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/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,