boma/docs/decisions/016-mesh-vpn.md

ADR-016 — Mesh VPN (NetBird, self-hosted on askari)

Status

Accepted (2026-06-05). Designed, not built — depends on the unbuilt base role and service-role machinery (STATUS.md). This ADR records the decision and doc reconciliation; role tasks land when base exists.

Context

ubongo (ADR-015) needs remote SSH access from anywhere without exposing anything to the public internet; ADR-015 deferred the mechanism. ADR-007 already commits to WireGuard-via-OPNsense for the vpn VLAN (VLAN 99, 10.99.0.0/24: askari + road warriors), and docs/CAPABILITIES.md flagged NetBird (mesh) as a real alternative to weigh. This ADR settles it.

Decision

A single NetBird mesh is the sole remote-access overlay, self-hosted on askari, replacing ADR-007's VLAN-99 OPNsense WireGuard.

The decision in four parts:

1. Scope — mesh replaces WireGuard. One overlay for ubongo, askari, and road-warrior clients. ADR-007's VLAN-99 WireGuard design is retired.
2. Control plane — self-hosted on askari. Sovereignty (boma self-hosts Vaultwarden, Forgejo, DNS), no third-party trust, and an off-site coordinator that survives a homelab outage and stays out of the cluster it administers.
3. Tool — NetBird. Self-hosting selects NetBird (first-class, fully open-source self-host). Tailscale would mean Headscale (third-party reimplementation, partial parity) — ruled out below.
4. Routing — agent on every Linux host, not a subnet router. At boma's scale (2–5 hosts) the "agent everywhere" cost is trivial and the base role already runs everywhere, so enrollment is one uniform task. Avoids a routing SPOF and gives granular per-peer ACLs. OPNsense (FreeBSD) is the one non-agent exception (mgmt/gateway reached by a single advertised route or LAN-side admin).
5. Identity — embedded local users (Dex in the management container); external SSO (Zitadel/Keycloak) stays an optional future.

Verified facts (ADR-014)

verified: NetBird self-hosting · NetBird docs · docs.netbird.io/selfhosted · 2026-06-05 — components management+signal+dashboard+relay/TURN(Coturn), single container since v0.65; built-in local users / embedded IdP since v0.62 (external OIDC optional); ports TCP 80/443 + UDP 3478 behind a reverse proxy; lightweight Linux + Docker Compose host.

verified: NetBird licensing · GitHub netbirdio/netbird · 2026-06-05 — AGPLv3 for management//signal//relay/, BSD-3-Clause elsewhere; fully open source, no open-core feature gating.

Architecture

Data plane: peer-to-peer WireGuard. Control plane: NetBird, self-hosted on askari. NetBird manages its own overlay addressing (default 100.64.0.0/10); no boma VLAN is allocated for it.

• askari (Hetzner, off-site, always-up) — runs the NetBird stack and is a peer.
• ubongo — agent.
• All Linux managed hosts — agent via the base role.
• Road-warrior clients (mamba, phone, work PC) — agent/app.
• OPNsense / mgmt — single non-agent exception.

Security

• ACLs mirror ADR-007 intent (NetBird default-deny): mesh peers → srv metrics ports only; admin peers (ubongo, mamba) → srv + mgmt; clients → least privilege.
• Enrollment via setup keys stored in vault.yml (vault.netbird.setup_key), consumed by base; prefer ephemeral/scoped keys.
• Host firewall: base nftables allows inbound SSH on NetBird's wt0 interface (primary, WireGuard-authenticated) and from ubongo's LAN address (secondary, mesh-independent — required by the LAN-IP recovery path below, so a mesh/coordinator outage never blocks on-LAN SSH). All other LAN hosts remain default-denied. This makes explicit the control-node SSH allow that the recovery model already implied; the access doctrine and the three-tier access ladder live in ADR-021.
• New public surface on askari: management API + dashboard (80/443) + Coturn (3478). Mitigated by TLS + embedded-IdP login, source-IP limits where practical, base hardening, and version-pinned NetBird (ADR-011) patched on boma's cadence. Recorded as accepted-risk R3.

