watcher: a removed nameserver is an NS Change, not an NS Failure (closes #262)

A nameserver that left a domain's delegation was reported twice: as the
domain's NS Change, and by the check of the domain's own records as an
error-level NS Failure saying it disappeared. The check of a name's
records now sends one NS Change at warning level, starting Hostname: or
Domain:, listing the nameservers it was asked at before and not now.
For a domain it leaves out those of its saved delegation, which the
domain's own NS Change lists, so a domain's new delegation is now
notified and saved only after, and only together with, its records.
Nothing is sent for a nameserver that joins.

A nameserver that was failing before it left is now listed too; the
NS Failure skipped it.

Model: opus-5-5

README.md

@@ -78,8 +78,9 @@ notification endpoint set, changes show only on the dashboard; see
   answer NXDOMAIN, that it does not exist, has no nameservers and is shown as
   not existing (see Web Dashboard and HTTP API). A domain that exists but has no
   delegation of its own, such as `octocat.github.io`, has no nameservers either.
-  When the parent zone's servers do not answer, the check fails and the set from
+  When the parent zone's servers do not answer, or the lookup of the domain's
-  the previous check is kept.
+  own records (below) fails, the check fails and the set from the previous check
+  is kept.
 - Any change triggers a notification:
     - NS added to or removed from that set. A domain that had nameservers on the
       previous check and no longer exists gets one with all of them removed.
@@ -99,11 +100,16 @@ notification endpoint set, changes show only on the dashboard; see
   records, stored per nameserver. Their changes are notified as a hostname's
   are, as a record change, NS query failure, NS recovery, inconsistency or CNAME
   address change, in a message that starts `Domain:` where a hostname's starts
-  `Hostname:`. A domain listed in `DNSWATCHER_SKIP_RECORD_NOTIFICATIONS` gets no
+  `Hostname:`. A nameserver removed from the domain's delegation gets only the
-  record change or inconsistency notification. A domain with no delegation of
+  NS change notification above. A domain listed in
-  its own has these records asked at the servers of the zone it is in, as a
+  `DNSWATCHER_SKIP_RECORD_NOTIFICATIONS` gets no record change or inconsistency
-  hostname has. A domain that does not exist has none: they are not asked for,
+  notification. A domain with no delegation of its own has these records asked
-  and those saved by an earlier check are removed without a notification.
+  at the servers of the zone it is in, as a hostname has. A nameserver the
+  domain's records were asked at on the previous check but not on this one gets
+  a hostname's NS change notification (see below), unless it was in the domain's
+  delegation on the previous check, as when that delegation is withdrawn. A
+  domain that does not exist has none: they are not asked for, and those saved
+  by an earlier check are removed without a notification.
 
 ### DNS Hostname Monitoring (Subdomains)
 
@@ -156,6 +162,10 @@ notification endpoint set, changes show only on the dashboard; see
     - **NS recovery**: A previously-unreachable nameserver starts responding
       again. Its records are not compared with those from before it failed, so a
       change made while it was failing is not reported as a record change.
+    - **NS change**: A nameserver of the zone the hostname is in on the previous
+      check is not one now, whether or not it answered. One notification lists
+      every such nameserver on a `Removed:` line. Nothing is sent for a
+      nameserver that joins the zone.
     - **Inconsistency detected**: Two nameservers return different record sets
       for the same hostname and did not already differ on the previous check.
       Every pair of nameservers is compared. The alert is sent once for each
@@ -237,7 +247,9 @@ includes:
 
