feat(api): rate limiting, webhook telemetry, server fingerprints, region

- per-client-IP token bucket on POST /api/traverse (EXPLOREDNS_RATE_LIMIT,
  default 30/1h; direct localhost exempt, proxied clients are not)
- optional usage webhooks (EXPLOREDNS_WEBHOOK_URL): start/complete JSON
  events, fire-and-forget with 5s timeout + one retry so a dead receiver
  never delays a job
- post-traversal version.bind fingerprinting exposed at
  GET /api/traverse/{id}/servers (pending until ready) and announced via
  an SSE "servers" event; never delays job completion
- /api/health reports the serving Fly region (FLY_REGION) for observing
  anycast routing from a roaming client

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/api/handler.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io/fs"
+	"net"
 	"net/http"
 	"os"
 	"sort"
@@ -16,6 +17,7 @@ import (
 
 	"gitea.hansenits.com.au/hits/ExploreDNS/internal/config"
 	idns "gitea.hansenits.com.au/hits/ExploreDNS/internal/dns"
+	"gitea.hansenits.com.au/hits/ExploreDNS/internal/fingerprint"
 	"gitea.hansenits.com.au/hits/ExploreDNS/internal/traverse"
 )
 
@@ -38,6 +40,14 @@ const (
 	defaultMaxRunningJobs = 8
 )
 
+// Fingerprinting runs after a traversal reaches a terminal state: bounded
+// concurrency across the unique server IPs, with its own overall deadline so
+// a timed-out or cancelled job context never blocks the server list.
+const (
+	fingerprintConcurrency = 8
+	fingerprintTimeout     = 15 * time.Second
+)
+
 // TraverseRequest is the JSON body for POST /api/traverse.
 type TraverseRequest struct {
 	Domain   string `json:"domain"`
@@ -106,6 +116,14 @@ type Summary struct {
 	ByStatus []SummaryStatus `json:"by_status,omitempty"`
 }
 
+// ServerInfo is one (server name, IP) pair queried during a traversal plus
+// its version.bind fingerprint ("" when the server didn't answer the probe).
+type ServerInfo struct {
+	Name    string `json:"name"`
+	IP      string `json:"ip"`
+	Version string `json:"version"`
+}
+
 // TraversalJob holds all state for a single asynchronous traversal.
 type TraversalJob struct {
 	ID        string          `json:"id"`
@@ -115,13 +133,18 @@ type TraversalJob struct {
 	Results   []ResultItem    `json:"results,omitempty"`
 	Summary   *Summary        `json:"summary,omitempty"`
 	Progress  []ProgressEvent `json:"progress,omitempty"`
+	Servers   []ServerInfo    `json:"servers,omitempty"`
 	Error     string          `json:"error,omitempty"`
 	StartedAt time.Time       `json:"started_at"`
 	DoneAt    *time.Time      `json:"done_at,omitempty"`
 
-	mu     sync.RWMutex
+	mu       sync.RWMutex
-	subs   []chan ProgressEvent
+	subs     []chan ProgressEvent
-	cancel context.CancelFunc
+	cancel   context.CancelFunc
+	clientIP string
+	// serversDone flips once the post-traversal fingerprinting step has
+	// stored Servers (or was skipped); until then GET …/servers is pending.
+	serversDone bool
 }
 
 // subscribeSnapshot atomically registers a subscriber and snapshots the
@@ -224,24 +247,42 @@ func (s *store) cleanup() {
 	}
 }
 
+// versionQuerier is the subset of fingerprint.Fingerprinter the handler
+// uses; tests substitute a fake so no probes leave the process.
+type versionQuerier interface {
+	Query(ctx context.Context, ip net.IP) string
+}
+
 // Handler wires together the HTTP routes and the job store.
 type Handler struct {
 	st         *store
 	mux        *http.ServeMux
 	jobTimeout time.Duration
 	maxRunning int
+	version    string
+	limiter    *rateLimiter
+	webhook    *webhookReporter
+	// fp fingerprints server IPs after each traversal; shared across jobs
+	// so its per-IP cache is reused.
+	fp versionQuerier
 }
 
 func newHandler(ctx context.Context) *Handler {
+	limit, window := parseRateLimit(os.Getenv("EXPLOREDNS_RATE_LIMIT"))
 	h := &Handler{
 		st:         newStore(),
 		mux:        http.NewServeMux(),
 		jobTimeout: envDuration("EXPLOREDNS_JOB_TIMEOUT", defaultJobTimeout),
 		maxRunning: envInt("EXPLOREDNS_MAX_JOBS", defaultMaxRunningJobs),
+		version:    "dev",
+		limiter:    newRateLimiter(limit, window),
+		webhook:    newWebhookReporter(os.Getenv("EXPLOREDNS_WEBHOOK_URL")),
+		fp:         fingerprint.New(),
 	}
 
 	h.mux.HandleFunc("POST /api/traverse", h.startTraversal)
 	h.mux.HandleFunc("GET /api/traverse/{id}/stream", h.streamTraversal)
+	h.mux.HandleFunc("GET /api/traverse/{id}/servers", h.getServers)
 	h.mux.HandleFunc("GET /api/traverse/{id}", h.getTraversal)
 	h.mux.HandleFunc("GET /api/health", h.health)
 
@@ -255,6 +296,7 @@ func newHandler(ctx context.Context) *Handler {
 				return
 			case <-t.C:
 				h.st.cleanup()
+				h.limiter.sweep()
 			}
 		}
 	}()
@@ -270,11 +312,24 @@ func (h *Handler) registerStatic(sub fs.FS) {
 
 // health handles GET /api/health.
 func (h *Handler) health(w http.ResponseWriter, _ *http.Request) {
-	writeJSON(w, http.StatusOK, map[string]string{"status": "ok"})
+	body := map[string]string{"status": "ok", "version": h.version}
+	// On Fly.io this identifies which machine served the request —
+	// useful for observing anycast routing and auto-start behaviour.
+	if region := os.Getenv("FLY_REGION"); region != "" {
+		body["region"] = region
+	}
+	writeJSON(w, http.StatusOK, body)
 }
 
 // startTraversal handles POST /api/traverse.
 func (h *Handler) startTraversal(w http.ResponseWriter, r *http.Request) {
+	ip := clientIP(r)
+	if h.limiter != nil && !rateLimitExempt(r) && !h.limiter.allow(ip) {
+		writeError(w, http.StatusTooManyRequests,
+			fmt.Sprintf("rate limit exceeded: %s per client IP", h.limiter))
+		return
+	}
+
 	r.Body = http.MaxBytesReader(w, r.Body, 1<<20) // 1 MB limit
 	var req TraverseRequest
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
@@ -310,9 +365,20 @@ func (h *Handler) startTraversal(w http.ResponseWriter, r *http.Request) {
 		QueryType: queryType,
 		StartedAt: time.Now(),
 		cancel:    cancel,
+		clientIP:  ip,
 	}
 	h.st.set(job)
 
