// Copyright 2018 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package sniffer provides the implementation of data-link layer endpoints that
// wrap another endpoint and logs inbound and outbound packets.
//
// Sniffer endpoints can be used in the networking stack by calling New(eID) to
// create a new endpoint, where eID is the ID of the endpoint being wrapped,
// and then passing it as an argument to Stack.CreateNIC().
package sniffer

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"io"
	"os"
	"sync/atomic"
	"time"

	"github.com/google/netstack/tcpip"
	"github.com/google/netstack/tcpip/buffer"
	"github.com/google/netstack/tcpip/header"
	"github.com/google/netstack/tcpip/stack"
	"log"
)

// LogPackets is a flag used to enable or disable packet logging via the log
// package. Valid values are 0 or 1.
//
// LogPackets must be accessed atomically.
var LogPackets uint32 = 1

// LogPacketsToFile is a flag used to enable or disable logging packets to a
// pcap file. Valid values are 0 or 1. A file must have been specified when the
// sniffer was created for this flag to have effect.
//
// LogPacketsToFile must be accessed atomically.
var LogPacketsToFile uint32 = 1

type endpoint struct {
	dispatcher stack.NetworkDispatcher
	lower      stack.LinkEndpoint
	file       *os.File
	maxPCAPLen uint32
}

// New creates a new sniffer link-layer endpoint. It wraps around another
// endpoint and logs packets and they traverse the endpoint.
func New(lower tcpip.LinkEndpointID) tcpip.LinkEndpointID {
	return stack.RegisterLinkEndpoint(&endpoint{
		lower: stack.FindLinkEndpoint(lower),
	})
}

func zoneOffset() (int32, error) {
	loc, err := time.LoadLocation("Local")
	if err != nil {
		return 0, err
	}
	date := time.Date(0, 0, 0, 0, 0, 0, 0, loc)
	_, offset := date.Zone()
	return int32(offset), nil
}

func writePCAPHeader(w io.Writer, maxLen uint32) error {
	offset, err := zoneOffset()
	if err != nil {
		return err
	}
	return binary.Write(w, binary.BigEndian, pcapHeader{
		// From https://wiki.wireshark.org/Development/LibpcapFileFormat
		MagicNumber: 0xa1b2c3d4,

		VersionMajor: 2,
		VersionMinor: 4,
		Thiszone:     offset,
		Sigfigs:      0,
		Snaplen:      maxLen,
		Network:      101, // LINKTYPE_RAW
	})
}

// NewWithFile creates a new sniffer link-layer endpoint. It wraps around
// another endpoint and logs packets and they traverse the endpoint.
//
// Packets can be logged to file in the pcap format. A sniffer created
// with this function will not emit packets using the standard log
// package.
//
// snapLen is the maximum amount of a packet to be saved. Packets with a length
// less than or equal too snapLen will be saved in their entirety. Longer
// packets will be truncated to snapLen.
func NewWithFile(lower tcpip.LinkEndpointID, file *os.File, snapLen uint32) (tcpip.LinkEndpointID, error) {
	if err := writePCAPHeader(file, snapLen); err != nil {
		return 0, err
	}
	return stack.RegisterLinkEndpoint(&endpoint{
		lower:      stack.FindLinkEndpoint(lower),
		file:       file,
		maxPCAPLen: snapLen,
	}), nil
}

// DeliverNetworkPacket implements the stack.NetworkDispatcher interface. It is
// called by the link-layer endpoint being wrapped when a packet arrives, and
// logs the packet before forwarding to the actual dispatcher.
func (e *endpoint) DeliverNetworkPacket(linkEP stack.LinkEndpoint, remoteLinkAddr, localLinkAddr tcpip.LinkAddress, protocol tcpip.NetworkProtocolNumber, vv buffer.VectorisedView) {
	if atomic.LoadUint32(&LogPackets) == 1 && e.file == nil {
		logPacket("recv", protocol, vv.First())
	}
	if e.file != nil && atomic.LoadUint32(&LogPacketsToFile) == 1 {
		vs := vv.Views()
		length := vv.Size()
		if length > int(e.maxPCAPLen) {
			length = int(e.maxPCAPLen)
		}

		buf := bytes.NewBuffer(make([]byte, 0, pcapPacketHeaderLen+length))
		if err := binary.Write(buf, binary.BigEndian, newPCAPPacketHeader(uint32(length), uint32(vv.Size()))); err != nil {
			panic(err)
		}
		for _, v := range vs {
			if length == 0 {
				break
			}
			if len(v) > length {
				v = v[:length]
			}
			if _, err := buf.Write([]byte(v)); err != nil {
				panic(err)
			}
			length -= len(v)
		}
		if _, err := e.file.Write(buf.Bytes()); err != nil {
			panic(err)
		}
	}
	e.dispatcher.DeliverNetworkPacket(e, remoteLinkAddr, localLinkAddr, protocol, vv)
}

