resolver, watcher: a record type whose query fails keeps its previous records (closes #231)

The resolver lists in FailedTypes each record type whose query to a
nameserver got no usable reply (no reply, a code other than NOERROR or
NXDOMAIN, a referral, or a truncated reply whose TCP retry failed) and
logs it unless shutdown cut it short. A nameserver that answered no
type has failed.
The watcher saves such a type in failedTypes with the previous check's
records, leaves it out of the comparison with other nameservers on that
check, and compares it with the next answer. When the previous check
did not know its records either, it is also in unknownTypes and not
compared until it answers. A nameserver whose A, AAAA or CNAME query
failed is no answer when following a CNAME or resolving addresses.

Model: opus-5-5

README.md

@@ -80,7 +80,8 @@ notification endpoint set, changes show only on the dashboard; see
       different addresses than on the previous check. A nameserver added or
       removed gets only the NS change notification. When the lookup of a
       nameserver's addresses fails or finds none, its previous addresses are
-      kept and nothing is sent.
+      kept and nothing is sent. The lookup fails when no nameserver it asks
+      answers every one of its queries, for A, AAAA and CNAME.
 
 ### DNS Hostname Monitoring (Subdomains)
 
@@ -91,6 +92,19 @@ notification endpoint set, changes show only on the dashboard; see
   its last two labels (a name under `co.uk`, or in a delegated subdomain).
 - 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
+  error reply such as SERVFAIL, a referral, or a reply too large for UDP whose
+  retry over TCP fails), the failure is logged with the reason, and the type is
+  listed in the nameserver's `failedTypes` and keeps the records saved for the
+  nameserver by the previous check. On that check those records are not compared
+  with the other nameservers', so no record change or inconsistency is reported
+  for the type; on the next check they are compared with the nameserver's answer
+  as usual. When the previous check did not know the type's records either,
+  because the nameserver was new or failing then or the type was already listed
+  in `unknownTypes`, the type is also listed in `unknownTypes` and left out of
+  every comparison until it answers. A nameserver 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
@@ -119,8 +133,9 @@ notification endpoint set, changes show only on the dashboard; see
       they keep disagreeing, including after a restart. A nameserver that was
       not in the previous check (newly added, or back after dropping out), or
       failed on it, and answers differently is reported on the check where it
-      answers. If a pair agrees again and later disagrees, the alert is sent
-      again.
+      answers. So is a pair that differs in a record type whose query to either
+      nameserver failed on the previous check. If a pair agrees again and later
+      disagrees, the alert is sent again.
     - **CNAME address change**: The addresses at the end of a name's CNAME chain
       differ from those of the previous check. They are found when its
       nameservers answer with a CNAME and no address; a name that answers with
@@ -525,7 +540,7 @@ reachability:
 
 | Status  | Meaning                                                  |
 | ------- | -------------------------------------------------------- |
-| `ok`    | Query succeeded, records are current                     |
+| `ok`    | Query succeeded, records are current except as below     |
 | `error` | Query failed (timeout, SERVFAIL, REFUSED, network error) |
 
 A nameserver that answers NXDOMAIN or with no records has status `ok` and empty
@@ -534,6 +549,13 @@ nameservers, has status `error`, empty `records`, and the reason in `error`. A
 certificate entry whose TLS connection or handshake failed likewise has status
 `error`, the reason in `error`, and the certificate fields left empty or zero.
 
+A nameserver with status `ok` whose query for one record type failed lists that
+type in `failedTypes` and holds its records from the previous check, which may
+not be current. When the previous check did not know the type's records either,
+because the nameserver was new or failing then or the type was already listed in
+`unknownTypes`, the type is also listed in `unknownTypes`, and `records` holds
+nothing for it. Both lists are left out when empty.
+
 `nameserverAddresses` lists, by nameserver, the sorted addresses its name
 resolves to. A state file without it loads, and the next check fills it in
 without a notification.
@@ -541,10 +563,10 @@ without a notification.
 `cnameAddresses` lists the sorted addresses at the end of the chain of every
 CNAME target a hostname's nameservers gave, found when they answered with a
 CNAME and no address; it is empty when they answered with an address. When a
-chain cannot be followed, or none of the name's nameservers answered, the
-previous check's list is kept, or `null` when no earlier check saved one. A
-state file without it loads, and the first check after that saves it without a
-notification.
+chain cannot be followed, or none of the name's nameservers answered its queries
+for A, AAAA and CNAME, the previous check's list is kept, or `null` when no
+earlier check saved one. A state file without it loads, and the first check
+after that saves it without a notification.
 
 A port entry in the older format, with one `hostname` instead of the `hostnames`
 list, loads as a list of that one name.

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-02: a record type whose query to a nameserver fails keeps its previous
+  records and alerts nothing; the other types are still saved (closes #231).
 - 2026-10-02: the dashboard and `/api/v1/status` show why a nameserver query or
   a certificate check failed, which only the state file showed (closes #225).
 - 2026-10-02: a name's CNAME is stored once per nameserver, not once per record

internal/resolver/errors.go

@@ -22,6 +22,12 @@ var (
 		"reply is an error or a referral that leads no closer",
 	)
 
+	// ErrTruncated is the reason given for a reply too large for UDP
+	// whose retry over TCP failed.
+	ErrTruncated = errors.New(
+		"reply truncated and its retry over TCP failed",
+	)
+
 	// ErrIntercepted is returned when every root server refused a
 	// query. Root servers refuse no query, so the refusals came from
 	// something on the network answering in their place.

internal/resolver/export_test.go

@@ -2,10 +2,21 @@ package resolver
 
 import (
 	"context"
+	"log/slog"
+	"time"
 
 	"github.com/miekg/dns"
 )
 
