resolver: a nameserver with a failed record type is not nodata (closes #253)

classifyResponse set nodata when every record type that answered had
no records, even when another type's query got no usable reply. That
type is listed in FailedTypes and its records are unknown, so the
nameserver has not said it has none. It now stays ok, the status
README describes for a nameserver with a failed type, and nodata is
set only when no type failed. The watcher saved nodata as ok already,
so saved state is unchanged; the live test that rejects nodata no
longer fails when one of a nameserver's queries is lost.

Model: opus-5-5

README.md

@@ -73,20 +73,36 @@ notification endpoint set, changes show only on the dashboard; see
   to discover all authoritative nameservers (NS records) for each domain.
 - Queries **every** discovered authoritative nameserver independently.
 - Stores the domain's NS record set, as its parent zone's servers delegate it,
-  and the IPv4 and IPv6 addresses each nameserver's name resolves to.
+  and the IPv4 and IPv6 addresses each nameserver's name resolves to. The set is
+  only ever the domain's own delegation. A domain whose parent zone's servers
+  answer NXDOMAIN, that it does not exist, has no nameservers and is shown as
+  not existing (see Web Dashboard and HTTP API). A domain that exists but has no
+  delegation of its own, such as `octocat.github.io`, has no nameservers either.
+  When the parent zone's servers do not answer, the check fails and the set from
+  the previous check is kept.
 - Any change triggers a notification:
-    - NS added to or removed from that set.
+    - NS added to or removed from that set. A domain that had nameservers on the
+      previous check and no longer exists gets one with all of them removed.
+      After an upgrade, a domain with no delegation of its own, for which an
+      earlier version saved its parent zone's nameservers, also gets one with
+      all of them removed, on its first check. That one does not mean the domain
+      stopped existing: it is not shown as not existing, and its records are
+      still watched.
     - NS address change: a nameserver that stays in the set resolves to
       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.
 - Also watches the domain's own records as a hostname's are watched (see DNS
   Hostname Monitoring below): its A, AAAA, CNAME, MX, TXT, SRV, CAA and NS
   records, stored per nameserver. Their changes are notified as a hostname's
   are, as a record change, NS query failure, NS recovery, inconsistency or CNAME
   address change, in a message that starts `Domain:` where a hostname's starts
-  `Hostname:`.
+  `Hostname:`. 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 domain that does not
+  exist has none: they are not asked for, and those saved by an earlier check
+  are removed without a notification.
 
 ### DNS Hostname Monitoring (Subdomains)
 
@@ -94,9 +110,26 @@ notification endpoint set, changes show only on the dashboard; see
   via the Public Suffix List).
 - Every **1 hour** by default, performs a full iterative trace to discover the
   authoritative nameservers of the zone the hostname is in, which is not always
-  its last two labels (a name under `co.uk`, or in a delegated subdomain).
+  its last two labels (a name under `co.uk`, or in a delegated subdomain). The
+  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.
 - 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
@@ -125,8 +158,9 @@ 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
+      answers. So is a pair that differs in a record type whose query to either
-      again.
+      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
@@ -232,9 +266,10 @@ clears them.
   corruption.
 - A name removed from `DNSWATCHER_TARGETS` is removed from the state at startup,
   before the first check, without a notification: its domain, hostname and
-  certificate entries go, so the dashboard and `/api/v1/status` no longer list
+  certificate entries go, it is taken off each port entry's list of names, and a
-  or count it. The first check's port checks remove the port entries of
+  port entry left with no name goes, so the dashboard and `/api/v1/status` no
-  addresses no configured name has.
+  longer list or count it. The first check's port checks remove the port entries
+  of addresses no configured name has.
 - Each port check also removes the certificate entries for an address their name
   no longer resolves to, except while none of the name's nameservers answer.
 
@@ -244,8 +279,9 @@ dnswatcher includes an unauthenticated, read-only web dashboard at the root URL
 (`/`). It displays:
 
 - **Summary counts** for monitored domains, hostnames, ports, and certificates.
-- **Domains** with their discovered nameservers, and each domain's own records
+- **Domains** with their discovered nameservers, or "does not exist" for a
-  per nameserver and status, shown as a hostname's are.
+  domain whose parent zone's servers answered NXDOMAIN, and each domain's own
+  records per nameserver and status, shown as a hostname's are.
 - **Hostnames** with per-nameserver DNS records and status. For a nameserver
   whose query failed, the reason is shown in place of the records.
 - **Ports** with open/closed state and the domains and hostnames that resolve to
@@ -282,9 +318,11 @@ In `/api/v1/status`, each nameserver entry and certificate entry whose `status`
 is `error` also has `error`, the reason, as in the state file (see State File
 Format). A domain's own records are in its entry in `domains`, under
 `recordsByNameserver`, in the form a hostname's entry in `hostnames` has them
-under `nameservers`; `hostnames` and `counts.hostnames` hold no domain. A port
+under `nameservers`; `hostnames` and `counts.hostnames` hold no domain. A domain
-entry lists the domains that resolve to its address in `domains`, and the
+entry's `nxdomain` is `true` when the domain's parent zone's servers answered
-hostnames in `hostnames`.
+NXDOMAIN, that it does not exist; its `nameservers` and `recordsByNameserver`
+are then empty. A port entry lists the domains that resolve to its address in
+`domains`, and the hostnames in `hostnames`.
 
 `/metrics` is served only when `DNSWATCHER_METRICS_USERNAME` is set, behind
 Basic Auth. It has the Prometheus Go client's default metrics only (Go runtime,
@@ -552,7 +590,7 @@ 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
@@ -561,17 +599,29 @@ 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.
 
+A domain entry has `"nxdomain": true` when the domain's parent zone's servers
+answered NXDOMAIN, that it does not exist. Its `nameservers` and
+`nameserverAddresses` are then empty, and `hostnames` holds no entry for it.
+`nxdomain` is left out when false.
+
 `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
+chain cannot be followed, or none of the name's nameservers answered its queries
-previous check's list is kept, or `null` when no earlier check saved one. A
+for A, AAAA and CNAME, the previous check's list is kept, or `null` when no
-state file without it loads, and the first check after that saves it without a
+earlier check saved one. A state file without it loads, and the first check
-notification.
+after that saves it without a notification.
 
