docs: architecture Part 06 — offsite-connectivity design-of-record (slice roadmap S1-S6)
Records the settled transport decisions (plain WG, host-side agent-managed pilot, one hub-driven endpoint VM running WG+PBS, hub source-of-truth over WireDesiredState, one datastore + per-customer namespaces, relay-through-DooPlex rejected). Grounded at file:line vs felhom.eu@bf099f6 + felhom-agent@4ba1b14. Backlog line resolved to cite spike + design doc. Docs-only; slices are separate future tasks. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6
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# Architecture Part 6 — Offsite Connectivity (the backup transport)
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> Status: **design-of-record** (2026-07-03). Records the settled offsite-backup-transport
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> decisions; grounded against felhom.eu @ `bf099f6` and felhom-agent @ `4ba1b14` (v0.63.0).
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> Evidence base: `documentation/audits/SPIKE-connectivity-wireguard-2026-07-03.md` (all
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> measurements cited below as "spike Pn"). This document ends in an implementation slice roadmap
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> (§8); **no slice is implemented yet** — each is a separate future task. Consistent with and
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> extending Part 01 (`01-topology-and-trust.md`); where this doc names a component's duty it uses
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> Part 01 §3 vocabulary.
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---
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## 1. Why (the gap)
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Offsite backup — the paid DR substrate (01 §8) — is structurally unreachable for a real customer
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today. Backup is an **outbound push to PBS** (01 §7: "Outbound only for control/report/backup"),
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but the only PBS lives at a **LAN address** — `internal/pbs/client.go:25-26` documents the
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`Server` field as literally `"192.168.0.180"`, port 8007. The operator's public edge is
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Cloudflare-Tunnel → `nginx-internal` (`felhom.eu/manifests/hub.yaml:233`,
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`ingressClassName: nginx-internal`) — HTTP(S) only; DooPlex has **no public IP and no public UDP**.
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So offsite backup currently works only because the demo box shares a LAN with the PBS host; a
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customer box cannot reach it at all. The DR crypto/identity side is done and drilled
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(`SPIKE-dr-recipe-2026-06-16.md`); **this transport is the last missing piece**, and the spike
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proved the cheapest transport suffices.
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---
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## 2. Decisions (settled — recorded, not re-litigated)
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| # | Decision | Rationale (one line) | Rejected alternative |
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|---|----------|----------------------|----------------------|
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| D1 | **Transport = plain WireGuard**, box-dials-outbound, `PersistentKeepalive=25` | Backup is hub-and-spoke to a *public* endpoint; outbound UDP + keepalive traverses any NAT tier — spike P2 (first-try up, 11.4-min idle hold), P4 (UDP 51820 *and* 443 pass) | Headscale/DERP **for traversal** — solves the p2p problem we don't have; a TLS control plane + DB + tailscaled-per-box + third-party DERP dependency for nothing (spike P5). Stays available as a *separable future fleet layer*. |
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| D2 | **Placement = customer host, AGENT-MANAGED** (`wg-felhom` systemd service reconciled from hub desired-state) | WG config is *ongoing reconciled desired-state* — the agent's job; the host-install script is one-shot and cannot react to hub changes (endpoint move, key rotation). Pattern exists: the agent already installs+enables host units via `configs/felhom-agent.sudoers:16-18` (`install … /etc/systemd/system/*.mount` + `systemctl enable --now`) | (a) install-script-owned static config — can't reconcile; (b) in-guest WG — *works* technically (spike P7: kernel WG needs no TUN) but violates 01 §3 (guest holds zero infra creds) and dies with the guest; backups originate host-side anyway. |
