fix missing context ID in datagrams

conn.go

@@ -3,6 +3,7 @@
 import (
 	"context"
 	"errors"
+	"fmt"
 	"io"
 	"log"
 	"net"
@@ -77,16 +78,26 @@
 		}
 		return 0, nil, os.ErrDeadlineExceeded
 	}
+	contextID, n, err := quicvarint.Parse(data)
+	if err != nil {
+		return 0, nil, fmt.Errorf("masque: malformed datagram: %w", err)
+	}
+	if contextID != 0 {
+		// Drop this datagram. We currently only support proxying of UDP payloads.
+		goto start
+	}
 	// If b is too small, additional bytes are discarded.
 	// This mirrors the behavior of large UDP datagrams received on a UDP socket (on Linux).
-	n = copy(b, data)
-	return n, c.remoteAddr, nil
+	return copy(b, data[n:]), c.remoteAddr, nil
 }
 
 // WriteTo sends a UDP datagram to the target.
 // The net.Addr parameter is ignored.
 func (c *proxiedConn) WriteTo(p []byte, _ net.Addr) (n int, err error) {
-	return len(p), c.str.SendDatagram(p)
+	data := make([]byte, 0, len(contextIDZero)+len(p))
+	data = append(data, contextIDZero...)
+	data = append(data, p...)
+	return len(p), c.str.SendDatagram(data)
 }
 
 func (c *proxiedConn) Close() error {

proxy.go

@@ -19,6 +19,8 @@
 	uriTemplateTargetPort = "target_port"
 )
 
+var contextIDZero = quicvarint.Append([]byte{}, 0)
+
 type proxyEntry struct {
 	str  http3.Stream
 	conn *net.UDPConn
@@ -126,7 +128,15 @@
 		if err != nil {
 			return err
 		}
-		if _, err := conn.Write(data); err != nil {
+		contextID, n, err := quicvarint.Parse(data)
+		if err != nil {
+			return err
+		}
+		if contextID != 0 {
+			// Drop this datagram. We currently only support proxying of UDP payloads.
+			continue
+		}
+		if _, err := conn.Write(data[n:]); err != nil {
 			return err
 		}
 	}
@@ -139,7 +149,10 @@
 		if err != nil {
 			return err
 		}
-		if err := str.SendDatagram(b[:n]); err != nil {
+		data := make([]byte, 0, len(contextIDZero)+n)
+		data = append(data, contextIDZero...)
+		data = append(data, b[:n]...)
+		if err := str.SendDatagram(data); err != nil {
 			return err
 		}
 	}

proxy_test.go

@@ -56,7 +56,7 @@ func TestProxyCloseProxiedConn(t *testing.T) {
 	done := make(chan struct{})
 	str := NewMockStream(gomock.NewController(t))
 	str.EXPECT().ReceiveDatagram(gomock.Any()).DoAndReturn(func(context.Context) ([]byte, error) {
-		return []byte("foo"), nil
+		return append(contextIDZero, []byte("foo")...), nil
 	})
 	// This datagram is received after the connection is closed.
 	// We expect that it won't get sent on.