+	h.webhook.send(webhookEventStart, webhookStartEvent{
+		Event:     webhookEventStart,
+		ID:        job.ID,
+		Domain:    job.Domain,
+		QueryType: job.QueryType,
+		AllRoots:  req.AllRoots,
+		ClientIP:  ip,
+		StartedAt: job.StartedAt,
+	})
+
 	go h.runTraversal(ctx, job, req.Domain, qtype, req.AllRoots)
 
 	writeJSON(w, http.StatusAccepted, TraverseStartResponse{ID: id, Status: statusRunning})
@@ -337,6 +403,7 @@ func (h *Handler) getTraversal(w http.ResponseWriter, r *http.Request) {
 		Results   []ResultItem    `json:"results,omitempty"`
 		Summary   *Summary        `json:"summary,omitempty"`
 		Progress  []ProgressEvent `json:"progress,omitempty"`
+		Servers   []ServerInfo    `json:"servers,omitempty"`
 		Error     string          `json:"error,omitempty"`
 		StartedAt time.Time       `json:"started_at"`
 		DoneAt    *time.Time      `json:"done_at,omitempty"`
@@ -348,6 +415,7 @@ func (h *Handler) getTraversal(w http.ResponseWriter, r *http.Request) {
 		Results:   job.Results,
 		Summary:   job.Summary,
 		Progress:  job.Progress,
+		Servers:   job.Servers,
 		Error:     job.Error,
 		StartedAt: job.StartedAt,
 		DoneAt:    job.DoneAt,
@@ -357,6 +425,35 @@ func (h *Handler) getTraversal(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, snapshot)
 }
 
+// getServers handles GET /api/traverse/{id}/servers. It answers 202 with a
+// pending body while the traversal or the post-traversal fingerprinting is
+// still in flight, then the fingerprinted server list.
+func (h *Handler) getServers(w http.ResponseWriter, r *http.Request) {
+	id := r.PathValue("id")
+	job, ok := h.st.get(id)
+	if !ok {
+		writeError(w, http.StatusNotFound, "traversal not found")
+		return
+	}
+
+	job.mu.RLock()
+	done := job.serversDone
+	servers := job.Servers
+	job.mu.RUnlock()
+
+	if !done {
+		writeJSON(w, http.StatusAccepted, map[string]string{"status": "pending"})
+		return
+	}
+	if servers == nil {
+		servers = []ServerInfo{}
+	}
+	writeJSON(w, http.StatusOK, struct {
+		Status  string       `json:"status"`
+		Servers []ServerInfo `json:"servers"`
+	}{Status: "complete", Servers: servers})
+}
+
 // streamTraversal handles GET /api/traverse/{id}/stream (SSE).
 func (h *Handler) streamTraversal(w http.ResponseWriter, r *http.Request) {
 	id := r.PathValue("id")
@@ -430,13 +527,21 @@ func (h *Handler) runTraversal(ctx context.Context, job *TraversalJob, domain st
 		if r := recover(); r != nil {
 			now := time.Now()
 			job.mu.Lock()
-			job.Status = statusError
+			if job.DoneAt == nil {
-			job.Error = fmt.Sprintf("panic: %v", r)
+				job.Status = statusError
-			job.DoneAt = &now
+				job.Error = fmt.Sprintf("panic: %v", r)
+				job.DoneAt = &now
+			}
 			job.mu.Unlock()
 		}
+		// Never leave GET …/servers pending: fingerprinting is skipped on
+		// the panic path, so flip the flag here (idempotent otherwise).
+		job.mu.Lock()
+		job.serversDone = true
+		job.mu.Unlock()
 		job.cancel()
 		job.closeSubscribers()
+		h.reportCompletion(job)
 	}()
 
 	cfg := traverse.DefaultTraverserConfig()
@@ -474,6 +579,25 @@ func (h *Handler) runTraversal(ctx context.Context, job *TraversalJob, domain st
 	tr := traverse.NewTraverser(cfg)
 	root, err := tr.Run(ctx, domain)
 
+	h.commitResult(ctx, job, root, err)
+
+	// Tell streaming clients the traversal reached a terminal state so they
+	// can fetch results now; the stream stays open for the servers event
+	// published once fingerprinting (below) finishes.
+	job.mu.Lock()
+	ev := ProgressEvent{Stage: "complete", Status: job.Status}
+	job.Progress = append(job.Progress, ev)
+	job.publishLocked(ev)
+	job.mu.Unlock()
+
+	// Fingerprint the servers queried during the traversal. This runs after
+	// the terminal status is committed, so results never wait on versions.
+	h.fingerprintServers(job, tr.ServersEncountered())
+}
+
+// commitResult stores the traversal outcome and moves the job to its
+// terminal status.
+func (h *Handler) commitResult(ctx context.Context, job *TraversalJob, root *traverse.Referral, err error) {
 	now := time.Now()
 	job.mu.Lock()
 	defer job.mu.Unlock()
@@ -507,6 +631,105 @@ func (h *Handler) runTraversal(ctx context.Context, job *TraversalJob, domain st
 	job.Status = statusComplete
 }
 
