header.go (5085B)
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 }