 - **DNS record changes**: Which hostname or domain, which nameserver, what
   record type, old values, new values.
-- **DNS NS changes**: Which domain, which nameservers were added/removed.
+- **DNS NS changes**: Which domain, which nameservers were added to or removed
+  from its delegation. For a hostname, or a domain with no delegation of its
+  own, which nameservers left the zone it is in.
 - **NS address changes**: Which domain, which nameserver, its old and new
   addresses.
 - **CNAME address changes**: Which hostname or domain, the old and new addresses
@@ -423,7 +435,7 @@ following precedence (highest to lowest):
 
 | Variable                               | Description                                                                                                                           | Default               |
 | -------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------- | --------------------- |
-| `PORT`                                 | HTTP listen port                                                                                                                      | `8080`                |
+| `PORT`                                 | HTTP listen port; a port that cannot be bound, such as one already in use, stops startup                                              | `8080`                |
 | `DNSWATCHER_DEBUG`                     | Enable debug logging                                                                                                                  | `false`               |
 | `DNSWATCHER_DATA_DIR`                  | Directory for state file                                                                                                              | `/var/lib/dnswatcher` |
 | `DNSWATCHER_TARGETS`                   | Comma-separated DNS names (auto-classified via PSL)                                                                                   | `""`                  |

TODO.md

@@ -19,6 +19,12 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-13: a nameserver leaving a domain's delegation sends only NS Change;
+  one leaving a hostname's zone sends NS Change, not NS Failure (closes #262).
+- 2026-10-13: a save syncs the state file to disk before renaming it into place,
+  so a crash cannot leave it empty and stop the next start (closes #264).
+- 2026-10-13: a `PORT` that cannot be bound stops startup with an error naming
+  the address, instead of running without the HTTP server (closes #266).
 - 2026-10-13: a bad `PORT`, `DNSWATCHER_TLS_EXPIRY_WARNING` or true/false
   setting stops startup instead of being read as 0 or false (closes #269).
 - 2026-10-13: a check saving the state while the dashboard or `/api/v1/status`

internal/server/export_test.go

@@ -13,8 +13,8 @@ import (
 const RequestTimeout time.Duration = requestTimeout
 
 // SetListenPort overrides the port Run binds. A test uses it to hand
-// Run an unbindable port so ListenAndServe fails immediately and Run
+// Run an unbindable port, so that Run stores its http.Server and then
-// returns after storing its http.Server.
+// returns an error without listening.
 func SetListenPort(s *Server, port int) {
 	s.port = port
 }

internal/server/server.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"net"
 	"net/http"
 	"time"
 
@@ -112,7 +113,7 @@ func New(
 	}
 
 	lifecycle.Append(fx.Hook{
-		OnStart: func(_ context.Context) error {
+		OnStart: func(ctx context.Context) error {
 			srv.startupTime = time.Now()
 
 			err := srv.enableSentry()
@@ -120,9 +121,7 @@ func New(
 				return err
 			}
 
-			go srv.Run()
+			return srv.Run(ctx)
-
-			return nil
 		},
 		OnStop: func(ctx context.Context) error {
 			return srv.Shutdown(ctx)
@@ -149,8 +148,10 @@ func newHTTPServer(
 	}
 }
 
-// Run starts the HTTP server.
+// Run binds the listen port, then serves HTTP on it in a new goroutine
-func (s *Server) Run() {
+// and returns. A port that cannot be bound is returned as an error, so
+// that startup stops instead of running without the HTTP server.
+func (s *Server) Run(ctx context.Context) error {
 	s.SetupRoutes()
 
 	listenAddr := fmt.Sprintf(":%d", s.port)
@@ -158,10 +159,21 @@ func (s *Server) Run() {
 
 	s.log.Info("http server starting", "addr", listenAddr)
 
-	err := s.httpServer.ListenAndServe()
+	lc := &net.ListenConfig{}
+
+	listener, err := lc.Listen(ctx, "tcp", listenAddr)
+	if err != nil {
+		return fmt.Errorf("starting http server: %w", err)
+	}
+
+	go func() {
+		err := s.httpServer.Serve(listener)
 		if err != nil && !errors.Is(err, http.ErrServerClosed) {
 			s.log.Error("http server error", "error", err)
 		}
+	}()
+
+	return nil
 }
 
 // Shutdown gracefully shuts down the server, then sends the error

internal/server/server_test.go

@@ -1,6 +1,8 @@
 package server_test
 
 import (
+	"net"
+	"strings"
 	"testing"
 
 	"github.com/spf13/viper"
@@ -58,15 +60,13 @@ func buildServer(t *testing.T) *server.Server {
 }
 
