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// 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 (
// A container for single LayerType->DecodingLayer mapping.
type decodingLayerElem struct {
typ LayerType
dec DecodingLayer
// 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
// DecodingLayerFunc decodes given packet and stores decoded LayerType
// values into specified slice. Returns either first encountered
// unsupported LayerType value or decoding error. In case of success,
// returns (LayerTypeZero, nil).
type DecodingLayerFunc func([]byte, *[]LayerType) (LayerType, error)
// DecodingLayerContainer stores all DecodingLayer-s and serves as a
// searching tool for DecodingLayerParser.
type DecodingLayerContainer interface {
// Put adds new DecodingLayer to container. The new instance of
// the same DecodingLayerContainer is returned so it may be
// implemented as a value receiver.
Put(DecodingLayer) DecodingLayerContainer
// Decoder returns DecodingLayer to decode given LayerType and
// true if it was found. If no decoder found, return false.
Decoder(LayerType) (DecodingLayer, bool)
// LayersDecoder returns DecodingLayerFunc which decodes given
// packet, starting with specified LayerType and DecodeFeedback.
LayersDecoder(first LayerType, df DecodeFeedback) DecodingLayerFunc
// DecodingLayerSparse is a sparse array-based implementation of
// DecodingLayerContainer. Each DecodingLayer is addressed in an
// allocated slice by LayerType value itself. Though this is the
// fastest container it may be memory-consuming if used with big
// LayerType values.
type DecodingLayerSparse []DecodingLayer
// Put implements DecodingLayerContainer interface.
func (dl DecodingLayerSparse) Put(d DecodingLayer) DecodingLayerContainer {
maxLayerType := LayerType(len(dl) - 1)
for _, typ := range d.CanDecode().LayerTypes() {
if typ > maxLayerType {
maxLayerType = typ
if extra := maxLayerType - LayerType(len(dl)) + 1; extra > 0 {
dl = append(dl, make([]DecodingLayer, extra)...)
for _, typ := range d.CanDecode().LayerTypes() {
dl[typ] = d
return dl
// LayersDecoder implements DecodingLayerContainer interface.
func (dl DecodingLayerSparse) LayersDecoder(first LayerType, df DecodeFeedback) DecodingLayerFunc {
return LayersDecoder(dl, first, df)
// Decoder implements DecodingLayerContainer interface.
func (dl DecodingLayerSparse) Decoder(typ LayerType) (DecodingLayer, bool) {
if int64(typ) < int64(len(dl)) {
decoder := dl[typ]
return decoder, decoder != nil
return nil, false
// DecodingLayerArray is an array-based implementation of
// DecodingLayerContainer. Each DecodingLayer is searched linearly in
// an allocated slice in one-by-one fashion.
type DecodingLayerArray []decodingLayerElem
// Put implements DecodingLayerContainer interface.
func (dl DecodingLayerArray) Put(d DecodingLayer) DecodingLayerContainer {
for _, typ := range d.CanDecode().LayerTypes() {
for i := range dl {
if dl[i].typ == typ {
dl[i].dec = d
continue TYPES
dl = append(dl, decodingLayerElem{typ, d})
return dl
// Decoder implements DecodingLayerContainer interface.
func (dl DecodingLayerArray) Decoder(typ LayerType) (DecodingLayer, bool) {
for i := range dl {
if dl[i].typ == typ {
return dl[i].dec, true
return nil, false
// LayersDecoder implements DecodingLayerContainer interface.
func (dl DecodingLayerArray) LayersDecoder(first LayerType, df DecodeFeedback) DecodingLayerFunc {
return LayersDecoder(dl, first, df)
// DecodingLayerMap is an map-based implementation of
// DecodingLayerContainer. Each DecodingLayer is searched in a map
// hashed by LayerType value.
type DecodingLayerMap map[LayerType]DecodingLayer
// Put implements DecodingLayerContainer interface.
func (dl DecodingLayerMap) Put(d DecodingLayer) DecodingLayerContainer {
for _, typ := range d.CanDecode().LayerTypes() {
if dl == nil {
dl = make(map[LayerType]DecodingLayer)
dl[typ] = d
return dl
// Decoder implements DecodingLayerContainer interface.
func (dl DecodingLayerMap) Decoder(typ LayerType) (DecodingLayer, bool) {
d, ok := dl[typ]
return d, ok
// LayersDecoder implements DecodingLayerContainer interface.
func (dl DecodingLayerMap) LayersDecoder(first LayerType, df DecodeFeedback) DecodingLayerFunc {
return LayersDecoder(dl, first, df)
// Static code check.
var (
_ = []DecodingLayerContainer{
// 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.
dlc DecodingLayerContainer
first LayerType
df DecodeFeedback
decodeFunc DecodingLayerFunc
// 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) {
// 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.
// NewDecodingLayerParser uses DecodingLayerMap container by
// default.
func NewDecodingLayerParser(first LayerType, decoders ...DecodingLayer) *DecodingLayerParser {
dlp := &DecodingLayerParser{first: first}
dlp.df = dlp // Cast this once to the interface
// default container
dlc := DecodingLayerContainer(DecodingLayerMap(make(map[LayerType]DecodingLayer)))
for _, d := range decoders {
dlc = dlc.Put(d)
return dlp
// SetDecodingLayerContainer specifies container with decoders. This
// call replaces all decoders already registered in given instance of
// DecodingLayerParser.
func (l *DecodingLayerParser) SetDecodingLayerContainer(dlc DecodingLayerContainer) {
l.dlc = dlc
l.decodeFunc = l.dlc.LayersDecoder(l.first, l.df)
// 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, err := l.decodeFunc(data, decoded)
if typ != LayerTypeZero {
// no decoder
if l.IgnoreUnsupported {
return nil
return UnsupportedLayerType(typ)
return err
// 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
// IgnoreUnsupported will stop parsing and return a nil error when it
// encounters a layer it doesn't have a parser for, instead of returning an
// UnsupportedLayerType error. If this is true, it's up to the caller to make
// sure that all expected layers have been parsed (by checking the decoded
// slice).
IgnoreUnsupported bool
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