 A port entry's `hostnames` lists every name that resolves to its address,
 domains included. A port entry in the older format, with one `hostname` instead

TODO.md

@@ -19,8 +19,16 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-02: a nameserver whose query for one record type failed while the
+  others answered with no records is `ok`, not `nodata` (closes #253).
+- 2026-10-02: a domain that does not exist is shown so, with no nameservers; no
+  name gets a parent's nameservers when its own did not answer (closes #222).
+- 2026-10-02: the refused-query test sends one query to four operators' public
+  resolvers in turn until one replies, not eight to one operator (closes #251).
 - 2026-10-02: a name removed from `DNSWATCHER_TARGETS` leaves the state, and so
   the dashboard and API, at startup, before the first check (closes #223).
+- 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: a Port Change notification lists the port's domains on a
   `Domains:` line and its hostnames on a `Hostnames:` line (closes #248).
 - 2026-10-02: the dashboard's Ports table and `/api/v1/status` port entries list

internal/handlers/dashboard_test.go

@@ -191,21 +191,39 @@ func TestDashboardShowsDomainRecordsUnderDomains(t *testing.T) {
 
 	words := strings.Join(strings.Fields(page), " ")
 
-	footer := "monitoring 1 domains + 1 hostnames"
+	footer := "monitoring 2 domains + 1 hostnames"
 	if !strings.Contains(words, footer) {
 		t.Errorf("dashboard does not say %q", footer)
 	}
 
-	// With the tags taken out, the summary bar starts "Domains 1
+	// With the tags taken out, the summary bar starts "Domains 2
 	// Hostnames 1".
 	text := regexp.MustCompile(`<[^>]*>`).ReplaceAllString(page, " ")
-	summary := "Domains 1 Hostnames 1"
+	summary := "Domains 2 Hostnames 1"
 
 	if !strings.Contains(strings.Join(strings.Fields(text), " "), summary) {
 		t.Errorf("summary bar does not say %q", summary)
 	}
 }
 
+// TestDashboardMarksDomainThatDoesNotExist checks that the Domains
+// section says a domain that does not exist does not exist, and does
+// not say so of a domain that exists.
+func TestDashboardMarksDomainThatDoesNotExist(t *testing.T) {
+	t.Parallel()
+
+	page := get(t, newHandlersWithFailures(t).HandleDashboard())
+	domains := dashboardSection(t, page, "Domains")
+
+	if !strings.Contains(dashboardRow(t, domains, missingDomain), "does not exist") {
+		t.Errorf("row of %s does not say it does not exist", missingDomain)
+	}
+
+	if strings.Contains(dashboardRow(t, domains, testDomain), "does not exist") {
+		t.Errorf("row of %s says it does not exist", testDomain)
+	}
+}
+
 // rowCells returns the text of each cell of a dashboard table row
 // whose cells start with tag, "<th" or "<td".
 func rowCells(row string, tag string) []string {

internal/handlers/status.go

@@ -10,10 +10,12 @@ import (
 
 // statusDomainInfo holds status information for a monitored domain.
 // RecordsByNameserver holds the domain's own records, in the form a
-// hostname's Nameservers holds the hostname's.
+// hostname's Nameservers holds the hostname's. NXDomain is true when
+// the domain's parent zone's servers answered that it does not exist.
 type statusDomainInfo struct {
 	Nameservers         []string                         `json:"nameservers"`
 	RecordsByNameserver map[string]*statusHostnameNSInfo `json:"recordsByNameserver"`
+	NXDomain            bool                             `json:"nxdomain"`
 	LastChecked         time.Time                        `json:"lastChecked"`
 }
 
@@ -155,6 +157,7 @@ func buildDomains(
 		resp.Domains[name] = &statusDomainInfo{
 			Nameservers:         ns,
 			RecordsByNameserver: records,
+			NXDomain:            ds.NXDomain,
 			LastChecked:         ds.LastChecked,
 		}
 	}

internal/handlers/status_test.go

@@ -23,7 +23,10 @@ import (
 // failed. example.net is an apex domain, whose own records are saved
 // with the hostnames' records, as the watcher saves them. Both names
 // resolve to domainAddress, whose port 443 entry lists them.
+// missingDomain is an apex domain whose parent zone's servers answered
+// that it does not exist, saved with no nameservers and no records.
 const (
+	missingDomain    = "does-not-exist.example"
 	testHostname     = "www.example.com"
 	answeringNS      = "ns1.example.com."
 	failedNS         = "ns2.example.com."
@@ -132,6 +135,12 @@ func setTestState(st *state.State) {
 		Hostnames:   []string{testDomain, testHostname},
 		LastChecked: now,
 	})
+
+	st.SetDomainState(missingDomain, &state.DomainState{
+		Nameservers: []string{},
+		NXDomain:    true,
+		LastChecked: now,
+	})
 }
 
 // get serves one GET request to handler and returns the response body.
@@ -233,6 +242,39 @@ func TestStatusGivesDomainRecordsUnderTheDomain(t *testing.T) {
 	}
 }
 
+// TestStatusMarksDomainThatDoesNotExist checks that /api/v1/status sets
+// nxdomain for a domain that does not exist, with no nameservers or
+// records, and not for a domain that exists.
+func TestStatusMarksDomainThatDoesNotExist(t *testing.T) {
+	t.Parallel()
+
+	body := get(t, newHandlersWithFailures(t).HandleStatus())
+
+	var resp struct {
+		Domains map[string]struct {
+			Nameservers         []string       `json:"nameservers"`
+			RecordsByNameserver map[string]any `json:"recordsByNameserver"`
+			NXDomain            bool           `json:"nxdomain"`
+		} `json:"domains"`
+	}
+
+	err := json.Unmarshal([]byte(body), &resp)
+	if err != nil {
+		t.Fatalf("decoding response: %v", err)
+	}
+
+	missing := resp.Domains[missingDomain]
+	if !missing.NXDomain || len(missing.Nameservers) != 0 ||
+		len(missing.RecordsByNameserver) != 0 {
+		t.Errorf("domain %s = %+v, want nxdomain and nothing else",
+			missingDomain, missing)
+	}
+
+	if resp.Domains[testDomain].NXDomain {
+		t.Errorf("domain %s has nxdomain set", testDomain)
+	}
+}
+
 // TestStatusPortsTellDomainsFromHostnames checks that a port entry in
 // /api/v1/status lists an apex domain in domains and a hostname in
 // hostnames when both resolve to its address.

internal/handlers/templates/dashboard.html

@@ -84,7 +84,11 @@
                   {{ $name }}
                 </td>
                 <td class="py-2 px-3 text-slate-400 break-all">
+                  {{ if $ds.NXDomain }}
+                  <span class="text-red-400">does not exist</span>
+                  {{ else }}
                   {{ joinStrings $ds.Nameservers ", " }}
+                  {{ end }}
                 </td>
                 <td class="py-2 px-3 text-slate-500 whitespace-nowrap">
                   {{ relTime $ds.LastChecked }}

internal/resolver/errors.go

@@ -10,6 +10,10 @@ var (
 		"no authoritative nameservers found",
 	)
 