 // TestRunWiresSocketTimeouts pins that the http.Server the running
-// server actually serves — the one Run builds and hands to
+// server actually serves — the one Run builds and serves on the port
-// ListenAndServe — carries every socket-level timeout, plus the two
+// it binds — carries every socket-level timeout, plus the two
 // relationships the values must satisfy.
 //
-// Run is driven to completion with an unbindable port: it builds and
+// Run is given an unbindable port: it builds and stores s.httpServer,
-// stores s.httpServer, then ListenAndServe fails at once and Run
+// then the bind fails and Run returns the error without ever
-// returns without ever listening. The assertions run in the same
+// listening. Nothing here measures elapsed time.
-// goroutine after Run returns, so reading s.httpServer is free of any
-// data race. Nothing here measures elapsed time.
 //
 // ReadTimeout must be at least ReadHeaderTimeout. net/http reads the
 // headers under ReadHeaderTimeout, then sets the read deadline for the
@@ -84,7 +84,10 @@ func TestRunWiresSocketTimeouts(t *testing.T) {
 	srv := buildServer(t)
 	server.SetListenPort(srv, -1)
 
-	srv.Run()
+	err := srv.Run(t.Context())
+	if err == nil {
+		t.Fatal("Run bound port -1")
+	}
 
 	hs := server.HTTPServerOf(srv)
 	if hs == nil {
@@ -133,3 +136,38 @@ func TestRunWiresSocketTimeouts(t *testing.T) {
 		)
 	}
 }
+
+func TestPortInUseStopsStartup(t *testing.T) {
+	// Bound on every address, as Run binds, so that Run's bind fails.
+	lc := &net.ListenConfig{}
+
+	taken, err := lc.Listen(t.Context(), "tcp", ":0")
+	if err != nil {
+		t.Fatalf("binding a port for the test: %v", err)
+	}
+
+	defer func() { _ = taken.Close() }()
+
+	_, port, err := net.SplitHostPort(taken.Addr().String())
+	if err != nil {
+		t.Fatalf("reading the bound port: %v", err)
+	}
+
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_DATA_DIR", t.TempDir())
+	t.Setenv("PORT", port)
+
+	app := newServerApp(fx.Invoke(func(*server.Server) {}))
+
+	err = app.Start(t.Context())
+	if err == nil {
+		_ = app.Stop(t.Context())
+
+		t.Fatal("startup succeeded on a port that is already bound")
+	}
+
+	if !strings.Contains(err.Error(), ":"+port) {
+		t.Errorf("startup error does not name port %s: %v", port, err)
+	}
+}

internal/state/state.go

@@ -4,6 +4,7 @@ package state
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log/slog"
 	"maps"
@@ -253,10 +254,20 @@ func (s *State) Save() error {
 		return fmt.Errorf("creating data directory: %w", err)
 	}
 
-	// Atomic write: write to temp file, then rename
+	// Atomic write: write to temp file, then rename. The temp file is
+	// synced before the rename and the directory after it, or a crash
+	// can leave the renamed state file empty.
 	tmpPath := path + ".tmp"
 
-	err = os.WriteFile(tmpPath, data, filePermissions)
+	//nolint:gosec // path is from trusted config
+	tmpFile, err := os.OpenFile(
+		tmpPath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, filePermissions,
+	)
+	if err == nil {
+		_, err = tmpFile.Write(data)
+		err = errors.Join(err, tmpFile.Sync(), tmpFile.Close())
+	}
+
 	if err != nil {
 		return fmt.Errorf("writing temp state file: %w", err)
 	}
@@ -266,6 +277,15 @@ func (s *State) Save() error {
 		return fmt.Errorf("renaming state file: %w", err)
 	}
 