+// fingerprintServers turns the traversal's (server, ip) pairs into
+// job.Servers, probing each unique IP's version.bind with bounded
+// concurrency, then publishes a {"stage":"servers"} event so streaming
+// clients know the list is ready without polling. Pseudo "key:" entries and
+// non-address entries are skipped.
+func (h *Handler) fingerprintServers(job *TraversalJob, seen map[string][]string) {
+	type pair struct{ name, ip string }
+	var pairs []pair
+	uniq := make(map[string]bool)
+	var ips []net.IP
+	for name, addrs := range seen {
+		for _, addr := range addrs {
+			if strings.HasPrefix(addr, "key:") {
+				continue
+			}
+			ip := net.ParseIP(addr)
+			if ip == nil {
+				continue
+			}
+			pairs = append(pairs, pair{name: name, ip: addr})
+			if !uniq[addr] {
+				uniq[addr] = true
+				ips = append(ips, ip)
+			}
+		}
+	}
+
+	versions := make(map[string]string, len(ips))
+	if len(ips) > 0 && h.fp != nil {
+		ctx, cancel := context.WithTimeout(context.Background(), fingerprintTimeout)
+		defer cancel()
+
+		var (
+			wg  sync.WaitGroup
+			mu  sync.Mutex
+			sem = make(chan struct{}, fingerprintConcurrency)
+		)
+		for _, ip := range ips {
+			wg.Add(1)
+			go func(ip net.IP) {
+				defer wg.Done()
+				sem <- struct{}{}
+				defer func() { <-sem }()
+				v := h.fp.Query(ctx, ip)
+				mu.Lock()
+				versions[ip.String()] = v
+				mu.Unlock()
+			}(ip)
+		}
+		wg.Wait()
+	}
+
+	servers := make([]ServerInfo, 0, len(pairs))
+	for _, p := range pairs {
+		servers = append(servers, ServerInfo{Name: p.name, IP: p.ip, Version: versions[p.ip]})
+	}
+	sort.Slice(servers, func(i, j int) bool {
+		if servers[i].Name != servers[j].Name {
+			return servers[i].Name < servers[j].Name
+		}
+		return servers[i].IP < servers[j].IP
+	})
+
+	ev := ProgressEvent{Stage: "servers"}
+	job.mu.Lock()
+	job.Servers = servers
+	job.serversDone = true
+	job.Progress = append(job.Progress, ev)
+	job.publishLocked(ev)
+	job.mu.Unlock()
+}
+
+// reportCompletion posts the webhook "complete" event for a job that has
+// reached a terminal state. Fire-and-forget; never blocks the caller.
+func (h *Handler) reportCompletion(job *TraversalJob) {
+	if h.webhook == nil {
+		return
+	}
+	job.mu.RLock()
+	ev := webhookCompleteEvent{
+		Event:       webhookEventComplete,
+		ID:          job.ID,
+		Domain:      job.Domain,
+		QueryType:   job.QueryType,
+		ClientIP:    job.clientIP,
+		StartedAt:   job.StartedAt,
+		Status:      job.Status,
+		Error:       job.Error,
+		ResultCount: len(job.Results),
+		Summary:     job.Summary,
+	}
+	if job.DoneAt != nil {
+		ev.DoneAt = *job.DoneAt
+		ev.DurationMS = job.DoneAt.Sub(job.StartedAt).Milliseconds()
+	}
+	job.mu.RUnlock()
+	h.webhook.send(webhookEventComplete, ev)
+}
+
 // envDuration reads a Go duration from the environment, falling back to
 // def when unset or unparsable.
 func envDuration(name string, def time.Duration) time.Duration {

web/api/handler_internal_test.go

@@ -2,6 +2,8 @@ package api
 
 import (
 	"context"
+	"encoding/json"
+	"net"
 	"net/http/httptest"
 	"strconv"
 	"strings"
@@ -147,3 +149,171 @@ func TestSubscribeSnapshot_NoDuplicates(t *testing.T) {
 		t.Error(msg)
 	}
 }
+
+// fakeVersionQuerier returns canned version strings without touching the
+// network.
+type fakeVersionQuerier struct {
+	mu       sync.Mutex
+	versions map[string]string
+	queried  []string
+}
+
+func (f *fakeVersionQuerier) Query(_ context.Context, ip net.IP) string {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.queried = append(f.queried, ip.String())
+	return f.versions[ip.String()]
+}
+
+// TestGetServers_PendingWhileRunning verifies the 202 pending shape while a
+// job has not finished fingerprinting (running or just-completed).
+func TestGetServers_PendingWhileRunning(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	h := newHandler(ctx)
+
+	now := time.Now()
+	jobs := []*TraversalJob{
+		{ID: "running", Status: statusRunning, StartedAt: now},
+		{ID: "fingerprinting", Status: statusComplete, StartedAt: now, DoneAt: &now},
+	}
+	for _, j := range jobs {
+		h.st.set(j)
+	}
+
+	for _, id := range []string{"running", "fingerprinting"} {
+		req := httptest.NewRequest("GET", "/api/traverse/"+id+"/servers", nil)
+		rec := httptest.NewRecorder()
+		h.mux.ServeHTTP(rec, req)
+
+		if rec.Code != 202 {
+			t.Fatalf("%s: want 202, got %d: %s", id, rec.Code, rec.Body.String())
+		}
+		var body map[string]string
+		if err := json.NewDecoder(rec.Body).Decode(&body); err != nil {
+			t.Fatalf("%s: decode: %v", id, err)
+		}
+		if body["status"] != "pending" {
+			t.Fatalf("%s: want status=pending, got %q", id, body["status"])
+		}
+	}
+}
+
+// TestFingerprintServers_StoresServersAndPublishes drives the fingerprint
+// step with a fake querier: (server, ip) pairs become sorted job.Servers with
+// versions, pseudo/invalid entries are skipped, a {"stage":"servers"} event
+// is published, and the endpoint flips from pending to the final list.
+func TestFingerprintServers_StoresServersAndPublishes(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	h := newHandler(ctx)
+	fake := &fakeVersionQuerier{versions: map[string]string{
+		"192.0.2.1": "TestDNS 1.0",
+		"192.0.2.2": "",
+	}}
+	h.fp = fake
+
+	now := time.Now()
+	job := &TraversalJob{ID: "j", Status: statusComplete, StartedAt: now, DoneAt: &now}
+	h.st.set(job)
+	sub, _, _ := job.subscribeSnapshot()
+	defer job.unsubscribe(sub)
+
+	h.fingerprintServers(job, map[string][]string{
+		"b.example.net": {"192.0.2.2"},
+		"a.example.net": {"192.0.2.1", "key:pseudo:entry", "not-an-ip"},
+	})
+
+	want := []ServerInfo{
+		{Name: "a.example.net", IP: "192.0.2.1", Version: "TestDNS 1.0"},
+		{Name: "b.example.net", IP: "192.0.2.2", Version: ""},
+	}
+	job.mu.RLock()
+	got := append([]ServerInfo(nil), job.Servers...)
+	done := job.serversDone
+	job.mu.RUnlock()
+	if !done {
+		t.Fatal("serversDone not set")
+	}
+	if len(got) != len(want) {
+		t.Fatalf("servers = %+v, want %+v", got, want)
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Fatalf("servers[%d] = %+v, want %+v", i, got[i], want[i])
+		}
+	}
+
+	select {
+	case ev := <-sub:
+		if ev.Stage != "servers" {
+			t.Fatalf("published stage = %q, want servers", ev.Stage)
+		}
+	default:
+		t.Fatal("no servers event published")
+	}
+
+	// Endpoint now serves the final list.
+	req := httptest.NewRequest("GET", "/api/traverse/j/servers", nil)
+	rec := httptest.NewRecorder()
+	h.mux.ServeHTTP(rec, req)
+	if rec.Code != 200 {
+		t.Fatalf("want 200, got %d: %s", rec.Code, rec.Body.String())
+	}
+	var body struct {
+		Status  string       `json:"status"`
+		Servers []ServerInfo `json:"servers"`
+	}
+	if err := json.NewDecoder(rec.Body).Decode(&body); err != nil {
+		t.Fatal(err)
+	}
+	if body.Status != "complete" || len(body.Servers) != 2 {
+		t.Fatalf("body = %+v", body)
+	}
+
+	// Snapshot includes servers too.
+	req = httptest.NewRequest("GET", "/api/traverse/j", nil)
+	rec = httptest.NewRecorder()
+	h.mux.ServeHTTP(rec, req)
+	if rec.Code != 200 {
+		t.Fatalf("snapshot: want 200, got %d", rec.Code)
+	}
+	var snap struct {
+		Servers []ServerInfo `json:"servers"`
+	}
+	if err := json.NewDecoder(rec.Body).Decode(&snap); err != nil {
+		t.Fatal(err)
+	}
+	if len(snap.Servers) != 2 {
+		t.Fatalf("snapshot servers = %+v, want 2 entries", snap.Servers)
+	}
+}
+
+// TestFingerprintServers_EmptySeen still terminates the pending state and
+// publishes the servers event for traversals that recorded no servers.
+func TestFingerprintServers_EmptySeen(t *testing.T) {
+	h := &Handler{st: newStore()}
+	job := &TraversalJob{ID: "e", Status: statusError}
+	h.st.set(job)
+	sub, _, _ := job.subscribeSnapshot()
+	defer job.unsubscribe(sub)
+
+	h.fingerprintServers(job, nil)
+
+	job.mu.RLock()
+	defer job.mu.RUnlock()
+	if !job.serversDone {
+		t.Fatal("serversDone not set")
+	}
+	if len(job.Servers) != 0 {
+		t.Fatalf("servers = %+v, want empty", job.Servers)
+	}
+	select {
+	case ev := <-sub:
+		if ev.Stage != "servers" {
+			t.Fatalf("published stage = %q, want servers", ev.Stage)
+		}
+	default:
+		t.Fatal("no servers event published")
+	}
+}