// Attach implements the stack.LinkEndpoint interface. It saves the dispatcher
// and registers with the lower endpoint as its dispatcher so that "e" is called
// for inbound packets.
func (e *endpoint) Attach(dispatcher stack.NetworkDispatcher) {
	e.dispatcher = dispatcher
	e.lower.Attach(e)
}

// IsAttached implements stack.LinkEndpoint.IsAttached.
func (e *endpoint) IsAttached() bool {
	return e.dispatcher != nil
}

// MTU implements stack.LinkEndpoint.MTU. It just forwards the request to the
// lower endpoint.
func (e *endpoint) MTU() uint32 {
	return e.lower.MTU()
}

// Capabilities implements stack.LinkEndpoint.Capabilities. It just forwards the
// request to the lower endpoint.
func (e *endpoint) Capabilities() stack.LinkEndpointCapabilities {
	return e.lower.Capabilities()
}

// MaxHeaderLength implements the stack.LinkEndpoint interface. It just forwards
// the request to the lower endpoint.
func (e *endpoint) MaxHeaderLength() uint16 {
	return e.lower.MaxHeaderLength()
}

func (e *endpoint) LinkAddress() tcpip.LinkAddress {
	return e.lower.LinkAddress()
}

// WritePacket implements the stack.LinkEndpoint interface. It is called by
// higher-level protocols to write packets; it just logs the packet and forwards
// the request to the lower endpoint.
func (e *endpoint) WritePacket(r *stack.Route, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.NetworkProtocolNumber) *tcpip.Error {
	if atomic.LoadUint32(&LogPackets) == 1 && e.file == nil {
		logPacket("send", protocol, hdr.View())
	}
	if e.file != nil && atomic.LoadUint32(&LogPacketsToFile) == 1 {
		hdrBuf := hdr.View()
		length := len(hdrBuf) + payload.Size()
		if length > int(e.maxPCAPLen) {
			length = int(e.maxPCAPLen)
		}

		buf := bytes.NewBuffer(make([]byte, 0, pcapPacketHeaderLen+length))
		if err := binary.Write(buf, binary.BigEndian, newPCAPPacketHeader(uint32(length), uint32(len(hdrBuf)+payload.Size()))); err != nil {
			panic(err)
		}
		if len(hdrBuf) > length {
			hdrBuf = hdrBuf[:length]
		}
		if _, err := buf.Write(hdrBuf); err != nil {
			panic(err)
		}
		length -= len(hdrBuf)
		if length > 0 {
			for _, v := range payload.Views() {
				if len(v) > length {
					v = v[:length]
				}
				n, err := buf.Write(v)
				if err != nil {
					panic(err)
				}
				length -= n
				if length == 0 {
					break
				}
			}
		}
		if _, err := e.file.Write(buf.Bytes()); err != nil {
			panic(err)
		}
	}
	return e.lower.WritePacket(r, hdr, payload, protocol)
}

func logPacket(prefix string, protocol tcpip.NetworkProtocolNumber, b buffer.View) {
	// Figure out the network layer info.
	var transProto uint8
	src := tcpip.Address("unknown")
	dst := tcpip.Address("unknown")
	id := 0
	size := uint16(0)
	switch protocol {
	case header.IPv4ProtocolNumber:
		ipv4 := header.IPv4(b)
		src = ipv4.SourceAddress()
		dst = ipv4.DestinationAddress()
		transProto = ipv4.Protocol()
		size = ipv4.TotalLength() - uint16(ipv4.HeaderLength())
		b = b[ipv4.HeaderLength():]
		id = int(ipv4.ID())

	case header.IPv6ProtocolNumber:
		ipv6 := header.IPv6(b)
		src = ipv6.SourceAddress()
		dst = ipv6.DestinationAddress()
		transProto = ipv6.NextHeader()
		size = ipv6.PayloadLength()
		b = b[header.IPv6MinimumSize:]

	case header.ARPProtocolNumber:
		arp := header.ARP(b)
		log.Printf(
			"%s arp %v (%v) -> %v (%v) valid:%v",
			prefix,
			tcpip.Address(arp.ProtocolAddressSender()), tcpip.LinkAddress(arp.HardwareAddressSender()),
			tcpip.Address(arp.ProtocolAddressTarget()), tcpip.LinkAddress(arp.HardwareAddressTarget()),
			arp.IsValid(),
		)
		return
	default:
		log.Printf("%s unknown network protocol", prefix)
		return
	}

