healthcheck: answer 503 when the last round of DNS checks reached no root server (closes #302)

The health check answered ok while every DNS lookup failed, as on a
network that intercepts DNS queries. A lookup whose root servers give no
usable reply now fails with ErrNoRootServerAnswered. At the end of each
round of DNS checks the watcher records whether every lookup failed with
it or with ErrIntercepted; the health check then gives status error, and
/.well-known/healthcheck and /health answer 503. Until the first round
finishes it stays ok.

The health check now depends on the watcher, so fx builds the watcher
first: its first round starts before the HTTP server listens, and the
server tests' fx graph includes it. NewWithRootServers lets tests start
a resolver's walks at real servers that are not root servers.

Model: opus-5-5

internal/handlers/healthcheck.go

@@ -2,16 +2,25 @@ package handlers
 
 import (
 	"net/http"
+
+	"sneak.berlin/go/dnswatcher/internal/healthcheck"
 )
 
-// HandleHealthCheck returns the health check handler.
+// HandleHealthCheck returns the health check handler. It answers 503
+// Service Unavailable when the health check's status is error, so that
+// a container health check fails, and 200 OK otherwise.
 func (h *Handlers) HandleHealthCheck() http.HandlerFunc {
 	return func(
 		writer http.ResponseWriter,
 		request *http.Request,
 	) {
-		h.respondJSON(
-			writer, request, h.hc.Check(), http.StatusOK,
-		)
+		resp := h.hc.Check()
+
+		code := http.StatusOK
+		if resp.Status == healthcheck.StatusError {
+			code = http.StatusServiceUnavailable
+		}
+
+		h.respondJSON(writer, request, resp, code)
 	}
 }