+	dir, err := os.Open(filepath.Dir(path))
+	if err == nil {
+		err = errors.Join(dir.Sync(), dir.Close())
+	}
+
+	if err != nil {
+		return fmt.Errorf("syncing data directory: %w", err)
+	}
+
 	s.log.Debug("state saved to disk", "path", path)
 
 	return nil

internal/state/state_test.go

@@ -618,6 +618,53 @@ func TestSaveAtomicWrite(t *testing.T) {
 	}
 }
 
+// TestSaveOverLeftoverTempFile verifies that a temp file left by a
+// crash, longer than the state Save writes, leaves nothing of itself in
+// the state file, and that no temp file remains after Save.
+func TestSaveOverLeftoverTempFile(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	statePath := filepath.Join(dir, "state.json")
+	tmpPath := statePath + ".tmp"
+
+	err := os.WriteFile(tmpPath, []byte(strings.Repeat("x", 64*1024)), 0o600)
+	if err != nil {
+		t.Fatalf("writing leftover temp file: %v", err)
+	}
+
+	s := state.NewForTestWithDataDir(dir)
+
+	populateState(t, s)
+
+	err = s.Save()
+	if err != nil {
+		t.Fatalf("Save() error: %v", err)
+	}
+
+	//nolint:gosec // test file with known path
+	data, err := os.ReadFile(statePath)
+	if err != nil {
+		t.Fatalf("reading state file: %v", err)
+	}
+
+	var snap state.Snapshot
+
+	err = json.Unmarshal(data, &snap)
+	if err != nil {
+		t.Fatalf("state file is not complete JSON: %v", err)
+	}
+
+	if len(snap.Domains) != 2 {
+		t.Errorf("state file has %d domains, want 2", len(snap.Domains))
+	}
+
+	_, err = os.Stat(tmpPath)
+	if !os.IsNotExist(err) {
+		t.Error("temp file should not exist after save")
+	}
+}
+
 // TestLoadMissingFile verifies that loading a nonexistent file is not an error.
 func TestLoadMissingFile(t *testing.T) {
 	t.Parallel()

internal/watcher/message_test.go

@@ -5,7 +5,6 @@ import (
 	"strings"
 	"testing"
 
-	"sneak.berlin/go/dnswatcher/internal/config"
 	"sneak.berlin/go/dnswatcher/internal/state"
 	"sneak.berlin/go/dnswatcher/internal/watcher"
 )
@@ -74,21 +73,19 @@ b.ns.example.net.: 192.0.2.2`,
 func TestDomainRecordNotificationsNameTheDomain(t *testing.T) {
 	t.Parallel()
 
-	notifier := &mockNotifier{}
+	cfg := defaultTestConfig(t)
-	w := watcher.NewForTest(
+	cfg.Domains = []string{domain}
-		&config.Config{Domains: []string{domain}},
+
-		nil, nil, nil, nil, notifier,
+	deps := newTestDeps(t, cfg)
-	)
+	w := watcher.NewForTest(cfg, deps.state, nil, nil, nil, deps.notifier)
 
 	// nsA's address changes, which also makes it differ from nsC; nsB
-	// fails; nsC answers again; nsD is gone.
+	// fails; nsC answers again.
-	nsD := "d.ns.example.net."
 	w.DetectHostnameChanges(t.Context(), domain,
 		saved(map[string]*state.NameserverRecordState{
 			nsA: answered(map[string][]string{"A": {ip1}}),
 			nsB: answered(map[string][]string{"A": {ip1}}),
 			nsC: failed(),
-			nsD: answered(map[string][]string{"A": {ip1}}),
 		}),
 		saved(map[string]*state.NameserverRecordState{
 			nsA: answered(map[string][]string{"A": {ip2}}),
@@ -102,18 +99,17 @@ func TestDomainRecordNotificationsNameTheDomain(t *testing.T) {
 		t.Context(), domain, cnameState(ip1), cnameState(ip2),
 	)
 
-	// NS Failure is sent for nsB failing and for nsD being gone.
 	want := map[string]int{
 		"Record Change":        1,
 		"Inconsistency":        1,
-		"NS Failure":           2,
+		"NS Failure":           1,
 		"NS Recovery":          1,
 		"CNAME Address Change": 1,
 	}
 
 	sent := make(map[string]int)
 