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| D3 | **Endpoint = ONE shared public dual-stack cloud VM** (Hetzner/EU class; HU optional later) running **WG server + the offsite PBS**. It is **dumb and hub-driven** — no agent on it; the hub is the only thing that mutates its WG peer list | Mirrors the proven "hub holds the credential and drives external infra" pattern (`hub/internal/cloudflare/unblock.go` — hub-held CF token mutating WAF rules). Zero-knowledge client-side encryption (01 §8) makes the VM's location privacy-neutral | An agent on the endpoint — a whole trust tier for a box that only needs a peer list and a datastore. |
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| D4 | **Hub = source of truth** for endpoint coordinates (DNS name, port, server pubkey) + the per-customer peer registry (customer pubkey → assigned tunnel `/32`), delivered over the **existing desired-state channel** | The channel is built for exactly this: heartbeat carries `DesiredGeneration` (`internal/hub/report.go:270`); the agent fetches the heavy state only on generation advance (`internal/desired/syncer.go:44-61`); `WireDesiredState` (`report.go:289-296`) already carries forward-compat sibling blocks (`RestoreDirective`, `StorageManifest`) — a `wireguard` block joins them | A parallel bespoke config channel. |
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| D5 | **PBS layout = one datastore, per-customer NAMESPACES**, each namespace under its own client-side encryption key (already escrowed — 01 §8 key custody) | PBS-native multi-tenancy; crypto isolation without datastore sprawl; the hub already holds "PBS namespace" in the customer record (01 §6) and `WireDesiredState.PBSNamespace` already exists on the wire (`report.go:294`) | Per-customer datastores — operational sprawl with no isolation gain over namespace+key. |
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| D6 | **Offsite PBS lives ON the cloud VM — NOT relayed through DooPlex** | Relaying funnels *every* customer's backup through the operator's one shared ~40 Mbit home uplink and couples all customer DR to home-connection uptime — non-scaling and a single point of failure. DC uplink is never the bottleneck (§5) | The spike's §4.4(b) rendezvous-relay shape — rejected by the operator for production. **DooPlex PBS → dev/demo only.** |
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---
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## 3. Architecture
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### 3.1 Components & trust (extends 01 §3 / §5)
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| | Customer host | Endpoint VM | Hub |
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|---|---|---|---|
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| Runs | `wg-felhom` systemd service (agent-reconciled) + the existing agent | WG server + PBS (one datastore, per-customer namespaces) + a minimal hub-driven peer-sync surface | peer registry + endpoint coords in the customer/host record |
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| Holds | its own WG **private key** (0600, agent state dir) + PBS token + the pinned PBS fingerprint | WG server private key; the PBS ciphertext (zero-knowledge — 01 §10) | every box's WG **public** key + assigned `/32`; the endpoint-mutation credential; escrowed key material (unchanged, 01 §8) |
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| Mutates | only its own WG interface/service | nothing on its own — peer list changes arrive from the hub | the endpoint's peer list; the desired-state the agent consumes |
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| Never | accepts inbound (dials out only) | initiates into a box or the hub; holds plaintext | connects into a box (01 §4 — unchanged) |
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New trust-boundary rows (extends 01 §5):
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| Boundary | What crosses | Mechanism | Blast radius if breached |
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|---|---|---|---|
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| box ↔ endpoint VM | WG-encapsulated, TLS-pinned, client-side-encrypted PBS traffic | outbound UDP; per-peer `/32`; no inter-peer routing (§4.5) | that box's tunnel; ciphertext only |
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| hub ↔ endpoint VM | peer-list mutations (add/remove pubkey+`/32`) | hub-held credential, minimal surface (mechanism = slice-1 design point, §7) | the peer list — an attacker can *disconnect* backups, not read them |
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### 3.2 Hub data model (conceptual fields — not SQL, not wire-final)
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- **Endpoint record** (one, for now): `dns_name`, `wg_port`, `server_pubkey`, `tunnel_subnet`,
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`pbs_tunnel_ip` (the endpoint's in-tunnel address the PBS client dials), plus the
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endpoint-mutation credential (out-of-band secret, never in desired-state).
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- **Per-host peer entry** (joins the customer/host record, 01 §6): `wg_pubkey` (box-generated,
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box-registered), `assigned_ip` (the box's tunnel `/32`, hub-assigned), `status`.