+// NewWithFailingTCP returns a Resolver whose TCP client gives up before
+// it can connect, so the retry over TCP of every truncated reply fails.
+func NewWithFailingTCP(log *slog.Logger) *Resolver {
+	r := NewFromLogger(log)
+	r.tcp = &tcpClient{timeout: time.Nanosecond}
+
+	return r
+}
+
 // ExtractRecordValue exports extractRecordValue for testing.
 func ExtractRecordValue(rr dns.RR) string {
 	return extractRecordValue(rr)

internal/resolver/iterative.go

@@ -89,6 +89,9 @@ func (r *Resolver) tryExchange(
 	return resp, err
 }
 
+// retryTCP returns the reply to msg over TCP when resp, its reply over
+// UDP, is truncated. When that fails it returns resp, still truncated,
+// which holds only the records that fit.
 func (r *Resolver) retryTCP(
 	ctx context.Context,
 	msg *dns.Msg,
@@ -632,14 +635,19 @@ func (r *Resolver) queryTypes(
 
 type queryState struct {
 	gotNXDomain bool
-	gotSERVFAIL bool
+	errorReply  string // code of an error reply, such as SERVFAIL
 	gotRefused  bool
 	gotTimeout  bool
 	gotReferral bool
 	netErr      error
 	hasRecords  bool
+	answered    bool
 }
 
+// queryEachType asks the nameserver at nsIP about hostname once for each
+// record type in qtypes, and lists in resp.FailedTypes the types whose
+// query got no usable reply, logging each with 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,
@@ -654,7 +662,34 @@ func (r *Resolver) queryEachType(
 			break
 		}
 
-		r.querySingleType(ctx, nsIP, hostname, qtype, resp, &state)
+		err := r.querySingleType(ctx, nsIP, hostname, qtype, resp, &state)
+		if err == nil {
+			state.answered = true
+
+			continue
+		}
+
+		rtype := dns.TypeToString[qtype]
+		resp.FailedTypes = append(resp.FailedTypes, rtype)
+
+		if errors.Is(ctx.Err(), context.Canceled) {
+			continue
+		}
+
+		r.log.Warn(
+			"record type query failed",
+			"hostname", hostname,
+			"nameserver", resp.Nameserver,
+			"type", rtype,
+			"error", err,
+		)
+	}
+
+	// The reply about another type can carry the name's CNAME. When the
+	// query for CNAME itself failed, that is left out too, so Records
+	// holds nothing for a failed type.
+	for _, rtype := range resp.FailedTypes {
+		delete(resp.Records, rtype)
 	}
 
 	for k := range resp.Records {
@@ -664,6 +699,9 @@ func (r *Resolver) queryEachType(
 	return state
 }
 
+// querySingleType asks the nameserver at nsIP about hostname's records
+// of type qtype. It returns nil when the nameserver answered: with
+// records, with none, or with NXDOMAIN; otherwise it returns why not.
 func (r *Resolver) querySingleType(
 	ctx context.Context,
 	nsIP string,
@@ -671,7 +709,7 @@ func (r *Resolver) querySingleType(
 	qtype uint16,
 	resp *NameserverResponse,
 	state *queryState,
-) {
+) error {
 	msg, err := r.queryDNS(ctx, nsIP, hostname, qtype)
 	if err != nil {
 		switch {
@@ -683,19 +721,36 @@ func (r *Resolver) querySingleType(
 			state.netErr = err
 		}
 
-		return
+		return err
 	}
 
+	return readReply(msg, resp, state)
+}
+
+// readReply adds to resp the records in msg, a nameserver's reply to a
+// query about one record type. It returns nil when the nameserver
+// answered: with records, with none, or with NXDOMAIN; otherwise it
+// returns why not.
+func readReply(
+	msg *dns.Msg,
+	resp *NameserverResponse,
+	state *queryState,
+) error {
 	if msg.Rcode == dns.RcodeNameError {
 		state.gotNXDomain = true
 
-		return
+		return nil
 	}
 
-	if msg.Rcode == dns.RcodeServerFailure {
-		state.gotSERVFAIL = true
+	// A reply with any other code but NOERROR, such as SERVFAIL, NOTIMP or
+	// FORMERR, is an error reply and says nothing about the name's
+	// records. usableReply takes the same codes as no usable reply.
+	if msg.Rcode != dns.RcodeSuccess {
+		state.errorReply = dns.RcodeToString[msg.Rcode]
 
-		return
+		return fmt.Errorf(
+			"server returned %s: %w", state.errorReply, ErrUnusableReply,
+		)
 	}
 
 	// A reply with no answer that lists other nameservers, from a server
@@ -708,10 +763,20 @@ func (r *Resolver) querySingleType(
 		len(extractNSSet(msg.Ns)) > 0 {
 		state.gotReferral = true
 
-		return
+		return fmt.Errorf("server returned a referral: %w", ErrUnusableReply)
+	}
+
+	// A reply still truncated is one whose TCP retry failed, and holds
+	// only the records that fit.
+	if msg.Truncated {
+		state.netErr = ErrTruncated
+
+		return ErrTruncated
 	}
 
 	collectAnswerRecords(msg, resp, state)
+
+	return nil
 }
 
 // collectAnswerRecords adds the records in msg's answer to resp, each
@@ -750,23 +815,26 @@ func isTimeout(err error) bool {
 	return false
 }
 
+// classifyResponse sets the nameserver's status. One that answered no
+// record type has failed, and Error says why; one that answered some has
+// the status of those answers.
 func classifyResponse(resp *NameserverResponse, state queryState) {
 	switch {
 	case state.gotNXDomain && !state.hasRecords:
 		resp.Status = StatusNXDomain
-	case state.gotTimeout && !state.hasRecords:
+	case state.gotTimeout && !state.answered:
 		resp.Status = StatusTimeout
 		resp.Error = "all queries timed out"
-	case state.gotSERVFAIL && !state.hasRecords:
+	case state.errorReply != "" && !state.answered:
 		resp.Status = StatusError
-		resp.Error = "server returned SERVFAIL"
-	case state.gotRefused && !state.hasRecords:
+		resp.Error = "server returned " + state.errorReply
+	case state.gotRefused && !state.answered:
 		resp.Status = StatusError
 		resp.Error = "server returned REFUSED"
-	case state.netErr != nil && !state.hasRecords:
+	case state.netErr != nil && !state.answered:
 		resp.Status = StatusError
 		resp.Error = "network error: " + state.netErr.Error()
-	case state.gotReferral && !state.hasRecords:
+	case state.gotReferral && !state.answered:
 		resp.Status = StatusError
 		resp.Error = "server returned a referral"
 	case !state.hasRecords && !state.gotNXDomain:
@@ -927,9 +995,11 @@ func (r *Resolver) resolveIPWithCNAME(
 }
 