	// Figure out the transport layer info.
	transName := "unknown"
	srcPort := uint16(0)
	dstPort := uint16(0)
	details := ""
	switch tcpip.TransportProtocolNumber(transProto) {
	case header.ICMPv4ProtocolNumber:
		transName = "icmp"
		icmp := header.ICMPv4(b)
		icmpType := "unknown"
		switch icmp.Type() {
		case header.ICMPv4EchoReply:
			icmpType = "echo reply"
		case header.ICMPv4DstUnreachable:
			icmpType = "destination unreachable"
		case header.ICMPv4SrcQuench:
			icmpType = "source quench"
		case header.ICMPv4Redirect:
			icmpType = "redirect"
		case header.ICMPv4Echo:
			icmpType = "echo"
		case header.ICMPv4TimeExceeded:
			icmpType = "time exceeded"
		case header.ICMPv4ParamProblem:
			icmpType = "param problem"
		case header.ICMPv4Timestamp:
			icmpType = "timestamp"
		case header.ICMPv4TimestampReply:
			icmpType = "timestamp reply"
		case header.ICMPv4InfoRequest:
			icmpType = "info request"
		case header.ICMPv4InfoReply:
			icmpType = "info reply"
		}
		log.Printf("%s %s %v -> %v %s len:%d id:%04x code:%d", prefix, transName, src, dst, icmpType, size, id, icmp.Code())
		return

	case header.ICMPv6ProtocolNumber:
		transName = "icmp"
		icmp := header.ICMPv6(b)
		icmpType := "unknown"
		switch icmp.Type() {
		case header.ICMPv6DstUnreachable:
			icmpType = "destination unreachable"
		case header.ICMPv6PacketTooBig:
			icmpType = "packet too big"
		case header.ICMPv6TimeExceeded:
			icmpType = "time exceeded"
		case header.ICMPv6ParamProblem:
			icmpType = "param problem"
		case header.ICMPv6EchoRequest:
			icmpType = "echo request"
		case header.ICMPv6EchoReply:
			icmpType = "echo reply"
		case header.ICMPv6RouterSolicit:
			icmpType = "router solicit"
		case header.ICMPv6RouterAdvert:
			icmpType = "router advert"
		case header.ICMPv6NeighborSolicit:
			icmpType = "neighbor solicit"
		case header.ICMPv6NeighborAdvert:
			icmpType = "neighbor advert"
		case header.ICMPv6RedirectMsg:
			icmpType = "redirect message"
		}
		log.Printf("%s %s %v -> %v %s len:%d id:%04x code:%d", prefix, transName, src, dst, icmpType, size, id, icmp.Code())
		return

	case header.UDPProtocolNumber:
		transName = "udp"
		udp := header.UDP(b)
		srcPort = udp.SourcePort()
		dstPort = udp.DestinationPort()
		size -= header.UDPMinimumSize

		details = fmt.Sprintf("xsum: 0x%x", udp.Checksum())

	case header.TCPProtocolNumber:
		transName = "tcp"
		tcp := header.TCP(b)
		offset := int(tcp.DataOffset())
		if offset < header.TCPMinimumSize {
			details += fmt.Sprintf("invalid packet: tcp data offset too small %d", offset)
			break
		}
		if offset > len(tcp) {
			details += fmt.Sprintf("invalid packet: tcp data offset %d larger than packet buffer length %d", offset, len(tcp))
			break
		}

		srcPort = tcp.SourcePort()
		dstPort = tcp.DestinationPort()
		size -= uint16(offset)

		// Initialize the TCP flags.
		flags := tcp.Flags()
		flagsStr := []byte("FSRPAU")
		for i := range flagsStr {
			if flags&(1<<uint(i)) == 0 {
				flagsStr[i] = ' '
			}
		}
		details = fmt.Sprintf("flags:0x%02x (%v) seqnum: %v ack: %v win: %v xsum:0x%x", flags, string(flagsStr), tcp.SequenceNumber(), tcp.AckNumber(), tcp.WindowSize(), tcp.Checksum())
		if flags&header.TCPFlagSyn != 0 {
			details += fmt.Sprintf(" options: %+v", header.ParseSynOptions(tcp.Options(), flags&header.TCPFlagAck != 0))
		} else {
			details += fmt.Sprintf(" options: %+v", tcp.ParsedOptions())
		}

	default:
		log.Printf("%s %v -> %v unknown transport protocol: %d", prefix, src, dst, transProto)
		return
	}

	log.Printf("%s %s %v:%v -> %v:%v len:%d id:%04x %s", prefix, transName, src, srcPort, dst, dstPort, size, id, details)
}