# Architecture Part 3 — The Host Agent

> Status: design draft (decision content). To be grounded by Claude Code against
> `docs/proxmox-platform.md` and `docs/architecture/02-controller-module-map.md`,
> then placed at `docs/architecture/03-host-agent.md`.
>
> Builds on Part 1 (`01-topology-and-trust.md`) and Part 2 (`02-controller-module-map.md`).
> Where this doc and the locked decisions disagree, the locked decisions win and this
> draft is wrong — flag it.

## 1. Purpose & scope

The **host agent** is the operator-tier component that runs on each Proxmox host and
owns *all* Proxmox interaction. It is the trusted host actor: it provisions and restores
guests, manages host storage, orchestrates backups and restore-tests, watches the host
and the tunnel, talks to the hub, and exposes a narrow local API to the in-guest
controllers it deploys.

It is the privileged tier. The controller deliberately holds **no** Proxmox credentials
(Part 1) — the privilege the controller shed by losing `storage/` did not disappear, it
**moved here**. That makes the agent's hardening and blast-radius discipline the most
security-sensitive part of the platform.

The agent manages a **set** of guests on its host (usually one customer = one guest, but
the multi-tenant/company case is not precluded — the agent's data model is per-host,
N-guests, never "the guest").

## 2. Responsibilities (and explicit non-responsibilities)

Owns:

1. **Proxmox lifecycle** — create/start/stop/destroy guests, snapshots, storage allocation. Via a scoped Proxmox API token (the **`FelhomAgent` operator role** — `proxmox-platform.md` §3.6, validated Phase 3 B3) for everything the API covers; raw host ops only where unavoidable.
2. **Storage management** — attach/classify targets, reconcile the storage manifest, mount USB-by-UUID, present mounts into guests.
3. **Backup/restore orchestration** — vzdump to the tiers, PBS, snapshot management, and the **self-restore-test**.
4. **Host & tunnel monitoring** — host metrics, guest up/down, storage-target status, and `cloudflared` health; reports the host domain to the hub.
5. **Provisioning** — provision a guest **by restoring the golden base image** (§9), deploy the controller into it, hand it its bootstrap config; also **build and refresh the golden base image** itself.
6. **Hub control loop** — poll for desired state + signed jobs, reconcile, execute, report, heartbeat.
7. **Local API** — the per-guest authorization gate the controller calls.
8. **Self-update** — update itself (carefully — it is a host service) and update the controllers it owns.

Explicitly does **not**:

- Serve application traffic or sit in the data path. **Control plane, not data plane**: if the agent dies, apps keep serving (Docker + LXC run without it); only *management* degrades — no new backups, no provisioning, hub loses the heartbeat.
- Hold or proxy customer application data.
- Run inside a guest. It is the thing that recovers guests and the host; it cannot be one of them.
- Manage **geo-restriction / the Cloudflare API**. Geo is hub-owned: the customer sets it in the controller UI, the controller reports the geo desired-state to the hub, and the **hub** (holding the CF API token) reconciles the WAF (S4). The agent manages only the *tunnel* service (`cloudflared`, §3/§5), never WAF rules.

## 3. Process model & host integration

- **Native Go binary, systemd service** on the host: boot-start, `Restart=always`, systemd watchdog (kill+restart on hang), journald logging, resource limits.
- **Root-minimized (boundary settled — Phase 3 B3).** The agent runs as a **non-root** service user with the scoped `FelhomAgent` token for all API-covered work + a **narrow `sudoers` allowlist** for true host ops. Per Phase 3 (B3) the boundary is settled: the entire per-customer guest lifecycle — provision (by restore, §9), config, start/stop, snapshot, backup, **restore**, destroy — is token-covered. Genuine OS-root is confined to: (1) building/refreshing the **golden base image** (`keyctl` create is `root@pam`-only — one-time at enrollment + a maintenance cadence, §9); (2) **host mounts** (USB mount-by-UUID, systemd mount units / fstab); (3) **SMART / hardware sensors**. Root therefore never sits on the per-customer path. See `proxmox-platform.md` §3.6 for the role + boundary table.
- **`cloudflared` is a separate systemd service**, not embedded in the agent. This is what makes the data path survive control-plane death by construction. The agent **manages and health-watches** it (see §5) but the tunnel does not live or die with the agent process.