 // collectIPs returns the addresses in the nameservers' answers and the
-// first CNAME target among them. It returns ErrNoNameserverAnswered when
-// every nameserver timed out, failed or returned a referral: that is not
-// a name with no addresses.
+// first CNAME target among them. A nameserver whose query for one of the
+// types failed gave only part of the addresses, and is left out. It
+// returns ErrNoNameserverAnswered when every nameserver timed out,
+// failed, returned a referral or was left out: that is not a name with
+// no addresses.
 func collectIPs(
 	results map[string]*NameserverResponse,
 ) ([]string, string, error) {
@@ -942,7 +1012,8 @@ func collectIPs(
 	answered := false
 
 	for _, resp := range results {
-		if resp.Status == StatusTimeout || resp.Status == StatusError {
+		if resp.Status == StatusTimeout || resp.Status == StatusError ||
+			len(resp.FailedTypes) > 0 {
 			continue
 		}
 

internal/resolver/iterative_internal_test.go

@@ -0,0 +1,116 @@
+package resolver
+
+import (
+	"syscall"
+	"testing"
+
+	"github.com/miekg/dns"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestClassifyResponse sets a nameserver's status from the results of
+// its queries, built here. One that answered some record types, even
+// with no records, has not failed when its query for another type got
+// no usable reply, whatever the reason; one whose every query got none
+// has.
+func TestClassifyResponse(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		results    queryState
+		wantStatus string
+		wantError  string
+	}{
+		{
+			"some types answered with no records, another timed out",
+			queryState{answered: true, gotTimeout: true},
+			StatusNoData, "",
+		},
+		{
+			"some types answered with no records, another got SERVFAIL",
+			queryState{answered: true, errorReply: "SERVFAIL"},
+			StatusNoData, "",
+		},
+		{
+			"some types answered with no records, another was refused",
+			queryState{answered: true, gotRefused: true},
+			StatusNoData, "",
+		},
+		{
+			"some types answered with no records, another got a network error",
+			queryState{answered: true, netErr: syscall.ECONNREFUSED},
+			StatusNoData, "",
+		},
+		{
+			"some types answered with no records, another's reply was " +
+				"truncated and its retry over TCP failed",
+			queryState{answered: true, netErr: ErrTruncated},
+			StatusNoData, "",
+		},
+		{
+			"some types answered with no records, another got a referral",
+			queryState{answered: true, gotReferral: true},
+			StatusNoData, "",
+		},
+		{
+			"every query timed out",
+			queryState{gotTimeout: true},
+			StatusTimeout, "all queries timed out",
+		},
+		{
+			"every query got NOTIMP",
+			queryState{errorReply: "NOTIMP"},
+			StatusError, "server returned NOTIMP",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			resp := &NameserverResponse{Status: StatusOK}
+			classifyResponse(resp, tt.results)
+
+			assert.Equal(t, tt.wantStatus, resp.Status)
+			assert.Equal(t, tt.wantError, resp.Error)
+		})
+	}
+}
+
+// TestReadReply checks which replies to a query about one record type,
+// built here, are an answer: one with the code NOERROR or NXDOMAIN. A
+// reply with any other code is not, and the type's query has failed.
+func TestReadReply(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		rcode    int
+		answered bool
+	}{
+		{dns.RcodeSuccess, true},
+		{dns.RcodeNameError, true},
+		{dns.RcodeServerFailure, false},
+		{dns.RcodeNotImplemented, false},
+		{dns.RcodeFormatError, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(dns.RcodeToString[tt.rcode], func(t *testing.T) {
+			t.Parallel()
+
+			msg := new(dns.Msg)
+			msg.Authoritative = true
+			msg.Rcode = tt.rcode
+
+			resp := &NameserverResponse{Records: map[string][]string{}}
+			err := readReply(msg, resp, &queryState{})
+
+			if tt.answered {
+				assert.NoError(t, err)
+			} else {
+				assert.ErrorIs(t, err, ErrUnusableReply)
+			}
+		})
+	}
+}

internal/resolver/iterative_test.go

@@ -43,6 +43,25 @@ func TestCollectIPs_FailedIsNoAnswer(t *testing.T) {
 	assert.Empty(t, ips)
 }
 
+// TestCollectIPs_FailedTypeIsNoAnswer checks that a nameserver whose
+// query for one of the types failed is no answer: its addresses are
+// only part of them.
+func TestCollectIPs_FailedTypeIsNoAnswer(t *testing.T) {
+	t.Parallel()
+
+	ips, _, err := resolver.CollectIPs(
+		map[string]*resolver.NameserverResponse{
+			nsExample1: {
+				Records:     map[string][]string{"A": {"192.0.2.1"}},
+				FailedTypes: []string{"AAAA"},
+				Status:      resolver.StatusOK,
+			},
+		},
+	)
+	require.ErrorIs(t, err, resolver.ErrNoNameserverAnswered)
+	assert.Empty(t, ips)
+}
+
 const (
 	// exampleCom is the zone most cases of TestUsableReply and
 	// TestNSSetFrom are about, and wwwExampleCom a name in it.

internal/resolver/resolver.go

@@ -31,9 +31,13 @@ type Params struct {
 }
 
 // NameserverResponse holds one nameserver's response for a query.
+// FailedTypes lists the record types whose query got no usable reply,
+// and Records holds nothing for them: their records are not known. When
+// no record type got one, Status and Error say the nameserver failed.
 type NameserverResponse struct {
 	Nameserver  string
 	Records     map[string][]string
+	FailedTypes []string
 	Status      string
 	Error       string
 }

internal/resolver/resolver_test.go

@@ -1,6 +1,7 @@
 package resolver_test
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"log/slog"
@@ -245,6 +246,30 @@ func TestQueryNameserver_TXT(t *testing.T) {
 	)
 }
 