internal/handlers/healthcheck_test.go

@@ -0,0 +1,158 @@
+package handlers_test
+
+import (
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"go.uber.org/fx/fxtest"
+
+	"sneak.berlin/go/dnswatcher/internal/config"
+	"sneak.berlin/go/dnswatcher/internal/globals"
+	"sneak.berlin/go/dnswatcher/internal/handlers"
+	"sneak.berlin/go/dnswatcher/internal/healthcheck"
+	"sneak.berlin/go/dnswatcher/internal/logger"
+	"sneak.berlin/go/dnswatcher/internal/notify"
+	"sneak.berlin/go/dnswatcher/internal/portcheck"
+	"sneak.berlin/go/dnswatcher/internal/resolver"
+	"sneak.berlin/go/dnswatcher/internal/state"
+	"sneak.berlin/go/dnswatcher/internal/tlscheck"
+	"sneak.berlin/go/dnswatcher/internal/watcher"
+)
+
+// TestHealthCheckAnswers503WhenDNSUnreachable serves the health check
+// before the watcher's first round of DNS checks, when it answers 200
+// with status ok, and after a round that could not reach the DNS, when
+// it answers 503 with status error. The watcher's resolver starts every
+// walk at public recursive resolvers of four operators, whose replies
+// are marked recursion available, as on a network that intercepts DNS
+// queries. If none of them replies, no root server answered, and the
+// round could not reach the DNS either.
+func TestHealthCheckAnswers503WhenDNSUnreachable(t *testing.T) {
+	t.Parallel()
+
+	hnd, w := newHandlersWithWatcher(t, resolver.NewWithRootServers(
+		slog.Default(),
+		[]string{"1.1.1.1", "8.8.8.8", "9.9.9.9", "208.67.222.222"},
+	))
+
+	assertHealthCheck(t, hnd, http.StatusOK, healthcheck.StatusOK)
+
+	w.RunOnce(t.Context())
+
+	assertHealthCheck(
+		t, hnd, http.StatusServiceUnavailable, healthcheck.StatusError,
+	)
+}
+
+// newHandlersWithWatcher builds real Handlers whose health check
+// follows the watcher it returns, which watches example.org with res.
+// The lifecycle is never started, so the watcher runs a round of checks
+// only when the test calls RunOnce.
+func newHandlersWithWatcher(
+	t *testing.T,
+	res *resolver.Resolver,
+) (*handlers.Handlers, *watcher.Watcher) {
+	t.Helper()
+
+	glob, err := globals.New(nil)
+	if err != nil {
+		t.Fatalf("globals.New: %v", err)
+	}
+
+	log, err := logger.New(nil, logger.Params{Globals: glob})
+	if err != nil {
+		t.Fatalf("logger.New: %v", err)
+	}
+
+	cfg := &config.Config{
+		Hostnames: []string{"example.org"},
+		DataDir:   t.TempDir(),
+	}
+	lifecycle := fxtest.NewLifecycle(t)
+
+	st, err := state.New(lifecycle, state.Params{Logger: log, Config: cfg})
+	if err != nil {
+		t.Fatalf("state.New: %v", err)
+	}
+
+	notifier, err := notify.New(
+		lifecycle, notify.Params{Logger: log, Config: cfg},
+	)
+	if err != nil {
+		t.Fatalf("notify.New: %v", err)
+	}
+
+	w, err := watcher.New(lifecycle, watcher.Params{
+		Logger:    log,
+		Config:    cfg,
+		State:     st,
+		Resolver:  res,
+		PortCheck: portcheck.NewStandalone(),
+		TLSCheck:  tlscheck.NewStandalone(),
+		Notify:    notifier,
+	})
+	if err != nil {
+		t.Fatalf("watcher.New: %v", err)
+	}
+
+	hc, err := healthcheck.New(lifecycle, healthcheck.Params{
+		Globals: glob,
+		Config:  cfg,
+		Logger:  log,
+		Watcher: w,
+	})
+	if err != nil {
+		t.Fatalf("healthcheck.New: %v", err)
+	}
+
+	hnd, err := handlers.New(nil, handlers.Params{
+		Logger:      log,
+		Globals:     glob,
+		Healthcheck: hc,
+		State:       st,
+		Notify:      notifier,
+	})
+	if err != nil {
+		t.Fatalf("handlers.New: %v", err)
+	}
+
+	return hnd, w
+}
+
+// assertHealthCheck serves one request to the health check and checks
+// that it answers with code and a JSON body whose status is status.
+func assertHealthCheck(
+	t *testing.T,
+	hnd *handlers.Handlers,
+	code int,
+	status string,
+) {
+	t.Helper()
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/.well-known/healthcheck", nil,
+	)
+
+	hnd.HandleHealthCheck()(rec, req)
+
+	if rec.Code != code {
+		t.Errorf("code = %d, want %d", rec.Code, code)
+	}
+
+	var body struct {
+		Status string `json:"status"`
+	}
+
+	err := json.Unmarshal(rec.Body.Bytes(), &body)
+	if err != nil {
+		t.Fatalf("decoding response: %v", err)
+	}
+
+	if body.Status != status {
+		t.Errorf("status = %q, want %q", body.Status, status)
+	}
+}

internal/healthcheck/healthcheck.go

@@ -11,6 +11,13 @@ import (
 	"sneak.berlin/go/dnswatcher/internal/config"
 	"sneak.berlin/go/dnswatcher/internal/globals"
 	"sneak.berlin/go/dnswatcher/internal/logger"
+	"sneak.berlin/go/dnswatcher/internal/watcher"
+)
+
+// Values of Response.Status.
+const (
+	StatusOK    = "ok"
+	StatusError = "error"
 )
 
 // Params contains dependencies for Healthcheck.
@@ -20,6 +27,7 @@ type Params struct {
 	Globals *globals.Globals
 	Config  *config.Config
 	Logger  *logger.Logger
+	Watcher *watcher.Watcher
 }
 
 // Healthcheck provides health status information.
@@ -61,10 +69,17 @@ func New(
 	return healthcheck, nil
 }
 
-// Check returns the current health status.
+// Check returns the current health status. Status is StatusError when
+// the watcher's most recent round of DNS checks could not reach the DNS
+// (see watcher.Watcher.DNSUnreachable), and StatusOK otherwise.
 func (h *Healthcheck) Check() *Response {
+	status := StatusOK
+	if h.params.Watcher.DNSUnreachable() {
+		status = StatusError
+	}
+
 	return &Response{
-		Status:        "ok",
+		Status:        status,
 		Now:           time.Now().UTC().Format(time.RFC3339Nano),
 		UptimeSeconds: int64(h.uptime().Seconds()),
 		UptimeHuman:   h.uptime().String(),

internal/healthcheck/healthcheck_test.go

@@ -3,10 +3,12 @@ package healthcheck_test
 import (
 	"context"
 	"encoding/json"
+	"log/slog"
 	"testing"
 	"time"
 
