hub v0.38.0: offsite SLICE 2 — capture the box host-key fingerprint

Descriptor.HostFingerprint (SHA256, non-secret), captured at provision via an
x/crypto/ssh keyscan (SSHHostKeyScanner — dials :23, grabs the host key from the
handshake, no ssh binary). Fail-closed: nil scanner or scan failure → error (don't
serve a descriptor the controller can't verify). Pairs with controller v0.106.0
which re-scans + refuses on mismatch (no blind TOFU).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

CONTEXT.md

@@ -3,6 +3,16 @@
 > Created with the REUSE.md rollout (2026-07-03). Authoritative history: `hub/CHANGELOG.md` (hub),
 > `website/CHANGELOG.md`, `scripts/CHANGELOG.md`; end-of-task detail in `REPORT.md`.
 
+- **2026-07-09 — offsite provisioning SLICE 2 (hub v0.38.0 + controller v0.106.0).** The controller apply-bridge:
+  on startup it reconciles the hub-served `offsite:` descriptor into a key-only offbox target
+  (`controller/internal/offsiteapply.Bridge`) — **verify-pin the box host key against the hub-captured
+  `host_fingerprint` (no blind TOFU)** → consume the one-time password (single-use) → `sshpass ssh-copy-id -s -f`
+  install → configure offbox → `EscrowState="pending"` → persist a descriptor-hash marker. Idempotent +
+  fail-safe; both red-proofs green. Hub v0.38.0 adds `Descriptor.HostFingerprint` captured via an
+  `x/crypto/ssh` keyscan (fail-closed). **NOT yet live-applied** — supervised end-to-end (hub provisions on
+  the new pool box → controller apply) is the next runbook, gated on the hub's new scoped `HETZNER_TOKEN`.
+  NEXT: SLICE 3 (escrow auto-confirm), SLICE 4 (soft-quota).
+
 - **2026-07-09 — hub offsite provisioning SLICE 1 (hub v0.37.0).** The hub can now provision the offsite tier
   on operator enable: `internal/hetznerapi` (typed client, base **api.hetzner.com/v1**, `CloudAPI` interface +
   exported `Fake`, `WaitAction`), `internal/offsite` (`Provisioner.ProvisionOffsite` — idempotent by label

hub/CHANGELOG.md

@@ -1,5 +1,17 @@
 # Felhom Hub — Changelog
 
+## v0.38.0 — offsite provisioning SLICE 2 (hub side): capture the box host-key fingerprint (2026-07-09)
+
+Pairs with controller v0.106.0. So the controller can VERIFY the box identity instead of blind-TOFU, the hub
+captures the box's SSH host-key fingerprint at provision and serves it in the descriptor.
+
+- `internal/offsite`: `Descriptor.HostFingerprint` (SHA256:…, non-secret). `ProvisionOffsite` now captures it
+  after the resource is ready via a `HostKeyScanner` seam (`SSHHostKeyScanner`, x/crypto/ssh — dials port 23
+  and grabs the host key from the handshake, no ssh binary needed). **Fail-closed:** a nil scanner or a scan
+  failure returns an error (don't serve a descriptor the controller can't verify). The controller re-scans and
+  refuses on mismatch (v0.106.0).
+- Tests: descriptor carries the fingerprint from a faked scanner; a scan failure fails-closed.
+
 ## v0.37.0 — offsite provisioning SLICE 1: Hetzner Cloud-API client + provisioning core (2026-07-09)
 
 Slice 1 of the offsite-provisioning epic. On operator enable, the hub provisions a Hetzner storage-box

hub/cmd/hub/main.go

@@ -279,7 +279,7 @@ func main() {
 		}
 		client := hetznerapi.NewClient(func() string { return os.Getenv("HETZNER_TOKEN") })
 		webServer.SetOffsiteProvisioner(&offsite.Provisioner{
-			API: client, Store: dataStore, PoolBoxID: poolBoxID, Location: location, Logger: logger,
+			API: client, Store: dataStore, Scanner: offsite.SSHHostKeyScanner{}, PoolBoxID: poolBoxID, Location: location, Logger: logger,
 		})
 		logger.Printf("[INFO] Offsite provisioning enabled (pool_box=%d, location=%s)", poolBoxID, location)
 	}

