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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 (
"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, ð, &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
}
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