resolver: a nameserver is asked at each of its addresses in turn (closes #285)

A nameserver's queries went only to the first IPv4 address its name
resolves to, so one unreachable address made it fail whenever that
address came first in the answer. Each record type is now asked at the
addresses one at a time until one gives a usable reply, and the next
type is asked first at the address that gave it. The README's DNS
Resolution Strategy says which addresses are asked.

Model: opus-5-5

README.md

@@ -104,9 +104,11 @@ notification endpoint set, changes show only on the dashboard; see
   NS change notification above. A domain listed in
   `DNSWATCHER_SKIP_RECORD_NOTIFICATIONS` gets no record change or inconsistency
   notification. A domain with no delegation of its own has these records asked
-  at the servers of the zone it is in, as a hostname has. A nameserver the
+  at the servers of the zone it is in, as a hostname has. When none of those
-  domain's records were asked at on the previous check but not on this one gets
+  servers answers, the lookup of its NS record set fails, and they are saved as
-  a hostname's NS change notification (see below), unless it was in the domain's
+  failed and notified as a hostname's are (see below). 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   delegation on the previous check, as when that delegation is withdrawn.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.
@@ -121,22 +123,29 @@ notification endpoint set, changes show only on the dashboard; see
   trace moves from a name to its parent only when the servers asked answer that
   the name has no delegation of its own, or does not exist. When they do not
   answer, the check fails and the hostname's records from the previous check are
-  kept.
+  kept, unless they are the nameservers the previous check saved for the
+  hostname. Each of those is then saved as failed, with the reason, as a
+  domain's nameservers are when none of them answers, so it gets an NS query
+  failure notification, and an NS recovery notification when it answers again
+  (see below). A trace that gets no answer before it reaches them, as when the
+  host running dnswatcher loses its network, fails the check without a
+  notification.
 - Queries **each** authoritative nameserver independently for **all** record
   types: A, AAAA, CNAME, MX, TXT, SRV, CAA, NS.
 - Each record type is a query of its own. When a nameserver answers some types
-  but the query for another gets no usable reply (no reply after two tries, an
+  but the query for another gets no usable reply at any of its addresses (no
-  error reply such as SERVFAIL, a referral, or a reply too large for UDP whose
+  reply after two tries, an error reply such as SERVFAIL, a referral, or a reply
-  retry over TCP fails), the failure is logged with the reason, and the type is
+  too large for UDP whose retry over TCP fails), the failure is logged with the
-  listed in the nameserver's `failedTypes` and keeps the records saved for the
+  reason, and the type is listed in the nameserver's `failedTypes` and keeps the
-  nameserver by the previous check. On that check those records are not compared
+  records saved for the nameserver by the previous check. On that check those
-  with the other nameservers', so no record change or inconsistency is reported
+  records are not compared with the other nameservers', so no record change or
-  for the type; on the next check they are compared with the nameserver's answer
+  inconsistency is reported for the type; on the next check they are compared
-  as usual. When the previous check did not know the type's records either,
+  with the nameserver's answer as usual. When the previous check did not know
-  because the nameserver was new or failing then or the type was already listed
+  the type's records either, because the nameserver was new or failing then or
-  in `unknownTypes`, the type is also listed in `unknownTypes` and left out of
+  the type was already listed in `unknownTypes`, the type is also listed in
-  every comparison until it answers. A nameserver none of whose queries got a
+  `unknownTypes` and left out of every comparison until it answers. A nameserver
-  usable reply has failed (see NS query failure below).
+  none of whose queries got a usable reply has failed (see NS query failure
+  below).
 - Stores results **per nameserver**. The state for a hostname is not a merged
   view — it is a map from nameserver to record set.
 - DNS names inside record values (CNAME, MX, SRV and NS targets) are stored in
@@ -541,6 +550,11 @@ name's nameservers and in the lookup of a nameserver's own address. Such a
 lookup can need others in turn; lookups go at most three deep, one inside
 another, so delegations that point at each other still end.
 