web/api/handler_test.go

@@ -29,6 +29,7 @@ func newTestServer(t *testing.T) *api.Server {
 }
 
 func TestHealth(t *testing.T) {
+	t.Setenv("FLY_REGION", "") // ensure region is absent regardless of host env
 	srv := newTestServer(t)
 	defer srv.Shutdown(5 * time.Second) //nolint:errcheck
 
@@ -48,6 +49,55 @@ func TestHealth(t *testing.T) {
 	if body["status"] != "ok" {
 		t.Fatalf("want status=ok, got %q", body["status"])
 	}
+	if body["version"] != "dev" {
+		t.Fatalf("want version=dev, got %q", body["version"])
+	}
+	if region, ok := body["region"]; ok {
+		t.Fatalf("region should be omitted outside Fly, got %q", region)
+	}
+}
+
+func TestHealthReportsFlyRegion(t *testing.T) {
+	t.Setenv("FLY_REGION", "syd")
+	srv := newTestServer(t)
+	defer srv.Shutdown(5 * time.Second) //nolint:errcheck
+
+	resp, err := http.Get("http://" + srv.Addr() + "/api/health")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+
+	var body map[string]string
+	if err := json.NewDecoder(resp.Body).Decode(&body); err != nil {
+		t.Fatal(err)
+	}
+	if body["region"] != "syd" {
+		t.Fatalf("want region=syd, got %q", body["region"])
+	}
+}
+
+func TestHealthReportsStampedVersion(t *testing.T) {
+	srv := api.NewServer("127.0.0.1:0")
+	srv.SetVersion("v1.2.3")
+	if err := srv.Start(); err != nil {
+		t.Fatalf("start server: %v", err)
+	}
+	defer srv.Shutdown(5 * time.Second) //nolint:errcheck
+
+	resp, err := http.Get("http://" + srv.Addr() + "/api/health")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+
+	var body map[string]string
+	if err := json.NewDecoder(resp.Body).Decode(&body); err != nil {
+		t.Fatal(err)
+	}
+	if body["version"] != "v1.2.3" {
+		t.Fatalf("want version=v1.2.3, got %q", body["version"])
+	}
 }
 
 func TestCORSDisabledByDefault(t *testing.T) {
@@ -205,6 +255,95 @@ func TestGetTraversal_Found(t *testing.T) {
 	}
 }
 
+func TestGetServers_NotFound(t *testing.T) {
+	srv := newTestServer(t)
+	defer srv.Shutdown(5 * time.Second) //nolint:errcheck
+
+	resp, err := http.Get("http://" + srv.Addr() + "/api/traverse/does-not-exist/servers")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusNotFound {
+		t.Fatalf("want 404, got %d", resp.StatusCode)
+	}
+}
+
+// TestGetServers_AvailableAfterCompletion drives a full job through the API:
+// the servers endpoint answers 202 pending while the traversal/fingerprinting
+// is in flight and the fingerprinted list once everything finished.
+func TestGetServers_AvailableAfterCompletion(t *testing.T) {
+	srv := newTestServer(t)
+	defer srv.Shutdown(5 * time.Second) //nolint:errcheck
+
+	startBody := bytes.NewBufferString(`{"domain":"example.com","type":"A"}`)
+	startResp, err := http.Post("http://"+srv.Addr()+"/api/traverse", "application/json", startBody)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer startResp.Body.Close()
+
+	var start struct {
+		ID string `json:"id"`
+	}
+	if err := json.NewDecoder(startResp.Body).Decode(&start); err != nil {
+		t.Fatal(err)
+	}
+
+	deadline := time.Now().Add(90 * time.Second)
+	for {
+		resp, err := http.Get("http://" + srv.Addr() + "/api/traverse/" + start.ID + "/servers")
+		if err != nil {
+			t.Fatal(err)
+		}
+		body, err := io.ReadAll(resp.Body)
+		resp.Body.Close()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		switch resp.StatusCode {
+		case http.StatusAccepted:
+			var pending struct {
+				Status string `json:"status"`
+			}
+			if err := json.Unmarshal(body, &pending); err != nil {
+				t.Fatalf("pending body: %v (%s)", err, body)
+			}
+			if pending.Status != "pending" {
+				t.Fatalf("want status=pending, got %q", pending.Status)
+			}
+		case http.StatusOK:
+			var done struct {
+				Status  string `json:"status"`
+				Servers []struct {
+					Name    string `json:"name"`
+					IP      string `json:"ip"`
+					Version string `json:"version"`
+				} `json:"servers"`
+			}
+			if err := json.Unmarshal(body, &done); err != nil {
+				t.Fatalf("servers body: %v (%s)", err, body)
+			}
+			if done.Status != "complete" {
+				t.Fatalf("want status=complete, got %q", done.Status)
+			}
+			if done.Servers == nil {
+				t.Fatalf("servers key missing or null: %s", body)
+			}
+			return
+		default:
+			t.Fatalf("unexpected status %d: %s", resp.StatusCode, body)
+		}
+
+		if time.Now().After(deadline) {
+			t.Fatal("timed out waiting for servers to become available")
+		}
+		time.Sleep(250 * time.Millisecond)
+	}
+}
+
 func TestStreamTraversal_NotFound(t *testing.T) {
 	srv := newTestServer(t)
 	defer srv.Shutdown(5 * time.Second) //nolint:errcheck
@@ -322,6 +461,71 @@ func TestStaticSPA_TypeOptions(t *testing.T) {
 	}
 }
 
