send: propagate DSCP bits to the outer tunnel

Like many, I am using WiFi a lot and often on congested networks.
 Without this, Wireguard strips the DSCP bits, preventing WME from
 kicking in and improving the audio/video experience.

Yes, it's technically an information leak. Who cares? It's not like if
traffic analysis based on packet sizes or timings wasn't a thing.

This is the first patch of the serie, more work has to happen on ECN

Signed-off-by: Florent Daigniere <nextgens@freenetproject.org>

conn.go

@@ -22,7 +22,7 @@ type Bind interface {
 	SetMark(value uint32) error
 	ReceiveIPv6(buff []byte) (int, Endpoint, error)
 	ReceiveIPv4(buff []byte) (int, Endpoint, error)
-	Send(buff []byte, end Endpoint) error
+	Send(buff []byte, end Endpoint, tos byte) error
 	Close() error
 }
 

conn_linux.go

@@ -258,18 +258,18 @@ func (bind *NativeBind) ReceiveIPv4(buff []byte) (int, Endpoint, error) {
 	return n, &end, err
 }
 
-func (bind *NativeBind) Send(buff []byte, end Endpoint) error {
+func (bind *NativeBind) Send(buff []byte, end Endpoint, tos byte) error {
 	nend := end.(*NativeEndpoint)
 	if !nend.isV6 {
 		if bind.sock4 == -1 {
 			return syscall.EAFNOSUPPORT
 		}
-		return send4(bind.sock4, nend, buff)
+		return send4(bind.sock4, nend, buff, tos)
 	} else {
 		if bind.sock6 == -1 {
 			return syscall.EAFNOSUPPORT
 		}
-		return send6(bind.sock6, nend, buff)
+		return send6(bind.sock6, nend, buff, tos)
 	}
 }
 
@@ -452,13 +452,18 @@ func create6(port uint16) (int, uint16, error) {
 	return fd, uint16(addr.Port), err
 }
 
-func send4(sock int, end *NativeEndpoint, buff []byte) error {
+func send4(sock int, end *NativeEndpoint, buff []byte, tos byte) error {
 
 	// construct message header
+	type ipTos struct {
+		tos byte
+	}
 
 	cmsg := struct {
 		cmsghdr unix.Cmsghdr
 		pktinfo unix.Inet4Pktinfo
+		cmsghdr2 unix.Cmsghdr
+		iptos   ipTos
 	}{
 		unix.Cmsghdr{
 			Level: unix.IPPROTO_IP,
@@ -469,6 +474,15 @@ func send4(sock int, end *NativeEndpoint, buff []byte) error {
 			Spec_dst: end.src4().src,
 			Ifindex:  end.src4().ifindex,
 		},
+		unix.Cmsghdr{
+			Level: unix.IPPROTO_IP,
+			Type:  unix.IP_TOS,
+			Len:   1 + unix.SizeofCmsghdr,
+		},
+		ipTos{
+			tos: tos,
+		},
+
 	}
 
 	_, err := unix.SendmsgN(sock, buff, (*[unsafe.Sizeof(cmsg)]byte)(unsafe.Pointer(&cmsg))[:], end.dst4(), 0)
@@ -488,13 +502,18 @@ func send4(sock int, end *NativeEndpoint, buff []byte) error {
 	return err
 }
 
-func send6(sock int, end *NativeEndpoint, buff []byte) error {
+func send6(sock int, end *NativeEndpoint, buff []byte, tos byte) error {
 
 	// construct message header
+	type ipTos struct {
+		tos byte
+	}
 
 	cmsg := struct {
 		cmsghdr unix.Cmsghdr
 		pktinfo unix.Inet6Pktinfo
+		cmsghdr2 unix.Cmsghdr
+		tclass   ipTos
 	}{
 		unix.Cmsghdr{
 			Level: unix.IPPROTO_IPV6,
@@ -505,6 +524,14 @@ func send6(sock int, end *NativeEndpoint, buff []byte) error {
 			Addr:    end.src6().src,
 			Ifindex: end.dst6().ZoneId,
 		},
+		unix.Cmsghdr{
+			Level: unix.IPPROTO_IPV6,
+			Type:  unix.IPV6_TCLASS,
+			Len:   1 + unix.SizeofCmsghdr,
+		},
+		ipTos{
+			tos: tos,
+		},
 	}
 
 	if cmsg.pktinfo.Addr == [16]byte{} {

peer.go

@@ -125,7 +125,7 @@ func (device *Device) NewPeer(pk NoisePublicKey) (*Peer, error) {
 	return peer, nil
 }
 
