package yggdrasil

// The ICMPv6 module implements functions to easily create ICMPv6
// packets. These functions, when mixed with the built-in Go IPv6
// and ICMP libraries, can be used to send control messages back
// to the host. Examples include:
// - NDP messages, when running in TAP mode
// - Packet Too Big messages, when packets exceed the session MTU
// - Destination Unreachable messages, when a session prohibits
//   incoming traffic

import (
	"encoding/binary"
	"errors"
	"net"

	"golang.org/x/net/icmp"
	"golang.org/x/net/ipv6"
)

type macAddress [6]byte

const len_ETHER = 14

type icmpv6 struct {
	tun        *tunDevice
	peermac    macAddress
	peerlladdr net.IP
	mylladdr   net.IP
	mymac      macAddress
}

// Marshal returns the binary encoding of h.
func ipv6Header_Marshal(h *ipv6.Header) ([]byte, error) {
	b := make([]byte, 40)
	b[0] |= byte(h.Version) << 4
	b[0] |= byte(h.TrafficClass) >> 4
	b[1] |= byte(h.TrafficClass) << 4
	b[1] |= byte(h.FlowLabel >> 16)
	b[2] = byte(h.FlowLabel >> 8)
	b[3] = byte(h.FlowLabel)
	binary.BigEndian.PutUint16(b[4:6], uint16(h.PayloadLen))
	b[6] = byte(h.NextHeader)
	b[7] = byte(h.HopLimit)
	copy(b[8:24], h.Src)
	copy(b[24:40], h.Dst)
	return b, nil
}

// Initialises the ICMPv6 module by assigning our link-local IPv6 address and
// our MAC address. ICMPv6 messages will always appear to originate from these
// addresses.
func (i *icmpv6) init(t *tunDevice) {
	i.tun = t

	// Our MAC address and link-local address
	copy(i.mymac[:], []byte{
		0x02, 0x00, 0x00, 0x00, 0x00, 0x02})
	i.mylladdr = net.IP{
		0xFE, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0xFE}
}

// Parses an incoming ICMPv6 packet. The packet provided may be either an
// ethernet frame containing an IP packet, or the IP packet alone. This is
// determined by whether the TUN/TAP adapter is running in TUN (layer 3) or
// TAP (layer 2) mode.
func (i *icmpv6) parse_packet(datain []byte) {
	var response []byte
	var err error

	// Parse the frame/packet
	if i.tun.iface.IsTAP() {
		response, err = i.parse_packet_tap(datain)
	} else {
		response, err = i.parse_packet_tun(datain)
	}

	if err != nil {
		return
	}

	// Write the packet to TUN/TAP
	i.tun.iface.Write(response)
}

// Unwraps the ethernet headers of an incoming ICMPv6 packet and hands off
// the IP packet to the parse_packet_tun function for further processing.
// A response buffer is also created for the response message, also complete
// with ethernet headers.
func (i *icmpv6) parse_packet_tap(datain []byte) ([]byte, error) {
	// Store the peer MAC address
	copy(i.peermac[:6], datain[6:12])

	// Ignore non-IPv6 frames
	if binary.BigEndian.Uint16(datain[12:14]) != uint16(0x86DD) {
		return nil, nil
	}

	// Hand over to parse_packet_tun to interpret the IPv6 packet
	ipv6packet, err := i.parse_packet_tun(datain[len_ETHER:])
	if err != nil {
		return nil, err
	}

	// Create the response buffer
	dataout := make([]byte, len_ETHER+ipv6.HeaderLen+32)

	// Populate the response ethernet headers
	copy(dataout[:6], datain[6:12])
	copy(dataout[6:12], i.mymac[:])
	binary.BigEndian.PutUint16(dataout[12:14], uint16(0x86DD))

	// Copy the returned packet to our response ethernet frame
	copy(dataout[len_ETHER:], ipv6packet)
	return dataout, nil
}

// Unwraps the IP headers of an incoming IPv6 packet and performs various
// sanity checks on the packet - i.e. is the packet an ICMPv6 packet, does the
// ICMPv6 message match a known expected type. The relevant handler function
// is then called and a response packet may be returned.
func (i *icmpv6) parse_packet_tun(datain []byte) ([]byte, error) {
	// Parse the IPv6 packet headers
	ipv6Header, err := ipv6.ParseHeader(datain[:ipv6.HeaderLen])
	if err != nil {
		return nil, err
	}

	// Check if the packet is IPv6
	if ipv6Header.Version != ipv6.Version {
		return nil, err
	}

	// Check if the packet is ICMPv6
	if ipv6Header.NextHeader != 58 {
		return nil, err
	}