+	// ErrNXDomain is returned when the servers of the zone a domain
+	// is in answer NXDOMAIN: the domain does not exist.
+	ErrNXDomain = errors.New("domain does not exist")
+
 	// ErrNoNameserverAnswered is returned when every nameserver
 	// asked about a name timed out, failed or returned a referral,
 	// so whether the name has addresses is unknown.
@@ -22,6 +26,12 @@ 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 +2,58 @@ 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
+}
+
+// NewWithQueryTimeout returns a Resolver whose queries over UDP give up
+// after timeout, so a test that asks an address where nothing answers
+// does not wait out the usual timeout.
+func NewWithQueryTimeout(log *slog.Logger, timeout time.Duration) *Resolver {
+	r := NewFromLogger(log)
+	r.client = &udpClient{timeout: timeout}
+
+	return r
+}
+
+// FollowDelegation exports followDelegation for testing.
+func (r *Resolver) FollowDelegation(
+	ctx context.Context,
+	domain string,
+	servers []string,
+) ([]string, error) {
+	return r.followDelegation(ctx, domain, servers)
+}
+
+// FindAuthoritativeNameserversFrom exports findAuthoritativeNameservers
+// for testing.
+func (r *Resolver) FindAuthoritativeNameserversFrom(
+	ctx context.Context,
+	domain string,
+	servers []string,
+) ([]string, error) {
+	return r.findAuthoritativeNameservers(ctx, domain, servers)
+}
+
+// ResolveNSIterative exports resolveNSIterative for testing.
+func (r *Resolver) ResolveNSIterative(
+	ctx context.Context,
+	domain string,
+) ([]string, error) {
+	return r.resolveNSIterative(ctx, domain)
+}
+
 // ExtractRecordValue exports extractRecordValue for testing.
 func ExtractRecordValue(rr dns.RR) string {
 	return extractRecordValue(rr)

internal/resolver/iterative.go

@@ -8,6 +8,7 @@ import (
 	"net"
 	"slices"
 	"sort"
+	"strconv"
 	"strings"
 	"time"
 
@@ -99,6 +100,9 @@ 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,
@@ -200,6 +204,12 @@ func glueIPs(nsNames []string, glue map[string][]net.IP) []string {
 	return ips
 }
 
+// followDelegation follows referrals from servers, the root servers, to
+// domain and returns the NS set of domain's delegation. 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.
 func (r *Resolver) followDelegation(
 	ctx context.Context,
 	domain string,
@@ -230,10 +240,15 @@ func (r *Resolver) followDelegation(
 		// An authoritative reply comes from the servers of the zone
 		// domain is in; it is not a referral, even when its authority
 		// section lists that zone's NS records. Without NS records in
-		// the answer, domain is not the zone's apex and has no
+		// the answer, domain has no nameservers of its own: it does
-		// nameservers of its own.
+		// not exist, when the reply is NXDOMAIN, or else it is not the
+		// zone's apex.
+		if resp.Authoritative && resp.Rcode == dns.RcodeNameError {
+			return nil, ErrNXDomain
+		}
+
 		if resp.Authoritative {
-			return nil, ErrNoNameservers
+			return []string{}, nil
 		}
 
 		authNS := extractNSSet(resp.Ns)
@@ -320,13 +335,20 @@ func (r *Resolver) queryServers(
 	return nil, fmt.Errorf("all servers failed: %w", lastErr)
 }
 
+// isErrorReply reports whether msg is an error reply: one with any code
+// but NOERROR and NXDOMAIN, such as SERVFAIL, NOTIMP or FORMERR. An error
+// reply says nothing about the name's records.
+func isErrorReply(msg *dns.Msg) bool {
+	return msg.Rcode != dns.RcodeSuccess && msg.Rcode != dns.RcodeNameError
+}
+
 // usableReply reports whether resp, a reply from one of the servers of
 // zone to a query about name, is usable. An error reply such as SERVFAIL
 // is not. Nor is a referral, unless it refers the query to a zone below
 // zone that name is in: a server that refers it back to zone, up or
 // sideways does not serve zone as it should.
 func usableReply(resp *dns.Msg, zone string, name string) bool {
-	if resp.Rcode != dns.RcodeSuccess && resp.Rcode != dns.RcodeNameError {
+	if isErrorReply(resp) {
 		return false
 	}
 
@@ -475,7 +497,8 @@ func (r *Resolver) resolveNSIPs(
 
 // resolveNSIterative queries for NS records using iterative
 // resolution as a fallback when followDelegation finds no
-// authoritative answer in the delegation chain.
+// authoritative answer in the delegation chain. Its result means what
+// followDelegation's does.
 func (r *Resolver) resolveNSIterative(
 	ctx context.Context,
 	domain string,
@@ -505,6 +528,16 @@ func (r *Resolver) resolveNSIterative(
 			return nsNames, nil
 		}
 
+		// As in followDelegation: domain has no nameservers of its
+		// own.
+		if resp.Authoritative && resp.Rcode == dns.RcodeNameError {
+			return nil, ErrNXDomain
+		}
+
+		if resp.Authoritative {
+			return []string{}, nil
+		}
+
 		// Follow delegation.
 		authNS := extractNSSet(resp.Ns)
 		if len(authNS) == 0 {
@@ -590,12 +623,23 @@ func (r *Resolver) resolveARecord(
 // FindAuthoritativeNameservers traces the delegation chain from
 // root servers to discover all authoritative nameservers for the
 // given domain, as the delegation from its parent zone's servers lists
-// them. For a name that is not a zone apex it tries each
+// them. When the servers asked answer that the name has no delegation
-// parent name in turn, so it returns the nameservers of the zone the
+// of its own, or does not exist, it tries each parent name in turn, so
-// name is in.
+// 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.
 func (r *Resolver) FindAuthoritativeNameservers(
 	ctx context.Context,
 	domain string,
+) ([]string, error) {
+	return r.findAuthoritativeNameservers(ctx, domain, rootServerList())
+}
+
+// findAuthoritativeNameservers is FindAuthoritativeNameservers with each
+// walk starting at servers, the root servers.
+func (r *Resolver) findAuthoritativeNameservers(
+	ctx context.Context,
+	domain string,
+	servers []string,
 ) ([]string, error) {
 	if checkCtx(ctx) != nil {
 		return nil, ErrContextCanceled
@@ -611,19 +655,16 @@ func (r *Resolver) FindAuthoritativeNameservers(
 
 		candidate := strings.Join(labels[i:], ".") + "."
 
