parser.go

// Copyright 2012 Google, Inc. All rights reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the LICENSE file in the root of the source
// tree.

package gopacket

import (
	"fmt"
)

// DecodingLayer is an interface for packet layers that can decode themselves.
//
// The important part of DecodingLayer is that they decode themselves in-place.
// Calling DecodeFromBytes on a DecodingLayer totally resets the entire layer to
// the new state defined by the data passed in.  A returned error leaves the
// DecodingLayer in an unknown intermediate state, thus its fields should not be
// trusted.
//
// Because the DecodingLayer is resetting its own fields, a call to
// DecodeFromBytes should normally not require any memory allocation.
type DecodingLayer interface {
	// DecodeFromBytes resets the internal state of this layer to the state
	// defined by the passed-in bytes.  Slices in the DecodingLayer may
	// reference the passed-in data, so care should be taken to copy it
	// first should later modification of data be required before the
	// DecodingLayer is discarded.
	DecodeFromBytes(data []byte, df DecodeFeedback) error
	// CanDecode returns the set of LayerTypes this DecodingLayer can
	// decode.  For Layers that are also DecodingLayers, this will most
	// often be that Layer's LayerType().
	CanDecode() LayerClass
	// NextLayerType returns the LayerType which should be used to decode
	// the LayerPayload.
	NextLayerType() LayerType
	// LayerPayload is the set of bytes remaining to decode after a call to
	// DecodeFromBytes.
	LayerPayload() []byte
}

// DecodingLayerParser parses a given set of layer types.  See DecodeLayers for
// more information on how DecodingLayerParser should be used.
type DecodingLayerParser struct {
	// DecodingLayerParserOptions is the set of options available to the
	// user to define the parser's behavior.
	DecodingLayerParserOptions
	first    LayerType
	decoders map[LayerType]DecodingLayer
	df       DecodeFeedback
	// Truncated is set when a decode layer detects that the packet has been
	// truncated.
	Truncated bool
}

// AddDecodingLayer adds a decoding layer to the parser.  This adds support for
// the decoding layer's CanDecode layers to the parser... should they be
// encountered, they'll be parsed.
func (l *DecodingLayerParser) AddDecodingLayer(d DecodingLayer) {
	for _, typ := range d.CanDecode().LayerTypes() {
		l.decoders[typ] = d
	}
}

// SetTruncated is used by DecodingLayers to set the Truncated boolean in the
// DecodingLayerParser.  Users should simply read Truncated after calling
// DecodeLayers.
func (l *DecodingLayerParser) SetTruncated() {
	l.Truncated = true
}

// NewDecodingLayerParser creates a new DecodingLayerParser and adds in all
// of the given DecodingLayers with AddDecodingLayer.
//
// Each call to DecodeLayers will attempt to decode the given bytes first by
// treating them as a 'first'-type layer, then by using NextLayerType on
// subsequently decoded layers to find the next relevant decoder.  Should a
// deoder not be available for the layer type returned by NextLayerType,
// decoding will stop.
func NewDecodingLayerParser(first LayerType, decoders ...DecodingLayer) *DecodingLayerParser {
	dlp := &DecodingLayerParser{
		decoders: make(map[LayerType]DecodingLayer),
		first:    first,
	}
	dlp.df = dlp // Cast this once to the interface
	for _, d := range decoders {
		dlp.AddDecodingLayer(d)
	}
	return dlp
}

// DecodeLayers decodes as many layers as possible from the given data.  It
// initially treats the data as layer type 'typ', then uses NextLayerType on
// each subsequent decoded layer until it gets to a layer type it doesn't know
// how to parse.
//
// For each layer successfully decoded, DecodeLayers appends the layer type to
// the decoded slice.  DecodeLayers truncates the 'decoded' slice initially, so
// there's no need to empty it yourself.
//
// This decoding method is about an order of magnitude faster than packet
// decoding, because it only decodes known layers that have already been
// allocated.  This means it doesn't need to allocate each layer it returns...
// instead it overwrites the layers that already exist.
//
// Example usage:
//    func main() {
//      var eth layers.Ethernet
//      var ip4 layers.IPv4
//      var ip6 layers.IPv6
//      var tcp layers.TCP
//      var udp layers.UDP
//      var payload gopacket.Payload
//      parser := gopacket.NewDecodingLayerParser(layers.LayerTypeEthernet, &eth, &ip4, &ip6, &tcp, &udp, &payload)
//      var source gopacket.PacketDataSource = getMyDataSource()
//      decodedLayers := make([]gopacket.LayerType, 0, 10)
//      for {
//        data, _, err := source.ReadPacketData()
//        if err == nil {
//          fmt.Println("Error reading packet data: ", err)
//          continue
//        }
//        fmt.Println("Decoding packet")
//        err = parser.DecodeLayers(data, &decodedLayers)
//        for _, typ := range decodedLayers {
//          fmt.Println("  Successfully decoded layer type", typ)
//          switch typ {
//            case layers.LayerTypeEthernet:
//              fmt.Println("    Eth ", eth.SrcMAC, eth.DstMAC)
//            case layers.LayerTypeIPv4:
//              fmt.Println("    IP4 ", ip4.SrcIP, ip4.DstIP)
//            case layers.LayerTypeIPv6:
//              fmt.Println("    IP6 ", ip6.SrcIP, ip6.DstIP)
//            case layers.LayerTypeTCP:
//              fmt.Println("    TCP ", tcp.SrcPort, tcp.DstPort)
//            case layers.LayerTypeUDP:
//              fmt.Println("    UDP ", udp.SrcPort, udp.DstPort)
//          }
//        }
//        if decodedLayers.Truncated {
//          fmt.Println("  Packet has been truncated")
//        }
//        if err != nil {
//          fmt.Println("  Error encountered:", err)
//        }
//      }
//    }
//
// If DecodeLayers is unable to decode the next layer type, it will return the
// error UnsupportedLayerType.
func (l *DecodingLayerParser) DecodeLayers(data []byte, decoded *[]LayerType) (err error) {
	l.Truncated = false
	if !l.IgnorePanic {
		defer panicToError(&err)
	}
	typ := l.first
	*decoded = (*decoded)[:0] // Truncated decoded layers.
	for len(data) > 0 {
		decoder, ok := l.decoders[typ]
		if !ok {
			return UnsupportedLayerType(typ)
		} else if err = decoder.DecodeFromBytes(data, l.df); err != nil {
			return err
		}
		*decoded = append(*decoded, typ)
		typ = decoder.NextLayerType()
		data = decoder.LayerPayload()
	}
	return nil
}

// UnsupportedLayerType is returned by DecodingLayerParser if DecodeLayers
// encounters a layer type that the DecodingLayerParser has no decoder for.
type UnsupportedLayerType LayerType

// Error implements the error interface, returning a string to say that the
// given layer type is unsupported.
func (e UnsupportedLayerType) Error() string {
	return fmt.Sprintf("No decoder for layer type %v", LayerType(e))
}

func panicToError(e *error) {
	if r := recover(); r != nil {
		*e = fmt.Errorf("panic: %v", r)
	}
}

// DecodingLayerParserOptions provides options to affect the behavior of a given
// DecodingLayerParser.
type DecodingLayerParserOptions struct {
	// IgnorePanic determines whether a DecodingLayerParser should stop
	// panics on its own (by returning them as an error from DecodeLayers)
	// or should allow them to raise up the stack.  Handling errors does add
	// latency to the process of decoding layers, but is much safer for
	// callers.  IgnorePanic defaults to false, thus if the caller does
	// nothing decode panics will be returned as errors.
	IgnorePanic bool
}