+// TestStaticSPA_DetailTree asserts the SPA ships the live detail tree with
+// its resolve-subtree toggle markup, plus the raw-log fallback feed so the
+// old flat progress view is still reachable for debugging.
+func TestStaticSPA_DetailTree(t *testing.T) {
+	srv := newTestServer(t)
+	defer srv.Shutdown(5 * time.Second) //nolint:errcheck
+
+	resp, err := http.Get("http://" + srv.Addr() + "/")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	page := string(body)
+
+	for _, want := range []string{
+		`id="detailTree"`,   // detail-tree container
+		`resolve-toggle`,    // per-node show/hide resolve markup
+		`show resolve`,      // toggle wording mirrors dns.squish.net
+		`id="progressFeed"`, // raw-log fallback feed still present
+		`id="rawToggle"`,    // toggle that reveals it
+	} {
+		if !strings.Contains(page, want) {
+			t.Errorf("index.html missing %q", want)
+		}
+	}
+}
+
+// TestStaticSPA_ServersSection asserts the SPA ships the server map/table
+// section: Leaflet lazy-loaded from unpkg, geojs.io client-side geolocation,
+// and the reference-style table headings.
+func TestStaticSPA_ServersSection(t *testing.T) {
+	srv := newTestServer(t)
+	defer srv.Shutdown(5 * time.Second) //nolint:errcheck
+
+	resp, err := http.Get("http://" + srv.Addr() + "/")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	page := string(body)
+
+	for _, want := range []string{
+		`unpkg.com/leaflet@1.9`, // map library CDN
+		`get.geojs.io`,          // client-side geolocation service
+		`id="serversCard"`,
+		`id="serverMap"`,
+		`<th>Country</th><th>City</th><th>Servers</th><th>Software guess</th>`,
+		`/servers`, // fetches the servers endpoint
+	} {
+		if !strings.Contains(page, want) {
+			t.Errorf("index.html missing %q", want)
+		}
+	}
+}
+
 func TestStaticSPA_FallbackToIndex(t *testing.T) {
 	srv := newTestServer(t)
 	defer srv.Shutdown(5 * time.Second) //nolint:errcheck

web/api/ratelimit.go

@@ -0,0 +1,120 @@
+package api
+
+import (
+	"fmt"
+	"net"
+	"net/http"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+// Default per-IP rate limit for POST /api/traverse, overridable via
+// EXPLOREDNS_RATE_LIMIT ("N/duration", e.g. "30/1h" or "10/10m").
+const (
+	defaultRateLimitCount  = 30
+	defaultRateLimitWindow = time.Hour
+)
+
+// rateLimiter is an in-memory token-bucket limiter keyed by client IP.
+// Each bucket starts full with limit tokens and refills continuously at
+// limit tokens per window.
+type rateLimiter struct {
+	limit  int
+	window time.Duration
+
+	mu      sync.Mutex
+	buckets map[string]*tokenBucket
+	now     func() time.Time // overridable in tests
+}
+
+type tokenBucket struct {
+	tokens float64
+	last   time.Time
+}
+
+func newRateLimiter(limit int, window time.Duration) *rateLimiter {
+	return &rateLimiter{
+		limit:   limit,
+		window:  window,
+		buckets: make(map[string]*tokenBucket),
+		now:     time.Now,
+	}
+}
+
+// allow reports whether one request from ip fits within the limit,
+// consuming a token when it does.
+func (rl *rateLimiter) allow(ip string) bool {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	now := rl.now()
+	b, ok := rl.buckets[ip]
+	if !ok {
+		b = &tokenBucket{tokens: float64(rl.limit), last: now}
+		rl.buckets[ip] = b
+	} else {
+		refill := now.Sub(b.last).Seconds() * float64(rl.limit) / rl.window.Seconds()
+		b.tokens = min(b.tokens+refill, float64(rl.limit))
+		b.last = now
+	}
+	if b.tokens < 1 {
+		return false
+	}
+	b.tokens--
+	return true
+}
+
+// sweep drops buckets idle for at least one full window; such buckets
+// would be full again anyway, so dropping them loses nothing.
+func (rl *rateLimiter) sweep() {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+	cutoff := rl.now().Add(-rl.window)
+	for ip, b := range rl.buckets {
+		if b.last.Before(cutoff) {
+			delete(rl.buckets, ip)
+		}
+	}
+}
+
+// String renders the limit for error messages, e.g. "30 requests per 1h0m0s".
+func (rl *rateLimiter) String() string {
+	return fmt.Sprintf("%d requests per %s", rl.limit, rl.window)
+}
+
+// rateLimitExempt reports whether r may bypass the rate limit: direct
+// connections from loopback (the SPA dev loop, tests, health tooling).
+// Proxied requests are never exempt — when Fly-Client-IP or
+// X-Forwarded-For is present, RemoteAddr is just the proxy, so the real
+// client IP must be limited even though the socket peer is local.
+func rateLimitExempt(r *http.Request) bool {
+	if r.Header.Get("Fly-Client-IP") != "" || r.Header.Get("X-Forwarded-For") != "" {
+		return false
+	}
+	host, _, err := net.SplitHostPort(r.RemoteAddr)
+	if err != nil {
+		host = r.RemoteAddr
+	}
+	ip := net.ParseIP(host)
+	return ip != nil && ip.IsLoopback()
+}
+
+// parseRateLimit parses "N/duration" (e.g. "30/1h"), falling back to the
+// defaults when v is empty or invalid.
+func parseRateLimit(v string) (int, time.Duration) {
+	parts := strings.SplitN(v, "/", 2)
+	if len(parts) != 2 {
+		return defaultRateLimitCount, defaultRateLimitWindow
+	}
+	n, err := strconv.Atoi(strings.TrimSpace(parts[0]))
+	if err != nil || n <= 0 {
+		return defaultRateLimitCount, defaultRateLimitWindow
+	}
+	d, err := time.ParseDuration(strings.TrimSpace(parts[1]))
+	if err != nil || d <= 0 {
+		return defaultRateLimitCount, defaultRateLimitWindow
+	}
+	return n, d
+}