+In step 4 each record type is asked at the IPv4 addresses the nameserver's name
+resolves to, one at a time, until one gives a usable reply; the next type is
+asked first at that address. A type's query fails only when none of them gives
+one, and the nameserver has failed only when that is so for every type.
+
 This approach ensures:
 
 - Independence from any upstream resolver's cache or filtering.

TODO.md

@@ -19,6 +19,10 @@ milestone: https://git.eeqj.de/sneak/dnswatcher/milestone/7
 
 # Completed Steps
 
+- 2026-10-15: a nameserver is asked at each of its IPv4 addresses in turn, so it
+  has failed only when none of them gives a usable reply (closes #285).
+- 2026-10-13: a hostname or a domain whose zone's nameservers all stop answering
+  gets an NS Failure for each, then an NS Recovery (closes #284).
 - 2026-10-13: an IPv6 address is written in brackets before its port, as in
   `[2001:db8::1]:443`, in keys and Port Change notifications (closes #270).
 - 2026-10-13: `TODO.md` names only open issues as next and future steps, and its

internal/resolver/errors.go

@@ -2,6 +2,23 @@ package resolver
 
 import "errors"
 
+// NoAnswerError is returned when the walk to a name's nameservers
+// reached a zone through a referral and none of that zone's
+// nameservers, Nameservers as the referral names them, answered. Err
+// says why they did not.
+type NoAnswerError struct {
+	Nameservers []string
+	Err         error
+}
+
+func (e *NoAnswerError) Error() string {
+	return e.Err.Error()
+}
+
+func (e *NoAnswerError) Unwrap() error {
+	return e.Err
+}
+
 // Sentinel errors returned by the resolver.
 var (
 	// ErrNoNameservers is returned when no authoritative NS

internal/resolver/export_test.go

@@ -31,9 +31,11 @@ func NewWithQueryTimeout(log *slog.Logger, timeout time.Duration) *Resolver {
 func (r *Resolver) FollowDelegation(
 	ctx context.Context,
 	domain string,
+	zone string,
+	nameservers []string,
 	servers []string,
 ) ([]string, error) {
-	return r.followDelegation(ctx, domain, servers)
+	return r.followDelegation(ctx, domain, zone, nameservers, servers)
 }
 
 // FindAuthoritativeNameserversFrom exports findAuthoritativeNameservers
@@ -94,6 +96,17 @@ func (r *Resolver) QueryServers(
 	return r.queryServers(ctx, servers, zone, name, qtype)
 }
 
+// QueryTypes exports queryTypes for testing.
+func (r *Resolver) QueryTypes(
+	ctx context.Context,
+	nsHostname string,
+	nsIPs []string,
+	hostname string,
+	qtypes []uint16,
+) (*NameserverResponse, error) {
+	return r.queryTypes(ctx, nsHostname, nsIPs, hostname, qtypes)
+}
+
 // QueryEachNS exports queryEachNS for testing.
 func (r *Resolver) QueryEachNS(
 	ctx context.Context,

internal/resolver/iterative.go

@@ -204,20 +204,24 @@ func glueIPs(nsNames []string, glue map[string][]net.IP) []string {
 	return ips
 }
 
-// followDelegation follows referrals from servers, the root servers, to
+// followDelegation follows referrals to domain from servers, the
-// domain and returns the NS set of domain's delegation. When the servers
+// addresses of the servers of zone, and returns the NS set of domain's
-// of the zone domain is in answer that domain does not exist, the error
+// delegation. nameservers are the names a referral gave the servers of
-// is ErrNXDomain. When they answer that it has no delegation of its own,
+// zone; the walk to a name's nameservers starts at the root servers, the
+// servers of zone ".", which no referral names. When the servers of the
+// zone domain is in answer that domain does not exist, the error is
+// ErrNXDomain. When they answer that it has no delegation of its own,
 // because it is not the zone's apex, the set is empty and there is no
-// error. Any other error means that no such answer came.
+// error. Any other error means that no such answer came; it is a
+// *NoAnswerError when the servers that did not answer are those of a
+// zone a referral named.
 func (r *Resolver) followDelegation(
 	ctx context.Context,
 	domain string,
+	zone string,
+	nameservers []string,
 	servers []string,
 ) ([]string, error) {
-	// servers are the root servers, the servers of zone ".".
-	zone := "."
-
 	var withoutAddresses []string
 
