Fix FakeIP returning error for unconfigured address family

Return SUCCESS with empty answers instead of an error when the
queried address family has no range configured. Reject configurations
where neither inet4_range nor inet6_range is set.

dns/transport/fakeip/fakeip.go

@@ -23,16 +23,25 @@ var _ adapter.FakeIPTransport = (*Transport)(nil)
 
 type Transport struct {
 	dns.TransportAdapter
-	logger logger.ContextLogger
+	logger       logger.ContextLogger
-	store  adapter.FakeIPStore
+	store        adapter.FakeIPStore
+	inet4Enabled bool
+	inet6Enabled bool
 }
 
 func NewTransport(ctx context.Context, logger log.ContextLogger, tag string, options option.FakeIPDNSServerOptions) (adapter.DNSTransport, error) {
-	store := NewStore(ctx, logger, options.Inet4Range.Build(netip.Prefix{}), options.Inet6Range.Build(netip.Prefix{}))
+	inet4Range := options.Inet4Range.Build(netip.Prefix{})
+	inet6Range := options.Inet6Range.Build(netip.Prefix{})
+	if !inet4Range.IsValid() && !inet6Range.IsValid() {
+		return nil, E.New("at least one of inet4_range or inet6_range must be set")
+	}
+	store := NewStore(ctx, logger, inet4Range, inet6Range)
 	return &Transport{
 		TransportAdapter: dns.NewTransportAdapter(C.DNSTypeFakeIP, tag, nil),
 		logger:           logger,
 		store:            store,
+		inet4Enabled:     inet4Range.IsValid(),
+		inet6Enabled:     inet6Range.IsValid(),
 	}, nil
 }
 
@@ -55,6 +64,9 @@ func (t *Transport) Exchange(ctx context.Context, message *mDNS.Msg) (*mDNS.Msg,
 	if question.Qtype != mDNS.TypeA && question.Qtype != mDNS.TypeAAAA {
 		return nil, E.New("only IP queries are supported by fakeip")
 	}
+	if question.Qtype == mDNS.TypeA && !t.inet4Enabled || question.Qtype == mDNS.TypeAAAA && !t.inet6Enabled {
+		return dns.FixedResponseStatus(message, mDNS.RcodeSuccess), nil
+	}
 	address, err := t.store.Create(dns.FqdnToDomain(question.Name), question.Qtype == mDNS.TypeAAAA)
 	if err != nil {
 		return nil, err