1// Copyright 2012 Google, Inc. All rights reserved. 2// 3// Use of this source code is governed by a BSD-style license 4// that can be found in the LICENSE file in the root of the source 5// tree. 6 7package layers 8 9import ( 10 "encoding/binary" 11 "errors" 12 "fmt" 13 14 "github.com/google/gopacket" 15) 16 17// Loopback contains the header for loopback encapsulation. This header is 18// used by both BSD and OpenBSD style loopback decoding (pcap's DLT_NULL 19// and DLT_LOOP, respectively). 20type Loopback struct { 21 BaseLayer 22 Family ProtocolFamily 23} 24 25// LayerType returns LayerTypeLoopback. 26func (l *Loopback) LayerType() gopacket.LayerType { return LayerTypeLoopback } 27 28// DecodeFromBytes decodes the given bytes into this layer. 29func (l *Loopback) DecodeFromBytes(data []byte, df gopacket.DecodeFeedback) error { 30 if len(data) < 4 { 31 return errors.New("Loopback packet too small") 32 } 33 34 // The protocol could be either big-endian or little-endian, we're 35 // not sure. But we're PRETTY sure that the value is less than 36 // 256, so we can check the first two bytes. 37 var prot uint32 38 if data[0] == 0 && data[1] == 0 { 39 prot = binary.BigEndian.Uint32(data[:4]) 40 } else { 41 prot = binary.LittleEndian.Uint32(data[:4]) 42 } 43 if prot > 0xFF { 44 return fmt.Errorf("Invalid loopback protocol %q", data[:4]) 45 } 46 47 l.Family = ProtocolFamily(prot) 48 l.BaseLayer = BaseLayer{data[:4], data[4:]} 49 return nil 50} 51 52// CanDecode returns the set of layer types that this DecodingLayer can decode. 53func (l *Loopback) CanDecode() gopacket.LayerClass { 54 return LayerTypeLoopback 55} 56 57// NextLayerType returns the layer type contained by this DecodingLayer. 58func (l *Loopback) NextLayerType() gopacket.LayerType { 59 return l.Family.LayerType() 60} 61 62// SerializeTo writes the serialized form of this layer into the 63// SerializationBuffer, implementing gopacket.SerializableLayer. 64func (l *Loopback) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.SerializeOptions) error { 65 bytes, err := b.PrependBytes(4) 66 if err != nil { 67 return err 68 } 69 binary.LittleEndian.PutUint32(bytes, uint32(l.Family)) 70 return nil 71} 72 73func decodeLoopback(data []byte, p gopacket.PacketBuilder) error { 74 l := Loopback{} 75 if err := l.DecodeFromBytes(data, gopacket.NilDecodeFeedback); err != nil { 76 return err 77 } 78 p.AddLayer(&l) 79 return p.NextDecoder(l.Family) 80} 81