From caeb0d53cc056e061a5406cbd7bc72d33889baac Mon Sep 17 00:00:00 2001
From: David Scott <dave@recoil.org>
Date: Fri, 22 Apr 2016 11:42:24 +0100
Subject: [PATCH] proxy: add an initial length to the marshalled frames

Since the header is variable length it's useful to write a length
field first, so the peer can read the rest of the packet as a block.

Signed-off-by: David Scott <dave.scott@docker.com>
---
 .../proxy/libproxy/udp_encapsulation.go       | 26 ++++++++++++++-----
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/alpine/packages/proxy/libproxy/udp_encapsulation.go b/alpine/packages/proxy/libproxy/udp_encapsulation.go
index 1891aa234..df837a896 100644
--- a/alpine/packages/proxy/libproxy/udp_encapsulation.go
+++ b/alpine/packages/proxy/libproxy/udp_encapsulation.go
@@ -91,30 +91,40 @@ type udpDatagram struct {
 }
 
 func (u *udpDatagram) Marshal(conn net.Conn) error {
+	// marshal the variable length header to a temporary buffer
+	var header bytes.Buffer
 	var length uint16
 	length = uint16(len(*u.IP))
-	if err := binary.Write(conn, binary.LittleEndian, &length); err != nil {
+	if err := binary.Write(&header, binary.LittleEndian, &length); err != nil {
 		return err
 	}
-	if err := binary.Write(conn, binary.LittleEndian, &u.IP); err != nil {
+	if err := binary.Write(&header, binary.LittleEndian, &u.IP); err != nil {
 		return err
 	}
-	if err := binary.Write(conn, binary.LittleEndian, &u.Port); err != nil {
+	if err := binary.Write(&header, binary.LittleEndian, &u.Port); err != nil {
 		return err
 	}
 	length = uint16(len(u.Zone))
-	if err := binary.Write(conn, binary.LittleEndian, &length); err != nil {
+	if err := binary.Write(&header, binary.LittleEndian, &length); err != nil {
 		return err
 	}
-	if err := binary.Write(conn, binary.LittleEndian, &u.Zone); err != nil {
+	if err := binary.Write(&header, binary.LittleEndian, &u.Zone); err != nil {
 		return nil
 	}
 	length = uint16(len(u.payload))
+	if err := binary.Write(&header, binary.LittleEndian, &length); err != nil {
+		return nil
+	}
+	length = uint16(header.Len() + len(u.payload))
 	if err := binary.Write(conn, binary.LittleEndian, &length); err != nil {
 		return nil
 	}
+	_, err := io.Copy(conn, &header)
+	if err != nil {
+		return err
+	}
 	payload := bytes.NewBuffer(u.payload)
-	_, err := io.Copy(conn, payload)
+	_, err = io.Copy(conn, payload)
 	if err != nil {
 		return err
 	}
@@ -123,6 +133,10 @@ func (u *udpDatagram) Marshal(conn net.Conn) error {
 
 func (u *udpDatagram) Unmarshal(conn net.Conn) error {
 	var length uint16
+	// frame length
+	if err := binary.Read(conn, binary.LittleEndian, &length); err != nil {
+		return err
+	}
 	if err := binary.Read(conn, binary.LittleEndian, &length); err != nil {
 		return err
 	}