## 4. Control model — reconcile + signed destructive ops

Two channels, split by **reversibility**, not by transport.

**(a) Desired-state reconciliation — steady state.**
The hub holds desired state for the host: which guests should exist (and at what spec),
the storage manifest, backup/retention policies, controller image versions. The agent
runs a reconcile loop converging actual Proxmox state → desired: idempotent, self-healing,
and tolerant of missed polls (drift is corrected on the next loop). Provisioning retries,
re-attach of a flapping USB target, redeploy of a crashed controller — all fall out of
reconciliation for free.

**(b) Signed one-shot jobs — operator actions.**
Restore-now, decommission, force-backup, break-glass-enable. Discrete, run-once
(idempotency key), written to the customer-visible audit log, and **outside** the reconcile
loop — they are point-in-time and often destructive, and a reconciler must never re-run a
restore because it "sees drift." A one-shot job names a **target** ("restore guest X from
snapshot S"), not a procedure; the agent owns the *how*.

**The reversibility gate (security-critical).**
"Signed jobs resist hub compromise" only holds if the agent also distrusts hub-supplied
*desired state* for destructive changes. The gate is by **provenance + data-bearing-ness, not
by verb**:

- **The reconciler MAY act without an operator signature** when: (a) creating/starting/restarting; (b) destroying resources it created earlier **within the same journaled transaction** (compensating rollback, §10); (c) destroying resources it **tagged ephemeral/scratch** (e.g. restore-test scratch guests, §8). The ephemeral/scratch tag is **agent-internal provenance and is never accepted from the hub** — else a compromised hub could relabel a data-bearing guest as scratch to walk the gate.
- **An operator signature is always required** to destroy/overwrite any resource holding the only/primary copy of customer data — live-guest destroy, storage detach/wipe, restore-overwrite, decommission — *regardless of whether it arrives as a job or as a desired-state delta*. A compromised hub cannot forge them because the signing key is **not held by the hub** (it lives with the operator / a separate signing path; the hub only queues opaque signed blobs).
- **Healing a crashed controller is non-destructive by construction:** it is reconstructable from its image + the guest's persistent volume, so "redeploy" = restart the LXC / `docker compose up -d` **inside the existing guest** — never a guest destroy. (v0.33 precedent: `watchdog.go` restarts stopped stacks, it never destroys the guest.)

Signed payloads carry a **nonce + expiry** (anti-replay: a captured "restore" job cannot be
re-injected later) and a target binding (host + guest id) so a signature can't be retargeted.
Notification-on-destructive-op is an **audit signal, never the guard** — a compromised hub
could both issue and suppress the notice, which is exactly why the *signature* (not the
notification) is the control.

## 5. Hub ↔ agent protocol (host domain)

**Box-initiated poll.** The hub never connects inbound. Each poll cycle exchanges:

- **Up:** heartbeat + a host-domain state report — host CPU/RAM/disk, per-guest up/down + spec, storage-target status (USB connected? NFS/CIFS reachable? PBS reachable?), last backup per target, last restore-test result, `cloudflared` health, agent + controller versions, audit-log tail.
- **Down:** the current desired state, any pending signed one-shot jobs, and config (poll interval, update window, policy changes).

**Dead-man's-switch (essential, not optional).** In a box-initiated model the heartbeat
*is* the liveness signal — a box that stops checking in is otherwise invisible. The hub
alerts the operator when an agent misses its expected check-in window. This is the worst
failure mode for a managed service, so it gets first-class treatment hub-side.

**Break-glass.** Standing inbound control is off. But when the poll loop *itself* is wedged
(agent hung, host sick) you cannot fix it through the poll loop. So there is an explicit,
**off-by-default, customer-consented, fully-audited** emergency path: SSH to the host via
the Cloudflare Tunnel behind Cloudflare Access (or on-site). Enabling it is itself a signed,
logged operation; it auto-expires.

## 6. Agent ↔ controller local API

The controller (in its LXC) reaches the agent (on the host) over the local bridge.