 	"go.uber.org/fx"
+	"go.uber.org/fx/fxtest"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -15,6 +17,12 @@ import (
 	"sneak.berlin/go/dnswatcher/internal/globals"
 	"sneak.berlin/go/dnswatcher/internal/healthcheck"
 	"sneak.berlin/go/dnswatcher/internal/logger"
+	"sneak.berlin/go/dnswatcher/internal/notify"
+	"sneak.berlin/go/dnswatcher/internal/portcheck"
+	"sneak.berlin/go/dnswatcher/internal/resolver"
+	"sneak.berlin/go/dnswatcher/internal/state"
+	"sneak.berlin/go/dnswatcher/internal/tlscheck"
+	"sneak.berlin/go/dnswatcher/internal/watcher"
 )
 
 // recordingLifecycle is a minimal fx.Lifecycle that records the hooks
@@ -30,12 +38,14 @@ func (l *recordingLifecycle) Append(hook fx.Hook) {
 
 // newHealthcheck builds a Healthcheck through the real constructor and
 // runs the registered OnStart hook so StartupTime is set the same way
-// the fx lifecycle would set it.
+// the fx lifecycle would set it. It follows the watcher it returns,
+// which watches example.org with res and has run no round of checks.
 func newHealthcheck(
 	t *testing.T,
+	res *resolver.Resolver,
 	maintenance bool,
 	version string,
-) *healthcheck.Healthcheck {
+) (*healthcheck.Healthcheck, *watcher.Watcher) {
 	t.Helper()
 
 	g := &globals.Globals{Appname: "dnswatcher", Version: version}
@@ -43,12 +53,21 @@ func newHealthcheck(
 	log, err := logger.New(nil, logger.Params{Globals: g})
 	require.NoError(t, err)
 
+	cfg := &config.Config{
+		MaintenanceMode: maintenance,
+		Hostnames:       []string{"example.org"},
+		DataDir:         t.TempDir(),
+	}
+
+	w := newWatcher(t, log, cfg, res)
+
 	lifecycle := &recordingLifecycle{}
 
 	hc, err := healthcheck.New(lifecycle, healthcheck.Params{
 		Globals: g,
-		Config:  &config.Config{MaintenanceMode: maintenance},
+		Config:  cfg,
 		Logger:  log,
+		Watcher: w,
 	})
 	require.NoError(t, err)
 
@@ -57,16 +76,53 @@ func newHealthcheck(
 	require.NotNil(t, lifecycle.hooks[0].OnStart)
 	require.NoError(t, lifecycle.hooks[0].OnStart(context.Background()))
 
-	return hc
+	return hc, w
+}
+
+// newWatcher builds a watcher through the real constructor, with res as
+// its resolver. Its lifecycle is never started, so it runs a round of
+// checks only when the test calls RunOnce.
+func newWatcher(
+	t *testing.T,
+	log *logger.Logger,
+	cfg *config.Config,
+	res *resolver.Resolver,
+) *watcher.Watcher {
+	t.Helper()
+
+	lifecycle := fxtest.NewLifecycle(t)
+
+	st, err := state.New(lifecycle, state.Params{Logger: log, Config: cfg})
+	require.NoError(t, err)
+
+	notifier, err := notify.New(
+		lifecycle, notify.Params{Logger: log, Config: cfg},
+	)
+	require.NoError(t, err)
+
+	w, err := watcher.New(lifecycle, watcher.Params{
+		Logger:    log,
+		Config:    cfg,
+		State:     st,
+		Resolver:  res,
+		PortCheck: portcheck.NewStandalone(),
+		TLSCheck:  tlscheck.NewStandalone(),
+		Notify:    notifier,
+	})
+	require.NoError(t, err)
+
+	return w
 }
 
 func TestCheckStatusAndPayloadShape(t *testing.T) {
 	t.Parallel()
 
-	hc := newHealthcheck(t, false, "v9.9.9")
+	hc, _ := newHealthcheck(
+		t, resolver.NewFromLogger(slog.Default()), false, "v9.9.9",
+	)
 	resp := hc.Check()
 