 	for range maxDelegation {
@@ -228,6 +232,12 @@ func (r *Resolver) followDelegation(
 		resp, err := r.queryZone(
 			ctx, servers, withoutAddresses, zone, domain, dns.TypeNS, 0,
 		)
+
+		// A query that ctx cut short says nothing about the servers.
+		if err != nil && len(nameservers) > 0 && checkCtx(ctx) == nil {
+			return nil, &NoAnswerError{Nameservers: nameservers, Err: err}
+		}
+
 		if err != nil {
 			return nil, err
 		}
@@ -256,6 +266,7 @@ func (r *Resolver) followDelegation(
 			return r.resolveNSIterative(ctx, domain)
 		}
 
+		nameservers = authNS
 		servers, withoutAddresses = referralNameservers(resp)
 		zone = referralZone(resp)
 	}
@@ -626,7 +637,8 @@ func (r *Resolver) resolveARecord(
 // them. When the servers asked answer that the name has no delegation
 // of its own, or does not exist, it tries each parent name in turn, so
 // it returns the nameservers of the zone the name is in. When they do
-// not answer, it returns the error and tries no parent name.
+// not answer, it returns the error and tries no parent name; when a
+// referral named them, the error is a *NoAnswerError naming them.
 func (r *Resolver) FindAuthoritativeNameservers(
 	ctx context.Context,
 	domain string,
@@ -655,7 +667,7 @@ func (r *Resolver) findAuthoritativeNameservers(
 
 		candidate := strings.Join(labels[i:], ".") + "."
 
-		nsNames, err := r.followDelegation(ctx, candidate, servers)
+		nsNames, err := r.followDelegation(ctx, candidate, ".", nil, servers)
 		if err != nil && !errors.Is(err, ErrNXDomain) {
 			return nil, err
 		}
@@ -696,8 +708,9 @@ func (r *Resolver) QueryNameserver(
 	return r.queryNameserver(ctx, nsHostname, hostname, recordTypes())
 }
 
-// queryNameserver queries a specific nameserver for the record types
+// queryNameserver queries a specific nameserver, at the IPv4 addresses
-// in qtypes and builds a NameserverResponse.
+// its name resolves to, for the record types in qtypes and builds a
+// NameserverResponse.
 func (r *Resolver) queryNameserver(
 	ctx context.Context,
 	nsHostname string,
@@ -715,7 +728,7 @@ func (r *Resolver) queryNameserver(
 
 	hostname = dns.Fqdn(hostname)
 
-	return r.queryTypes(ctx, nsHostname, nsIPs[0], hostname, qtypes)
+	return r.queryTypes(ctx, nsHostname, nsIPs, hostname, qtypes)
 }
 
 // QueryNameserverIP queries a nameserver by its IP address directly,
@@ -732,13 +745,18 @@ func (r *Resolver) QueryNameserverIP(
 
 	hostname = dns.Fqdn(hostname)
 
-	return r.queryTypes(ctx, nsHostname, nsIP, hostname, recordTypes())
+	return r.queryTypes(
+		ctx, nsHostname, []string{nsIP}, hostname, recordTypes(),
+	)
 }
 
+// queryTypes asks the nameserver nsHostname, at its addresses nsIPs,
+// about hostname's records of each type in qtypes and builds a
+// NameserverResponse.
 func (r *Resolver) queryTypes(
 	ctx context.Context,
 	nsHostname string,
-	nsIP string,
+	nsIPs []string,
 	hostname string,
 	qtypes []uint16,
 ) (*NameserverResponse, error) {
@@ -748,7 +766,7 @@ func (r *Resolver) queryTypes(
 		Status:     StatusOK,
 	}
 
-	state := r.queryEachType(ctx, nsIP, hostname, qtypes, resp)
+	state := r.queryEachType(ctx, nsIPs, hostname, qtypes, resp)
 	classifyResponse(resp, state)
 
 	return resp, nil
@@ -766,28 +784,37 @@ type queryState struct {
 	answered      bool
 }
 