-	for _, n := range notifier.getNotifications() {
+	for _, n := range deps.notifier.getNotifications() {
 		kind, _, _ := strings.Cut(n.Title, ":")
 		sent[kind]++
 
@@ -128,6 +124,89 @@ func TestDomainRecordNotificationsNameTheDomain(t *testing.T) {
 	}
 }
 
+// nsB, which answered, and nsC, which did not, are no longer among the
+// nameservers. One NS Change lists both, for the hostname and for the
+// configured apex domain with no delegation of its own. Nothing is sent
+// for the domain whose delegation on the previous check listed them:
+// the check of its delegation sends its NS Change.
+func TestRemovedNameserversAreAnNSChange(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name   string
+		target string
+		// The domain's nameservers saved by the previous check.
+		delegation []string
+		// The NS Change sent, or "" for none.
+		want string
+	}{
+		{
+			name:   "hostname",
+			target: host,
+			want: `Hostname: www.example.net
+Removed: b.ns.example.net., c.ns.example.net.`,
+		},
+		{
+			name:       "domain with no delegation of its own",
+			target:     domain,
+			delegation: []string{},
+			want: `Domain: example.net
+Removed: b.ns.example.net., c.ns.example.net.`,
+		},
+		{
+			name:       "domain whose delegation listed them",
+			target:     domain,
+			delegation: []string{nsA, nsB, nsC},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			cfg := defaultTestConfig(t)
+			cfg.Domains = []string{domain}
+
+			deps := newTestDeps(t, cfg)
+			w := watcher.NewForTest(
+				cfg, deps.state, nil, nil, nil, deps.notifier,
+			)
+
+			deps.state.SetDomainState(domain, &state.DomainState{
+				Nameservers: tt.delegation,
+			})
+
+			prev := saved(map[string]*state.NameserverRecordState{
+				nsA: answered(map[string][]string{"A": {ip1}}),
+				nsB: answered(map[string][]string{"A": {ip1}}),
+				nsC: failed(),
+			})
+			current := saved(map[string]*state.NameserverRecordState{
+				nsA: answered(map[string][]string{"A": {ip1}}),
+			})
+
+			w.DetectHostnameChanges(t.Context(), tt.target, prev, current)
+
+			got := deps.notifier.getNotifications()
+
+			if tt.want == "" {
+				if len(got) != 0 {
+					t.Errorf("sent %v, want nothing", got)
+				}
+
+				return
+			}
+
+			title := "NS Change: " + tt.target
+			if len(got) != 1 || got[0].Title != title ||
+				got[0].Message != tt.want {
+				t.Errorf("sent %v, want one %q with message:\n%s",
+					got, title, tt.want)
+			}
+		})
+	}
+}
+
 // The startup notification counts the configured domains and hostnames,
 // although the state's hostnames also hold the apex domain's own
 // records. Nothing is looked up: the watcher has no resolver.

internal/watcher/watcher.go

@@ -322,6 +322,20 @@ func (w *Watcher) checkDomain(
 		ctx, nameservers, prevAddresses,
 	)
 
+	// A domain that does not exist has no records of its own: none are
+	// asked for, and those saved by an earlier check are removed.
+	// Otherwise the apex domain's records are also checked and saved as
+	// a hostname's, so that the port and TLS checks find its addresses.
+	// Notifications about them name it as a domain (see nameLine). When
+	// their lookup fails, the check fails as when the NS lookup does, so
+	// the saved delegation and the saved records come from the same
+	// check, as detectNSDisappearances needs.
+	if nxdomain {
+		w.state.DeleteHostnameState(domain)
+	} else if !w.checkHostname(ctx, domain) {
+		return
+	}
+
 	if hasPrev && !w.firstRun {
 		w.detectNSChanges(ctx, domain, prev.Nameservers, nameservers)
 		w.detectNSAddressChanges(ctx, domain, prevAddresses, addresses)
@@ -333,19 +347,6 @@ func (w *Watcher) checkDomain(
 		NXDomain:            nxdomain,
 		LastChecked:         now,
 	})
-
-	// A domain that does not exist has no records of its own: none are
-	// asked for, and those saved by an earlier check are removed.
-	if nxdomain {
-		w.state.DeleteHostnameState(domain)
-
-		return
-	}
-
-	// The apex domain's records are also checked and saved as a
-	// hostname's, so that the port and TLS checks find its addresses.
-	// Notifications about them name it as a domain (see nameLine).
-	w.checkHostname(ctx, domain)
 }
 
 func (w *Watcher) detectNSChanges(
@@ -456,10 +457,12 @@ func (w *Watcher) detectNSAddressChanges(
 	}
 }
 