-	assert.Equal(t, "ok", resp.Status)
+	assert.Equal(t, healthcheck.StatusOK, resp.Status)
 
 	// The JSON shape and field names are part of the contract for the
 	// /health and /.well-known/healthcheck routes, so assert on the
@@ -114,7 +170,10 @@ func TestCheckMaintenanceModeReflectsConfig(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			hc := newHealthcheck(t, tt.maintenance, "test")
+			hc, _ := newHealthcheck(
+				t, resolver.NewFromLogger(slog.Default()), tt.maintenance,
+				"test",
+			)
 			resp := hc.Check()
 			assert.Equal(t, tt.maintenance, resp.Maintenance,
 				"maintenanceMode must mirror Config.MaintenanceMode")
@@ -125,10 +184,38 @@ func TestCheckMaintenanceModeReflectsConfig(t *testing.T) {
 func TestCheckSurfacesVersionAndAppname(t *testing.T) {
 	t.Parallel()
 
-	hc := newHealthcheck(t, false, "surfaced-version-123")
+	hc, _ := newHealthcheck(
+		t, resolver.NewFromLogger(slog.Default()), false,
+		"surfaced-version-123",
+	)
 	resp := hc.Check()
 
 	assert.Equal(t, "surfaced-version-123", resp.Version,
 		"version from globals must appear in the payload")
 	assert.Equal(t, "dnswatcher", resp.Appname)
 }
+
+// TestCheckStatusFollowsTheDNSChecks checks that the status is ok
+// before the watcher's first round of DNS checks, and error after a
+// round that could not reach the DNS. The watcher's resolver starts
+// every walk at public recursive resolvers of four operators, whose
+// replies are marked recursion available, as on a network that
+// intercepts DNS queries. If none of them replies, no root server
+// answered, and the round could not reach the DNS either.
+func TestCheckStatusFollowsTheDNSChecks(t *testing.T) {
+	t.Parallel()
+
+	res := resolver.NewWithRootServers(
+		slog.Default(),
+		[]string{"1.1.1.1", "8.8.8.8", "9.9.9.9", "208.67.222.222"},
+	)
+	hc, w := newHealthcheck(t, res, false, "test")
+
+	assert.Equal(t, healthcheck.StatusOK, hc.Check().Status,
+		"status before the first round")
+
+	w.RunOnce(t.Context())
+
+	assert.Equal(t, healthcheck.StatusError, hc.Check().Status,
+		"status after a round that could not reach the DNS")
+}

internal/resolver/errors.go

@@ -36,6 +36,12 @@ var (
 	// so whether the name has addresses is unknown.
 	ErrNoNameserverAnswered = errors.New("no nameserver answered")
 
+	// ErrNoRootServerAnswered is returned when the root servers were
+	// asked and none of them gave a usable reply, as when the host
+	// running dnswatcher has lost its network. When they all refused,
+	// the error is ErrIntercepted instead.
+	ErrNoRootServerAnswered = errors.New("no root server answered")
+
 	// ErrNoNameserverAddress is returned when a nameserver is to be
 	// asked and no address for it was given.
 	ErrNoNameserverAddress = errors.New("nameserver has no address")

internal/resolver/iterative.go

@@ -293,7 +293,8 @@ func shuffled(
 // place. Otherwise a server that times out, refuses or gives a reply
 // that is not usable is passed over for the next. When every server
 // refused, the error says so, and when they are the root servers it is
-// ErrIntercepted.
+// ErrIntercepted. When the root servers gave no usable reply otherwise,
+// it is ErrNoRootServerAnswered.
 func (r *Resolver) queryServers(
 	ctx context.Context,
 	servers []string,
@@ -352,6 +353,10 @@ func (r *Resolver) queryServers(
 		)
 	}
 
+	if zone == "." {
+		return nil, fmt.Errorf("%w: %w", ErrNoRootServerAnswered, lastErr)
+	}
+
 	return nil, fmt.Errorf("all servers failed: %w", lastErr)
 }
 
@@ -528,7 +533,7 @@ func (r *Resolver) resolveNSIterative(
 	}
 
 	domain = dns.Fqdn(domain)
-	servers := rootServerList()
+	servers := r.rootServers
 	zone := "."
 
