wgengine/filter: add full IPv6 support.

Part of #19.

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

net/packet/ip6.go

@@ -5,13 +5,14 @@
 package packet
 
 import (
+	"encoding/binary"
 	"fmt"
 
 	"inet.af/netaddr"
 )
 
 // IP6 is an IPv6 address.
-type IP6 [16]byte
+type IP6 [16]byte // TODO: maybe 2x uint64 would be faster for the type of ops we do?
 
 // IP6FromNetaddr converts a netaddr.IP to an IP6. Panics if !ip.Is6.
 func IP6FromNetaddr(ip netaddr.IP) IP6 {
@@ -30,5 +31,73 @@ func (ip IP6) String() string {
 	return ip.Netaddr().String()
 }
 
+func (ip IP6) IsMulticast() bool {
+	return ip[0] == 0xFF
+}
+
+func (ip IP6) IsLinkLocalUnicast() bool {
+	return ip[0] == 0xFE && ip[1] == 0x80
+}
+
 // ip6HeaderLength is the length of an IPv6 header with no IP options.
 const ip6HeaderLength = 40
+
+// IP6Header represents an IPv6 packet header.
+type IP6Header struct {
+	IPProto IPProto
+	IPID    uint32 // only lower 20 bits used
+	SrcIP   IP6
+	DstIP   IP6
+}
+
+// Len implements Header.
+func (h IP6Header) Len() int {
+	return ip6HeaderLength
+}
+
+// Marshal implements Header.
+func (h IP6Header) Marshal(buf []byte) error {
+	if len(buf) < h.Len() {
+		return errSmallBuffer
+	}
+	if len(buf) > maxPacketLength {
+		return errLargePacket
+	}
+
+	binary.BigEndian.PutUint32(buf[:4], h.IPID&0x000FFFFF)
+	buf[0] = 0x60
+	binary.BigEndian.PutUint16(buf[4:6], uint16(len(buf)-ip6HeaderLength)) // Total length
+	buf[6] = uint8(h.IPProto)                                              // Inner protocol
+	buf[7] = 64                                                            // TTL
+	copy(buf[8:24], h.SrcIP[:])
+	copy(buf[24:40], h.DstIP[:])
+
+	return nil
+}
+
+// ToResponse implements Header.
+func (h *IP6Header) ToResponse() {
+	h.SrcIP, h.DstIP = h.DstIP, h.SrcIP
+	// Flip the bits in the IPID. If incoming IPIDs are distinct, so are these.
+	h.IPID = (^h.IPID) & 0x000FFFFF
+}
+
+// marshalPseudo serializes h into buf in the "pseudo-header" form
+// required when calculating UDP checksums.
+func (h IP6Header) marshalPseudo(buf []byte) error {
+	if len(buf) < h.Len() {
+		return errSmallBuffer
+	}
+	if len(buf) > maxPacketLength {
+		return errLargePacket
+	}
+
+	copy(buf[:16], h.SrcIP[:])
+	copy(buf[16:32], h.DstIP[:])
+	binary.BigEndian.PutUint32(buf[32:36], uint32(len(buf)-h.Len()))
+	buf[36] = 0
+	buf[37] = 0
+	buf[38] = 0
+	buf[39] = 17 // NextProto
+	return nil
+}