+// TestQueryNameserver_TruncatedReplyWhoseTCPRetryFails asks a google.com
+// nameserver about google.com with a resolver whose retries over TCP
+// fail. google.com's TXT records do not fit in a reply over UDP, so TXT
+// is reported as failed, holding none of the records that fit, and
+// logged with the reason, while the nameserver, which answered the other
+// types, is ok.
+func TestQueryNameserver_TruncatedReplyWhoseTCPRetryFails(t *testing.T) {
+	t.Parallel()
+
+	ns := findOneNSForDomain(t, newTestResolver(t), "google.com")
+
+	var logs bytes.Buffer
+
+	r := resolver.NewWithFailingTCP(slog.New(slog.NewTextHandler(&logs, nil)))
+	resp := liveQueryNameserver(t, r, ns, "google.com")
+
+	assert.Equal(t, resolver.StatusOK, resp.Status)
+	assert.Contains(t, resp.FailedTypes, "TXT")
+	assert.NotContains(t, resp.Records, "TXT")
+	assert.Contains(t, logs.String(),
+		"hostname=google.com. nameserver="+ns+" type=TXT error=",
+	)
+}
+
 func TestQueryNameserver_NXDomain(t *testing.T) {
 	t.Parallel()
 
@@ -865,6 +890,29 @@ func TestQueryNameserverIP_Timeout(t *testing.T) {
 	assert.NotEmpty(t, resp.Error)
 }
 
+// TestQueryNameserverIP_CancelledLogsNothing cancels the context while
+// a query to 192.0.2.1, where nothing answers, is waiting for a reply,
+// as shutdown does. The query was cut short, not failed, so nothing is
+// logged.
+func TestQueryNameserverIP_CancelledLogsNothing(t *testing.T) {
+	t.Parallel()
+
+	var logs bytes.Buffer
+
+	r := resolver.NewFromLogger(slog.New(slog.NewTextHandler(&logs, nil)))
+
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+	time.AfterFunc(100*time.Millisecond, cancel)
+
+	_, err := r.QueryNameserverIP(
+		ctx, "unreachable.test.", "192.0.2.1", "example.com",
+	)
+	require.NoError(t, err)
+
+	assert.Empty(t, logs.String())
+}
+
 // TestCollectIPs_NoNameserverAnswered takes the response of a
 // nameserver at 192.0.2.1, where nothing answers, as
 // TestQueryNameserverIP_Timeout does. Addresses collected from

internal/state/state.go

@@ -46,8 +46,15 @@ type DomainState struct {
 }
 
 // NameserverRecordState holds one NS's response for a hostname.
+// FailedTypes lists the record types whose query to the nameserver
+// failed on this check: Records holds for them the records saved by the
+// previous check, which are kept. UnknownTypes lists those of them whose
+// records the previous check did not know either, as when the
+// nameserver was new or failing then: Records holds nothing for them.
 type NameserverRecordState struct {
 	Records      map[string][]string `json:"records"`
+	FailedTypes  []string            `json:"failedTypes,omitempty"`
+	UnknownTypes []string            `json:"unknownTypes,omitempty"`
 	Status       string              `json:"status"`
 	Error        string              `json:"error,omitempty"`
 	LastChecked  time.Time           `json:"lastChecked"`

internal/state/state_test.go

@@ -236,6 +236,61 @@ func TestSaveLoadRoundTrip_CNAMEAddresses(t *testing.T) {
 	}
 }
 
+// TestSaveLoadRoundTrip_FailedTypes checks that a nameserver's
+// failedTypes and unknownTypes survive a save and load. Without
+// unknownTypes, a type whose records were not known would load as one
+// with no records.
+func TestSaveLoadRoundTrip_FailedTypes(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	s := state.NewForTestWithDataDir(dir)
+
+	failed := []string{"TXT", "CAA"}
+	unknown := []string{"CAA"}
+
+	s.SetHostnameState(testHostname, &state.HostnameState{
+		RecordsByNameserver: map[string]*state.NameserverRecordState{
+			testNS1: {
+				Records:      map[string][]string{"TXT": {"v=spf1 -all"}},
+				FailedTypes:  failed,
+				UnknownTypes: unknown,
+				Status:       "ok",
+			},
+		},
+	})
+
+	err := s.Save()
+	if err != nil {
+		t.Fatalf("Save() error: %v", err)
+	}
+
+	loaded := state.NewForTestWithDataDir(dir)
+
+	err = loaded.Load()
+	if err != nil {
+		t.Fatalf("Load() error: %v", err)
+	}
+
+	hs, ok := loaded.GetHostnameState(testHostname)
+	if !ok {
+		t.Fatal("missing hostname " + testHostname)
+	}
+
+	ns1 := hs.RecordsByNameserver[testNS1]
+	if ns1 == nil {
+		t.Fatal("missing nameserver " + testNS1)
+	}
+
+	if !reflect.DeepEqual(ns1.FailedTypes, failed) {
+		t.Errorf("failedTypes: got %#v", ns1.FailedTypes)
+	}
+
+	if !reflect.DeepEqual(ns1.UnknownTypes, unknown) {
+		t.Errorf("unknownTypes: got %#v", ns1.UnknownTypes)
+	}
+}
+
 // TestLoadStateFromBeforeCNAMEAddresses loads a state file written
 // before the addresses at the end of a hostname's CNAME chain were
 // saved. They load as not known (nil), not as none.

internal/watcher/cname_test.go

@@ -202,6 +202,42 @@ func TestCNAMEWhoseNameserversAllFailedKeepsPrevious(t *testing.T) {
 	}
 }
 
