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