resolver: a domain's nameservers are only its own delegation (closes #222)

LookupNS now follows the delegation for the domain alone. When the parent
zone's servers answer that it has no delegation, as for a domain that does
not exist, the set is empty, and the watcher's NS comparison reports every
nameserver removed.

FindAuthoritativeNameservers, used for hostnames, still moves to a parent
name when the servers answer that the name has no delegation of its own,
but returns the error when they do not answer, where it used to take a
parent zone's nameservers. The fallback walk treats an authoritative
answer the same way.

A domain with no delegation still has its own records asked at the
servers of the zone it is in.

Model: opus-5-5

README.md

@@ -73,9 +73,13 @@ 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 that its parent zone's servers
+  answer does not exist, or has no delegation, has no nameservers. When they 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.
     - 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
@@ -87,7 +91,9 @@ notification endpoint set, changes show only on the dashboard; see
   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 nameservers of its own has these records asked
+  at the servers of the zone it is in, as a hostname has: for a `.com` domain
+  that does not exist, the `.com` servers, which answer that it does not exist.
 
 ### DNS Hostname Monitoring (Subdomains)
 
@@ -95,7 +101,10 @@ 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. 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

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 domain that does not exist has no nameservers, not its parent
+  zone's; no name gets a parent's when its servers did not answer (closes #222).
 - 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

internal/resolver/iterative.go

@@ -204,6 +204,11 @@ 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 it has no delegation of its own,
+// because it is not the zone's apex or does not exist, the set is empty
+// and there is no error. An error means that no such answer came.
 func (r *Resolver) followDelegation(
 	ctx context.Context,
 	domain string,
@@ -234,10 +239,10 @@ 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
-		// nameservers of its own.
+		// the answer, domain is not the zone's apex, or does not
+		// exist, and has no nameservers of its own.
 		if resp.Authoritative {
-			return nil, ErrNoNameservers
+			return []string{}, nil
 		}
 
 		authNS := extractNSSet(resp.Ns)
@@ -486,7 +491,8 @@ 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,
@@ -516,6 +522,12 @@ func (r *Resolver) resolveNSIterative(
 			return nsNames, nil
 		}
 
+		// As in followDelegation: domain has no nameservers of its
+		// own.
+		if resp.Authoritative {
+			return []string{}, nil
+		}
+
 		// Follow delegation.
 		authNS := extractNSSet(resp.Ns)
 		if len(authNS) == 0 {
@@ -601,9 +613,10 @@ 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
-// parent name in turn, so it returns the nameservers of the zone the
-// name is in.
+// them. When the servers asked answer that the name has no delegation
+// of its own, it tries each parent name in turn, so it returns the
+// nameservers of the zone the name is in. When they do not answer, it
+// returns the error and tries no parent name.
 func (r *Resolver) FindAuthoritativeNameservers(
 	ctx context.Context,
 	domain string,
@@ -625,16 +638,15 @@ func (r *Resolver) FindAuthoritativeNameservers(
 		nsNames, err := r.followDelegation(
 			ctx, candidate, rootServerList(),
 		)
-		if err == nil && len(nsNames) > 0 {
+		if err != nil {
+			return nil, err
+		}
+
+		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
@@ -852,9 +864,7 @@ func readReply(
 	// 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
-	// when FindAuthoritativeNameservers found no delegation for the
-	// name's zone and moved on to a parent name.
+	// does not hold the zone may send one.
 	if !msg.Authoritative && len(msg.Answer) == 0 &&
 		len(extractNSSet(msg.Ns)) > 0 {
 		state.gotReferral = true
@@ -1022,12 +1032,21 @@ 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 has no delegation of its own, as when it
+// does not exist, 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

internal/resolver/livedns_test.go

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

internal/resolver/resolver_test.go

@@ -88,6 +88,26 @@ 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)
+}
+
 func TestFindAuthoritativeNameservers_ReturnsSorted(
 	t *testing.T,
 ) {
@@ -831,8 +851,7 @@ 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,
-// which a.ntpns.org is not one of.
+// would fail.
 func TestLookupNS_ParentZoneDelegatedWithoutAddresses(t *testing.T) {
 	t.Parallel()
 
@@ -842,6 +861,33 @@ 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 that it does not
+// exist, so it has none, and does not get theirs.
+func TestLookupNS_DomainThatDoesNotExist(t *testing.T) {
+	t.Parallel()
+
+	const domain = "dnswatcher-test-does-not-exist.com"
+
+	r := newTestResolver(t)
+
+	var nameservers []string
+
+	livednstest.Retry(
+		t,
+		"LookupNS("+domain+")",
+		func(ctx context.Context) error {
+			var err error
+
+			nameservers, err = r.LookupNS(ctx, domain)
+
+			return err
+		},
+	)
+
+	assert.Empty(t, nameservers)
+}
+
 // ----------------------------------------------------------------
 // ResolveIPAddresses tests
 // ----------------------------------------------------------------

internal/watcher/interfaces.go

@@ -11,7 +11,8 @@ 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.
 	LookupNS(
 		ctx context.Context,
 		domain string,