 	for range maxDelegation {
@@ -592,7 +597,7 @@ func (r *Resolver) resolveARecord(
 	}
 
 	hostname = dns.Fqdn(hostname)
-	servers := rootServerList()
+	servers := r.rootServers
 	zone := "."
 
 	var withoutAddresses []string
@@ -652,7 +657,7 @@ func (r *Resolver) FindAuthoritativeNameservers(
 	ctx context.Context,
 	domain string,
 ) ([]string, error) {
-	return r.findAuthoritativeNameservers(ctx, domain, rootServerList())
+	return r.findAuthoritativeNameservers(ctx, domain, r.rootServers)
 }
 
 // findAuthoritativeNameservers is FindAuthoritativeNameservers with each
@@ -1127,7 +1132,7 @@ func (r *Resolver) LookupNS(
 	}
 
 	return r.followDelegation(
-		ctx, dns.Fqdn(strings.ToLower(domain)), ".", nil, rootServerList(),
+		ctx, dns.Fqdn(strings.ToLower(domain)), ".", nil, r.rootServers,
 	)
 }
 

internal/resolver/resolver.go

@@ -43,10 +43,12 @@ type NameserverResponse struct {
 }
 
 // Resolver performs iterative DNS resolution from root servers.
+// rootServers are the addresses every walk from the root starts at.
 type Resolver struct {
-	log    *slog.Logger
-	client DNSClient
-	tcp    DNSClient
+	log         *slog.Logger
+	client      DNSClient
+	tcp         DNSClient
+	rootServers []string
 }
 
 // New creates a new Resolver instance for use with uber/fx.
@@ -55,9 +57,10 @@ func New(
 	params Params,
 ) (*Resolver, error) {
 	return &Resolver{
-		log:    params.Logger.Get(),
-		client: &udpClient{timeout: queryTimeoutDuration},
-		tcp:    &tcpClient{timeout: queryTimeoutDuration},
+		log:         params.Logger.Get(),
+		client:      &udpClient{timeout: queryTimeoutDuration},
+		tcp:         &tcpClient{timeout: queryTimeoutDuration},
+		rootServers: rootServerList(),
 	}, nil
 }
 
@@ -65,10 +68,22 @@ func New(
 // useful for testing without the fx lifecycle.
 func NewFromLogger(log *slog.Logger) *Resolver {
 	return &Resolver{
-		log:    log,
-		client: &udpClient{timeout: queryTimeoutDuration},
-		tcp:    &tcpClient{timeout: queryTimeoutDuration},
+		log:         log,
+		client:      &udpClient{timeout: queryTimeoutDuration},
+		tcp:         &tcpClient{timeout: queryTimeoutDuration},
+		rootServers: rootServerList(),
 	}
 }
 
+// NewWithRootServers creates a Resolver as NewFromLogger does, except
+// that every walk from the root starts at servers instead of at the root
+// servers. Tests use it to show what dnswatcher does when no root server
+// answers, or when something on the network answers in their place.
+func NewWithRootServers(log *slog.Logger, servers []string) *Resolver {
+	r := NewFromLogger(log)
+	r.rootServers = servers
+
+	return r
+}
+
 // Method implementations are in iterative.go.

internal/resolver/resolver_test.go

@@ -815,6 +815,7 @@ func TestQueryServers_EveryRootServerRefused(t *testing.T) {
 // but nothing answers at 192.0.2.1, a documentation address, so not
 // every server refused, wherever 192.0.2.1 falls in the random order:
 // the error is not ErrIntercepted and does not say every server refused.
+// No root server gave a usable reply, so it is ErrNoRootServerAnswered.
 func TestQueryServers_NotEveryRootServerRefused(t *testing.T) {
 	t.Parallel()
 