-		nsNames, err := r.followDelegation(
+		nsNames, err := r.followDelegation(ctx, candidate, servers)
-			ctx, candidate, rootServerList(),
+		if err != nil && !errors.Is(err, ErrNXDomain) {
-		)
+			return nil, err
-		if err == nil && len(nsNames) > 0 {
+		}
+
+		if len(nsNames) > 0 {
 			sort.Strings(nsNames)
 
 			return nsNames, nil
 		}
-
-		// The root servers would refuse every parent name too.
-		if errors.Is(err, ErrIntercepted) {
-			return nil, err
-		}
 	}
 
 	return nil, ErrNoNameservers
@@ -714,15 +755,21 @@ func (r *Resolver) queryTypes(
 }
 
 type queryState struct {
-	gotNXDomain bool
+	gotNXDomain   bool
-	gotSERVFAIL bool
+	gotErrorReply bool
-	gotRefused  bool
+	errorReply    string // its code, such as SERVFAIL, or number if unnamed
-	gotTimeout  bool
+	gotRefused    bool
-	gotReferral bool
+	gotTimeout    bool
-	netErr      error
+	gotReferral   bool
-	hasRecords  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,
@@ -737,7 +784,34 @@ 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 {
@@ -747,6 +821,9 @@ 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,
@@ -754,7 +831,7 @@ func (r *Resolver) querySingleType(
 	qtype uint16,
 	resp *NameserverResponse,
 	state *queryState,
-) {
+) error {
 	msg, err := r.queryDNS(ctx, nsIP, hostname, qtype)
 	if err != nil {
 		switch {
@@ -766,35 +843,64 @@ 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 {
+	if isErrorReply(msg) {
-		state.gotSERVFAIL = true
+		state.gotErrorReply = true
 
-		return
+		code, named := dns.RcodeToString[msg.Rcode]
+		if !named {
+			code = strconv.Itoa(msg.Rcode)
+		}
+
+		state.errorReply = code
+
+		return fmt.Errorf(
+			"server returned %s: %w", state.errorReply, ErrUnusableReply,
+		)
 	}
 
 	// A reply with no answer that lists other nameservers, from a server
 	// that does not hold the name's zone, is a referral and says nothing
 	// about the name's records. A server named in the delegation that
-	// does not hold the zone may send one, as do a parent zone's servers
+	// does not hold the zone may send one.
-	// when FindAuthoritativeNameservers found no delegation for the
-	// name's zone and moved on to a parent name.
 	if !msg.Authoritative && len(msg.Answer) == 0 &&
 		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
@@ -833,26 +939,32 @@ 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. It has no data only when every type
+// answered with no records: a type in FailedTypes may have records, so a
+// nameserver with one stays ok.
 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.gotErrorReply && !state.answered:
 		resp.Status = StatusError
-		resp.Error = "server returned SERVFAIL"
+		resp.Error = "server returned " + state.errorReply
-	case state.gotRefused && !state.hasRecords:
+	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:
+	// An NXDOMAIN reply with no records was taken by the first case.
+	case !state.hasRecords && len(resp.FailedTypes) == 0:
 		resp.Status = StatusNoData
 	}
 }
@@ -941,12 +1053,22 @@ func (r *Resolver) queryEachNS(
 	return results, nil
 }
 
-// LookupNS returns the NS record set for a domain.
+// LookupNS returns the NS record set of a domain, as the delegation from
+// its parent zone's servers lists it, and never a parent name's. When
+// they answer that the domain does not exist, the error is ErrNXDomain.
+// When they answer that it has no delegation of its own, the set is
+// empty and there is no error.
 func (r *Resolver) LookupNS(
 	ctx context.Context,
 	domain string,
 ) ([]string, error) {
-	return r.FindAuthoritativeNameservers(ctx, domain)
+	if checkCtx(ctx) != nil {
+		return nil, ErrContextCanceled
+	}
+
+	return r.followDelegation(
+		ctx, dns.Fqdn(strings.ToLower(domain)), rootServerList(),
+	)
 }
 
 // LookupAllRecords performs iterative resolution to find all DNS
@@ -1010,9 +1132,11 @@ func (r *Resolver) resolveIPWithCNAME(
 }
 
 // collectIPs returns the addresses in the nameservers' answers and the
-// first CNAME target among them. It returns ErrNoNameserverAnswered when
+// first CNAME target among them. A nameserver whose query for one of the
-// every nameserver timed out, failed or returned a referral: that is not
+// types failed gave only part of the addresses, and is left out. It
-// a name with no addresses.
+// 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) {
@@ -1025,7 +1149,8 @@ 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 +1,149 @@
+package resolver
+
+import (
+	"strconv"
+	"syscall"
+	"testing"
+
+	"github.com/miekg/dns"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestClassifyResponse sets a nameserver's status from the results of
+// its queries and the record types whose query failed, 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, and is ok, not nodata: that type may have records. One whose
+// every query got none has failed. Only one whose every type answered
+// with no records is nodata.
+func TestClassifyResponse(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		results     queryState
+		failedTypes []string
+		wantStatus  string
+		wantError   string
+	}{
+		{
+			"every type answered with no records",
+			queryState{answered: true},
+			nil,
+			StatusNoData, "",
+		},
+		{
+			"some types answered with no records, another timed out",
+			queryState{answered: true, gotTimeout: true},
+			[]string{"A"},
+			StatusOK, "",
+		},
+		{
+			"some types answered with no records, another got SERVFAIL",
+			queryState{
+				answered: true, gotErrorReply: true, errorReply: "SERVFAIL",
+			},
+			[]string{"A"},
+			StatusOK, "",
+		},
+		{
+			"some types answered with no records, another was refused",
+			queryState{answered: true, gotRefused: true},
+			[]string{"A"},
+			StatusOK, "",
+		},
+		{
+			"some types answered with no records, another got a network error",
+			queryState{answered: true, netErr: syscall.ECONNREFUSED},
+			[]string{"A"},
+			StatusOK, "",
+		},
+		{
+			"some types answered with no records, another's reply was " +
+				"truncated and its retry over TCP failed",
+			queryState{answered: true, netErr: ErrTruncated},
+			[]string{"TXT"},
+			StatusOK, "",
+		},
+		{
+			"some types answered with no records, another got a referral",
+			queryState{answered: true, gotReferral: true},
+			[]string{"A"},
+			StatusOK, "",
+		},
+		{
+			"every query timed out",
+			queryState{gotTimeout: true},
+			[]string{"A", "AAAA", "CNAME"},
+			StatusTimeout, "all queries timed out",
+		},
+		{
+			"every query got NOTIMP",
+			queryState{gotErrorReply: true, errorReply: "NOTIMP"},
+			[]string{"A", "AAAA", "CNAME"},
+			StatusError, "server returned NOTIMP",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			resp := &NameserverResponse{Status: StatusOK, FailedTypes: tt.failedTypes}
+			classifyResponse(resp, tt.results)
+
+			assert.Equal(t, tt.wantStatus, resp.Status)
+			assert.Equal(t, tt.wantError, resp.Error)
+			assert.Equal(t, tt.failedTypes, resp.FailedTypes)
+		})
+	}
+}
+
+// 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; a
+// nameserver whose only reply it is has failed, and Error gives the
+// code, or its number when the code has no name.
+func TestReadReply(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		rcode      int
+		wantStatus string
+		wantError  string
+	}{
+		{dns.RcodeSuccess, StatusNoData, ""},
+		{dns.RcodeNameError, StatusNXDomain, ""},
+		{dns.RcodeServerFailure, StatusError, "server returned SERVFAIL"},
+		{dns.RcodeNotImplemented, StatusError, "server returned NOTIMP"},
+		{dns.RcodeFormatError, StatusError, "server returned FORMERR"},
+		{12, StatusError, "server returned 12"}, // unassigned, no name
+	}
+
+	for _, tt := range tests {
+		t.Run(strconv.Itoa(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{}}
+
+			var state queryState
+
+			err := readReply(msg, resp, &state)
+			classifyResponse(resp, state)
+
+			if tt.wantStatus == StatusError {
+				require.ErrorIs(t, err, ErrUnusableReply)
+			} else {
+				require.NoError(t, err)
+			}
+
+			assert.Equal(t, tt.wantStatus, resp.Status)
+			assert.Equal(t, tt.wantError, resp.Error)
+		})
+	}
+}