+// TestCNAMEWhoseAddressQueryFailedKeepsPrevious checks a name whose
+// nameserver answered, but whose query for A, AAAA or CNAME failed with
+// nothing kept for it. That is not an answer with no address: the
+// addresses the previous check saved from following its CNAME are kept,
+// and nothing is looked up, the watcher having no resolver.
+func TestCNAMEWhoseAddressQueryFailedKeepsPrevious(t *testing.T) {
+	t.Parallel()
+
+	for _, rtype := range []string{"A", "AAAA", "CNAME"} {
+		t.Run(rtype, func(t *testing.T) {
+			t.Parallel()
+
+			w := watcher.NewForTest(nil, nil, nil, nil, nil, nil)
+
+			current := saved(map[string]*state.NameserverRecordState{
+				nsA: {
+					Records:      map[string][]string{},
+					FailedTypes:  []string{rtype},
+					UnknownTypes: []string{rtype},
+					Status:       "ok",
+				},
+			})
+			prev := cnameState(oldIP)
+
+			w.ResolveCNAMEAddresses(t.Context(), host, current, prev)
+
+			if !slices.Equal(current.CNAMEAddresses, prev.CNAMEAddresses) {
+				t.Errorf(
+					"saved %v, want %v",
+					current.CNAMEAddresses, prev.CNAMEAddresses,
+				)
+			}
+		})
+	}
+}
+
 // cnameTo builds the records of a nameserver that answered with a CNAME
 // to target and no address.
 func cnameTo(target string) map[string][]string {

internal/watcher/export_test.go

@@ -109,7 +109,8 @@ func (w *Watcher) RunTLSChecks(ctx context.Context) {
 // BuildHostnameState exports buildHostnameState for testing.
 func BuildHostnameState(
 	results map[string]*resolver.NameserverResponse,
+	prev *state.HostnameState,
 	now time.Time,
 ) *state.HostnameState {
-	return buildHostnameState(results, now)
+	return buildHostnameState(results, prev, now)
 }

