// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package ipv4
import (
"encoding/binary"
"fmt"
"net"
"runtime"
"syscall"
)
const (
Version = 4 // protocol version
HeaderLen = 20 // header length without extension headers
maxHeaderLen = 60 // sensible default, revisit if later RFCs define new usage of version and header length fields
)
type HeaderFlags int
const (
MoreFragments HeaderFlags = 1 << iota // more fragments flag
DontFragment // don't fragment flag
)
// A Header represents an IPv4 header.
type Header struct {
Version int // protocol version
Len int // header length
TOS int // type-of-service
TotalLen int // packet total length
ID int // identification
Flags HeaderFlags // flags
FragOff int // fragment offset
TTL int // time-to-live
Protocol int // next protocol
Checksum int // checksum
Src net.IP // source address
Dst net.IP // destination address
Options []byte // options, extension headers
}
func (h *Header) String() string {
if h == nil {
return "<nil>"
}
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)
}
// Marshal returns the binary encoding of the IPv4 header h.
func (h *Header) Marshal() ([]byte, error) {
if h == nil {
return nil, syscall.EINVAL
}
if h.Len < HeaderLen {
return nil, errHeaderTooShort
}
hdrlen := HeaderLen + len(h.Options)
b := make([]byte, hdrlen)
b[0] = byte(Version<<4 | (hdrlen >> 2 & 0x0f))
b[1] = byte(h.TOS)
flagsAndFragOff := (h.FragOff & 0x1fff) | int(h.Flags<<13)
switch runtime.GOOS {
case "darwin", "dragonfly", "freebsd", "netbsd":
nativeEndian.PutUint16(b[2:4], uint16(h.TotalLen))
nativeEndian.PutUint16(b[6:8], uint16(flagsAndFragOff))
default:
binary.BigEndian.PutUint16(b[2:4], uint16(h.TotalLen))
binary.BigEndian.PutUint16(b[6:8], uint16(flagsAndFragOff))
}
binary.BigEndian.PutUint16(b[4:6], uint16(h.ID))
b[8] = byte(h.TTL)
b[9] = byte(h.Protocol)
binary.BigEndian.PutUint16(b[10:12], uint16(h.Checksum))
if ip := h.Src.To4(); ip != nil {
copy(b[12:16], ip[:net.IPv4len])
}
if ip := h.Dst.To4(); ip != nil {
copy(b[16:20], ip[:net.IPv4len])
} else {
return nil, errMissingAddress
}
if len(h.Options) > 0 {
copy(b[HeaderLen:], h.Options)
}
return b, nil
}
// ParseHeader parses b as an IPv4 header.
func ParseHeader(b []byte) (*Header, error) {
if len(b) < HeaderLen {
return nil, errHeaderTooShort
}
hdrlen := int(b[0]&0x0f) << 2
if hdrlen > len(b) {
return nil, errBufferTooShort
}
h := &Header{
Version: int(b[0] >> 4),
Len: hdrlen,
TOS: int(b[1]),
ID: int(binary.BigEndian.Uint16(b[4:6])),
TTL: int(b[8]),
Protocol: int(b[9]),
Checksum: int(binary.BigEndian.Uint16(b[10:12])),
Src: net.IPv4(b[12], b[13], b[14], b[15]),
Dst: net.IPv4(b[16], b[17], b[18], b[19]),
}
switch runtime.GOOS {
case "darwin", "dragonfly", "netbsd":
h.TotalLen = int(nativeEndian.Uint16(b[2:4])) + hdrlen
h.FragOff = int(nativeEndian.Uint16(b[6:8]))
case "freebsd":
h.TotalLen = int(nativeEndian.Uint16(b[2:4]))
if freebsdVersion < 1000000 {
h.TotalLen += hdrlen
}
h.FragOff = int(nativeEndian.Uint16(b[6:8]))
default:
h.TotalLen = int(binary.BigEndian.Uint16(b[2:4]))
h.FragOff = int(binary.BigEndian.Uint16(b[6:8]))
}
h.Flags = HeaderFlags(h.FragOff&0xe000) >> 13
h.FragOff = h.FragOff & 0x1fff
if hdrlen-HeaderLen > 0 {
h.Options = make([]byte, hdrlen-HeaderLen)
copy(h.Options, b[HeaderLen:])
}
return h, nil
}