web/api/ratelimit_test.go

@@ -0,0 +1,169 @@
+package api
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestParseRateLimit(t *testing.T) {
+	tests := []struct {
+		in     string
+		limit  int
+		window time.Duration
+	}{
+		{"", defaultRateLimitCount, defaultRateLimitWindow},
+		{"30/1h", 30, time.Hour},
+		{"10/10m", 10, 10 * time.Minute},
+		{"5 / 30s", 5, 30 * time.Second},
+		{"bogus", defaultRateLimitCount, defaultRateLimitWindow},
+		{"0/1h", defaultRateLimitCount, defaultRateLimitWindow},
+		{"-3/1h", defaultRateLimitCount, defaultRateLimitWindow},
+		{"10/-1h", defaultRateLimitCount, defaultRateLimitWindow},
+		{"10/soon", defaultRateLimitCount, defaultRateLimitWindow},
+		{"/1h", defaultRateLimitCount, defaultRateLimitWindow},
+	}
+	for _, tc := range tests {
+		limit, window := parseRateLimit(tc.in)
+		if limit != tc.limit || window != tc.window {
+			t.Errorf("parseRateLimit(%q) = %d, %s; want %d, %s",
+				tc.in, limit, window, tc.limit, tc.window)
+		}
+	}
+}
+
+func TestNewHandlerReadsRateLimitEnv(t *testing.T) {
+	t.Setenv("EXPLOREDNS_RATE_LIMIT", "5/10m")
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	h := newHandler(ctx)
+	if h.limiter.limit != 5 || h.limiter.window != 10*time.Minute {
+		t.Fatalf("limiter = %d/%s, want 5/10m", h.limiter.limit, h.limiter.window)
+	}
+}
+
+// postTraverse sends POST /api/traverse with an empty JSON body so requests
+// that pass the rate limiter fail validation (400) instead of spawning a
+// real traversal. remoteAddr and headers shape the client identity.
+func postTraverse(t *testing.T, h *Handler, remoteAddr string, headers map[string]string) *httptest.ResponseRecorder {
+	t.Helper()
+	req := httptest.NewRequest(http.MethodPost, "/api/traverse", strings.NewReader(`{}`))
+	req.RemoteAddr = remoteAddr
+	for k, v := range headers {
+		req.Header.Set(k, v)
+	}
+	rec := httptest.NewRecorder()
+	h.mux.ServeHTTP(rec, req)
+	return rec
+}
+
+func TestRateLimit_OverLimitReturns429(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	h := newHandler(ctx)
+	h.limiter = newRateLimiter(2, time.Hour)
+
+	hdr := map[string]string{"X-Forwarded-For": "203.0.113.9"}
+	for i := 0; i < 2; i++ {
+		if rec := postTraverse(t, h, "10.0.0.1:1234", hdr); rec.Code != http.StatusBadRequest {
+			t.Fatalf("request %d: want 400 (under limit), got %d: %s", i, rec.Code, rec.Body.String())
+		}
+	}
+	rec := postTraverse(t, h, "10.0.0.1:1234", hdr)
+	if rec.Code != http.StatusTooManyRequests {
+		t.Fatalf("want 429 over limit, got %d: %s", rec.Code, rec.Body.String())
+	}
+	if body := rec.Body.String(); !strings.Contains(body, "2 requests per 1h0m0s") {
+		t.Fatalf("429 body should name the limit, got %s", body)
+	}
+
+	// A distinct client IP has its own bucket and is unaffected.
+	other := map[string]string{"X-Forwarded-For": "203.0.113.10"}
+	if rec := postTraverse(t, h, "10.0.0.1:1234", other); rec.Code != http.StatusBadRequest {
+		t.Fatalf("distinct IP: want 400, got %d: %s", rec.Code, rec.Body.String())
+	}
+}
+
+func TestRateLimit_RefillsContinuously(t *testing.T) {
+	rl := newRateLimiter(2, time.Second)
+	now := time.Now()
+	rl.now = func() time.Time { return now }
+
+	if !rl.allow("a") || !rl.allow("a") {
+		t.Fatal("first two requests should be allowed")
+	}
+	if rl.allow("a") {
+		t.Fatal("third request should be denied")
+	}
+	// Half a window refills half the bucket: one token.
+	now = now.Add(500 * time.Millisecond)
+	if !rl.allow("a") {
+		t.Fatal("request after refill should be allowed")
+	}
+	if rl.allow("a") {
+		t.Fatal("bucket should hold only the refilled token")
+	}
+}
+
+func TestRateLimit_SweepDropsIdleBuckets(t *testing.T) {
+	rl := newRateLimiter(1, time.Minute)
+	now := time.Now()
+	rl.now = func() time.Time { return now }
+
+	rl.allow("stale")
+	now = now.Add(2 * time.Minute)
+	rl.allow("fresh")
+	rl.sweep()
+
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+	if _, ok := rl.buckets["stale"]; ok {
+		t.Fatal("idle bucket should have been swept")
+	}
+	if _, ok := rl.buckets["fresh"]; !ok {
+		t.Fatal("active bucket should survive the sweep")
+	}
+}
+
+func TestRateLimit_LocalhostExempt(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	h := newHandler(ctx)
+	h.limiter = newRateLimiter(1, time.Hour)
+
+	for _, addr := range []string{"127.0.0.1:5555", "[::1]:5555"} {
+		for i := 0; i < 3; i++ {
+			rec := postTraverse(t, h, addr, nil)
+			if rec.Code != http.StatusBadRequest {
+				t.Fatalf("%s request %d: localhost should be exempt, got %d: %s",
+					addr, i, rec.Code, rec.Body.String())
+			}
+		}
+	}
+}
+
+// TestRateLimit_ProxiedLocalhostNotExempt verifies that a request arriving
+// from a local proxy (RemoteAddr loopback) is still limited by the real
+// client IP carried in Fly-Client-IP / X-Forwarded-For.
+func TestRateLimit_ProxiedLocalhostNotExempt(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	h := newHandler(ctx)
+	h.limiter = newRateLimiter(1, time.Hour)
+
+	hdr := map[string]string{"Fly-Client-IP": "198.51.100.4"}
+	if rec := postTraverse(t, h, "127.0.0.1:5555", hdr); rec.Code != http.StatusBadRequest {
+		t.Fatalf("first proxied request: want 400, got %d", rec.Code)
+	}
+	if rec := postTraverse(t, h, "127.0.0.1:5555", hdr); rec.Code != http.StatusTooManyRequests {
+		t.Fatalf("second proxied request: want 429, got %d", rec.Code)
+	}
+	// The same proxy forwarding a different client is unaffected.
+	other := map[string]string{"Fly-Client-IP": "198.51.100.5"}
+	if rec := postTraverse(t, h, "127.0.0.1:5555", other); rec.Code != http.StatusBadRequest {
+		t.Fatalf("other client via proxy: want 400, got %d", rec.Code)
+	}
+}

