Improve DNS truncate behavior

client_truncate.go

@@ -0,0 +1,37 @@
+package dns
+
+import "github.com/miekg/dns"
+
+func TruncateDNSMessage(request *dns.Msg, response *dns.Msg) (*dns.Msg, int) {
+	maxLen := 512
+	if edns0Option := request.IsEdns0(); edns0Option != nil {
+		if udpSize := int(edns0Option.UDPSize()); udpSize > 0 {
+			maxLen = udpSize
+		}
+	}
+	return truncateDNSMessage(response, maxLen)
+}
+
+func truncateDNSMessage(response *dns.Msg, maxLen int) (*dns.Msg, int) {
+	responseLen := response.Len()
+	if responseLen <= maxLen {
+		return response, responseLen
+	}
+	newResponse := *response
+	response = &newResponse
+	response.Compress = true
+	responseLen = response.Len()
+	if responseLen <= maxLen {
+		return response, responseLen
+	}
+	for len(response.Answer) > 0 && responseLen > maxLen {
+		response.Answer = response.Answer[:len(response.Answer)-1]
+		response.Truncated = true
+		responseLen = response.Len()
+	}
+	if responseLen > maxLen {
+		response.Ns = nil
+		response.Extra = nil
+	}
+	return response, response.Len()
+}

transport_https.go

@@ -57,6 +57,7 @@ func (t *HTTPSTransport) Start() error {
 
 func (t *HTTPSTransport) Reset() {
 	t.transport.CloseIdleConnections()
+	t.transport = t.transport.Clone()
 }
 
 func (t *HTTPSTransport) Close() error {

transport_udp.go

@@ -8,6 +8,7 @@ import (
 	"github.com/sagernet/sing/common"
 	"github.com/sagernet/sing/common/buf"
 	E "github.com/sagernet/sing/common/exceptions"
+	"github.com/sagernet/sing/common/logger"
 	M "github.com/sagernet/sing/common/metadata"
 
 	"github.com/miekg/dns"
@@ -25,6 +26,8 @@ func init() {
 
 type UDPTransport struct {
 	myTransportAdapter
+	tcpTransport *TCPTransport
+	logger       logger.ContextLogger
 }
 
 func NewUDPTransport(options TransportOptions) (*UDPTransport, error) {
@@ -40,13 +43,31 @@ func NewUDPTransport(options TransportOptions) (*UDPTransport, error) {
 	if serverAddr.Port == 0 {
 		serverAddr.Port = 53
 	}
+	tcpTransport, err := NewTCPTransport(options)
+	if err != nil {
+		return nil, err
+	}
 	transport := &UDPTransport{
 		newAdapter(options, serverAddr),
+		tcpTransport,
+		options.Logger,
 	}
 	transport.handler = transport
 	return transport, nil
 }
 
+func (t *UDPTransport) Exchange(ctx context.Context, message *dns.Msg) (*dns.Msg, error) {
+	response, err := t.myTransportAdapter.Exchange(ctx, message)
+	if err != nil {
+		return nil, err
+	}
+	if response.Truncated {
+		t.logger.InfoContext(ctx, "response truncated, retrying with TCP")
+		return t.tcpTransport.Exchange(ctx, message)
+	}
+	return response, nil
+}
+
 func (t *UDPTransport) DialContext(ctx context.Context) (net.Conn, error) {
 	return t.dialer.DialContext(ctx, "udp", t.serverAddr)
 }