-// queryEachType asks the nameserver at nsIP about hostname once for each
+// queryEachType asks the nameserver at its addresses nsIPs about hostname
-// record type in qtypes, and lists in resp.FailedTypes the types whose
+// once for each record type in qtypes, and lists in resp.FailedTypes the
-// query got no usable reply, logging each with the reason unless ctx was
+// types whose query got no usable reply at any of them, logging each with
-// cancelled: shutdown cancels it, and a query it cut short did not fail.
+// the reason unless ctx was cancelled: shutdown cancels it, and a query
+// it cut short did not fail.
 func (r *Resolver) queryEachType(
 	ctx context.Context,
-	nsIP string,
+	nsIPs []string,
 	hostname string,
 	qtypes []uint16,
 	resp *NameserverResponse,
 ) queryState {
 	var state queryState
 
+	// Reordered below; the caller's list is left as it was.
+	nsIPs = slices.Clone(nsIPs)
+
 	for _, qtype := range qtypes {
 		if checkCtx(ctx) != nil {
 			break
 		}
 
-		err := r.querySingleType(ctx, nsIP, hostname, qtype, resp, &state)
+		answeredAt, err := r.querySingleType(
+			ctx, nsIPs, hostname, qtype, resp, &state,
+		)
 		if err == nil {
 			state.answered = true
 
+			// The next type is asked first at the address that answered.
+			nsIPs[0], nsIPs[answeredAt] = nsIPs[answeredAt], nsIPs[0]
+
 			continue
 		}
 
@@ -821,20 +848,29 @@ func (r *Resolver) queryEachType(
 	return state
 }
 
-// querySingleType asks the nameserver at nsIP about hostname's records
+// querySingleType asks the nameserver about hostname's records of type
-// of type qtype. It returns nil when the nameserver answered: with
+// qtype at its addresses nsIPs, one at a time, until one answers: with
-// records, with none, or with NXDOMAIN; otherwise it returns why not.
+// records, with none, or with NXDOMAIN. It returns the index in nsIPs of
+// the address that answered; when none did, it returns why the last one
+// did not.
 func (r *Resolver) querySingleType(
 	ctx context.Context,
-	nsIP string,
+	nsIPs []string,
 	hostname string,
 	qtype uint16,
 	resp *NameserverResponse,
 	state *queryState,
-) error {
+) (int, error) {
-	msg, err := r.queryDNS(ctx, nsIP, hostname, qtype)
+	var err error
-	if err != nil {
+
+	for i, nsIP := range nsIPs {
+		var msg *dns.Msg
+
+		msg, err = r.queryDNS(ctx, nsIP, hostname, qtype)
+
 		switch {
+		case err == nil:
+			err = readReply(msg, resp, state)
 		case isTimeout(err):
 			state.gotTimeout = true
 		case errors.Is(err, ErrRefused):
@@ -843,10 +879,12 @@ func (r *Resolver) querySingleType(
 			state.netErr = err
 		}
 
-		return err
+		if err == nil {
+			return i, nil
+		}
 	}
 
-	return readReply(msg, resp, state)
+	return 0, err
 }
 
 // readReply adds to resp the records in msg, a nameserver's reply to a
@@ -1067,7 +1105,7 @@ func (r *Resolver) LookupNS(
 	}
 
 	return r.followDelegation(
-		ctx, dns.Fqdn(strings.ToLower(domain)), rootServerList(),
+		ctx, dns.Fqdn(strings.ToLower(domain)), ".", nil, rootServerList(),
 	)
 }
 

internal/resolver/resolver_test.go

@@ -121,7 +121,9 @@ func TestFindAuthoritativeNameservers_DelegatedSubdomain(
 // delegation of its own, so the lookup returns that walk's error, about
 // www.google.com, and tries no parent name: trying google.com and com
 // would end in ErrNoNameservers, or in the error of a walk for one of
-// them.
+// them. The servers that did not answer are those the walk started at,
+// which no referral named, so the error is no NoAnswerError, as when
+// the host running dnswatcher has lost its network.
 func TestFindAuthoritativeNameservers_NoAnswer(t *testing.T) {
 	t.Parallel()
 