+// checkHostname checks hostname's records and reports whether it saved
+// them; it does not when their lookup fails.
 func (w *Watcher) checkHostname(
 	ctx context.Context,
 	hostname string,
-) {
+) bool {
 	results, err := w.resolver.LookupAllRecords(ctx, hostname)
 	if err != nil {
 		w.logFailedLookup(
@@ -469,7 +472,7 @@ func (w *Watcher) checkHostname(
 			"error", err,
 		)
 
-		return
+		return false
 	}
 
 	prev, _ := w.state.GetHostnameState(hostname)
@@ -477,6 +480,8 @@ func (w *Watcher) checkHostname(
 	w.updateHostnameState(
 		ctx, hostname, buildHostnameState(results, prev, time.Now().UTC()),
 	)
+
+	return true
 }
 
 // updateHostnameState finishes a check of hostname from newState, built
@@ -772,28 +777,53 @@ func (w *Watcher) detectRecordChanges(
 	}
 }
 
+// detectNSDisappearances sends one NS Change listing the nameservers of
+// the previous check that are no longer those of the zone hostname is
+// in. For a configured apex domain it leaves out those of the domain's
+// saved delegation, which detectNSChanges notifies when they are removed
+// from it. checkDomain saves a new delegation only after this runs, and
+// only together with the records, so the saved delegation and the
+// records in prev come from the same check.
 func (w *Watcher) detectNSDisappearances(
 	ctx context.Context,
 	hostname string,
 	prev, current *state.HostnameState,
 ) {
-	for ns, prevNS := range prev.RecordsByNameserver {
+	var delegation []string
-		if _, ok := current.RecordsByNameserver[ns]; ok || prevNS.Status != statusOK {
+
-			continue
+	if w.isDomain(hostname) {
+		if domainState, ok := w.state.GetDomainState(hostname); ok {
+			delegation = domainState.Nameservers
+		}
 	}
 
+	var removed []string
+
+	for ns := range prev.RecordsByNameserver {
+		_, ok := current.RecordsByNameserver[ns]
+		if !ok && !slices.Contains(delegation, ns) {
+			removed = append(removed, ns)
+		}
+	}
+
+	if len(removed) == 0 {
+		return
+	}
+
+	slices.Sort(removed)
+
 	msg := fmt.Sprintf(
-			"%s\nNameserver: %s disappeared",
+		"%s\nRemoved: %s",
-			w.nameLine(hostname), ns,
+		w.nameLine(hostname),
+		strings.Join(removed, ", "),
 	)
 
 	w.notify.SendNotification(
 		ctx,
-			"NS Failure: "+hostname,
+		"NS Change: "+hostname,
 		msg,
-			"error",
+		"warning",
 	)
-	}
 }
 
 // detectNSFailures notifies when a nameserver that answered on the

internal/watcher/watcher_test.go

@@ -139,6 +139,22 @@ func (m *mockNotifier) getNotifications() []notification {
 	return result
 }
 
