header.go raw
1 // Copyright 2012 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
4
5 package ipv4
6
7 import (
8 "encoding/binary"
9 "fmt"
10 "net"
11 "runtime"
12
13 "golang.org/x/net/internal/socket"
14 )
15
16 const (
17 Version = 4 // protocol version
18 HeaderLen = 20 // header length without extension headers
19 )
20
21 type HeaderFlags int
22
23 const (
24 MoreFragments HeaderFlags = 1 << iota // more fragments flag
25 DontFragment // don't fragment flag
26 )
27
28 // A Header represents an IPv4 header.
29 type Header struct {
30 Version int // protocol version
31 Len int // header length
32 TOS int // type-of-service
33 TotalLen int // packet total length
34 ID int // identification
35 Flags HeaderFlags // flags
36 FragOff int // fragment offset
37 TTL int // time-to-live
38 Protocol int // next protocol
39 Checksum int // checksum
40 Src net.IP // source address
41 Dst net.IP // destination address
42 Options []byte // options, extension headers
43 }
44
45 func (h *Header) String() string {
46 if h == nil {
47 return "<nil>"
48 }
49 return fmt.Sprintf("ver=%d hdrlen=%d tos=%#x totallen=%d id=%#x flags=%#x fragoff=%#x ttl=%d proto=%d cksum=%#x src=%v dst=%v", h.Version, h.Len, h.TOS, h.TotalLen, h.ID, h.Flags, h.FragOff, h.TTL, h.Protocol, h.Checksum, h.Src, h.Dst)
50 }
51
52 // Marshal returns the binary encoding of h.
53 //
54 // The returned slice is in the format used by a raw IP socket on the
55 // local system.
56 // This may differ from the wire format, depending on the system.
57 func (h *Header) Marshal() ([]byte, error) {
58 if h == nil {
59 return nil, errNilHeader
60 }
61 if h.Len < HeaderLen {
62 return nil, errHeaderTooShort
63 }
64 hdrlen := HeaderLen + len(h.Options)
65 b := make([]byte, hdrlen)
66 b[0] = byte(Version<<4 | (hdrlen >> 2 & 0x0f))
67 b[1] = byte(h.TOS)
68 flagsAndFragOff := (h.FragOff & 0x1fff) | int(h.Flags<<13)
69 switch runtime.GOOS {
70 case "darwin", "ios", "dragonfly", "netbsd":
71 socket.NativeEndian.PutUint16(b[2:4], uint16(h.TotalLen))
72 socket.NativeEndian.PutUint16(b[6:8], uint16(flagsAndFragOff))
73 case "freebsd":
74 if freebsdVersion < 1100000 {
75 socket.NativeEndian.PutUint16(b[2:4], uint16(h.TotalLen))
76 socket.NativeEndian.PutUint16(b[6:8], uint16(flagsAndFragOff))
77 } else {
78 binary.BigEndian.PutUint16(b[2:4], uint16(h.TotalLen))
79 binary.BigEndian.PutUint16(b[6:8], uint16(flagsAndFragOff))
80 }
81 default:
82 binary.BigEndian.PutUint16(b[2:4], uint16(h.TotalLen))
83 binary.BigEndian.PutUint16(b[6:8], uint16(flagsAndFragOff))
84 }
85 binary.BigEndian.PutUint16(b[4:6], uint16(h.ID))
86 b[8] = byte(h.TTL)
87 b[9] = byte(h.Protocol)
88 binary.BigEndian.PutUint16(b[10:12], uint16(h.Checksum))
89 if ip := h.Src.To4(); ip != nil {
90 copy(b[12:16], ip[:net.IPv4len])
91 }
92 if ip := h.Dst.To4(); ip != nil {
93 copy(b[16:20], ip[:net.IPv4len])
94 } else {
95 return nil, errMissingAddress
96 }
97 if len(h.Options) > 0 {
98 copy(b[HeaderLen:], h.Options)
99 }
100 return b, nil
101 }
102
103 // Parse parses b as an IPv4 header and stores the result in h.
104 //
105 // The provided b must be in the format used by a raw IP socket on the
106 // local system.
107 // This may differ from the wire format, depending on the system.
108 func (h *Header) Parse(b []byte) error {
109 if h == nil || b == nil {
110 return errNilHeader
111 }
112 if len(b) < HeaderLen {
113 return errHeaderTooShort
114 }
115 hdrlen := int(b[0]&0x0f) << 2
116 if len(b) < hdrlen {
117 return errExtHeaderTooShort
118 }
119 h.Version = int(b[0] >> 4)
120 h.Len = hdrlen
121 h.TOS = int(b[1])
122 h.ID = int(binary.BigEndian.Uint16(b[4:6]))
123 h.TTL = int(b[8])
124 h.Protocol = int(b[9])
125 h.Checksum = int(binary.BigEndian.Uint16(b[10:12]))
126 h.Src = net.IPv4(b[12], b[13], b[14], b[15])
127 h.Dst = net.IPv4(b[16], b[17], b[18], b[19])
128 switch runtime.GOOS {
129 case "darwin", "ios", "dragonfly", "netbsd":
130 h.TotalLen = int(socket.NativeEndian.Uint16(b[2:4])) + hdrlen
131 h.FragOff = int(socket.NativeEndian.Uint16(b[6:8]))
132 case "freebsd":
133 if freebsdVersion < 1100000 {
134 h.TotalLen = int(socket.NativeEndian.Uint16(b[2:4]))
135 if freebsdVersion < 1000000 {
136 h.TotalLen += hdrlen
137 }
138 h.FragOff = int(socket.NativeEndian.Uint16(b[6:8]))
139 } else {
140 h.TotalLen = int(binary.BigEndian.Uint16(b[2:4]))
141 h.FragOff = int(binary.BigEndian.Uint16(b[6:8]))
142 }
143 default:
144 h.TotalLen = int(binary.BigEndian.Uint16(b[2:4]))
145 h.FragOff = int(binary.BigEndian.Uint16(b[6:8]))
146 }
147 h.Flags = HeaderFlags(h.FragOff&0xe000) >> 13
148 h.FragOff = h.FragOff & 0x1fff
149 optlen := hdrlen - HeaderLen
150 if optlen > 0 && len(b) >= hdrlen {
151 if cap(h.Options) < optlen {
152 h.Options = make([]byte, optlen)
153 } else {
154 h.Options = h.Options[:optlen]
155 }
156 copy(h.Options, b[HeaderLen:hdrlen])
157 }
158 return nil
159 }
160
161 // ParseHeader parses b as an IPv4 header.
162 //
163 // The provided b must be in the format used by a raw IP socket on the
164 // local system.
165 // This may differ from the wire format, depending on the system.
166 func ParseHeader(b []byte) (*Header, error) {
167 h := new(Header)
168 if err := h.Parse(b); err != nil {
169 return nil, err
170 }
171 return h, nil
172 }
173