@@ -134,6 +136,9 @@ func TestFindAuthoritativeNameservers_NoAnswer(t *testing.T) {
 	require.NotErrorIs(t, err, resolver.ErrNoNameservers)
 	assert.Contains(t, err.Error(), "query www.google.com. @"+noAnswerAddress)
 	assert.Empty(t, nameservers)
+
+	var noAnswer *resolver.NoAnswerError
+	assert.NotErrorAs(t, err, &noAnswer)
 }
 
 func TestFindAuthoritativeNameservers_ReturnsSorted(
@@ -355,6 +360,58 @@ func TestQueryNameserver_ZoneDelegatedWithoutAddresses(t *testing.T) {
 	assert.Equal(t, resolver.StatusOK, resp.Status)
 }
 
+// TestQueryTypes_FirstAddressNeverAnswers asks a google.com nameserver
+// about google.com's A and AAAA records at a list of addresses that puts
+// 192.0.2.1, where nothing answers, before the nameserver's own. The
+// nameserver answers at its own address, so it is ok, with its records.
+// Each attempt has time for one round of tries at 192.0.2.1, two tries
+// of two seconds, and not for two, so AAAA must be asked first at the
+// address that answered A.
+func TestQueryTypes_FirstAddressNeverAnswers(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+	ns := findOneNSForDomain(t, r, "google.com")
+	nsIPs := append(
+		[]string{noAnswerAddress}, liveResolveNSIPs(t, r, []string{ns}, 1)...,
+	)
+
+	var resp *resolver.NameserverResponse
+
+	livednstest.Retry(
+		t,
+		"QueryTypes("+ns+" at "+strings.Join(nsIPs, ", ")+", google.com)",
+		func(ctx context.Context) error {
+			ctx, cancel := context.WithTimeout(ctx, 6*time.Second)
+			defer cancel()
+
+			var err error
+
+			resp, err = r.QueryTypes(
+				ctx, ns, nsIPs, "google.com.",
+				[]uint16{dns.TypeA, dns.TypeAAAA},
+			)
+			if err != nil {
+				return err
+			}
+
+			if len(resp.FailedTypes) > 0 {
+				return fmt.Errorf(
+					"%w: %s gave no usable reply for %s",
+					livednstest.ErrNoAnswer, ns,
+					strings.Join(resp.FailedTypes, ", "),
+				)
+			}
+
+			return nil
+		},
+	)
+
+	assert.Equal(t, resolver.StatusOK, resp.Status)
+	assert.NotEmpty(t, resp.Records["A"])
+	assert.NotEmpty(t, resp.Records["AAAA"])
+}
+
 func TestQueryNameserver_AAAA(t *testing.T) {
 	t.Parallel()
 
@@ -954,12 +1011,34 @@ func TestFollowDelegation_NoAnswer(t *testing.T) {
 	r := resolver.NewWithQueryTimeout(slog.Default(), 100*time.Millisecond)
 
 	nameservers, err := r.FollowDelegation(
-		t.Context(), "google.com.", []string{noAnswerAddress},
+		t.Context(), "google.com.", ".", nil, []string{noAnswerAddress},
 	)
 	require.Error(t, err)
 	assert.Empty(t, nameservers)
 }
 