@@ -848,6 +849,34 @@ func TestQueryServers_NotEveryRootServerRefused(t *testing.T) {
 	require.NotErrorIs(t, err, resolver.ErrIntercepted)
 	// Both errors for every server refusing say "refused a query for".
 	require.NotContains(t, err.Error(), "refused a query for")
+	require.ErrorIs(t, err, resolver.ErrNoRootServerAnswered)
+}
+
+// TestLookupsWhenNoRootServerAnswers starts every walk at 192.0.2.1, a
+// documentation address where nothing answers, as if no root server
+// answered. The lookups the watcher makes, LookupNS for a domain and
+// LookupAllRecords for a hostname, both fail with
+// ErrNoRootServerAnswered, which the watcher reads.
+func TestLookupsWhenNoRootServerAnswers(t *testing.T) {
+	t.Parallel()
+
+	r := resolver.NewWithRootServers(slog.Default(), []string{noAnswerAddress})
+
+	t.Run("LookupNS", func(t *testing.T) {
+		t.Parallel()
+
+		_, err := r.LookupNS(t.Context(), "google.com")
+		require.ErrorIs(t, err, resolver.ErrNoRootServerAnswered)
+		require.NotErrorIs(t, err, resolver.ErrIntercepted)
+	})
+
+	t.Run("LookupAllRecords", func(t *testing.T) {
+		t.Parallel()
+
+		_, err := r.LookupAllRecords(t.Context(), "www.google.com")
+		require.ErrorIs(t, err, resolver.ErrNoRootServerAnswered)
+		require.NotErrorIs(t, err, resolver.ErrIntercepted)
+	})
 }
 
 // TestQueryServers_RecursiveResolversAsRootServers passes public

internal/server/server_test.go

@@ -15,14 +15,21 @@ import (
 	"sneak.berlin/go/dnswatcher/internal/logger"
 	"sneak.berlin/go/dnswatcher/internal/middleware"
 	"sneak.berlin/go/dnswatcher/internal/notify"
+	"sneak.berlin/go/dnswatcher/internal/portcheck"
+	"sneak.berlin/go/dnswatcher/internal/resolver"
 	"sneak.berlin/go/dnswatcher/internal/server"
 	"sneak.berlin/go/dnswatcher/internal/state"
+	"sneak.berlin/go/dnswatcher/internal/tlscheck"
+	"sneak.berlin/go/dnswatcher/internal/watcher"
 )
 
 // newServerApp builds an fx app holding a *server.Server wired exactly
-// as cmd/dnswatcher wires it, minus the watcher/resolver subtree that
-// would touch live DNS, plus the given option. config.New reads viper,
-// so the caller must first configure it, which is also why the caller
+// as cmd/dnswatcher wires it, plus the given option. The health check
+// follows the watcher, so the watcher is in the app too. Starting the
+// app starts the watcher's first round of checks, against live DNS,
+// before the server; the tests that start it expect the server to fail
+// to start, which stops the watcher again. config.New reads viper, so
+// the caller must first configure it, which is also why the caller
 // cannot run in parallel.
 func newServerApp(option fx.Option) *fx.App {
 	return fx.New(
@@ -33,11 +40,29 @@ func newServerApp(option fx.Option) *fx.App {
 			config.New,
 			state.New,
 			healthcheck.New,
+			resolver.New,
+			portcheck.New,
+			tlscheck.New,
 			notify.New,
+			watcher.New,
 			middleware.New,
 			handlers.New,
 			server.New,
 		),
+		fx.Provide(
+			func(r *resolver.Resolver) watcher.DNSResolver {
+				return r
+			},
+			func(p *portcheck.Checker) watcher.PortChecker {
+				return p
+			},
+			func(t *tlscheck.Checker) watcher.TLSChecker {
+				return t
+			},
+			func(n *notify.Service) watcher.Notifier {
+				return n
+			},
+		),
 		option,
 	)
 }