web/api/webhook.go

@@ -0,0 +1,137 @@
+package api
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"log"
+	"net"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// Webhook event names, sent both in the JSON body and in the
+// X-ExploreDNS-Event request header.
+const (
+	webhookEventStart    = "start"
+	webhookEventComplete = "complete"
+)
+
+// webhookStartEvent is posted when a traversal job is accepted.
+type webhookStartEvent struct {
+	Event     string    `json:"event"`
+	ID        string    `json:"id"`
+	Domain    string    `json:"domain"`
+	QueryType string    `json:"query_type"`
+	AllRoots  bool      `json:"all_roots"`
+	ClientIP  string    `json:"client_ip"`
+	StartedAt time.Time `json:"started_at"`
+}
+
+// webhookCompleteEvent is posted when a traversal job reaches a terminal
+// state. Summary reuses the API Summary shape.
+type webhookCompleteEvent struct {
+	Event       string    `json:"event"`
+	ID          string    `json:"id"`
+	Domain      string    `json:"domain"`
+	QueryType   string    `json:"query_type"`
+	ClientIP    string    `json:"client_ip"`
+	StartedAt   time.Time `json:"started_at"`
+	DoneAt      time.Time `json:"done_at"`
+	DurationMS  int64     `json:"duration_ms"`
+	Status      string    `json:"status"`
+	Error       string    `json:"error,omitempty"`
+	ResultCount int       `json:"result_count"`
+	Summary     *Summary  `json:"summary"`
+}
+
+// webhookReporter posts usage events to a configured URL. Sends are
+// fire-and-forget: each runs in its own goroutine with a timeout and a
+// single retry, and failures are logged but never surface to callers.
+type webhookReporter struct {
+	url        string
+	client     *http.Client
+	timeout    time.Duration
+	retryDelay time.Duration
+}
+
+// newWebhookReporter returns a reporter for url, or nil when url is empty
+// (webhook reporting disabled). A nil reporter is safe to call.
+func newWebhookReporter(url string) *webhookReporter {
+	if url == "" {
+		return nil
+	}
+	return &webhookReporter{
+		url:        url,
+		client:     &http.Client{},
+		timeout:    5 * time.Second,
+		retryDelay: 2 * time.Second,
+	}
+}
+
+// send marshals payload and posts it asynchronously with one retry on
+// failure. Errors are logged and never affect the caller.
+func (wr *webhookReporter) send(event string, payload any) {
+	if wr == nil {
+		return
+	}
+	body, err := json.Marshal(payload)
+	if err != nil {
+		log.Printf("webhook: marshal %s event: %v", event, err)
+		return
+	}
+	go func() {
+		err := wr.post(event, body)
+		if err == nil {
+			return
+		}
+		log.Printf("webhook: %s event failed, retrying in %s: %v", event, wr.retryDelay, err)
+		time.Sleep(wr.retryDelay)
+		if err := wr.post(event, body); err != nil {
+			log.Printf("webhook: %s event failed after retry: %v", event, err)
+		}
+	}()
+}
+
+// post performs one synchronous webhook delivery attempt.
+func (wr *webhookReporter) post(event string, body []byte) error {
+	ctx, cancel := context.WithTimeout(context.Background(), wr.timeout)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, wr.url, bytes.NewReader(body))
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("X-ExploreDNS-Event", event)
+
+	resp, err := wr.client.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode < 200 || resp.StatusCode > 299 {
+		return fmt.Errorf("webhook returned status %d", resp.StatusCode)
+	}
+	return nil
+}
+
+// clientIP resolves the requesting client's IP: the Fly-Client-IP header if
+// present, else the first entry of X-Forwarded-For, else the host part of
+// RemoteAddr.
+func clientIP(r *http.Request) string {
+	if ip := strings.TrimSpace(r.Header.Get("Fly-Client-IP")); ip != "" {
+		return ip
+	}
+	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
+		if first := strings.TrimSpace(strings.Split(xff, ",")[0]); first != "" {
+			return first
+		}
+	}
+	if host, _, err := net.SplitHostPort(r.RemoteAddr); err == nil {
+		return host
+	}
+	return r.RemoteAddr
+}