hub/internal/offsite/offsite.go

@@ -35,6 +35,14 @@ type Descriptor struct {
 	RepoPath string `json:"repo_path,omitempty"` // /home/<repo>
 	QuotaGB  int    `json:"quota_gb,omitempty"`  // shared soft-quota (Felhom-enforced; no native lever)
 	BoxType  string `json:"box_type,omitempty"`  // dedicated (Hetzner-hard quota via the type)
+	// HostFingerprint is the box's SSH host-key fingerprint (SHA256:…), captured at provision so the
+	// controller VERIFIES the box identity instead of blind-TOFU (SLICE 2). Non-secret.
+	HostFingerprint string `json:"host_fingerprint,omitempty"`
+}
+
+// HostKeyScanner returns a box's SSH host-key fingerprint (SHA256:…). Seam'd so tests inject a fake.
+type HostKeyScanner interface {
+	Fingerprint(ctx context.Context, host string, port int) (string, error)
 }
 
 // Input is the operator's offsite choice.
@@ -49,8 +57,9 @@ type Input struct {
 type Provisioner struct {
 	API       hetznerapi.CloudAPI
 	Store     *store.Store
-	PoolBoxID int64  // the shared-pool storage-box id (e.g. 611421)
-	Location  string // dedicated-box location, e.g. "fsn1"
+	Scanner   HostKeyScanner // captures the box host-key fingerprint (fail-closed if nil/scan-fails)
+	PoolBoxID int64          // the shared-pool storage-box id (e.g. 611421)
+	Location  string         // dedicated-box location, e.g. "fsn1"
 	Logger    *log.Logger
 }
 
@@ -75,14 +84,34 @@ func (p *Provisioner) ProvisionOffsite(ctx context.Context, customerID string, i
 	if !in.Enabled {
 		return &Descriptor{Enabled: false}, nil
 	}
+	var d *Descriptor
+	var err error
 	switch in.Type {
 	case "shared":
-		return p.provisionShared(ctx, customerID, in)
+		d, err = p.provisionShared(ctx, customerID, in)
 	case "dedicated":
-		return p.provisionDedicated(ctx, customerID, in)
+		d, err = p.provisionDedicated(ctx, customerID, in)
 	default:
 		return nil, fmt.Errorf("offsite: unknown type %q (want shared|dedicated)", in.Type)
 	}
+	if err != nil {
+		return nil, err
+	}
+	// Capture the box host-key fingerprint so the controller verifies (no blind TOFU). Fail-closed: don't
+	// serve a descriptor the controller can't verify. Applies to both fresh and idempotent paths.
+	if p.Scanner == nil {
+		return nil, fmt.Errorf("offsite: no host-key scanner configured (cannot capture the pin)")
+	}
+	port := d.Port
+	if port == 0 {
+		port = sftpPort
+	}
+	fp, err := p.Scanner.Fingerprint(ctx, d.Host, port)
+	if err != nil {
+		return nil, fmt.Errorf("offsite: host-key scan %s: %w", d.Host, err)
+	}
+	d.HostFingerprint = fp
+	return d, nil
 }
 
 func (p *Provisioner) provisionShared(ctx context.Context, customerID string, in Input) (*Descriptor, error) {

hub/internal/offsite/offsite_test.go

@@ -23,7 +23,17 @@ func newTestProvisioner(t *testing.T) (*Provisioner, *hetznerapi.Fake, *store.St
 	}
 	t.Cleanup(func() { st.Close() })
 	fake := hetznerapi.NewFake()
-	return &Provisioner{API: fake, Store: st, PoolBoxID: 611421, Location: "fsn1", Logger: log.New(io.Discard, "", 0)}, fake, st
+	return &Provisioner{API: fake, Store: st, Scanner: &fakeScanner{fp: "SHA256:testfp"}, PoolBoxID: 611421, Location: "fsn1", Logger: log.New(io.Discard, "", 0)}, fake, st
+}
+
+// fakeScanner returns a fixed fingerprint (or an error) — no live SSH in tests.
+type fakeScanner struct {
+	fp  string
+	err error
+}
+
+func (f *fakeScanner) Fingerprint(_ context.Context, _ string, _ int) (string, error) {
+	return f.fp, f.err
 }
 