+// TestFollowDelegation_NoAnswerAfterReferral starts the walk for
+// www.example.net at the servers of example.net as a referral names
+// them, ns1.example.net and ns2.example.net, at 192.0.2.1, where nothing
+// answers. The error is a NoAnswerError naming them, which the watcher
+// compares with the nameservers it saved for the name.
+func TestFollowDelegation_NoAnswerAfterReferral(t *testing.T) {
+	t.Parallel()
+
+	r := resolver.NewWithQueryTimeout(slog.Default(), 100*time.Millisecond)
+	nameservers := []string{"ns1.example.net.", "ns2.example.net."}
+
+	_, err := r.FollowDelegation(
+		t.Context(), "www.example.net.", "example.net.", nameservers,
+		[]string{noAnswerAddress},
+	)
+
+	var noAnswer *resolver.NoAnswerError
+	require.ErrorAs(t, err, &noAnswer)
+	assert.Equal(t, nameservers, noAnswer.Nameservers)
+	assert.Contains(t, err.Error(), "query www.example.net. @"+noAnswerAddress)
+}
+
 // TestResolveNSIterative_NoDelegationOfItsOwn walks to the nameservers
 // of www.google.com as the fallback walk does. As in
 // TestLookupNS_NoDelegationOfItsOwn, the set is empty, with no error.

internal/watcher/export_test.go

@@ -122,6 +122,34 @@ func (w *Watcher) RunTLSChecks(ctx context.Context) {
 	w.runTLSChecks(ctx)
 }
 
+// CheckHostnameResult exports checkHostnameResult for testing.
+func (w *Watcher) CheckHostnameResult(
+	ctx context.Context,
+	hostname string,
+	results map[string]*resolver.NameserverResponse,
+	err error,
+) bool {
+	return w.checkHostnameResult(ctx, hostname, results, err)
+}
+
+// CheckDomainResult exports checkDomainResult for testing.
+func (w *Watcher) CheckDomainResult(
+	ctx context.Context,
+	domain string,
+	nameservers []string,
+	err error,
+) {
+	w.checkDomainResult(ctx, domain, nameservers, err)
+}
+
+// FailedNameservers exports failedNameservers for testing.
+func FailedNameservers(
+	err error,
+	prev *state.HostnameState,
+) map[string]*resolver.NameserverResponse {
+	return failedNameservers(err, prev)
+}
+
 // BuildHostnameState exports buildHostnameState for testing.
 func BuildHostnameState(
 	results map[string]*resolver.NameserverResponse,

internal/watcher/interfaces.go

@@ -20,7 +20,10 @@ type DNSResolver interface {
 	) ([]string, error)
 
 	// LookupAllRecords queries all record types for a hostname,
-	// returning each nameserver's response keyed by nameserver.
+	// returning each nameserver's response keyed by nameserver. When
+	// the walk to the nameservers reached a zone through a referral and
+	// none of that zone's nameservers answered, the error is a
+	// *resolver.NoAnswerError naming them.
 	LookupAllRecords(
 		ctx context.Context,
 		hostname string,

internal/watcher/nsfailure_test.go

@@ -2,6 +2,7 @@ package watcher_test
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log/slog"
 	"strings"
@@ -14,6 +15,10 @@ import (
 	"sneak.berlin/go/dnswatcher/internal/watcher"
 )
 
+// errNoReply is why a walk that got no reply from a zone's servers
+// failed.
+var errNoReply = errors.New("all servers failed: i/o timeout")
+
 // answered is what a check saves for a nameserver that answered with
 // these records.
 func answered(records map[string][]string) *state.NameserverRecordState {
@@ -183,6 +188,134 @@ func TestNSFailureAlertNamesHostnameNameserverAndReason(t *testing.T) {
 	}
 }
 