web/api/webhook_test.go

@@ -0,0 +1,241 @@
+package api
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestClientIP(t *testing.T) {
+	tests := []struct {
+		name       string
+		remoteAddr string
+		headers    map[string]string
+		want       string
+	}{
+		{"remote addr only", "192.0.2.7:4711", nil, "192.0.2.7"},
+		{"remote addr no port", "192.0.2.7", nil, "192.0.2.7"},
+		{"fly header wins", "127.0.0.1:80",
+			map[string]string{"Fly-Client-IP": "203.0.113.1", "X-Forwarded-For": "198.51.100.1"},
+			"203.0.113.1"},
+		{"xff first entry", "127.0.0.1:80",
+			map[string]string{"X-Forwarded-For": " 198.51.100.1 , 10.0.0.1"},
+			"198.51.100.1"},
+		{"ipv6 remote", "[::1]:9999", nil, "::1"},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			r := httptest.NewRequest(http.MethodPost, "/api/traverse", nil)
+			r.RemoteAddr = tc.remoteAddr
+			for k, v := range tc.headers {
+				r.Header.Set(k, v)
+			}
+			if got := clientIP(r); got != tc.want {
+				t.Fatalf("clientIP = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+type webhookHit struct {
+	event string
+	body  map[string]any
+}
+
+// newWebhookHandler builds a Handler whose traversals fail instantly (the
+// per-job context is already expired) and whose webhook posts to url.
+func newWebhookHandler(t *testing.T, url string) *Handler {
+	t.Helper()
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+	h := newHandler(ctx)
+	h.jobTimeout = time.Nanosecond
+	h.webhook = &webhookReporter{
+		url:        url,
+		client:     &http.Client{},
+		timeout:    2 * time.Second,
+		retryDelay: 10 * time.Millisecond,
+	}
+	return h
+}
+
+func TestWebhook_StartAndCompleteEventsDelivered(t *testing.T) {
+	hits := make(chan webhookHit, 4)
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if ct := r.Header.Get("Content-Type"); ct != "application/json" {
+			t.Errorf("Content-Type = %q, want application/json", ct)
+		}
+		raw, _ := io.ReadAll(r.Body)
+		var body map[string]any
+		if err := json.Unmarshal(raw, &body); err != nil {
+			t.Errorf("unmarshal webhook body: %v (%s)", err, raw)
+		}
+		hits <- webhookHit{event: r.Header.Get("X-ExploreDNS-Event"), body: body}
+	}))
+	defer ts.Close()
+
+	h := newWebhookHandler(t, ts.URL)
+
+	req := httptest.NewRequest(http.MethodPost, "/api/traverse",
+		strings.NewReader(`{"domain":"example.com","type":"A","all_roots":true}`))
+	req.Header.Set("X-Forwarded-For", "198.51.100.7")
+	rec := httptest.NewRecorder()
+	h.mux.ServeHTTP(rec, req)
+	if rec.Code != http.StatusAccepted {
+		t.Fatalf("want 202, got %d: %s", rec.Code, rec.Body.String())
+	}
+	var start TraverseStartResponse
+	if err := json.NewDecoder(rec.Body).Decode(&start); err != nil {
+		t.Fatal(err)
+	}
+
+	// Both events are fired asynchronously; collect them by event name.
+	got := map[string]map[string]any{}
+	for len(got) < 2 {
+		select {
+		case hit := <-hits:
+			got[hit.event] = hit.body
+		case <-time.After(10 * time.Second):
+			t.Fatalf("timed out waiting for webhook events, have %v", got)
+		}
+	}
+
+	startEv := got["start"]
+	if startEv == nil {
+		t.Fatal("no start event received")
+	}
+	for k, want := range map[string]any{
+		"event": "start", "id": start.ID, "domain": "example.com",
+		"query_type": "A", "all_roots": true, "client_ip": "198.51.100.7",
+	} {
+		if startEv[k] != want {
+			t.Errorf("start event %s = %v, want %v", k, startEv[k], want)
+		}
+	}
+	if s, _ := startEv["started_at"].(string); s == "" {
+		t.Error("start event missing started_at")
+	}
+
+	compEv := got["complete"]
+	if compEv == nil {
+		t.Fatal("no complete event received")
+	}
+	for k, want := range map[string]any{
+		"event": "complete", "id": start.ID, "domain": "example.com",
+		"query_type": "A", "client_ip": "198.51.100.7", "status": statusError,
+	} {
+		if compEv[k] != want {
+			t.Errorf("complete event %s = %v, want %v", k, compEv[k], want)
+		}
+	}
+	if msg, _ := compEv["error"].(string); !strings.Contains(msg, "timed out") {
+		t.Errorf("complete event error = %v, want timeout message", compEv["error"])
+	}
+	if _, ok := compEv["duration_ms"].(float64); !ok {
+		t.Errorf("complete event duration_ms = %v, want a number", compEv["duration_ms"])
+	}
+	if d, _ := compEv["done_at"].(string); d == "" {
+		t.Error("complete event missing done_at")
+	}
+	if _, ok := compEv["result_count"].(float64); !ok {
+		t.Errorf("complete event result_count = %v, want a number", compEv["result_count"])
+	}
+	if _, ok := compEv["summary"]; !ok {
+		t.Error("complete event missing summary key")
+	}
+}
+
+// TestWebhook_SlowReceiverDoesNotDelayJob verifies that a webhook receiver
+// stuck for longer than the whole traversal never delays job completion.
+func TestWebhook_SlowReceiverDoesNotDelayJob(t *testing.T) {
+	release := make(chan struct{})
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		<-release // hold every delivery until the test finishes
+	}))
+	defer ts.Close()
+	defer close(release)
+
+	h := newWebhookHandler(t, ts.URL)
+
+	startedAt := time.Now()
+	req := httptest.NewRequest(http.MethodPost, "/api/traverse",
+		strings.NewReader(`{"domain":"example.com"}`))
+	rec := httptest.NewRecorder()
+	h.mux.ServeHTTP(rec, req)
+	if rec.Code != http.StatusAccepted {
+		t.Fatalf("want 202, got %d: %s", rec.Code, rec.Body.String())
+	}
+	var start TraverseStartResponse
+	if err := json.NewDecoder(rec.Body).Decode(&start); err != nil {
+		t.Fatal(err)
+	}
+
+	job, ok := h.st.get(start.ID)
+	if !ok {
+		t.Fatal("job not found")
+	}
+	deadline := time.After(5 * time.Second)
+	for {
+		job.mu.RLock()
+		done := job.DoneAt != nil
+		job.mu.RUnlock()
+		if done {
+			break
+		}
+		select {
+		case <-deadline:
+			t.Fatal("job did not reach a terminal state while webhook was stalled")
+		case <-time.After(5 * time.Millisecond):
+		}
+	}
+	// The traversal fails instantly (expired context); reaching terminal
+	// state must not have waited on the stalled webhook receiver.
+	if elapsed := time.Since(startedAt); elapsed > 3*time.Second {
+		t.Fatalf("job completion took %s, webhook receiver must not delay it", elapsed)
+	}
+}
+
+func TestWebhook_RetriesOnceOnFailure(t *testing.T) {
+	var calls atomic.Int32
+	done := make(chan struct{})
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if calls.Add(1) == 1 {
+			w.WriteHeader(http.StatusInternalServerError)
+			return
+		}
+		close(done)
+	}))
+	defer ts.Close()
+
+	wr := &webhookReporter{
+		url:        ts.URL,
+		client:     &http.Client{},
+		timeout:    2 * time.Second,
+		retryDelay: 10 * time.Millisecond,
+	}
+	wr.send("start", map[string]string{"event": "start"})
+
+	select {
+	case <-done:
+	case <-time.After(5 * time.Second):
+		t.Fatal("webhook was not retried after a failed delivery")
+	}
+	if n := calls.Load(); n != 2 {
+		t.Fatalf("webhook deliveries = %d, want 2 (initial + one retry)", n)
+	}
+}
+
+// TestWebhook_NilReporterSafe covers the disabled (no URL) path.
+func TestWebhook_NilReporterSafe(t *testing.T) {
+	var wr *webhookReporter
+	wr.send("start", map[string]string{"event": "start"}) // must not panic
+	if newWebhookReporter("") != nil {
+		t.Fatal("empty URL should disable the webhook reporter")
+	}
+}