internal/watcher/failedtype_test.go

@@ -0,0 +1,364 @@
+package watcher_test
+
+import (
+	"maps"
+	"slices"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/dnswatcher/internal/resolver"
+	"sneak.berlin/go/dnswatcher/internal/state"
+	"sneak.berlin/go/dnswatcher/internal/watcher"
+)
+
+const (
+	// txt is the record type whose query fails in these tests.
+	txt  = "TXT"
+	spf1 = "v=spf1 -all"
+	spf2 = "v=spf1 include:example.net -all"
+)
+
+// response is a nameserver's response with these records, whose queries
+// for failedTypes failed.
+func response(
+	records map[string][]string,
+	failedTypes ...string,
+) *resolver.NameserverResponse {
+	return &resolver.NameserverResponse{
+		Records:     records,
+		FailedTypes: failedTypes,
+		Status:      resolver.StatusOK,
+	}
+}
+
+// savedChecks saves the state of each check in turn from the
+// nameservers' responses, each from the state the check before saved.
+func savedChecks(
+	checks ...map[string]*resolver.NameserverResponse,
+) []*state.HostnameState {
+	states := make([]*state.HostnameState, 0, len(checks))
+
+	var prev *state.HostnameState
+
+	for _, results := range checks {
+		prev = watcher.BuildHostnameState(results, prev, time.Now())
+		states = append(states, prev)
+	}
+
+	return states
+}
+
+// TestFailedTypeKeepsPreviousRecords saves a check in which nsA's query
+// for TXT failed, after previous checks of several kinds. TXT is always
+// saved in FailedTypes, and in UnknownTypes when there was nothing to
+// keep.
+func TestFailedTypeKeepsPreviousRecords(t *testing.T) {
+	t.Parallel()
+
+	aOnly := map[string][]string{"A": {ip1}}
+	withTXT := map[string][]string{"A": {ip1}, txt: {spf1}}
+	txtKept := &state.NameserverRecordState{
+		Records: withTXT, FailedTypes: []string{txt}, Status: "ok",
+	}
+	txtNotKnown := &state.NameserverRecordState{
+		Records:      aOnly,
+		FailedTypes:  []string{txt},
+		UnknownTypes: []string{txt},
+		Status:       "ok",
+	}
+
+	tests := []struct {
+		name        string
+		prev        *state.HostnameState
+		wantRecords map[string][]string
+		wantUnknown []string
+	}{
+		{
+			"previous TXT records are kept",
+			saved(map[string]*state.NameserverRecordState{nsA: answered(withTXT)}),
+			withTXT, nil,
+		},
+		{
+			"previous check had no TXT records",
+			saved(map[string]*state.NameserverRecordState{nsA: answered(aOnly)}),
+			aOnly, nil,
+		},
+		{
+			"TXT failed on the previous check, which kept its records",
+			saved(map[string]*state.NameserverRecordState{nsA: txtKept}),
+			withTXT, nil,
+		},
+		{"first check", nil, aOnly, []string{txt}},
+		{
+			"nameserver new on this check",
+			saved(map[string]*state.NameserverRecordState{nsB: answered(withTXT)}),
+			aOnly, []string{txt},
+		},
+		{
+			"nameserver failed on the previous check",
+			saved(map[string]*state.NameserverRecordState{nsA: failed()}),
+			aOnly, []string{txt},
+		},
+		{
+			"TXT failed on the previous check with nothing to keep",
+			saved(map[string]*state.NameserverRecordState{nsA: txtNotKnown}),
+			aOnly, []string{txt},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			hs := watcher.BuildHostnameState(
+				map[string]*resolver.NameserverResponse{
+					nsA: response(map[string][]string{"A": {ip1}}, txt),
+				},
+				tt.prev, time.Now(),
+			)
+
+			got := hs.RecordsByNameserver[nsA]
+			if got.Status != "ok" ||
+				!maps.EqualFunc(got.Records, tt.wantRecords, slices.Equal) ||
+				!slices.Equal(got.FailedTypes, []string{txt}) ||
+				!slices.Equal(got.UnknownTypes, tt.wantUnknown) {
+				t.Errorf(
+					"saved status %q, records %v, failed types %v, "+
+						"unknown types %v; want ok, %v, [%s], %v",
+					got.Status, got.Records, got.FailedTypes,
+					got.UnknownTypes, tt.wantRecords, txt, tt.wantUnknown,
+				)
+			}
+		})
+	}
+}
+
+// TestFailedTypeAlerts saves the checks of each case in turn from the
+// nameservers' responses, the first being the state loaded at startup,
+// and counts the alerts sent. nsB's TXT query fails on one check, and
+// nothing changes.
+func TestFailedTypeAlerts(t *testing.T) {
+	t.Parallel()
+
+	records := map[string][]string{"A": {ip1}, txt: {spf1}}
+	aOnly := map[string][]string{"A": {ip1}}
+
+	bothAnswer := map[string]*resolver.NameserverResponse{
+		nsA: response(records), nsB: response(records),
+	}
+	bTXTFails := map[string]*resolver.NameserverResponse{
+		nsA: response(records), nsB: response(aOnly, txt),
+	}
+	onlyA := map[string]*resolver.NameserverResponse{
+		nsA: response(records),
+	}
+	bFails := map[string]*resolver.NameserverResponse{
+		nsA: response(records),
+		nsB: {
+			Records: map[string][]string{},
+			Status:  resolver.StatusTimeout,
+			Error:   "all queries timed out",
+		},
+	}
+
+	tests := []struct {
+		name   string
+		checks []map[string]*resolver.NameserverResponse
+		want   alertCounts
+	}{
+		{
+			"type failing at one nameserver alerts nothing, nor its next answer",
+			[]map[string]*resolver.NameserverResponse{
+				bothAnswer, bTXTFails, bothAnswer,
+			},
+			alertCounts{},
+		},
+		{
+			"type failing on the first check alerts nothing on the next",
+			[]map[string]*resolver.NameserverResponse{bTXTFails, bothAnswer},
+			alertCounts{},
+		},
+		{
+			"type failing at a nameserver new on that check alerts nothing",
+			[]map[string]*resolver.NameserverResponse{
+				onlyA, bTXTFails, bothAnswer,
+			},
+			alertCounts{},
+		},
+		{
+			"type failing at a recovering nameserver alerts the recovery",
+			[]map[string]*resolver.NameserverResponse{
+				bFails, bTXTFails, bothAnswer,
+			},
+			alertCounts{recoveries: 1},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			states := savedChecks(tt.checks...)
+
+			got := countAlerts(t, states[0], states[1:])
+			if got != tt.want {
+				t.Errorf("sent %+v, want %+v", got, tt.want)
+			}
+		})
+	}
+}
+
+// TestFailedTypeComparedOnceItAnswers saves the checks of each case in
+// turn as TestFailedTypeAlerts does. nsB's TXT query fails on one check,
+// and the TXT record changes: the change is sent as a Record Change for
+// each nameserver on the check where it answers it, and an Inconsistency
+// only when nsB still answers the old record.
+func TestFailedTypeComparedOnceItAnswers(t *testing.T) {
+	t.Parallel()
+
+	records := map[string][]string{"A": {ip1}, txt: {spf1}}
+	changed := map[string][]string{"A": {ip1}, txt: {spf2}}
+	aOnly := map[string][]string{"A": {ip1}}
+
+	bothAnswer := map[string]*resolver.NameserverResponse{
+		nsA: response(records), nsB: response(records),
+	}
+	bTXTFails := map[string]*resolver.NameserverResponse{
+		nsA: response(records), nsB: response(aOnly, txt),
+	}
+	bothChange := map[string]*resolver.NameserverResponse{
+		nsA: response(changed), nsB: response(changed),
+	}
+	aChangesBTXTFails := map[string]*resolver.NameserverResponse{
+		nsA: response(changed), nsB: response(aOnly, txt),
+	}
+	bStillOld := map[string]*resolver.NameserverResponse{
+		nsA: response(changed), nsB: response(records),
+	}
+
+	tests := []struct {
+		name   string
+		checks []map[string]*resolver.NameserverResponse
+		want   alertCounts
+	}{
+		{
+			"change made while the type failed",
+			[]map[string]*resolver.NameserverResponse{
+				bothAnswer, bTXTFails, bothChange,
+			},
+			alertCounts{recordChanges: 2},
+		},
+		{
+			"change seen at one nameserver while the other's type failed",
+			[]map[string]*resolver.NameserverResponse{
+				bothAnswer, aChangesBTXTFails, bothChange,
+			},
+			alertCounts{recordChanges: 2},
+		},
+		{
+			"old record answered after the type failed",
+			[]map[string]*resolver.NameserverResponse{
+				bothAnswer, aChangesBTXTFails, bStillOld,
+			},
+			alertCounts{recordChanges: 1, inconsistencies: 1},
+		},
+		{
+			"change after the type failed on the first check and answered",
+			[]map[string]*resolver.NameserverResponse{
+				bTXTFails, bothAnswer, bothChange,
+			},
+			alertCounts{recordChanges: 2},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			states := savedChecks(tt.checks...)
+
+			got := countAlerts(t, states[0], states[1:])
+			if got != tt.want {
+				t.Errorf("sent %+v, want %+v", got, tt.want)
+			}
+		})
+	}
+}
+
+// TestFailedTypeLeftOutOfMessages checks that a Record Change and an
+// Inconsistency name only the record types they compared. nsB's TXT
+// records are not known on the first check, and on the second either
+// answered or still not known; nsB's A record changes, so both alerts
+// are sent and name the A record alone.
+func TestFailedTypeLeftOutOfMessages(t *testing.T) {
+	t.Parallel()
+
+	withTXT := map[string][]string{"A": {ip1}, txt: {spf1}}
+	txtNotKnown := func(address string) *state.NameserverRecordState {
+		return &state.NameserverRecordState{
+			Records:      map[string][]string{"A": {address}},
+			FailedTypes:  []string{txt},
+			UnknownTypes: []string{txt},
+			Status:       "ok",
+		}
+	}
+
+	before := saved(map[string]*state.NameserverRecordState{
+		nsA: answered(withTXT), nsB: txtNotKnown(ip1),
+	})
+
+	tests := []struct {
+		name  string
+		after *state.HostnameState
+	}{
+		{
+			"TXT answers",
+			saved(map[string]*state.NameserverRecordState{
+				nsA: answered(withTXT),
+				nsB: answered(map[string][]string{"A": {ip2}, txt: {spf1}}),
+			}),
+		},
+		{
+			"TXT still not known",
+			saved(map[string]*state.NameserverRecordState{
+				nsA: answered(withTXT), nsB: txtNotKnown(ip2),
+			}),
+		},
+	}
+
+	want := map[string]string{
+		"Record Change: " + host: "Hostname: " + host +
+			"\nNameserver: " + nsB + "\nType: A\nOld: " + ip1 + "\nNew: " + ip2,
+		"Inconsistency: " + host: "Hostname: " + host +
+			"\nType: A\n" + nsA + ": " + ip1 + "\n" + nsB + ": " + ip2,
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			// The hostname change detection uses only the notifier.
+			notifier := &mockNotifier{}
+			w := watcher.NewForTest(nil, nil, nil, nil, nil, notifier)
+
+			w.DetectHostnameChanges(t.Context(), host, before, tt.after)
+
+			notifications := notifier.getNotifications()
+			if len(notifications) != len(want) {
+				t.Fatalf(
+					"sent %d notifications, want %d: %v",
+					len(notifications), len(want), notifications,
+				)
+			}
+
+			for _, n := range notifications {
+				if n.Message != want[n.Title] {
+					t.Errorf(
+						"%s message:\n%s\nwant:\n%s",
+						n.Title, n.Message, want[n.Title],
+					)
+				}
+			}
+		})
+	}
+}