- **Transport:** HTTPS to the host's bridge IP on a fixed port.
- **Auth:** a per-guest local token, minted by the agent when it deploys the controller and written into the guest's bootstrap config. The agent maps token → guest and **authorizes per guest**: a controller can only act on *its own* guest. This is the agent acting as the per-guest authorization gate from Part 1.
- **Surface (minimal, all scoped to the caller's own guest):**
  - `GET /storage` — mounts available to this guest and their **class** (fast/slow), so the controller can place hot vs bulk volumes per `.felhom.yml`. (The agent owns the actual mounts; the controller just binds to the paths it's given.)
  - `POST /snapshot` — snapshot *this* guest (the snapshot-before-deploy primitive).
  - `POST /rollback` — roll *this* guest back to a named snapshot (post-deploy failure recovery).
  - `POST /backup` — request a backup-now of *this* guest (enqueued; non-destructive).
  - `GET /backup/due` — whether a policy-scheduled backup is due for *this* guest, so the controller can quiesce then call `POST /backup` (the app-consistent path, §8).
  - `GET /backup/status`, `GET /restore-test/status` — read-only status for the controller's UI.

Note what is *absent*: nothing here lets a controller touch another guest, the host, storage
attachment, or restore-overwrite. Destructive/cross-guest power stays operator-signed (§4).

A controller can only `POST /rollback` (or snapshot/backup) **its own** guest — the agent maps
token → guest and authorizes per guest, so a compromised controller's blast radius is
**self-scoped and bounded** to its own guest.

## 7. Storage manifest & reconciliation

The manifest is the load-bearing contract. It absorbs the **persisted** disk-state fields that
`settings.StoragePath` carries today **and adds** `durable_id`/UUID — today the controller
re-derives the UUID from fstab each boot (Part 2 / Phase-3), so persisting it is an
improvement. Held in the hub, reconciled by the agent.

Per target:

| field | meaning |
|---|---|
| `type` | `local-dir` / `usb` / `nfs` / `cifs` / `pbs` |
| `durable_id` | UUID (USB), `server:export` (NFS/CIFS), `repo+fingerprint` (PBS) — survives box loss |
| `class` | `fast` or `slow`, set **once at attach**, with an IOPS marker; no runtime speed-test |
| `role` | `primary` / `vzdump-target` / `pbs-offsite` / `bulk-data` |
| `creds` | encrypted (NFS/CIFS/PBS); USB has none |
| `policy` | schedule + retention for this target |
| `state` | `attached` / `disconnected` / `decommissioned` |

Reconciliation: ensure each `attached` target is mounted (USB-by-UUID via the sudoers
allowlist), each Proxmox storage entry matches, and `disconnected` targets are surfaced to
the hub (the storage watchdog — detect a USB drop in seconds, not at the next health cycle).

**Placement is per-volume, not per-app.** Hot volumes (DB/config) → a `fast` target,
**enforced**; bulk volumes (media) → may live on `slow`, declared in `.felhom.yml`.

A `bulk` volume **MUST** be realized as a `backup=0` **volume mount point** (or an external
bind mount) — **never** a Docker named volume in rootfs, which `vzdump` always captures
(verified, `phase3-findings.md` B2). Proven recipe: attach
`-mpN <storage>:<size>,mp=/mnt/bulk,backup=0`, then
`docker volume create --driver local -o type=none -o o=bind -o device=/mnt/bulk <vol>` (or a
compose bind). The per-volume placement component (Part 2 §5(2)) enforces this at deploy. The
**DR consequence** of excluding bulk is covered in §8.

**Field re-homing (from `settings.StoragePath`, Part 2):** `Label` → manifest (canonical);
`IsDefault`/`Schedulable` → manifest `policy`; `MigratedTo` + decommission → manifest `state`;
`StoppedStacks` → the **controller's `settings`** (app-domain: which apps to restart on
reconnect, not a host concern).

## 8. Backup/restore orchestration

Tiers double as backup *and* restore-source priority (fastest surviving source first),
per Part 1: **snapshot** (LVM-thin, transient, whole-guest rollback — not a backup) →
**local second storage** (vzdump to dir/NFS/CIFS) → **PBS offsite** (the DR substrate).