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/livedns_test.go

@@ -187,8 +187,8 @@ func liveFindAuthoritative(
 	return out
 }
 
-// liveLookupNS is liveFindAuthoritative through the LookupNS entry
+// liveLookupNS looks up the NS record set of domain, a domain that has
-// point, so that both entry points stay independently exercised.
+// one, retrying until the delegation chain can be walked.
 func liveLookupNS(
 	t *testing.T,
 	r *resolver.Resolver,

internal/resolver/resolver.go

@@ -31,11 +31,15 @@ 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
+	Nameserver  string
-	Records    map[string][]string
+	Records     map[string][]string
-	Status     string
+	FailedTypes []string
-	Error      string
+	Status      string
+	Error       string
 }
 
 // Resolver performs iterative DNS resolution from root servers.

internal/resolver/resolver_test.go

@@ -1,6 +1,7 @@
 package resolver_test
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -24,6 +25,13 @@ import (
 // Test helpers
 // ----------------------------------------------------------------
 
+// nonexistentDomain is a .com domain that does not exist.
+const nonexistentDomain = "dnswatcher-test-does-not-exist.com"
+
+// noAnswerAddress is 192.0.2.1, a documentation address: nothing
+// answers there.
+const noAnswerAddress = "192.0.2.1"
+
 func newTestResolver(t *testing.T) *resolver.Resolver {
 	t.Helper()
 
@@ -87,6 +95,47 @@ func TestFindAuthoritativeNameservers_Subdomain(
 	assert.Equal(t, fromZone, fromHost)
 }
 
+// TestFindAuthoritativeNameservers_DelegatedSubdomain looks up the
+// nameservers of www.cs.cmu.edu, a name in cs.cmu.edu, a zone that
+// cmu.edu delegates to other servers. The servers of cs.cmu.edu answer
+// that the name has no delegation of its own, so it gets their names,
+// not those of the cmu.edu servers. Every referral on the way gives the
+// nameservers' addresses, so the walk sends few queries.
+func TestFindAuthoritativeNameservers_DelegatedSubdomain(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+	fromHost := liveFindAuthoritative(t, r, "www.cs.cmu.edu")
+	fromZone := liveLookupNS(t, r, "cs.cmu.edu")
+	fromParent := liveLookupNS(t, r, "cmu.edu")
+
+	assert.Equal(t, fromZone, fromHost)
+	assert.NotEqual(t, fromParent, fromHost)
+}
+
+// TestFindAuthoritativeNameservers_NoAnswer starts each walk for
+// www.google.com at 192.0.2.1, a documentation address where nothing
+// answers. A walk that got no answer does not say that the name has no
+// 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.
+func TestFindAuthoritativeNameservers_NoAnswer(t *testing.T) {
+	t.Parallel()
+
+	r := resolver.NewWithQueryTimeout(slog.Default(), 100*time.Millisecond)
+
+	nameservers, err := r.FindAuthoritativeNameserversFrom(
+		t.Context(), "www.google.com", []string{noAnswerAddress},
+	)
+	require.Error(t, err)
+	require.NotErrorIs(t, err, resolver.ErrNoNameservers)
+	assert.Contains(t, err.Error(), "query www.google.com. @"+noAnswerAddress)
+	assert.Empty(t, nameservers)
+}
+
 func TestFindAuthoritativeNameservers_ReturnsSorted(
 	t *testing.T,
 ) {
@@ -366,6 +415,30 @@ 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()
 
@@ -417,48 +490,45 @@ func TestQueryNameserver_Refused(t *testing.T) {
 	assert.Equal(t, "server returned REFUSED", resp.Error)
 }
 
-// TestQueryNameserverIP_RecursiveResolverRefused asks Quad9, a public
+// TestQueryServers_RecursiveResolverRefused passes a public recursive
-// recursive resolver, about google.com at both of its addresses. Quad9
+// resolver to QueryServers as the server of google.com. These resolvers
-// refuses a query that does not ask for recursion and answers one that
+// refuse a query that does not ask for recursion and answer one that
-// does. The resolver never asks for recursion, so it must be reported
+// does. The resolver never asks for recursion, so the query must be
-// as refusing, never as answering.
+// reported as refused, never answered. Each resolver is run by a
-func TestQueryNameserverIP_RecursiveResolverRefused(t *testing.T) {
+// different operator, and they are asked in turn until one replies, so
+// one operator not answering does not fail the test.
+func TestQueryServers_RecursiveResolverRefused(t *testing.T) {
 	t.Parallel()
 