internal/watcher/nsfailure_test.go

@@ -201,7 +201,7 @@ func TestNameserverThatNeverAnswers(t *testing.T) {
 	}
 
 	hs := watcher.BuildHostnameState(
-		map[string]*resolver.NameserverResponse{nsA: resp}, time.Now(),
+		map[string]*resolver.NameserverResponse{nsA: resp}, nil, time.Now(),
 	)
 
 	got := hs.RecordsByNameserver[nsA]
@@ -256,7 +256,7 @@ func TestNameserverThatAnswersNXDOMAIN(t *testing.T) {
 	}
 
 	hs := watcher.BuildHostnameState(
-		map[string]*resolver.NameserverResponse{ns: resp}, time.Now(),
+		map[string]*resolver.NameserverResponse{ns: resp}, nil, time.Now(),
 	)
 
 	got := hs.RecordsByNameserver[ns]
@@ -320,7 +320,7 @@ func TestNameserverThatRefuses(t *testing.T) {
 	}
 
 	hs := watcher.BuildHostnameState(
-		map[string]*resolver.NameserverResponse{ns: resp}, time.Now(),
+		map[string]*resolver.NameserverResponse{ns: resp}, nil, time.Now(),
 	)
 
 	got := hs.RecordsByNameserver[ns]

internal/watcher/watcher.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"maps"
 	"slices"
 	"sort"
 	"strings"
@@ -400,8 +401,10 @@ func (w *Watcher) checkHostname(
 		return
 	}
 
+	prev, _ := w.state.GetHostnameState(hostname)
+
 	w.updateHostnameState(
-		ctx, hostname, buildHostnameState(results, time.Now().UTC()),
+		ctx, hostname, buildHostnameState(results, prev, time.Now().UTC()),
 	)
 }
 
@@ -431,8 +434,9 @@ func (w *Watcher) updateHostnameState(
 // the addresses found for all of them are saved, so nameservers that
 // disagree on the target do not change the result from check to check.
 // The addresses saved in prev, which may be nil, are kept when none of
-// the name's nameservers answered, and when a target cannot be
-// followed, as when no nameserver of a zone in its chain answers.
+// the name's nameservers answered its queries for A, AAAA and CNAME,
+// and when a target cannot be followed, as when no nameserver of a zone
+// in its chain answers.
 func (w *Watcher) resolveCNAMEAddresses(
 	ctx context.Context,
 	hostname string,
@@ -450,7 +454,10 @@ func (w *Watcher) resolveCNAMEAddresses(
 	targets := make(map[string]bool)
 
 	for _, nsState := range current.RecordsByNameserver {
-		if nsState.Status != statusOK {
+		if nsState.Status != statusOK ||
+			slices.Contains(nsState.FailedTypes, "A") ||
+			slices.Contains(nsState.FailedTypes, "AAAA") ||
+			slices.Contains(nsState.FailedTypes, "CNAME") {
 			continue
 		}
 
@@ -496,11 +503,13 @@ func (w *Watcher) resolveCNAMEAddresses(
 }
 
 // buildHostnameState saves each nameserver's response. A nameserver
-// that answered, even with NXDOMAIN or no records, is saved as ok; one
-// that timed out or failed is saved as error with the reason, and its
-// empty record set is not an answer.
+// that answered, even with NXDOMAIN or no records, is saved as ok, with
+// the record types whose query failed; one that timed out or failed is
+// saved as error with the reason, and its empty record set is not an
+// answer. prev is the hostname's state from the previous check, or nil.
 func buildHostnameState(
 	results map[string]*resolver.NameserverResponse,
+	prev *state.HostnameState,
 	now time.Time,
 ) *state.HostnameState {
 	hs := &state.HostnameState{
@@ -512,7 +521,7 @@ func buildHostnameState(
 
 	for ns, resp := range results {
 		nsState := &state.NameserverRecordState{
-			Records:     resp.Records,
+			Records:     maps.Clone(resp.Records),
 			Status:      statusOK,
 			LastChecked: now,
 		}
@@ -521,6 +530,15 @@ func buildHostnameState(
 			resp.Status == resolver.StatusError {
 			nsState.Status = statusError
 			nsState.Error = resp.Error
+		} else {
+			nsState.FailedTypes = resp.FailedTypes
+
+			var prevNS *state.NameserverRecordState
+			if prev != nil {
+				prevNS = prev.RecordsByNameserver[ns]
+			}
+
+			keepFailedTypes(nsState, prevNS)
 		}
 
 		hs.RecordsByNameserver[ns] = nsState
@@ -529,6 +547,27 @@ func buildHostnameState(
 	return hs
 }
 