	// Store the peer link local address, it will come in useful later
	copy(i.peerlladdr[:], ipv6Header.Src[:])

	// Parse the ICMPv6 message contents
	icmpv6Header, err := icmp.ParseMessage(58, datain[ipv6.HeaderLen:])
	if err != nil {
		return nil, err
	}

	// Check for a supported message type
	switch icmpv6Header.Type {
	case ipv6.ICMPTypeNeighborSolicitation:
		{
			response, err := i.handle_ndp(datain[ipv6.HeaderLen:])
			if err == nil {
				// Create our ICMPv6 response
				responsePacket, err := i.create_icmpv6_tun(
					ipv6Header.Src, i.mylladdr,
					ipv6.ICMPTypeNeighborAdvertisement, 0,
					&icmp.DefaultMessageBody{Data: response})
				if err != nil {
					return nil, err
				}

				// Send it back
				return responsePacket, nil
			} else {
				return nil, err
			}
		}
	}

	return nil, errors.New("ICMPv6 type not matched")
}

// Creates an ICMPv6 packet based on the given icmp.MessageBody and other
// parameters, complete with ethernet and IP headers, which can be written
// directly to a TAP adapter.
func (i *icmpv6) create_icmpv6_tap(dstmac macAddress, dst net.IP, src net.IP, mtype ipv6.ICMPType, mcode int, mbody icmp.MessageBody) ([]byte, error) {
	// Pass through to create_icmpv6_tun
	ipv6packet, err := i.create_icmpv6_tun(dst, src, mtype, mcode, mbody)
	if err != nil {
		return nil, err
	}

	// Create the response buffer
	dataout := make([]byte, len_ETHER+len(ipv6packet))

	// Populate the response ethernet headers
	copy(dataout[:6], dstmac[:6])
	copy(dataout[6:12], i.mymac[:])
	binary.BigEndian.PutUint16(dataout[12:14], uint16(0x86DD))

	// Copy the returned packet to our response ethernet frame
	copy(dataout[len_ETHER:], ipv6packet)
	return dataout, nil
}

// Creates an ICMPv6 packet based on the given icmp.MessageBody and other
// parameters, complete with IP headers only, which can be written directly to
// a TUN adapter, or called directly by the create_icmpv6_tap function when
// generating a message for TAP adapters.
func (i *icmpv6) create_icmpv6_tun(dst net.IP, src net.IP, mtype ipv6.ICMPType, mcode int, mbody icmp.MessageBody) ([]byte, error) {
	// Create the ICMPv6 message
	icmpMessage := icmp.Message{
		Type: mtype,
		Code: mcode,
		Body: mbody,
	}

	// Convert the ICMPv6 message into []byte
	icmpMessageBuf, err := icmpMessage.Marshal(icmp.IPv6PseudoHeader(src, dst))
	if err != nil {
		return nil, err
	}

	// Create the IPv6 header
	ipv6Header := ipv6.Header{
		Version:    ipv6.Version,
		NextHeader: 58,
		PayloadLen: len(icmpMessageBuf),
		HopLimit:   255,
		Src:        src,
		Dst:        dst,
	}

	// Convert the IPv6 header into []byte
	ipv6HeaderBuf, err := ipv6Header_Marshal(&ipv6Header)
	if err != nil {
		return nil, err
	}

	// Construct the packet
	responsePacket := make([]byte, ipv6.HeaderLen+ipv6Header.PayloadLen)
	copy(responsePacket[:ipv6.HeaderLen], ipv6HeaderBuf)
	copy(responsePacket[ipv6.HeaderLen:], icmpMessageBuf)

	// Send it back
	return responsePacket, nil
}

// Generates a response to an NDP discovery packet. This is effectively called
// when the host operating system generates an NDP request for any address in
// the fd00::/8 range, so that the operating system knows to route that traffic
// to the Yggdrasil TAP adapter.
func (i *icmpv6) handle_ndp(in []byte) ([]byte, error) {
	// Ignore NDP requests for anything outside of fd00::/8
	var source address
	copy(source[:], in[8:])
	var snet subnet
	copy(snet[:], in[8:])
	switch {
	case source.isValid():
	case snet.isValid():
	default:
		return nil, errors.New("Not an NDP for 0200::/7")
	}

	// Create our NDP message body response
	body := make([]byte, 28)
	binary.BigEndian.PutUint32(body[:4], uint32(0x20000000))
	copy(body[4:20], in[8:24]) // Target address
	body[20] = uint8(2)
	body[21] = uint8(1)
	copy(body[22:28], i.mymac[:6])

	// Send it back
	return body, nil
}