- **Quiescing (controller-driven for app-consistency):** an LXC has no fsfreeze
  (`proxmox-platform.md` §4.2), so app-consistency is the controller's job: it learns a backup
  is due (`GET /backup/due`, §6, or via its hub channel) → **quiesces** the app stack →
  `POST /backup` → polls `GET /backup/status` → unquiesces. **An agent-initiated vzdump is
  crash-consistent only** (there is no inbound-to-guest channel to trigger a quiesce — §3/§5).
  Every Proxmox op is async → the agent polls `task exitstatus`, never trusts the POST return.
- **Bulk volumes have no DR coverage from the guest vzdump** — they are excluded (§7). Every
  `bulk` volume needs an explicit own-backup decision: its own backup target per the manifest
  `policy`, **or deliberately none** when the data is re-downloadable (customer informed). On
  host-loss, un-backed-up bulk is gone; a **bind-mounted** bulk volume re-attaches only on the
  *same* host, so cross-host DR needs the separate backup. A deliberate per-volume choice,
  never a silent loss.
- **Key custody (PBS):** the **live** PBS key sits on the box so the agent can both back up
  *and* run restore-tests. The hub holds only the **recovery-code-wrapped escrow** copy it
  cannot open (zero-knowledge default). So: the box can restore-test; the operator cannot
  read the data; the customer's offsite recovery code is the irreducible residual.
- **Self-restore-test:** the closing of the "tested restore is the critical gap" theme. The
  agent periodically restores a backup into a **throwaway scratch guest**, boots it, runs
  health checks, reports pass/fail, and tears it down. Zero-knowledge backups can *only* be
  restore-tested by the box (the operator lacks the key) — so this lives in the agent by
  necessity, not just convenience. Integrity-verify (cheap, ciphertext-level) runs more often
  as the lighter check.

## 9. Provisioning & DR flows

**Provisioning (reconcile-driven, by restore).** Fresh creation of a Docker-capable LXC needs
the `keyctl=1` feature flag, which Proxmox permits only for `root@pam` (Phase 3, B3) — not the
scoped token. But a token-authorized **restore preserves `keyctl`** (Phase 3, B3), so the agent
provisions **by restoring a golden base image**, never by `pct create` on the per-customer path:

- A **golden base archive** — minimal Debian + Docker, `nesting=1,keyctl=1`, overlayfs — is
  built once as `root@pam` **at enrollment** (when the agent legitimately holds root to mint its
  Proxmox token) and refreshed on a maintenance cadence. This is the one place `keyctl`/root
  provisioning lives — off the per-customer path.
- To provision guest G: restore the golden archive → new VMID (token-covered: `VM.Allocate` +
  `Datastore.AllocateSpace`; `keyctl` preserved) → reset identity (MAC/hostname) → size the guest
  (CPU/mem config + `pct resize` rootfs, token-covered) → attach storage mounts per the manifest
  → deploy the controller → hand it bootstrap config. A mid-flight failure is journaled and
  compensating-rolled-back (destroy the just-restored guest — allowed without a signature per §4,
  same-transaction provenance).

**Unified bring-up primitive.** Provisioning and DR-restore share the same token-covered front
half — *restore an archive → reset identity* — and differ only in the archive and the back half:
provisioning restores the **golden base** then deploys a fresh controller; DR-restore restores
the **customer's backup** (already containing controller + data), brings it up, and reattaches
external storage. One code path, exercised by every restore-test (§8).

**Guest loss.** Agent restores G from the fastest surviving tier and resets identity
(MAC/hostname) so the restored guest rejoins cleanly — this *is* the unified restore primitive
above (customer-backup archive, DR back half).

**Host/hardware loss.** Re-enroll the new host in **restore mode**; the hub — the durable
source of truth that survives box death — hands the new agent the existing identity, PBS
namespace, tunnel token, storage manifest, and a restore directive. Tunnel is reused from
the hub record, so DNS stays intact.

## 10. Concurrency, crash-safety, idempotency

- **Per-guest serialization.** Reconcile, one-shot jobs, and local-API calls all feed a
  work queue that serializes mutations **per guest** (Proxmox dislikes concurrent conflicting
  ops on the same guest). Independent guests proceed in parallel.
- **Operation journaling.** Multi-step async ops (provision, restore, controller-update, agent
  self-update) are journaled with their in-flight Proxmox task ids. On agent restart, the
  journal is replayed: resume-or-rollback, so a crash mid-restore never leaves a corrupt or
  half-built guest.
- **Idempotency keys** on one-shot jobs (run-once across retries and restarts).

