wgengine/filter: add full IPv6 support.

Part of #19.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/packet/udp6.go

@@ -0,0 +1,51 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package packet
+
+import "encoding/binary"
+
+// UDP6Header is an IPv6+UDP header.
+type UDP6Header struct {
+	IP6Header
+	SrcPort uint16
+	DstPort uint16
+}
+
+// Len implements Header.
+func (h UDP6Header) Len() int {
+	return h.IP6Header.Len() + udpHeaderLength
+}
+
+// Marshal implements Header.
+func (h UDP6Header) Marshal(buf []byte) error {
+	if len(buf) < h.Len() {
+		return errSmallBuffer
+	}
+	if len(buf) > maxPacketLength {
+		return errLargePacket
+	}
+	// The caller does not need to set this.
+	h.IPProto = UDP
+
+	length := len(buf) - h.IP6Header.Len()
+	binary.BigEndian.PutUint16(buf[40:42], h.SrcPort)
+	binary.BigEndian.PutUint16(buf[42:44], h.DstPort)
+	binary.BigEndian.PutUint16(buf[44:46], uint16(length))
+	binary.BigEndian.PutUint16(buf[46:48], 0) // blank checksum
+
+	// UDP checksum with IP pseudo header.
+	h.IP6Header.marshalPseudo(buf)
+	binary.BigEndian.PutUint16(buf[46:48], ip4Checksum(buf[:]))
+
+	h.IP6Header.Marshal(buf)
+
+	return nil
+}
+
+// ToResponse implements Header.
+func (h *UDP6Header) ToResponse() {
+	h.SrcPort, h.DstPort = h.DstPort, h.SrcPort
+	h.IP6Header.ToResponse()
+}