (ADR-002). + +"Shared catalog, each layer renders its own" 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: a port opened on the host but not at OPNsense, or vice versa). + +## OPNsense automation — owned here, mechanism deferred + +OPNsense is **Ansible-managed** (CLAUDE.md: "OPNsense is entirely Ansible; do not reach +for a Terraform OPNsense provider"). It renders the **cross-VLAN slice** of the catalog +(every `from: ` rule) plus the static ADR-007 facts (WAN edge, per-VLAN +egress, mgmt access, inter-VLAN defaults). + +This ADR pins **what** OPNsense owns and that it renders from the shared catalog. The +**how** — config-XML templating vs the OPNsense API vs a plugin — is a substantial, +separate tooling decision, **deferred to the OPNsense-as-code follow-up spec**. Recorded +here as an explicit open sub-decision so it is not lost. + +## Guardrails & enforcement + +- **The catalog is authoritative.** If a port is not in the catalog, it does not exist. + This hardens the existing CLAUDE.md guardrail ("never open a firewall port ad-hoc on a + host") into a positive contract. +- **The `firewall` tag** (ADR-019) marks firewall tasks, so `--tags firewall` re-renders + rules on `base` and any service role that contributes them. +- **Drift detection (aspiration).** A deterministic check — in the spirit of + `scripts/check-tags.py` — compares each host's actual listening ports / live `nft` + ruleset against the catalog and flags anything undeclared. Ties to TODO 8.5 + (`/security-review`) and the "undeclared open ports" pre-scan idea. Listed as a + consequence and future guardrail; not necessarily built in the first implementation. + +## Consequences + +- "Per-host vs central" is answered: **both**, with clear ownership — central perimeter + (OPNsense) + per-host default-deny with east-west allowlisting, fed by one catalog. +- Lateral movement within `srv` is constrained (the gap OPNsense can't close). +- One declarative catalog means no ad-hoc ports and no cross-layer drift on the shared + facts (ports, IPs, sources). +- Cost: the catalog and the render-per-layer machinery must be built and maintained; + east-west allowlisting adds per-service ingress declarations (mitigated by the + proxied-by-default pattern, which keeps most entries to a single line). + +## Scope + +**This ADR decides:** the two-layer model and each layer's responsibilities; host +nftables = default-deny inbound + east-west allowlist + permissive egress + guaranteed +management plane + Docker `iptables:false`; the shared `group_vars` service 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` — exact catalog schema, nftables template + structure, Docker `iptables:false` integration, fail-safe ordering, Molecule tests. + The natural next spec. +2. **OPNsense-as-code** — the tooling mechanism + cross-VLAN rule rendering. +3. **Drift-detection check** — if/when we build it. + +## 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).