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- **On the wire:** a `wireguard` block on `WireDesiredState` (`report.go:289` — sibling to
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`Guests`/`RestoreDirective`, same forward-compat carriage rule as `report.go:284-288`: carried +
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cached by old agents, acted on by new ones): endpoint coords + this box's assignment. Delivery
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needs **zero new plumbing** — bump `DesiredGeneration`, the agent's existing syncer fetches
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(`syncer.go:44-61`).
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### 3.3 Provisioning handshake (Day-0 join)
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Plugs into the existing Day-0 flow (`runbooks/day0-install.md`) after agent enrollment (01 §6 —
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"the agent enrolls first"); offsite stays retrofittable via the same path for boxes installed
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before this ships (the drilled `--rescope-acl` posture is unaffected).
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1. **Box (agent):** generates the WG keypair; private key 0600 in the agent state dir, never
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leaves the box (except escrowed — §3.5).
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2. **Box → hub:** registers `wg_pubkey` (new box-facing endpoint, slice-2; authenticated by the
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box's existing hub identity).
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3. **Hub:** assigns the `/32` from the tunnel subnet, stores the peer entry, bumps
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`DesiredGeneration`.
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4. **Hub → endpoint VM:** adds `{pubkey, /32}` to the WG server peer list (slice-1 surface).
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5. **Box (agent):** next heartbeat sees the generation advance → fetches desired-state → writes
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the WG config → `install` + `systemctl enable --now wg-felhom` (the sudoers `*.mount` pattern
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of `felhom-agent.sudoers:16-18`, extended to this unit — slice-3).
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6. **Box:** tunnel handshakes (outbound, keepalive); PBS storage is (re)pointed at
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`pbs_tunnel_ip:8007` (§3.4); first backup runs.
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Steps 3-4 and 5-6 are order-independent (WG tolerates either side appearing first); the reconcile
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loop retries until both halves exist.
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### 3.4 PBS over the tunnel
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The PBS client config (`internal/pbs/client.go:24-31`) changes **only its `Server` value**: the
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endpoint's tunnel-internal address (`pbs_tunnel_ip`) instead of a LAN IP. The TLS exact-cert pin
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(`internal/pbs/pin.go:17-35`) sits **above** WG and is unchanged through the tunnel —
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spike-proven both ways (P2b: pin validated through the tunnel; wrong pin rejected). Per-customer
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namespace + per-customer escrowed encryption key exactly as today (D5); the endpoint sees
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ciphertext only (01 §10).
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### 3.5 Key custody + DR re-establishment
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The WG private key joins the **escrowed IdentityBundle**
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(`internal/escrow/identity.go:24-27`, today `{TunnelToken, PBSToken}` → `+ WGPrivateKey`) — the
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same R-wrapped, zero-knowledge age blob the DR recipe already consumes. On host-loss
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re-provisioning (01 §9 restore mode): the box recovers the bundle with the customer's recovery
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code, restores the WG key, and re-establishes the tunnel; hub-side the peer entry still exists
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(or is re-added on re-registration — same handshake, §3.3). Revocation = hub deletes the peer at
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the endpoint: the box is off the offsite network immediately, auditable, no box cooperation
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needed.
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---
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## 4. Robustness (production details beyond the spike)
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- **4.1 Customer IP change = free, and explicitly NOT a DynDNS dependency.** The box dials out;
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WG roaming re-handshakes from the new source address automatically. Nothing anywhere references
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the customer's IP — the hub peer registry stores pubkey+`/32` only (§3.2). No customer-side DNS,
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ever.
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- **4.2 Endpoint referenced by DNS + a re-resolve watchdog.** The endpoint is `dns_name`, not a
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bare IP, so it can move. Trap to design around: `wg-quick` resolves `Endpoint=` **once** at
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service start — a later endpoint re-IP would strand running tunnels. The agent's reconcile loop
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therefore owns re-resolution: on tunnel-unhealthy (no recent handshake), re-resolve and re-apply
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`wg set … endpoint=` (slice-3). Endpoint moves are rare, hub-announced (coords in desired-state),
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and self-heal even without the watchdog on service restart.
