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// 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 (
"errors"
"fmt"
"net"
"runtime"
"syscall"
"unsafe"
)
var (
errMissingAddress = errors.New("missing address")
errMissingHeader = errors.New("missing header")
errHeaderTooShort = errors.New("header too short")
errBufferTooShort = errors.New("buffer too short")
errInvalidConnType = errors.New("invalid conn type")
)
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: %v, hdrlen: %v, tos: %#x, totallen: %v, id: %#x, flags: %#x, fragoff: %#x, ttl: %v, proto: %v, 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":
// TODO(mikio): fix potential misaligned memory access
*(*uint16)(unsafe.Pointer(&b[2:3][0])) = uint16(h.TotalLen)
*(*uint16)(unsafe.Pointer(&b[6:7][0])) = uint16(flagsAndFragOff)
default:
b[2], b[3] = byte(h.TotalLen>>8), byte(h.TotalLen)
b[6], b[7] = byte(flagsAndFragOff>>8), byte(flagsAndFragOff)
}
b[4], b[5] = byte(h.ID>>8), byte(h.ID)
b[8] = byte(h.TTL)
b[9] = byte(h.Protocol)
b[10], b[11] = byte(h.Checksum>>8), byte(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
}
// See http://www.freebsd.org/doc/en/books/porters-handbook/freebsd-versions.html.
var freebsdVersion uint32
// 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(b[4])<<8 | int(b[5]),
TTL: int(b[8]),
Protocol: int(b[9]),
Checksum: int(b[10])<<8 | int(b[11]),
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":
// TODO(mikio): fix potential misaligned memory access
h.TotalLen = int(*(*uint16)(unsafe.Pointer(&b[2:3][0]))) + hdrlen
// TODO(mikio): fix potential misaligned memory access
h.FragOff = int(*(*uint16)(unsafe.Pointer(&b[6:7][0])))
case "freebsd":
// TODO(mikio): fix potential misaligned memory access
h.TotalLen = int(*(*uint16)(unsafe.Pointer(&b[2:3][0])))
if freebsdVersion < 1000000 {
h.TotalLen += hdrlen
}
// TODO(mikio): fix potential misaligned memory access
h.FragOff = int(*(*uint16)(unsafe.Pointer(&b[6:7][0])))
default:
h.TotalLen = int(b[2])<<8 | int(b[3])
h.FragOff = int(b[6])<<8 | int(b[7])
}
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
}
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