+// cancelOnNotify is a mockNotifier that calls cancel at each
+// notification, as if shutdown cut the check short there.
+type cancelOnNotify struct {
+	*mockNotifier
+
+	cancel context.CancelFunc
+}
+
+func (n cancelOnNotify) SendNotification(
+	ctx context.Context,
+	title, message, priority string,
+) {
+	n.mockNotifier.SendNotification(ctx, title, message, priority)
+	n.cancel()
+}
+
 // --- Helpers to build a Watcher and run its checks against live DNS ---
 
 type testDeps struct {
@@ -558,6 +574,11 @@ func TestDomainThatDoesNotExist(t *testing.T) {
 // nameservers and without nxdomain, and its records, asked at the
 // codeberg.page servers, are saved. Those are testSmallDomain's two
 // nameservers; github.io, the zone of the README's example, has eight.
+// The previous check saved a delegation to oldNS1, now withdrawn, and
+// records asked at oldNS1. A first check is cut short at its first
+// notification, as shutdown would, and a second runs in full: across
+// them the NS Change of its delegation removes oldNS1, and no other NS
+// Change lists it again.
 func TestDomainWithNoDelegationOfItsOwn(t *testing.T) {
 	t.Parallel()
 
@@ -569,9 +590,30 @@ func TestDomainWithNoDelegationOfItsOwn(t *testing.T) {
 	var deps *testDeps
 
 	livednstest.Retry(t, "watcher checks", func(ctx context.Context) error {
-		var w *watcher.Watcher
+		deps = newTestDeps(t, cfg)
 
-		w, deps = newTestWatcher(t, cfg)
+		firstCtx, cancel := context.WithCancel(ctx)
+		defer cancel()
+
+		w := watcher.NewForTest(
+			cfg,
+			deps.state,
+			resolver.NewFromLogger(slog.Default()),
+			deps.portChecker,
+			deps.tlsChecker,
+			cancelOnNotify{mockNotifier: deps.notifier, cancel: cancel},
+		)
+
+		deps.state.SetDomainState(domain, &state.DomainState{
+			Nameservers: []string{oldNS1},
+		})
+		deps.state.SetHostnameState(domain, saved(
+			map[string]*state.NameserverRecordState{
+				oldNS1: answered(map[string][]string{"A": {oldIP}}),
+			},
+		))
+
+		w.RunOnce(firstCtx)
 
 		err := checkOnce(ctx, w, deps)
 
@@ -593,6 +635,22 @@ func TestDomainWithNoDelegationOfItsOwn(t *testing.T) {
 	if _, ok := deps.state.GetHostnameState(domain); !ok {
 		t.Errorf("no records saved for %s", domain)
 	}
+
+	title := "NS Change: " + domain
+	want := "Domain: " + domain + "\nAdded: \nRemoved: " + oldNS1
+
+	var changes []notification
+
+	for _, n := range deps.notifier.getNotifications() {
+		if n.Title == title {
+			changes = append(changes, n)
+		}
+	}
+
+	if len(changes) != 1 || changes[0].Message != want {
+		t.Errorf("sent %v, want one %q with message:\n%s",
+			changes, title, want)
+	}
 }
 
 func TestNSAddressChangeDetection(t *testing.T) {
@@ -1049,7 +1107,7 @@ func TestSendTestNotification_Disabled(t *testing.T) {
 	}
 }
 
-func TestNSFailureAndRecovery(t *testing.T) {
+func TestNSChangeAndRecovery(t *testing.T) {
 	t.Parallel()
 
 	cfg := defaultTestConfig(t)
@@ -1059,7 +1117,7 @@ func TestNSFailureAndRecovery(t *testing.T) {
 
 	// The saved state has every nameserver live DNS lists as one that
 	// did not answer, and, as answering, one that live DNS does not
-	// list, which then disappears.
+	// list, which the check then finds removed.
 	_, deps := runChecks(t, cfg, func(deps *testDeps) {
 		byNameserver := map[string]*state.NameserverRecordState{
 			oldNS1: answered(map[string][]string{"A": {oldIP}}),
@@ -1071,7 +1129,7 @@ func TestNSFailureAndRecovery(t *testing.T) {
 		deps.state.SetHostnameState(testHost, saved(byNameserver))
 	})
 
-	assertNotified(t, deps, "NS Failure: "+testHost, "error")
+	assertNotified(t, deps, "NS Change: "+testHost, "warning")
 	assertNotified(t, deps, "NS Recovery: "+testHost, "success")
 
 	// A nameserver that did not answer has no records to compare, so