resolver: a reply marked recursion available from a root server's address stops the lookup (closes #300)

dnswatcher never asks for recursion and a root server never offers it,
so a reply marked recursion available from a root server's address
came from a resolver on the network answering in its place. The first
such reply, a refusal included, now stops the lookup at once with
"this network intercepts DNS queries", naming the address, whether or
not that resolver had the name cached. queryDNS returns a refused reply
along with ErrRefused so the mark can be seen; a refusal without the
mark is handled as before. On such a network dnswatcher and its tests
fail with that error, by design, as the owner ruled on the issue.

Model: opus-5-5

internal/resolver/errors.go

@@ -53,9 +53,13 @@ var (
 		"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.
+	// ErrIntercepted is returned when a root server's address gave a
+	// reply marked recursion available, a refusal included, which ends
+	// the lookup at once, or when every root server refused a query.
+	// Root servers do neither, so those replies came from something on
+	// the network answering in their place. On such a network
+	// dnswatcher fails with it, by design: nothing works around the
+	// interception.
 	ErrIntercepted = errors.New("this network intercepts DNS queries")
 
 	// ErrCNAMEDepthExceeded is returned when a CNAME chain

internal/resolver/iterative.go

@@ -115,7 +115,8 @@ func (r *Resolver) retryTCP(
 }
 
 // queryDNS sends a DNS query to a specific server IP, never asking it
-// for recursion. A reply of REFUSED is returned as ErrRefused.
+// for recursion. A reply of REFUSED is returned together with
+// ErrRefused, so that the caller can still read the reply's flags.
 func (r *Resolver) queryDNS(
 	ctx context.Context,
 	serverIP string,
@@ -139,7 +140,7 @@ func (r *Resolver) queryDNS(
 	}
 
 	if resp.Rcode == dns.RcodeRefused {
-		return nil, fmt.Errorf(
+		return resp, fmt.Errorf(
 			"query %s @%s: %w", name, serverIP, ErrRefused,
 		)
 	}
@@ -284,10 +285,15 @@ func shuffled(
 }
 
 // queryServers asks servers, the servers of zone, about name in a random
-// order until one gives a usable reply. A server that times out, refuses
-// or gives a reply that is not usable is passed over for the next. When
-// every server refused, the error says so, and when they are the root
-// servers it is ErrIntercepted.
+// order until one gives a usable reply. When they are the root servers,
+// the first reply marked recursion available, a refusal included, ends
+// the lookup with ErrIntercepted, naming the address it came from, and
+// no other root server is asked: root servers offer no recursion, so
+// that reply came from something on the network answering in their
+// place. Otherwise a server that times out, refuses or gives a reply
+// that is not usable is passed over for the next. When every server
+// refused, the error says so, and when they are the root servers it is
+// ErrIntercepted.
 func (r *Resolver) queryServers(
 	ctx context.Context,
 	servers []string,
@@ -304,7 +310,17 @@ func (r *Resolver) queryServers(
 			return nil, ErrContextCanceled
 		}
 
+		// queryDNS returns a refusal together with its reply, so a
+		// refusal's mark is checked here too.
 		resp, err := r.queryDNS(ctx, ip, name, qtype)
+		if resp != nil && zone == "." && resp.RecursionAvailable {
+			return nil, fmt.Errorf(
+				"query %s @%s: reply marked recursion available, "+
+					"so not from a root server: %w",
+				name, ip, ErrIntercepted,
+			)
+		}
+
 		if err == nil && !usableReply(resp, zone, name) {
 			err = fmt.Errorf(
 				"query %s @%s: %w", name, ip, ErrUnusableReply,

internal/resolver/resolver_test.go

@@ -850,6 +850,74 @@ func TestQueryServers_NotEveryRootServerRefused(t *testing.T) {
 	require.NotContains(t, err.Error(), "refused a query for")
 }
 
+// TestQueryServers_RecursiveResolversAsRootServers passes public
+// recursive resolvers to QueryServers as the servers of the root zone,
+// as a network that sends every DNS query to a resolver would. Their
+// replies are marked recursion available, which no root server's is:
+// SERVFAIL, REFUSED, or an answer from a resolver's cache, as 1.1.1.1
+// gives for com., which would be used if a root server gave it. The
+// first such reply ends the lookup with ErrIntercepted, naming the
+// resolver that gave it. In the first group the replies differ; every
+// resolver in the second refuses a query that does not ask for
+// recursion, as 9.9.9.9 does, so there a refusal ends the lookup
+// instead of being passed over. Each group has resolvers of four
+// operators, and a resolver that gives no reply is passed over for the
+// next, so one of them answering is enough.
+func TestQueryServers_RecursiveResolversAsRootServers(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+	groups := [][]string{
+		{"1.1.1.1", "8.8.8.8", "9.9.9.9", "208.67.222.222"},
+		{"64.6.64.6", "185.222.222.222", "4.2.2.1", "94.140.14.14"},
+	}
+
+	for _, resolvers := range groups {
+		for _, name := range []string{"com.", "desec.io."} {
+			var err error
+
+			livednstest.Retry(
+				t,
+				"QueryServers("+strings.Join(resolvers, ", ")+
+					" as root servers, "+name+")",
+				func(ctx context.Context) error {
+					_, err = r.QueryServers(
+						ctx, resolvers, ".", name, dns.TypeNS,
+					)
+
+					// When no resolver gave a reply marked recursion
+					// available, none may have replied at all, so the
+					// attempt is tried again.
+					if err != nil && !strings.Contains(
+						err.Error(), "marked recursion available",
+					) {
+						return fmt.Errorf(
+							"%w: %w", livednstest.ErrNoAnswer, err,
+						)
+					}
+
+					return nil
+				},
+			)
+
+			require.ErrorIs(t, err, resolver.ErrIntercepted)
+
+			// The error names the resolver whose reply ended the
+			// lookup.
+			messages := make([]string, 0, len(resolvers))
+			for _, ip := range resolvers {
+				messages = append(messages,
+					"query "+name+" @"+ip+": reply marked recursion "+
+						"available, so not from a root server: "+
+						"this network intercepts DNS queries",
+				)
+			}
+
+			require.Contains(t, messages, err.Error())
+		}
+	}
+}
+
 func TestQueryNameserver_RecordsSorted(t *testing.T) {
 	t.Parallel()