+// keepFailedTypes copies into nsState, for each record type in its
+// FailedTypes, the records prevNS, the nameserver's state from the
+// previous check, holds for that type, which may be none. When prevNS
+// does not know them either, because the nameserver was new or failing
+// then or the type was in its UnknownTypes, the type goes in
+// nsState.UnknownTypes instead.
+func keepFailedTypes(nsState, prevNS *state.NameserverRecordState) {
+	for _, rtype := range nsState.FailedTypes {
+		if prevNS == nil || prevNS.Status != statusOK ||
+			slices.Contains(prevNS.UnknownTypes, rtype) {
+			nsState.UnknownTypes = append(nsState.UnknownTypes, rtype)
+
+			continue
+		}
+
+		if records, ok := prevNS.Records[rtype]; ok {
+			nsState.Records[rtype] = records
+		}
+	}
+}
+
 func (w *Watcher) detectHostnameChanges(
 	ctx context.Context,
 	hostname string,
@@ -573,7 +612,10 @@ func (w *Watcher) detectCNAMEAddressChanges(
 
 // detectRecordChanges compares each nameserver's records with those of
 // the previous check. Only answers are compared: a nameserver that
-// failed on either check has no records to compare.
+// failed on either check has no records to compare. The records kept
+// for a record type whose query failed are compared too, but not those
+// of a type in UnknownTypes on either check, which the message leaves
+// out as well.
 func (w *Watcher) detectRecordChanges(
 	ctx context.Context,
 	hostname string,
@@ -585,7 +627,11 @@ func (w *Watcher) detectRecordChanges(
 			continue
 		}
 
-		if recordsEqual(prevNS.Records, cur.Records) {
+		unknown := slices.Concat(prevNS.UnknownTypes, cur.UnknownTypes)
+		oldRecords := withoutTypes(prevNS.Records, unknown)
+		newRecords := withoutTypes(cur.Records, unknown)
+
+		if recordsEqual(oldRecords, newRecords) {
 			continue
 		}
 
@@ -593,8 +639,8 @@ func (w *Watcher) detectRecordChanges(
 			"Hostname: %s\nNameserver: %s\n%s",
 			hostname, ns,
 			recordDifferences(
-				"Old", prevNS.Records,
-				"New", cur.Records,
+				"Old", oldRecords,
+				"New", newRecords,
 			),
 		)
 
@@ -681,13 +727,19 @@ func (w *Watcher) detectInconsistencies(
 ) {
 	for _, pair := range newlyDisagreeingPairs(prev, current) {
 		ns1, ns2 := pair[0], pair[1]
+		state1 := current.RecordsByNameserver[ns1]
+		state2 := current.RecordsByNameserver[ns2]
+
+		// The record types left out of the comparison are left out of
+		// the message too.
+		failed := slices.Concat(state1.FailedTypes, state2.FailedTypes)
 
 		msg := fmt.Sprintf(
 			"Hostname: %s\n%s",
 			hostname,
 			recordDifferences(
-				ns1, current.RecordsByNameserver[ns1].Records,
-				ns2, current.RecordsByNameserver[ns2].Records,
+				ns1, withoutTypes(state1.Records, failed),
+				ns2, withoutTypes(state2.Records, failed),
 			),
 		)
 
@@ -705,7 +757,9 @@ func (w *Watcher) detectInconsistencies(
 // except pairs where both nameservers answered in prev and already
 // differed there. A nameserver missing from prev, or that failed there,
 // is paired with every nameserver it differs from. A nameserver that
-// failed in current has no records to compare and is in no pair.
+// failed in current has no records to compare and is in no pair. In
+// both checks, a record type whose query failed at either nameserver is
+// not compared.
 func newlyDisagreeingPairs(
 	prev, current *state.HostnameState,
 ) [][2]string {
@@ -722,9 +776,9 @@ func newlyDisagreeingPairs(
 
 	for i, ns1 := range nameservers {
 		for _, ns2 := range nameservers[i+1:] {
-			if recordsEqual(
-				current.RecordsByNameserver[ns1].Records,
-				current.RecordsByNameserver[ns2].Records,
+			if nameserversAgree(
+				current.RecordsByNameserver[ns1],
+				current.RecordsByNameserver[ns2],
 			) {
 				continue
 			}
@@ -734,7 +788,7 @@ func newlyDisagreeingPairs(
 
 			if ok1 && ok2 &&
 				prev1.Status == statusOK && prev2.Status == statusOK &&
-				!recordsEqual(prev1.Records, prev2.Records) {
+				!nameserversAgree(prev1, prev2) {
 				continue
 			}
 
@@ -1221,6 +1275,33 @@ func toSet(items []string) map[string]bool {
 	return set
 }
 
+// nameserversAgree reports whether two nameservers' states from the same
+// check hold the same records, leaving out the record types either lists
+// in FailedTypes: the records held for those are kept from an earlier
+// check, or not known.
+func nameserversAgree(a, b *state.NameserverRecordState) bool {
+	failed := slices.Concat(a.FailedTypes, b.FailedTypes)
+
+	return recordsEqual(
+		withoutTypes(a.Records, failed), withoutTypes(b.Records, failed),
+	)
+}
+
+// withoutTypes returns a copy of records without the record types in
+// types.
+func withoutTypes(
+	records map[string][]string,
+	types []string,
+) map[string][]string {
+	records = maps.Clone(records)
+
+	for _, rtype := range types {
+		delete(records, rtype)
+	}
+
+	return records
+}
+
 func recordsEqual(
 	a, b map[string][]string,
 ) bool {