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
5package ipv4
6
7import (
8	"encoding/binary"
9	"fmt"
10	"net"
11	"runtime"
12
13	"golang.org/x/net/internal/socket"
14)
15
16const (
17	Version      = 4  // protocol version
18	HeaderLen    = 20 // header length without extension headers
19	maxHeaderLen = 60 // sensible default, revisit if later RFCs define new usage of version and header length fields
20)
21
22type HeaderFlags int
23
24const (
25	MoreFragments HeaderFlags = 1 << iota // more fragments flag
26	DontFragment                          // don't fragment flag
27)
28
29// A Header represents an IPv4 header.
30type Header struct {
31	Version  int         // protocol version
32	Len      int         // header length
33	TOS      int         // type-of-service
34	TotalLen int         // packet total length
35	ID       int         // identification
36	Flags    HeaderFlags // flags
37	FragOff  int         // fragment offset
38	TTL      int         // time-to-live
39	Protocol int         // next protocol
40	Checksum int         // checksum
41	Src      net.IP      // source address
42	Dst      net.IP      // destination address
43	Options  []byte      // options, extension headers
44}
45
46func (h *Header) String() string {
47	if h == nil {
48		return "<nil>"
49	}
50	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)
51}
52
53// Marshal returns the binary encoding of h.
54//
55// The returned slice is in the format used by a raw IP socket on the
56// local system.
57// This may differ from the wire format, depending on the system.
58func (h *Header) Marshal() ([]byte, error) {
59	if h == nil {
60		return nil, errNilHeader
61	}
62	if h.Len < HeaderLen {
63		return nil, errHeaderTooShort
64	}
65	hdrlen := HeaderLen + len(h.Options)
66	b := make([]byte, hdrlen)
67	b[0] = byte(Version<<4 | (hdrlen >> 2 & 0x0f))
68	b[1] = byte(h.TOS)
69	flagsAndFragOff := (h.FragOff & 0x1fff) | int(h.Flags<<13)
70	switch runtime.GOOS {
71	case "darwin", "dragonfly", "netbsd":
72		socket.NativeEndian.PutUint16(b[2:4], uint16(h.TotalLen))
73		socket.NativeEndian.PutUint16(b[6:8], uint16(flagsAndFragOff))
74	case "freebsd":
75		if freebsdVersion < 1100000 {
76			socket.NativeEndian.PutUint16(b[2:4], uint16(h.TotalLen))
77			socket.NativeEndian.PutUint16(b[6:8], uint16(flagsAndFragOff))
78		} else {
79			binary.BigEndian.PutUint16(b[2:4], uint16(h.TotalLen))
80			binary.BigEndian.PutUint16(b[6:8], uint16(flagsAndFragOff))
81		}
82	default:
83		binary.BigEndian.PutUint16(b[2:4], uint16(h.TotalLen))
84		binary.BigEndian.PutUint16(b[6:8], uint16(flagsAndFragOff))
85	}
86	binary.BigEndian.PutUint16(b[4:6], uint16(h.ID))
87	b[8] = byte(h.TTL)
88	b[9] = byte(h.Protocol)
89	binary.BigEndian.PutUint16(b[10:12], uint16(h.Checksum))
90	if ip := h.Src.To4(); ip != nil {
91		copy(b[12:16], ip[:net.IPv4len])
92	}
93	if ip := h.Dst.To4(); ip != nil {
94		copy(b[16:20], ip[:net.IPv4len])
95	} else {
96		return nil, errMissingAddress
97	}
98	if len(h.Options) > 0 {
99		copy(b[HeaderLen:], h.Options)
100	}
101	return b, nil
102}
103
104// Parse parses b as an IPv4 header and stores the result in h.
105//
106// The provided b must be in the format used by a raw IP socket on the
107// local system.
108// This may differ from the wire format, depending on the system.
109func (h *Header) Parse(b []byte) error {
110	if h == nil || b == nil {
111		return errNilHeader
112	}
113	if len(b) < HeaderLen {
114		return errHeaderTooShort
115	}
116	hdrlen := int(b[0]&0x0f) << 2
117	if len(b) < hdrlen {
118		return errExtHeaderTooShort
119	}
120	h.Version = int(b[0] >> 4)
121	h.Len = hdrlen
122	h.TOS = int(b[1])
123	h.ID = int(binary.BigEndian.Uint16(b[4:6]))
124	h.TTL = int(b[8])
125	h.Protocol = int(b[9])
126	h.Checksum = int(binary.BigEndian.Uint16(b[10:12]))
127	h.Src = net.IPv4(b[12], b[13], b[14], b[15])
128	h.Dst = net.IPv4(b[16], b[17], b[18], b[19])
129	switch runtime.GOOS {
130	case "darwin", "dragonfly", "netbsd":
131		h.TotalLen = int(socket.NativeEndian.Uint16(b[2:4])) + hdrlen
132		h.FragOff = int(socket.NativeEndian.Uint16(b[6:8]))
133	case "freebsd":
134		if freebsdVersion < 1100000 {
135			h.TotalLen = int(socket.NativeEndian.Uint16(b[2:4]))
136			if freebsdVersion < 1000000 {
137				h.TotalLen += hdrlen
138			}
139			h.FragOff = int(socket.NativeEndian.Uint16(b[6:8]))
140		} else {
141			h.TotalLen = int(binary.BigEndian.Uint16(b[2:4]))
142			h.FragOff = int(binary.BigEndian.Uint16(b[6:8]))
143		}
144	default:
145		h.TotalLen = int(binary.BigEndian.Uint16(b[2:4]))
146		h.FragOff = int(binary.BigEndian.Uint16(b[6:8]))
147	}
148	h.Flags = HeaderFlags(h.FragOff&0xe000) >> 13
149	h.FragOff = h.FragOff & 0x1fff
150	optlen := hdrlen - HeaderLen
151	if optlen > 0 && len(b) >= hdrlen {
152		if cap(h.Options) < optlen {
153			h.Options = make([]byte, optlen)
154		} else {
155			h.Options = h.Options[:optlen]
156		}
157		copy(h.Options, b[HeaderLen:hdrlen])
158	}
159	return nil
160}
161
162// ParseHeader parses b as an IPv4 header.
163//
164// The provided b must be in the format used by a raw IP socket on the
165// local system.
166// This may differ from the wire format, depending on the system.
167func ParseHeader(b []byte) (*Header, error) {
168	h := new(Header)
169	if err := h.Parse(b); err != nil {
170		return nil, err
171	}
172	return h, nil
173}
174