+// TestZoneNameserversAllStopAnswering runs three checks of host from
+// the point where its lookup has returned. On the first, nsA and nsB,
+// the nameservers of its zone, answer. On the second, the lookup fails
+// with the error the walk to them returns when neither answers (see
+// the resolver's TestFollowDelegation_NoAnswerAfterReferral), given
+// here because no live zone has nameservers that never answer. For a
+// hostname it is the lookup of its records that fails, and for a domain
+// with no delegation of its own the lookup of its delegation. Both are
+// saved as failed, with the reason, and each gets an NS Failure. On the
+// third, both answer, and each gets an NS Recovery. A domain's records
+// are checked as a hostname's, so its first and third checks are too.
+func TestZoneNameserversAllStopAnswering(t *testing.T) {
+	t.Parallel()
+
+	records := map[string][]string{"A": {ip1}}
+	bothAnswer := map[string]*resolver.NameserverResponse{
+		nsA: response(records), nsB: response(records),
+	}
+	noAnswer := &resolver.NoAnswerError{
+		Nameservers: []string{nsA, nsB}, Err: errNoReply,
+	}
+
+	tests := []struct {
+		name     string
+		isDomain bool
+	}{
+		{"hostname", false},
+		{"domain with no delegation of its own", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			cfg := defaultTestConfig(t)
+			if tt.isDomain {
+				cfg.Domains = []string{host}
+			} else {
+				cfg.Hostnames = []string{host}
+			}
+
+			// The watcher is given each lookup's result, so it has no
+			// resolver.
+			deps := newTestDeps(t, cfg)
+			w := watcher.NewForTest(
+				cfg, deps.state, nil,
+				deps.portChecker, deps.tlsChecker, deps.notifier,
+			)
+			w.SetFirstRun(false)
+
+			w.CheckHostnameResult(t.Context(), host, bothAnswer, nil)
+
+			if tt.isDomain {
+				w.CheckDomainResult(t.Context(), host, nil, noAnswer)
+			} else {
+				w.CheckHostnameResult(t.Context(), host, nil, noAnswer)
+			}
+
+			current, _ := deps.state.GetHostnameState(host)
+
+			for _, ns := range []string{nsA, nsB} {
+				got := current.RecordsByNameserver[ns]
+				if got.Status != failed().Status || got.Error != errNoReply.Error() {
+					t.Errorf(
+						"%s saved with status %q, error %q; want %q, %q",
+						ns, got.Status, got.Error, failed().Status, errNoReply,
+					)
+				}
+			}
+
+			if n := countNotifications(deps, "NS Failure: "+host); n != 2 {
+				t.Errorf("sent %d NS Failure notifications, want 2", n)
+			}
+
+			w.CheckHostnameResult(t.Context(), host, bothAnswer, nil)
+
+			if n := countNotifications(deps, "NS Recovery: "+host); n != 2 {
+				t.Errorf("sent %d NS Recovery notifications, want 2", n)
+			}
+		})
+	}
+}
+
+// TestFailedNameserversOnlyForThoseOfThePreviousCheck gives
+// FailedNameservers lookup errors after a check in which nsA and nsB
+// answered. Only an error that names both makes them failed.
+func TestFailedNameserversOnlyForThoseOfThePreviousCheck(t *testing.T) {
+	t.Parallel()
+
+	records := map[string][]string{"A": {ip1}}
+	bothAnswered := saved(map[string]*state.NameserverRecordState{
+		nsA: answered(records), nsB: answered(records),
+	})
+	noAnswer := func(nameservers ...string) error {
+		return &resolver.NoAnswerError{Nameservers: nameservers, Err: errNoReply}
+	}
+
+	tests := []struct {
+		name       string
+		err        error
+		prev       *state.HostnameState
+		wantFailed bool
+	}{
+		{"both did not answer", noAnswer(nsA, nsB), bothAnswered, true},
+		{
+			"the servers of a parent zone did not answer",
+			noAnswer(nsC), bothAnswered, false,
+		},
+		{
+			"the zone's delegation lists only nsA now",
+			noAnswer(nsA), bothAnswered, false,
+		},
+		{"no root server answered", errNoReply, bothAnswered, false},
+		{"no previous check", noAnswer(nsA, nsB), nil, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got := watcher.FailedNameservers(tt.err, tt.prev)
+			if (got != nil) != tt.wantFailed {
+				t.Errorf("returned %v, want failed: %v", got, tt.wantFailed)
+			}
+		})
+	}
+}
+
 // TestNameserverThatNeverAnswers asks a nameserver address where
 // nothing answers, 192.0.2.1, and checks what the watcher saves for it.
 // The deadline outlasts the resolver's first two-second try, as in the

internal/watcher/watcher.go

@@ -291,12 +291,25 @@ func (w *Watcher) logFailedLookup(
 	w.log.Error(msg, args...)
 }
 
+// checkDomain looks up domain's delegation and checks domain with it
+// (see checkDomainResult).
 func (w *Watcher) checkDomain(
 	ctx context.Context,
 	domain string,
 ) {
 	nameservers, err := w.resolver.LookupNS(ctx, domain)
 