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- **4.3 MTU/MSS.** WG costs 60 bytes on v4 (interface MTU 1420 — what the spike ran end-to-end
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under PBS TLS with zero fragmentation stalls, P3). The unit ships explicit `MTU 1420`; since
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only the box's own PBS client uses the tunnel (no forwarded flows), no MSS clamp is needed —
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noted here so nobody adds forwarding without one.
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- **4.4 Keepalive.** `PersistentKeepalive=25` — spike-proven to hold the NAT mapping through an
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11.4-minute fully-idle window (P2) at ~150 B/s of overhead traffic.
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- **4.5 Isolation.** Per-peer `/32` `AllowedIPs`; IP forwarding stays **off** on the endpoint; its
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firewall admits, from the WG interface, only the PBS port — so a box can reach the PBS API and
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nothing else, and boxes cannot see each other **by topology** (spike P6). Public surface: SSH
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(operator) + the WG UDP port, nothing more. PBS tenancy on top: namespace + per-customer token +
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per-customer key (D5).
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- **4.6 Tunnel health → hub.** The tunnel is a storage dependency, so it reports like one — the
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storage-manifest model (01 §8: agent "continuously checks presence/reachability, and reports
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per-target status; a disconnected target → actionable notification") gains a tunnel-health
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input: no handshake within ~3 keepalive periods → the offsite target reports unreachable → the
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existing alerting path carries it. No new alarm channel.
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---
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## 5. The endpoint VM (the one new managed-infra piece)
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Runs three things, nothing else: the WG server (peer list = hub-written), PBS (one datastore,
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per-customer namespaces), and the minimal peer-sync surface the hub drives. **How** the hub
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mutates the peer list — a tiny authenticated push API on the VM vs the VM pulling a signed peer
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manifest from the hub — is deliberately **left as the slice-1 design point** (§7); the trust
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shape is fixed either way (hub holds the credential; the VM trusts only the hub; mirroring
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`hub/internal/cloudflare/unblock.go`). Firewall posture per §4.5. Dual-stack (spike P1: keep v6
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available even though the pilot line had none). No agent, no controller, no customer plaintext.
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Bandwidth reality (why D6 is safe): the operator line measured 1 Gbit-class down / ~40 Mbit up
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(spike P3) — a customer backup is bound by *that customer's own uplink*, and a restore by their
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downlink; a DC-grade endpoint uplink is never the bottleneck, and customers never contend with
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the operator's home line (the rejected relay's flaw).
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Spike-earned deploy note for slice-1: a PBS datastore under `/root` fails (`backup` user cannot
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traverse `/root`) — create datastores under a world-traversable parent (`/srv`).
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---
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## 6. Trust-model conformance (01 §3/§4 preserved)
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- The box **only ever dials out** (UDP to the endpoint; the same posture as hub-poll and
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PBS-push, 01 §4/§7). No inbound listener is added to any customer box; the hub still never
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initiates into a box.
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- The endpoint VM cannot reach into boxes either: it terminates tunnels whose per-peer routes
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(§4.5) admit only box→PBS traffic; it holds no box credentials.
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- **Out of scope, explicitly:** (a) operator-inbound access to boxes — a *separate* future
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problem with its own trust decision; nothing here creates or presumes it; (b) game-server
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ingress — public players are not tunnel peers; that is a different ingress problem (public
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reachability), not a backup-transport one.
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---
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## 7. Open validation + open sub-decisions (honest ledger)
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**Open validation (one):** true-CGNAT traversal is argued from mechanism (outbound-initiated +
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keepalive holds a mapping on any NAT tier), **not yet measured** — the spike's line proved to be
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plain single-NAT with a public IPv4 (spike P0). Cheap closure: a mobile-hotspot smoke test
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(phone-tethered box → endpoint; mobile networks are true CGNAT); the deferred second-ISP vantage
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(Peti VM 110) remains the thorough confirmation. Do this before or during slice-3; it does not
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block slices 1-2.
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**Open sub-decisions (deferred by design):**
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- Endpoint peer-sync mechanism (push-API vs signed-manifest pull) — **slice-1 design point** (§5).