 	r := newTestResolver(t)
-
+	resolvers := []string{
-	for _, ip := range []string{"9.9.9.9", "149.112.112.112"} {
+		"64.6.64.6", "185.222.222.222", "4.2.2.1", "9.9.9.9",
-		var resp *resolver.NameserverResponse
-
-		livednstest.Retry(
-			t,
-			"QueryNameserverIP("+ip+", google.com)",
-			func(ctx context.Context) error {
-				var err error
-
-				resp, err = r.QueryNameserverIP(
-					ctx, ip, ip, "google.com",
-				)
-				if err != nil {
-					return err
-				}
-
-				// A timeout or a network error is no reply at all.
-				if resp.Status == resolver.StatusTimeout ||
-					strings.HasPrefix(resp.Error, "network error") {
-					return fmt.Errorf(
-						"%w: %s: %s",
-						livednstest.ErrNoAnswer, ip, resp.Error,
-					)
-				}
-
-				return nil
-			},
-		)
-
-		assert.Equal(t, resolver.StatusError, resp.Status, ip)
-		assert.Equal(t, "server returned REFUSED", resp.Error, ip)
 	}
+
+	var err error
+
+	livednstest.Retry(
+		t,
+		"QueryServers(public recursive resolvers, google.com)",
+		func(ctx context.Context) error {
+			for _, ip := range resolvers {
+				_, err = r.QueryServers(
+					ctx, []string{ip}, "google.com.", "google.com.",
+					dns.TypeA,
+				)
+
+				// A refusal or an answer is a reply; anything else may
+				// be no reply at all, so the next resolver is asked.
+				if err == nil || errors.Is(err, resolver.ErrRefused) {
+					return nil
+				}
+			}
+
+			return fmt.Errorf("%w: %w", livednstest.ErrNoAnswer, err)
+		},
+	)
+
+	require.ErrorIs(t, err, resolver.ErrRefused)
 }
 
 // googleNameserverIPv4s returns the IPv4 addresses of google.com's
@@ -806,8 +876,7 @@ func TestLookupNS_MatchesFindAuthoritative(t *testing.T) {
 // nameservers of g.ntpns.org. The org servers delegate its parent zone,
 // ntpns.org, without the addresses of its nameservers, so the walk has
 // to look them up to ask them. If it did not, the walk for g.ntpns.org
-// would fail and LookupNS would return the nameservers of ntpns.org,
+// would fail.
-// which a.ntpns.org is not one of.
 func TestLookupNS_ParentZoneDelegatedWithoutAddresses(t *testing.T) {
 	t.Parallel()
 
@@ -817,6 +886,131 @@ func TestLookupNS_ParentZoneDelegatedWithoutAddresses(t *testing.T) {
 	assert.Contains(t, nameservers, "a.ntpns.org.")
 }
 
+// TestLookupNS_DomainThatDoesNotExist looks up the nameservers of a .com
+// domain that does not exist. The .com servers answer NXDOMAIN, so the
+// error is ErrNXDomain, and the domain does not get their names.
+func TestLookupNS_DomainThatDoesNotExist(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+
+	var (
+		nameservers []string
+		err         error
+	)
+
+	livednstest.Retry(
+		t,
+		"LookupNS("+nonexistentDomain+")",
+		func(ctx context.Context) error {
+			nameservers, err = r.LookupNS(ctx, nonexistentDomain)
+			if errors.Is(err, resolver.ErrNXDomain) {
+				return nil
+			}
+
+			return err
+		},
+	)
+
+	require.ErrorIs(t, err, resolver.ErrNXDomain)
+	assert.Empty(t, nameservers)
+}
+
+// TestLookupNS_NoDelegationOfItsOwn looks up the nameservers of
+// www.google.com, a name in the google.com zone with no delegation of
+// its own, as a domain such as octocat.github.io is. The google.com
+// servers answer with no NS records for it: the set is empty, and it is
+// not ErrNXDomain.
+func TestLookupNS_NoDelegationOfItsOwn(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+
+	var nameservers []string
+
+	livednstest.Retry(
+		t,
+		"LookupNS(www.google.com)",
+		func(ctx context.Context) error {
+			var err error
+
+			nameservers, err = r.LookupNS(ctx, "www.google.com")
+
+			return err
+		},
+	)
+
+	assert.Empty(t, nameservers)
+}
+
+// TestFollowDelegation_NoAnswer starts the walk LookupNS uses, for
+// google.com, at 192.0.2.1, a documentation address where nothing
+// answers. A walk that got no answer is an error, not an empty set,
+// which the watcher would report as an NS Change with every nameserver
+// removed.
+func TestFollowDelegation_NoAnswer(t *testing.T) {
+	t.Parallel()
+
+	r := resolver.NewWithQueryTimeout(slog.Default(), 100*time.Millisecond)
+
+	nameservers, err := r.FollowDelegation(
+		t.Context(), "google.com.", []string{noAnswerAddress},
+	)
+	require.Error(t, err)
+	assert.Empty(t, nameservers)
+}
+
+// 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.
+func TestResolveNSIterative_NoDelegationOfItsOwn(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+
+	var nameservers []string
+
+	livednstest.Retry(
+		t,
+		"ResolveNSIterative(www.google.com)",
+		func(ctx context.Context) error {
+			var err error
+
+			nameservers, err = r.ResolveNSIterative(ctx, "www.google.com")
+
+			return err
+		},
+	)
+
+	assert.Empty(t, nameservers)
+}
+
+// TestResolveNSIterative_DomainThatDoesNotExist walks to the nameservers
+// of a .com domain that does not exist as the fallback walk does. As in
+// TestLookupNS_DomainThatDoesNotExist, the error is ErrNXDomain.
+func TestResolveNSIterative_DomainThatDoesNotExist(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+
+	var err error
+
+	livednstest.Retry(
+		t,
+		"ResolveNSIterative("+nonexistentDomain+")",
+		func(ctx context.Context) error {
+			_, err = r.ResolveNSIterative(ctx, nonexistentDomain)
+			if errors.Is(err, resolver.ErrNXDomain) {
+				return nil
+			}
+
+			return err
+		},
+	)
+
+	require.ErrorIs(t, err, resolver.ErrNXDomain)
+}
+
 // ----------------------------------------------------------------
 // ResolveIPAddresses tests
 // ----------------------------------------------------------------
@@ -1001,6 +1195,29 @@ 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

@@ -38,19 +38,29 @@ type Params struct {
 // DomainState holds the monitoring state for an apex domain.
 // NameserverAddresses holds the sorted addresses each nameserver's name
 // resolves to, by nameserver name. A state file written before it
-// existed loads with it nil.
+// existed loads with it nil. NXDomain is true when the domain's parent
+// zone's servers answered that it does not exist; it then has no
+// nameservers.
 type DomainState struct {
 	Nameservers         []string            `json:"nameservers"`
 	NameserverAddresses map[string][]string `json:"nameserverAddresses"`
+	NXDomain            bool                `json:"nxdomain,omitempty"`
 	LastChecked         time.Time           `json:"lastChecked"`
 }
 
 // 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"`
+	Records      map[string][]string `json:"records"`
-	Status      string              `json:"status"`
+	FailedTypes  []string            `json:"failedTypes,omitempty"`
-	Error       string              `json:"error,omitempty"`
+	UnknownTypes []string            `json:"unknownTypes,omitempty"`
-	LastChecked time.Time           `json:"lastChecked"`
+	Status       string              `json:"status"`
+	Error        string              `json:"error,omitempty"`
+	LastChecked  time.Time           `json:"lastChecked"`
 }
 
 // HostnameState holds per-nameserver monitoring state for a hostname.