+	w.checkDomainResult(ctx, domain, nameservers, err)
+}
+
+// checkDomainResult finishes a check of domain from nameservers and err,
+// what the lookup of its delegation returned.
+func (w *Watcher) checkDomainResult(
+	ctx context.Context,
+	domain string,
+	nameservers []string,
+	err error,
+) {
 	// A domain that does not exist has no nameservers.
 	nxdomain := errors.Is(err, resolver.ErrNXDomain)
 	if nxdomain {
@@ -311,6 +324,12 @@ func (w *Watcher) checkDomain(
 			"error", err,
 		)
 
+		// For a domain with no delegation of its own, the servers that
+		// did not answer can be those of the zone it is in, at which
+		// its records are asked. They are then saved as failed, as a
+		// hostname's are.
+		w.checkHostnameResult(ctx, domain, nil, err)
+
 		return
 	}
 
@@ -464,8 +483,8 @@ func (w *Watcher) detectNSAddressChanges(
 	}
 }
 
-// checkHostname checks hostname's records and reports whether it saved
+// checkHostname looks up hostname's records and checks them (see
-// them; it does not when their lookup fails.
+// checkHostnameResult). It reports whether it saved them.
 func (w *Watcher) checkHostname(
 	ctx context.Context,
 	hostname string,
@@ -478,12 +497,31 @@ func (w *Watcher) checkHostname(
 			"hostname", hostname,
 			"error", err,
 		)
-
-		return false
 	}
 
+	return w.checkHostnameResult(ctx, hostname, results, err)
+}
+
+// checkHostnameResult finishes a check of hostname from results and err,
+// what the lookup of its records returned, and reports whether it saved
+// them. When the lookup failed, it saves nothing, unless the nameservers
+// that did not answer are those of the previous check (see
+// failedNameservers).
+func (w *Watcher) checkHostnameResult(
+	ctx context.Context,
+	hostname string,
+	results map[string]*resolver.NameserverResponse,
+	err error,
+) bool {
 	prev, _ := w.state.GetHostnameState(hostname)
 
+	if err != nil {
+		results = failedNameservers(err, prev)
+		if results == nil {
+			return false
+		}
+	}
+
 	w.updateHostnameState(
 		ctx, hostname, buildHostnameState(results, prev, time.Now().UTC()),
 	)
@@ -491,6 +529,40 @@ func (w *Watcher) checkHostname(
 	return true
 }
 
+// failedNameservers returns a failed response for each nameserver in
+// prev, a name's state from the previous check, with err as the reason,
+// when err says that the walk to the name's nameservers reached their
+// zone and none of them answered. For any other error, as when the walk
+// got no answer before it reached their zone, or when prev is nil, it
+// returns nil.
+func failedNameservers(
+	err error,
+	prev *state.HostnameState,
+) map[string]*resolver.NameserverResponse {
+	var noAnswer *resolver.NoAnswerError
+	if !errors.As(err, &noAnswer) || prev == nil {
+		return nil
+	}
+
+	saved := slices.Sorted(maps.Keys(prev.RecordsByNameserver))
+	if !slices.Equal(noAnswer.Nameservers, saved) {
+		return nil
+	}
+
+	results := make(map[string]*resolver.NameserverResponse)
+
+	for _, ns := range saved {
+		results[ns] = &resolver.NameserverResponse{
+			Nameserver: ns,
+			Records:    make(map[string][]string),
+			Status:     resolver.StatusError,
+			Error:      err.Error(),
+		}
+	}
+
+	return results
+}
+
 // updateHostnameState finishes a check of hostname from newState, built
 // from its nameservers' answers: it follows the CNAME in them, notifies
 // what changed since the previous check, and saves newState.
@@ -790,7 +862,7 @@ func (w *Watcher) detectRecordChanges(
 // 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.
+// nameservers in prev come from the same check.
 func (w *Watcher) detectNSDisappearances(
 	ctx context.Context,
 	hostname string,