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- WG key rotation cadence/mechanics — the reconcile channel can carry it (new pubkey → hub →
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endpoint swap); design when fleet size warrants, possibly the moment the "separable fleet
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layer" (D1) conversation reopens.
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- Endpoint scaling/sharding (N customers per VM, second region, HU-hosted option) — capacity
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planning, not architecture; the peer registry already keys per-host.
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- Backup/DR **of the endpoint VM itself** (datastore redundancy, re-provision runbook) — slice-1
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must at least write the re-provision runbook; PBS-side redundancy is a later economics call.
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---
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## 8. Implementation slice roadmap (each = a separate future task)
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| Slice | Scope | Done = |
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|---|---|---|
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| **S1 — endpoint provisioning + peer-sync surface** | provision the production VM (WG server, PBS, firewall, `/srv` datastore, namespaces); resolve the push-vs-pull peer-sync design point; re-provision runbook | hub can add/remove a peer on the live endpoint via its credential; runbook re-creates the VM from nothing |
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| **S2 — hub schema + desired-state** | endpoint coords + peer registry; `wireguard` block on `WireDesiredState`; box-facing pubkey-registration endpoint; generation bump on peer changes | a registered box's desired-state carries its assignment; hub UI shows the peer registry |
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| **S3 — agent WG service (the agent-managed-unit pilot)** | keygen + registration; reconcile `wg-felhom` from desired-state (sudoers additions on the `:16-18` pattern); re-resolve watchdog (§4.2); MTU 1420; IdentityBundle `+ WGPrivateKey`; tunnel-health into the report. **Risky/supervised class** (host service + sudoers + escrow change) | fresh box joins by §3.3 end-to-end; tunnel survives agent restart + endpoint re-IP; unhealthy tunnel alerts |
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| **S4 — PBS over the tunnel** | point the box's PBS storage at `pbs_tunnel_ip:8007`; verify pin-through-tunnel | real backup lands in the box's namespace over the tunnel (nearly free — spike-proven path) |
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| **S5 — DR re-establishment** | consume `WGPrivateKey` in restore mode; re-peer on re-provision | host-loss drill restores offsite connectivity with only the recovery code. *Merges with the DR-completeness arc.* |
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| **S6 — monitoring + isolation hardening** | endpoint-side alerting, per-peer firewall audit, CGNAT smoke-test closure if not yet done | tunnel-down and peer-anomaly both alert; isolation verified adversarially |
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**MVP that puts offsite backup in a real customer's hands = S1 → S2 → S3; then S4.** S5/S6
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complete the story. The cloudflared service (`internal/hub/cloudflared.go:9-12` — today a
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read-only prober that names itself "the seam for the tunnel-management slice") adopts the
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S3-piloted agent-managed-service pattern in a later, separate slice.
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@@ -119,11 +119,12 @@ Notes:
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Felhom-pool guests — other guests on a shared box are invisible there. Always pick from
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`pct list` + `qm list` on the box (`documentation/runbooks/provisioning.md`).
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- Offsite backup (`felhom-pbs`) is NOT part of a go-live today: the PBS host is LAN-only. The
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transport decision is now MADE — plain WireGuard, host-side, box-dials-out to a small public
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endpoint (spike-proven: `documentation/audits/SPIKE-connectivity-wireguard-2026-07-03.md`);
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what remains is the production connectivity spec + endpoint VM. Go-live standard = **local
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backups only** (that is what the Part C command configures); offsite DR is retrofitted later
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via `--rescope-acl` / `--acl-storages` without reinstalling.
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transport is DECIDED and DESIGNED — plain WireGuard, host-side agent-managed, box-dials-out to
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a public endpoint VM running the offsite PBS. Evidence:
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`documentation/audits/SPIKE-connectivity-wireguard-2026-07-03.md`; design-of-record + slice
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roadmap: `documentation/architecture/06-offsite-connectivity.md` (MVP = slices S1→S2→S3, then
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S4). Go-live standard = **local backups only** (that is what the Part C command configures);
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offsite DR is retrofitted later via `--rescope-acl` / `--acl-storages` without reinstalling.
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---
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