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

@@ -125,7 +125,8 @@ 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/interfaces.go

@@ -11,7 +11,9 @@ import (
 
 // DNSResolver performs iterative DNS resolution.
 type DNSResolver interface {
-	// LookupNS discovers authoritative nameservers for a domain.
+	// LookupNS returns a domain's NS record set, as its parent zone's
+	// servers delegate it: empty when they answer that it has none, and
+	// resolver.ErrNXDomain when they answer that it does not exist.
 	LookupNS(
 		ctx context.Context,
 		domain string,

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/removed_test.go

@@ -87,6 +87,68 @@ func TestRemovedTargetsLeaveTheState(t *testing.T) {
 	}
 }
 
+// TestRemovedTargetsLeaveThePortEntries loads a state whose port
+// entries name a domain and a hostname now removed from the
+// configuration, and runs the removal that Run does before the first
+// check. The removed names are off each port entry's list of names, the
+// entry only they had is gone, the entry that also names configured
+// names is kept for the port checks, and nothing is notified.
+func TestRemovedTargetsLeaveThePortEntries(t *testing.T) {
+	t.Parallel()
+
+	const (
+		removedDomain = "example.com"
+		removedHost   = "www.example.com"
+	)
+
+	cfg := defaultTestConfig(t)
+	cfg.Domains = []string{domain}
+	cfg.Hostnames = []string{host}
+
+	deps := newTestDeps(t, cfg)
+	w := watcher.NewForTest(
+		cfg, deps.state, nil,
+		deps.portChecker, deps.tlsChecker, deps.notifier,
+	)
+
+	// The port 443 entries a check of all four names saves: each name
+	// at ip1, except the removed hostname, at ip2.
+	deps.state.SetPortState(ip1+":443", &state.PortState{
+		Open: true, Hostnames: []string{removedDomain, domain, host},
+	})
+	deps.state.SetPortState(ip2+":443", &state.PortState{
+		Open: true, Hostnames: []string{removedHost},
+	})
+
+	err := deps.state.Save()
+	if err != nil {
+		t.Fatalf("saving the state: %v", err)
+	}
+
+	err = deps.state.Load()
+	if err != nil {
+		t.Fatalf("loading the state: %v", err)
+	}
+
+	w.CleanupRemovedTargets()
+
+	if sent := deps.notifier.getNotifications(); len(sent) != 0 {
+		t.Errorf("sent %v, want nothing", sent)
+	}
+
+	snap := deps.state.GetSnapshot()
+
+	got := slices.Sorted(maps.Keys(snap.Ports))
+	if want := []string{ip1 + ":443"}; !slices.Equal(got, want) {
+		t.Fatalf("port entries %v, want %v", got, want)
+	}
+
+	got = snap.Ports[ip1+":443"].Hostnames
+	if want := []string{domain, host}; !slices.Equal(got, want) {
+		t.Errorf("names of port entry %s:443 %v, want %v", ip1, got, want)
+	}
+}
+
 // TestCertificateStateForAnAddressGone runs the port checks on hostname
 // state built here for a configured hostname, with certificate entries
 // saved for it at ip1, ip2 and an IPv6 address. When its nameservers

internal/watcher/watcher.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"maps"
 	"slices"
 	"sort"
 	"strings"
@@ -203,9 +204,10 @@ func (w *Watcher) detectFirstRun() {
 
 // cleanupRemovedTargets removes from the loaded state the domain,
 // hostname and certificate entries of names no longer in the
-// configuration, which changes only at a restart. Nothing is notified.
+// configuration, which changes only at a restart, and takes those names
-// A configured domain's own records are saved as a hostname entry under
+// off each port entry's list of names, removing a port entry left with
-// its name, which is kept.
+// none. Nothing is notified. A configured domain's own records are
+// saved as a hostname entry under its name, which is kept.
 func (w *Watcher) cleanupRemovedTargets() {
 	for _, name := range w.state.GetAllDomainNames() {
 		if !w.isDomain(name) {
@@ -224,6 +226,33 @@ func (w *Watcher) cleanupRemovedTargets() {
 			w.state.DeleteCertificateState(key)
 		}
 	}
+
+	for _, key := range w.state.GetAllPortKeys() {
+		ps, ok := w.state.GetPortState(key)
+		if !ok {
+			continue
+		}
+
+		var names []string
+
+		for _, name := range ps.Hostnames {
+			if w.isConfigured(name) {
+				names = append(names, name)
+			}
+		}
+
+		if len(names) == 0 {
+			w.state.DeletePortState(key)
+
+			continue
+		}
+
+		w.state.SetPortState(key, &state.PortState{
+			Open:        ps.Open,
+			Hostnames:   names,
+			LastChecked: ps.LastChecked,
+		})
+	}
 }
 
 // runDNSChecks performs DNS resolution for all configured domains
@@ -260,6 +289,13 @@ func (w *Watcher) checkDomain(
 	domain string,
 ) {
 	nameservers, err := w.resolver.LookupNS(ctx, domain)
+
+	// A domain that does not exist has no nameservers.
+	nxdomain := errors.Is(err, resolver.ErrNXDomain)
+	if nxdomain {
+		nameservers, err = []string{}, nil
+	}
+
 	if err != nil {
 		w.logFailedLookup(
 			ctx,
@@ -294,9 +330,18 @@ func (w *Watcher) checkDomain(
 	w.state.SetDomainState(domain, &state.DomainState{
 		Nameservers:         nameservers,
 		NameserverAddresses: addresses,
+		NXDomain:            nxdomain,
 		LastChecked:         now,
 	})
 
+	// A domain that does not exist has no records of its own: none are
+	// asked for, and those saved by an earlier check are removed.
+	if nxdomain {
+		w.state.DeleteHostnameState(domain)
+
+		return
+	}
+
 	// The apex domain's records are also checked and saved as a
 	// hostname's, so that the port and TLS checks find its addresses.
 	// Notifications about them name it as a domain (see nameLine).
@@ -427,8 +472,10 @@ 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()),
 	)
 }
 