 // Scenario A — enable shared → sub-account provisioned, descriptor built, one-time password stored (NOT in
@@ -62,6 +72,30 @@ func TestProvision_Shared(t *testing.T) {
 	}
 }
 
+// Part 0 — the descriptor carries the box host-key fingerprint (captured at provision).
+func TestProvision_HostFingerprint(t *testing.T) {
+	p, _, _ := newTestProvisioner(t)
+	d, err := p.ProvisionOffsite(context.Background(), "cust-fp", Input{Enabled: true, Type: "shared", QuotaGB: 10})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if d.HostFingerprint != "SHA256:testfp" {
+		t.Fatalf("descriptor must carry the host fingerprint, got %q", d.HostFingerprint)
+	}
+}
+
+// Part 0 — a host-key scan failure is fail-closed (no descriptor served).
+func TestProvision_ScanFailClosed(t *testing.T) {
+	p, _, st := newTestProvisioner(t)
+	p.Scanner = &fakeScanner{err: errors.New("keyscan timeout")}
+	d, err := p.ProvisionOffsite(context.Background(), "cust-sf", Input{Enabled: true, Type: "shared", QuotaGB: 10})
+	if err == nil || d != nil {
+		t.Fatalf("a keyscan failure must fail-closed, got d=%+v err=%v", d, err)
+	}
+	// the resource may have been created + password stored, but no verifiable descriptor is served
+	_ = st
+}
+
 // Scenario B — enable dedicated → box provisioned.
 func TestProvision_Dedicated(t *testing.T) {
 	p, fake, st := newTestProvisioner(t)

hub/internal/offsite/scanner.go

@@ -0,0 +1,57 @@
+package offsite
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"time"
+
+	"golang.org/x/crypto/ssh"
+)
+
+// SSHHostKeyScanner captures a box's SSH host-key fingerprint by dialing port 23 and grabbing the host key
+// from the handshake — BEFORE any auth (there is no credential; the callback aborts once the key is seen).
+// This is the hub-side equivalent of `ssh-keyscan`, using x/crypto/ssh so no ssh binary is needed in the
+// container. The captured fingerprint is non-secret (a public host-key identifier).
+type SSHHostKeyScanner struct {
+	Timeout time.Duration
+}
+
+// errCaptured aborts the handshake once we have the host key (we never intended to authenticate).
+var errCaptured = errors.New("host key captured")
+
+func (s SSHHostKeyScanner) Fingerprint(ctx context.Context, host string, port int) (string, error) {
+	timeout := s.Timeout
+	if timeout == 0 {
+		timeout = 10 * time.Second
+	}
+	var fp string
+	cfg := &ssh.ClientConfig{
+		User:    "felhom-keyscan",
+		Auth:    nil, // no auth — we abort in the host-key callback
+		Timeout: timeout,
+		HostKeyCallback: func(_ string, _ net.Addr, key ssh.PublicKey) error {
+			fp = ssh.FingerprintSHA256(key)
+			return errCaptured
+		},
+	}
+	d := net.Dialer{Timeout: timeout}
+	conn, err := d.DialContext(ctx, "tcp", fmt.Sprintf("%s:%d", host, port))
+	if err != nil {
+		return "", fmt.Errorf("dial: %w", err)
+	}
+	defer conn.Close()
+	// ssh.NewClientConn runs the handshake; our callback captures the key then returns errCaptured, so this
+	// returns an error — but fp is set. Any OTHER error (or no key) is a real failure.
+	c, chans, reqs, herr := ssh.NewClientConn(conn, host, cfg)
+	if c != nil {
+		c.Close()
+	}
+	_ = chans
+	_ = reqs
+	if fp != "" {
+		return fp, nil
+	}
+	return "", fmt.Errorf("host-key handshake: %w", herr)
+}