-func (peer *Peer) SendBuffer(buffer []byte) error {
+func (peer *Peer) SendBuffer(buffer []byte, tos byte) error {
 	peer.device.net.RLock()
 	defer peer.device.net.RUnlock()
 
@@ -140,7 +140,7 @@ func (peer *Peer) SendBuffer(buffer []byte) error {
 		return errors.New("no known endpoint for peer")
 	}
 
-	return peer.device.net.bind.Send(buffer, peer.endpoint)
+	return peer.device.net.bind.Send(buffer, peer.endpoint, tos)
 }
 
 func (peer *Peer) String() string {

send.go

@@ -41,6 +41,10 @@ import (
  * (to allow the construction of transport messages in-place)
  */
 
+const (
+	HandshakeDSCP = 0x88 // AF41, plus 00 ECN
+)
+
 type QueueOutboundElement struct {
 	dropped int32
 	sync.Mutex
@@ -49,6 +53,7 @@ type QueueOutboundElement struct {
 	nonce   uint64                // nonce for encryption
 	keypair *Keypair              // keypair for encryption
 	peer    *Peer                 // related peer
+	tos     byte                  // Type of Service (DSCP + ECN bits)
 }
 
 func (device *Device) NewOutboundElement() *QueueOutboundElement {
@@ -159,7 +164,7 @@ func (peer *Peer) SendHandshakeInitiation(isRetry bool) error {
 	peer.timersAnyAuthenticatedPacketTraversal()
 	peer.timersAnyAuthenticatedPacketSent()
 
-	err = peer.SendBuffer(packet)
+	err = peer.SendBuffer(packet, HandshakeDSCP)
 	if err != nil {
 		peer.device.log.Error.Println(peer, "- Failed to send handshake initiation", err)
 	}
@@ -197,7 +202,7 @@ func (peer *Peer) SendHandshakeResponse() error {
 	peer.timersAnyAuthenticatedPacketTraversal()
 	peer.timersAnyAuthenticatedPacketSent()
 
-	err = peer.SendBuffer(packet)
+	err = peer.SendBuffer(packet, HandshakeDSCP)
 	if err != nil {
 		peer.device.log.Error.Println(peer, "- Failed to send handshake response", err)
 	}
@@ -218,7 +223,7 @@ func (device *Device) SendHandshakeCookie(initiatingElem *QueueHandshakeElement)
 	var buff [MessageCookieReplySize]byte
 	writer := bytes.NewBuffer(buff[:0])
 	binary.Write(writer, binary.LittleEndian, reply)
-	device.net.bind.Send(writer.Bytes(), initiatingElem.endpoint)
+	device.net.bind.Send(writer.Bytes(), initiatingElem.endpoint, HandshakeDSCP)
 	if err != nil {
 		device.log.Error.Println("Failed to send cookie reply:", err)
 	}
@@ -294,14 +299,14 @@ func (device *Device) RoutineReadFromTUN() {
 			}
 			dst := elem.packet[IPv4offsetDst : IPv4offsetDst+net.IPv4len]
 			peer = device.allowedips.LookupIPv4(dst)
-
+			elem.tos = elem.packet[1];
 		case ipv6.Version:
 			if len(elem.packet) < ipv6.HeaderLen {
 				continue
 			}
 			dst := elem.packet[IPv6offsetDst : IPv6offsetDst+net.IPv6len]
 			peer = device.allowedips.LookupIPv6(dst)
-
+			elem.tos = elem.packet[1];
 		default:
 			logDebug.Println("Received packet with unknown IP version")
 		}
@@ -600,7 +605,7 @@ func (peer *Peer) RoutineSequentialSender() {
 			// send message and return buffer to pool
 
 			length := uint64(len(elem.packet))
-			err := peer.SendBuffer(elem.packet)
+			err := peer.SendBuffer(elem.packet, elem.tos)
 			device.PutMessageBuffer(elem.buffer)
 			device.PutOutboundElement(elem)
 			if err != nil {