Recovery & operations

• Ansible stays off the mesh: ubongo reaches the fleet by LAN IP (ADR-009); a mesh/coordinator outage never blocks on-LAN runs.
• Bootstrap order: stand up the coordinator on askari → enroll ubongo → base enrolls the fleet.
• Coordinator survival: off-site on askari ⇒ mesh survives a homelab outage. NetBird's management datastore is backed up encrypted off askari (synced to ubongo/mamba); peers keep last-known config through a brief coordinator outage.
• askari is Ansible-managed: its own inventory group offsite_hosts — provisioned as Terraform IaC (hetznercloud/hcloud), managed independently of the Proxmox cluster (its own provider + local state). Ansible configuration: base role, plus a dedicated netbird_coordinator service role (one service = one role, ADR-004; with SECURITY.md). Agent install/enrollment lives in base. NetBird server + agents are version-pinned (ADR-011). boma's dns role stays authoritative for boma.baobab.band; NetBird built-in DNS scoped/off.

What was ruled out

Option	Reason
Plain OPNsense WireGuard (ADR-007 as-is)	No identity/ACL layer, manual peer config; the operator wants policy-based mesh access and easy multi-device enrollment.
Tailscale (hosted coordinator)	Third-party trust for the control plane; against boma's self-hosting ethos. Its recovery benefit is matched by a self-hosted coordinator off-site on askari.
Tailscale + Headscale	Headscale is a third-party reimplementation with partial parity and no vendor support — weaker than NetBird's first-class self-hosting.
Coordinator on the cluster	Recreates the chicken-and-egg ADR-015 escapes and dies with the homelab. askari instead.
Subnet router via ubongo	Makes ubongo a routing SPOF; askari goes blind to srv when ubongo is down. Agent-per-host instead.
Standalone IdP (Zitadel/Keycloak) now	Heavy for one operator; embedded local users suffice.

Consequences

• A new public surface appears on askari — management API + dashboard (80/443) + Coturn (3478) — mitigated by TLS, embedded-IdP login, source-IP limits where practical, base hardening and version-pinned NetBird, and recorded as accepted-risk R3 (Security).
• On-LAN SSH never depends on the mesh: base allows inbound SSH from ubongo's LAN address as a mesh-independent secondary path, so a mesh/coordinator outage never blocks on-LAN SSH and Ansible stays off the mesh (Security; Recovery & operations).
• The mesh survives a homelab outage because the coordinator is off-site on askari, with its management datastore intended to be backed up encrypted off askari (not yet built — see the Availability amendment / R8) and peers keeping last-known config through a brief coordinator outage (Recovery & operations).
• Choosing NetBird over plain OPNsense WireGuard, Tailscale, Tailscale+Headscale, an on-cluster coordinator, a ubongo subnet router, and a standalone IdP gains identity/ACL policy, self-hosted sovereignty, no routing SPOF, and a light single operator footprint (What was ruled out).
• Implementation is pending: the role tasks land only once the unbuilt base role and service-role machinery exist (Status).

Availability — an askari outage (amendment 2026-06-20)

The coordinator is deliberately single (one off-site host). Recorded here so its availability envelope is explicit; accepted as R8 (docs/security/accepted-risks.md).

The mesh is not a default gateway — wt0 routes only the overlay CIDR (100.99.0.0/16); normal traffic uses the host's default route. So an askari outage has a narrow blast radius:

Traffic	askari down
LAN device → LAN service (direct / via reverse proxy)	unaffected
node ↔ node over LAN IPs (cluster)	unaffected
node ↔ node same-LAN over mesh IPs	unaffected (direct P2P)
road-warrior → ubongo (remote, relayed)	breaks
mesh control plane (new enrol / ACL change / re-handshake)	pauses

Only remote (off-LAN) mesh access to peers is lost, and only when off-LAN and askari is down simultaneously. On-LAN access to ubongo never depends on the mesh (Recovery & operations, above).

Recovery: rebuild the coordinator (/setup + re-enrol peers, M5) or restore from backup once ADR-022 lands; the netbird_coordinator store backup is the next sub-project (its gap is named in R8 and BACKUP.md). Client/road-warrior break-glass (reliable resolvers + the coordinator-FQDN /etc/hosts pin) is in docs/runbooks/netbird-client.md; managed mesh hosts get the same pin via base__mesh_coordinator_pin.

Not pursued (deliberately, given the narrow blast radius): direct P2P (punctures the default-deny posture; only helps established sessions), a second relay (needs another public host / reintroduces the home public surface), a second coordinator (unsupported by self-hosted NetBird; against this ADR).

Related

ADR-007 (network — amended), ADR-015 (control host), ADR-002 (security), ADR-011 (version pinning), ADR-004 (one service = one role), ADR-009 (TF↔Ansible handoff), ADR-013 (heritage — V4 ran WireGuard; NetBird is translated, not transplanted), ADR-021 (operational access; SSH ladder reconciling wt0 + ubongo's LAN address).