@@ -458,8 +505,9 @@ 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
+// the name's nameservers answered its queries for A, AAAA and CNAME,
-// followed, as when no nameserver of a zone in its chain answers.
+// 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,
@@ -477,7 +525,10 @@ 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
 		}
 
@@ -523,11 +574,13 @@ 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 answered, even with NXDOMAIN or no records, is saved as ok, with
-// that timed out or failed is saved as error with the reason, and its
+// the record types whose query failed; one that timed out or failed is
-// empty record set is not an answer.
+// 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{
@@ -539,7 +592,7 @@ func buildHostnameState(
 
 	for ns, resp := range results {
 		nsState := &state.NameserverRecordState{
-			Records:     resp.Records,
+			Records:     maps.Clone(resp.Records),
 			Status:      statusOK,
 			LastChecked: now,
 		}
@@ -548,6 +601,15 @@ 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
@@ -556,6 +618,27 @@ 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,
@@ -644,7 +727,10 @@ 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,
@@ -656,7 +742,11 @@ 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
 		}
 
@@ -664,8 +754,8 @@ func (w *Watcher) detectRecordChanges(
 			"%s\nNameserver: %s\n%s",
 			w.nameLine(hostname), ns,
 			recordDifferences(
-				"Old", prevNS.Records,
+				"Old", oldRecords,
-				"New", cur.Records,
+				"New", newRecords,
 			),
 		)
 
@@ -752,13 +842,19 @@ 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(
 			"%s\n%s",
 			w.nameLine(hostname),
 			recordDifferences(
-				ns1, current.RecordsByNameserver[ns1].Records,
+				ns1, withoutTypes(state1.Records, failed),
-				ns2, current.RecordsByNameserver[ns2].Records,
+				ns2, withoutTypes(state2.Records, failed),
 			),
 		)
 
@@ -776,7 +872,9 @@ 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 {
@@ -793,9 +891,9 @@ func newlyDisagreeingPairs(
 
 	for i, ns1 := range nameservers {
 		for _, ns2 := range nameservers[i+1:] {
-			if recordsEqual(
+			if nameserversAgree(
-				current.RecordsByNameserver[ns1].Records,
+				current.RecordsByNameserver[ns1],
-				current.RecordsByNameserver[ns2].Records,
+				current.RecordsByNameserver[ns2],
 			) {
 				continue
 			}
@@ -805,7 +903,7 @@ func newlyDisagreeingPairs(
 
 			if ok1 && ok2 &&
 				prev1.Status == statusOK && prev2.Status == statusOK &&
-				!recordsEqual(prev1.Records, prev2.Records) {
+				!nameserversAgree(prev1, prev2) {
 				continue
 			}
 
@@ -1330,6 +1428,33 @@ 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 {

internal/watcher/watcher_test.go

@@ -482,6 +482,119 @@ func TestNSChangeDetection(t *testing.T) {
 	}
 }
 
+// TestDomainThatDoesNotExist checks a .com domain that does not exist,
+// 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.
+func TestDomainThatDoesNotExist(t *testing.T) {
+	t.Parallel()
+
+	const domain = "dnswatcher-test-does-not-exist.com"
+
+	cfg := defaultTestConfig(t)
+	cfg.Domains = []string{domain}
+
+	var 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{
+			Nameservers: []string{oldNS1, oldNS2},
+		})
+		deps.state.SetHostnameState(domain, &state.HostnameState{
+			RecordsByNameserver: map[string]*state.NameserverRecordState{
+				oldNS1: {
+					Records: map[string][]string{"A": {oldIP}},
+					Status:  "ok",
+				},
+			},
+		})
+
+		started := time.Now()
+
+		w.RunOnce(ctx)
+
+		// When no server answered, the domain's state is not saved.
+		ds, _ := deps.state.GetDomainState(domain)
+		if ds.LastChecked.Before(started) {
+			return fmt.Errorf("%s: %w", domain, livednstest.ErrNoAnswer)
+		}
+
+		return nil
+	})
+
+	ds, _ := deps.state.GetDomainState(domain)
+	if !ds.NXDomain || len(ds.Nameservers) != 0 {
+		t.Errorf("saved nxdomain %v and nameservers %v, want true and none",
+			ds.NXDomain, ds.Nameservers)
+	}
+
+	if hs, ok := deps.state.GetHostnameState(domain); ok {
+		t.Errorf("records saved for %s: %v", domain, hs.RecordsByNameserver)
+	}
+
+	assertNotified(t, deps, "NS Change: "+domain, "warning")
+
+	// That is the only notification, and it removes both nameservers,
+	// in either order.
+	for _, n := range deps.notifier.getNotifications() {
+		removed := strings.TrimPrefix(
+			n.Message, "Domain: "+domain+"\nAdded: \nRemoved: ",
+		)
+		if removed != oldNS1+", "+oldNS2 && removed != oldNS2+", "+oldNS1 {
+			t.Errorf("unexpected notification: %v", n)
+		}
+	}
+}
+
+// TestDomainWithNoDelegationOfItsOwn checks a domain with no delegation
+// of its own: codeberg.page is on the public suffix list, so
+// docs.codeberg.page is a domain, but the .page servers delegate only
+// codeberg.page, whose servers answer for it. It is saved with no
+// nameservers and without nxdomain, and its records, asked at the
+// codeberg.page servers, are saved. Those are testSmallDomain's two
+// nameservers; github.io, the zone of the README's example, has eight.
+func TestDomainWithNoDelegationOfItsOwn(t *testing.T) {
+	t.Parallel()
+
+	const domain = "docs.codeberg.page"
+
+	cfg := defaultTestConfig(t)
+	cfg.Domains = []string{domain}
+
+	var deps *testDeps
+
+	livednstest.Retry(t, "watcher checks", func(ctx context.Context) error {
+		var w *watcher.Watcher
+
+		w, deps = newTestWatcher(t, cfg)
+
+		err := checkOnce(ctx, w, deps)
+
+		// A domain saved as not existing has no records to wait for;
+		// the checks below fail on it.
+		if ds, ok := deps.state.GetDomainState(domain); ok && ds.NXDomain {
+			return nil
+		}
+
+		return err
+	})
+
+	ds, _ := deps.state.GetDomainState(domain)
+	if ds.NXDomain || len(ds.Nameservers) != 0 {
+		t.Errorf("saved nxdomain %v and nameservers %v, want false and none",
+			ds.NXDomain, ds.Nameservers)
+	}
+
+	if _, ok := deps.state.GetHostnameState(domain); !ok {
+		t.Errorf("no records saved for %s", domain)
+	}
+}
+
 func TestNSAddressChangeDetection(t *testing.T) {
 	t.Parallel()