internal/watcher/dnsunreachable_test.go

@@ -0,0 +1,65 @@
+package watcher_test
+
+import (
+	"log/slog"
+	"testing"
+
+	"sneak.berlin/go/dnswatcher/internal/resolver"
+	"sneak.berlin/go/dnswatcher/internal/watcher"
+)
+
+// TestDNSUnreachableWhenNoRootServerReplies runs one round of checks of
+// a domain and a hostname with every walk starting at real servers that
+// are not root servers. At 192.0.2.1, a documentation address, nothing
+// answers, as when no root server answers. Public recursive resolvers of
+// four operators mark their replies recursion available, as on a network
+// that intercepts DNS queries; one that gives no reply is passed over
+// for the next, and if none replies, no root server answered. Either
+// way no lookup got a reply from a root server, so the round could not
+// reach the DNS. TestFirstRunBaseline and TestDomainThatDoesNotExist
+// check rounds in which the root servers answered.
+func TestDNSUnreachableWhenNoRootServerReplies(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		servers []string
+	}{
+		{"no root server answers", []string{"192.0.2.1"}},
+		{
+			"the network intercepts DNS queries",
+			[]string{"1.1.1.1", "8.8.8.8", "9.9.9.9", "208.67.222.222"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			cfg := defaultTestConfig(t)
+			cfg.Domains = []string{testSmallDomain}
+			cfg.Hostnames = []string{testHost}
+
+			deps := newTestDeps(t, cfg)
+			w := watcher.NewForTest(
+				cfg,
+				deps.state,
+				resolver.NewWithRootServers(slog.Default(), tt.servers),
+				deps.portChecker,
+				deps.tlsChecker,
+				deps.notifier,
+			)
+
+			if w.DNSUnreachable() {
+				t.Error("DNSUnreachable is true before the first round")
+			}
+
+			w.RunOnce(t.Context())
+
+			if !w.DNSUnreachable() {
+				t.Error("DNSUnreachable is false after a round in " +
+					"which no lookup got a reply from a root server")
+			}
+		})
+	}
+}

internal/watcher/watcher.go

@@ -11,6 +11,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"sync/atomic"
 	"time"
 
 	"go.uber.org/fx"
@@ -62,6 +63,11 @@ type Watcher struct {
 	cancel    context.CancelFunc
 	done      chan struct{} // closed when Run returns
 	firstRun  bool
+
+	// dnsUnreachable is what DNSUnreachable reports. Each round of DNS
+	// checks sets it, and the HTTP server reads it from its own
+	// goroutines.
+	dnsUnreachable atomic.Bool
 }
 
 // New creates a new Watcher instance wired into the fx lifecycle.
@@ -197,6 +203,18 @@ func (w *Watcher) RunOnce(ctx context.Context) {
 	w.firstRun = false
 }
 
+// DNSUnreachable reports whether the most recent round of DNS checks
+// could not reach the DNS at all: no lookup in it got a reply from a
+// root server, because none of them answered or because the network
+// intercepts DNS queries. A round in which the root servers answered
+// reached the DNS, even when a name's own nameservers did not answer or
+// a domain does not exist. It is false until the first round finishes,
+// and a round that shutdown cut short changes nothing. It may be called
+// while a round runs.
+func (w *Watcher) DNSUnreachable() bool {
+	return w.dnsUnreachable.Load()
+}
+
 func (w *Watcher) detectFirstRun() {
 	snap := w.state.GetSnapshot()
 	hasState := len(snap.Domains) > 0 ||
@@ -265,15 +283,40 @@ func (w *Watcher) cleanupRemovedTargets() {
 // runDNSChecks performs DNS resolution for all configured domains
 // and hostnames, updating state with freshly resolved records.
 // This must complete before port or TLS checks run so those
-// checks operate on current IP addresses.
+// checks operate on current IP addresses. Unless ctx was cut short, it
+// then records whether this round could reach the DNS (see
+// DNSUnreachable).
 func (w *Watcher) runDNSChecks(ctx context.Context) {
+	errs := make([]error, 0, len(w.config.Domains)+len(w.config.Hostnames))
+
 	for _, domain := range w.config.Domains {
-		w.checkDomain(ctx, domain)
+		errs = append(errs, w.checkDomain(ctx, domain))
 	}
 
 	for _, hostname := range w.config.Hostnames {
-		w.checkHostname(ctx, hostname)
+		_, err := w.checkHostname(ctx, hostname)
+		errs = append(errs, err)
 	}
+
+	if ctx.Err() == nil {
+		w.dnsUnreachable.Store(noRootServerReplied(errs))
+	}
+}
+
+// noRootServerReplied reports whether errs, the errors of the lookups in
+// a round of DNS checks, nil for a lookup that succeeded, show that no
+// lookup got a reply from a root server: each failed because none of
+// them answered, or because the network intercepts DNS queries. A round
+// with no lookups shows nothing.
+func noRootServerReplied(errs []error) bool {
+	for _, err := range errs {
+		if !errors.Is(err, resolver.ErrNoRootServerAnswered) &&
+			!errors.Is(err, resolver.ErrIntercepted) {
+			return false
+		}
+	}
+
+	return len(errs) > 0
 }
 
