resolver: a root server's reply marked recursion available means interception (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. Such a
reply is now passed over, and when every root server refused or gave
one, the lookup fails with the existing "this network intercepts DNS
queries", whether or not that resolver had the name cached. Before, a
cached name was checked against the resolver's second-hand answer.

On such a network dnswatcher and its tests fail with that error, by
design; nothing asks again another way or works around it. A live test
passes public resolvers as the root servers and expects the error.

Model: opus-5-5

internal/resolver/errors.go

@@ -53,9 +53,19 @@ var (
 		"reply truncated and its retry over TCP failed",
 	)
 
+	// ErrRecursionAvailable is the reason a reply from a root server's
+	// address marked recursion available is not used. Root servers offer
+	// no recursion, so the reply came from something on the network
+	// answering in their place.
+	ErrRecursionAvailable = errors.New(
+		"reply marked recursion available, so not from a root server",
+	)
+
 	// 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.
+	// query or gave a reply marked recursion available. 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

@@ -284,10 +284,11 @@ 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 reply it uses (see replyError). A server that
+// times out, refuses or gives a reply it does not use is passed over for
+// the next. When every server refused, the error says so. When they are
+// the root servers and each one refused or gave a reply marked recursion
+// available, the error is ErrIntercepted.
 func (r *Resolver) queryServers(
 	ctx context.Context,
 	servers []string,
@@ -298,6 +299,7 @@ func (r *Resolver) queryServers(
 	var lastErr error
 
 	refused := 0
+	recursionAvailable := 0
 
 	for _, ip := range shuffled(servers, rand.Shuffle) {
 		if checkCtx(ctx) != nil {
@@ -305,10 +307,8 @@ func (r *Resolver) queryServers(
 		}
 
 		resp, err := r.queryDNS(ctx, ip, name, qtype)
-		if err == nil && !usableReply(resp, zone, name) {
-			err = fmt.Errorf(
-				"query %s @%s: %w", name, ip, ErrUnusableReply,
-			)
+		if err == nil {
+			err = replyError(resp, ip, zone, name)
 		}
 
 		if err == nil {
@@ -319,6 +319,10 @@ func (r *Resolver) queryServers(
 			refused++
 		}
 
+		if errors.Is(err, ErrRecursionAvailable) {
+			recursionAvailable++
+		}
+
 		lastErr = err
 	}
 
@@ -329,6 +333,14 @@ func (r *Resolver) queryServers(
 		)
 	}
 
+	if refused+recursionAvailable == len(servers) && zone == "." {
+		return nil, fmt.Errorf(
+			"every root server refused a query for %s or gave a reply "+
+				"marked recursion available: %w",
+			name, ErrIntercepted,
+		)
+	}
+
 	if refused == len(servers) {
 		return nil, fmt.Errorf(
 			"every server of %s refused a query for %s: %w",
@@ -339,6 +351,27 @@ func (r *Resolver) queryServers(
 	return nil, fmt.Errorf("all servers failed: %w", lastErr)
 }
 
+// replyError returns why resp, the reply of the server at ip, one of the
+// servers of zone, to a query about name, is not used, or nil when it
+// is. A reply that is not usable is not used. Nor is a reply from a root
+// server marked recursion available: root servers offer no recursion,
+// so it came from something on the network answering in their place.
+func replyError(resp *dns.Msg, ip string, zone string, name string) error {
+	if zone == "." && resp.RecursionAvailable {
+		return fmt.Errorf(
+			"query %s @%s: %w", name, ip, ErrRecursionAvailable,
+		)
+	}
+
+	if !usableReply(resp, zone, name) {
+		return fmt.Errorf(
+			"query %s @%s: %w", name, ip, ErrUnusableReply,
+		)
+	}
+
+	return nil
+}
+
 // 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.

internal/resolver/resolver_test.go

@@ -850,6 +850,55 @@ func TestQueryServers_NotEveryRootServerRefused(t *testing.T) {
 	require.NotContains(t, err.Error(), "refused a query for")
 }
 
+// TestQueryServers_RecursiveResolversAsRootServers passes public
+// recursive resolvers of four operators to QueryServers as the servers
+// of the root zone, as a network that sends every DNS query to a
+// resolver would. Each refuses the query or gives a reply marked
+// recursion available, which no root server does: SERVFAIL, or an
+// answer from its cache, as 1.1.1.1 gives for com., which would be used
+// if a root server gave it. Either way the error is ErrIntercepted, and
+// since 1.1.1.1 and 8.8.8.8 refuse no such query, it says a reply was
+// marked recursion available.
+func TestQueryServers_RecursiveResolversAsRootServers(t *testing.T) {
+	t.Parallel()
+
+	r := newTestResolver(t)
+	resolvers := []string{
+		"1.1.1.1", "8.8.8.8", "9.9.9.9", "208.67.222.222",
+	}
+
+	for _, name := range []string{"com.", "desec.io."} {
+		var err error
+
+		livednstest.Retry(
+			t,
+			"QueryServers(public recursive resolvers as root servers, "+
+				name+")",
+			func(ctx context.Context) error {
+				_, err = r.QueryServers(ctx, resolvers, ".", name, dns.TypeNS)
+
+				// When the error is another, one resolver may have given
+				// no reply at all, so the attempt is tried again.
+				if err != nil && !errors.Is(err, resolver.ErrIntercepted) {
+					return fmt.Errorf(
+						"%w: %w", livednstest.ErrNoAnswer, err,
+					)
+				}
+
+				return nil
+			},
+		)
+
+		require.ErrorIs(t, err, resolver.ErrIntercepted)
+		require.EqualError(
+			t, err,
+			"every root server refused a query for "+name+" or gave a "+
+				"reply marked recursion available: "+
+				"this network intercepts DNS queries",
+		)
+	}
+}
+
 func TestQueryNameserver_RecordsSorted(t *testing.T) {
 	t.Parallel()