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:

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):

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

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).

6.3 KiB Raw Blame History