 // logFailedLookup logs a failed DNS lookup at error level, unless ctx
@@ -292,14 +335,16 @@ func (w *Watcher) logFailedLookup(
 }
 
 // checkDomain looks up domain's delegation and checks domain with it
-// (see checkDomainResult).
+// (see checkDomainResult). It returns the lookup's error.
 func (w *Watcher) checkDomain(
 	ctx context.Context,
 	domain string,
-) {
+) error {
 	nameservers, err := w.resolver.LookupNS(ctx, domain)
 
 	w.checkDomainResult(ctx, domain, nameservers, err)
+
+	return err
 }
 
 // checkDomainResult finishes a check of domain from nameservers and err,
@@ -358,7 +403,7 @@ func (w *Watcher) checkDomainResult(
 	// check, as detectNSDisappearances needs.
 	if nxdomain {
 		w.state.DeleteHostnameState(domain)
-	} else if !w.checkHostname(ctx, domain) {
+	} else if saved, _ := w.checkHostname(ctx, domain); !saved {
 		return
 	}
 
@@ -484,11 +529,12 @@ func (w *Watcher) detectNSAddressChanges(
 }
 
 // checkHostname looks up hostname's records and checks them (see
-// checkHostnameResult). It reports whether it saved them.
+// checkHostnameResult). It reports whether it saved them, and returns
+// the lookup's error.
 func (w *Watcher) checkHostname(
 	ctx context.Context,
 	hostname string,
-) bool {
+) (bool, error) {
 	results, err := w.resolver.LookupAllRecords(ctx, hostname)
 	if err != nil {
 		w.logFailedLookup(
@@ -499,7 +545,7 @@ func (w *Watcher) checkHostname(
 		)
 	}
 
-	return w.checkHostnameResult(ctx, hostname, results, err)
+	return w.checkHostnameResult(ctx, hostname, results, err), err
 }
 
 // checkHostnameResult finishes a check of hostname from results and err,

internal/watcher/watcher_test.go

@@ -385,11 +385,16 @@ func TestFirstRunBaseline(t *testing.T) {
 	cfg.Domains = []string{testSmallDomain}
 	cfg.Hostnames = []string{testHost}
 
-	_, deps := runChecks(t, cfg, nil)
+	w, deps := runChecks(t, cfg, nil)
 
 	assertNoNotifications(t, deps)
 	assertStatePopulated(t, deps)
 
+	if w.DNSUnreachable() {
+		t.Error("DNSUnreachable is true after a round whose lookups " +
+			"succeeded")
+	}
+
 	// testHost answers with an address, so the check saves an empty list
 	// of CNAME addresses for it; nil would mean the check did not look
 	// at whether to follow a CNAME.
@@ -551,7 +556,9 @@ func TestNSChangeDetection(t *testing.T) {
 // with nameservers and records saved by an earlier check. The .com
 // servers answer that it does not exist, so it is saved with nxdomain
 // set and no nameservers, an NS Change removes them all, and its saved
-// records are removed rather than asked for at the .com servers.
+// records are removed rather than asked for at the .com servers. The
+// round's only lookup failed, but the root servers and the .com servers
+// answered, so the round reached the DNS.
 func TestDomainThatDoesNotExist(t *testing.T) {
 	t.Parallel()
 
@@ -560,11 +567,12 @@ func TestDomainThatDoesNotExist(t *testing.T) {
 	cfg := defaultTestConfig(t)
 	cfg.Domains = []string{domain}
 
-	var deps *testDeps
+	var (
+		w    *watcher.Watcher
+		deps *testDeps
+	)
 
 	livednstest.Retry(t, "watcher checks", func(ctx context.Context) error {
-		var w *watcher.Watcher
-
 		w, deps = newTestWatcher(t, cfg)
 
 		deps.state.SetDomainState(domain, &state.DomainState{
@@ -602,6 +610,11 @@ func TestDomainThatDoesNotExist(t *testing.T) {
 		t.Errorf("records saved for %s: %v", domain, hs.RecordsByNameserver)
 	}
 
+	if w.DNSUnreachable() {
+		t.Error("DNSUnreachable is true after a round that reached " +
+			"the .com servers")
+	}
+
 	assertNotified(t, deps, "NS Change: "+domain, "warning")
 
 	// That is the only notification, and it removes both nameservers,