## 11. Self-update

- **Agent (the hard case — a host service, no snapshot-rollback).** **A/B layout:** download →
  verify signature → stage as the inactive slot → flip a `current → good|new` symlink → restart.
  **Revert authority lives outside the swapped binary** — `Restart=always` alone just
  crash-loops a bad binary — so a **separate health-gate** (a systemd oneshot `ExecStartPost`
  probe, or a tiny supervisor unit) flips `current` back to last-good and restarts on a failed
  health window. The new version is **committed as "good" only after a clean health window**.
  Triggered by a hub signed job within the update window; manual always allowed. Journaled (§10).
- **Controller (the easy case — it's a guest).** The agent owns the controller's lifecycle,
  so the **agent updates the controller**: snapshot-before-update (free rollback, because the
  controller *is* a snapshottable guest) → pull new image → redeploy → health-check → rollback
  on failure. This resolves the Part-2 `selfupdate/` open: the controller is **agent-managed**,
  not self-updating; the controller's old self-update path is removed.

## 12. Secrets at rest on the host

The agent holds, root-only on the host fs: the scoped Proxmox token, the hub API key, the
operator's **public** verify key (for §4 signatures — public, low-risk), the Cloudflare
tunnel token, encrypted storage creds (NFS/CIFS/PBS), and the **live PBS key**. The privilege
and the secret footprint that left the controller now concentrate here — which is the whole
argument for §3's root-minimization and a small, auditable agent.

## 13. Open items / what this unblocks

Resolved here: tunnel placement (host, agent-managed, own systemd service), the
reconcile-vs-jobs fork (hybrid, gated by reversibility), agent process model, self-update
ownership, the local-API surface, the storage-manifest schema, **provision-by-restore**, and
the **root-vs-API boundary** (Phase 3, B3).

Still open:

- Multi-tenant **resource fairness** on a shared host (per-guest cgroup limits, noisy-neighbor) — deferred to the company-case pass.
- Operator-side **signing tooling** — where the operator signing key lives operationally and how a destructive op gets signed without undue friction (offline key vs. a small signing service; the security floor is "not in the hub").
- Hub-side **desired-state editing UX** and the host-domain report schema details — belong to the hub architecture doc.
- **Golden base image** refresh cadence + fleet versioning — who triggers a rebuild, how the per-host image version is tracked (operational detail, not blocking; §9).

This doc hands the implementation three contracts it was waiting on:

1. the **local-API surface** (§6) → the controller's NEW local-API client, snapshot-before-deploy, and self-restore-test wiring (Part 2);
2. the **storage-manifest schema** (§7) → the `settings.StoragePath` reshape and per-volume hot/bulk placement (Part 2);
3. the **backup contract** (§7–8) → the destination for the app-data-backup package extracted in the Part-2 refactor.

---

## Changelog — design-review + Phase-3 fold-in (2026-06-08)

- **NEW provision-by-restore** (§9): the agent provisions by **restoring a golden base image**
  (token-covered, preserves `keyctl`), never `pct create` on the per-customer path; one unified
  restore primitive shared with DR. §2 responsibility + §3 boundary updated.
- **B3** (§2/§3): replaced "Phase-1 minimal role" with the validated **`FelhomAgent`** operator
  role; root-vs-API boundary **settled** (root only for golden-image build, host mounts, SMART).
- **B1** (§4): reversibility gate rewritten as **provenance + data-bearing** (scratch tag is
  agent-internal, never hub-supplied; crashed-controller heal is non-destructive in-place).
- **B2** (§7/§8): validated bulk-as-`backup=0`-mountpoint recipe + the **bulk-DR consequence**
  (excluded bulk needs its own backup decision).
- **S1** (§6/§8): `GET /backup/due` added; controller-driven quiescing; agent vzdump is
  crash-consistent only. **S2** (§10/§11): A/B self-update with external revert authority;
  controller-update + agent self-update journaled. **S3** (§7): `StoragePath` field re-homing.
  **S4:** geo non-responsibility added (§2). **M2** (§7): manifest "absorbs + adds durable_id".
  **§6:** rollback is self-scoped/bounded. **§13:** golden-image refresh cadence added as open.