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20 .TH PCAP-SAVEFILE @MAN_FILE_FORMATS@ "29 July 2013"
21 .SH NAME
22 pcap-savefile \- libpcap savefile format
23 .SH DESCRIPTION
24 NOTE: applications and libraries should, if possible, use libpcap to
25 read savefiles, rather than having their own code to read savefiles.
26 If, in the future, a new file format is supported by libpcap,
27 applications and libraries using libpcap to read savefiles will be able
28 to read the new format of savefiles, but applications and libraries
29 using their own code to read savefiles will have to be changed to
30 support the new file format.
31 .PP
32 ``Savefiles'' read and written by libpcap and applications using libpcap
33 start with a per-file header. The format of the per-file header is:
34 .RS
35 .TS
36 box;
37 c s
38 c | c
39 c s.
40 Magic number
41 _
42 Major version Minor version
43 _
44 Time zone offset
45 _
46 Time stamp accuracy
47 _
48 Snapshot length
49 _
50 Link-layer header type
51 .TE
52 .RE
53 .PP
54 All fields in the per-file header are in the byte order of the host
55 writing the file. Normally, the first field in the per-file header is a
56 4-byte magic number, with the value 0xa1b2c3d4. The magic number, when
57 read by a host with the same byte order as the host that wrote the file,
58 will have the value 0xa1b2c3d4, and, when read by a host with the
59 opposite byte order as the host that wrote the file, will have the value
60 0xd4c3b2a1. That allows software reading the file to determine whether
61 the byte order of the host that wrote the file is the same as the byte
62 order of the host on which the file is being read, and thus whether the
63 values in the per-file and per-packet headers need to be byte-swapped.
64 .PP
65 If the magic number has the value 0xa1b23c4d (with the two nibbles of
66 the two lower-order bytes of the magic number swapped), which would be
67 read as 0xa1b23c4d by a host with the same byte order as the host that
68 wrote the file and as 0x4d3cb2a1 by a host with the opposite byte order
69 as the host that wrote the file, the file format is the same as for
70 regular files, except that the time stamps for packets are given in
71 seconds and nanoseconds rather than seconds and microseconds.
72 .PP
73 Following this are:
74 .IP
75 A 2-byte file format major version number; the current version number is
76 2.
77 .IP
78 A 2-byte file format minor version number; the current version number is
79 4.
80 .IP
81 A 4-byte time zone offset; this is always 0.
82 .IP
83 A 4-byte number giving the accuracy of time stamps in the file; this is
84 always 0.
85 .IP
86 A 4-byte number giving the "snapshot length" of the capture; packets
87 longer than the snapshot length are truncated to the snapshot length, so
88 that, if the snapshot length is
89 .IR N ,
90 only the first
91 .I N
92 bytes of a packet longer than
93 .I N
94 bytes will be saved in the capture.
95 .IP
96 a 4-byte number giving the link-layer header type for packets in the
97 capture; see
98 .BR pcap-linktype (@MAN_MISC_INFO@)
99 for the
100 .B LINKTYPE_
101 values that can appear in this field.
102 .PP
103 Following the per-file header are zero or more packets; each packet
104 begins with a per-packet header, which is immediately followed by the
105 raw packet data. The format of the per-packet header is:
106 .RS
107 .TS
108 box;
109 c.
110 Time stamp, seconds value
111 _
112 Time stamp, microseconds or nanoseconds value
113 _
114 Length of captured packet data
115 _
116 Un-truncated length of the packet data
117 .TE
118 .RE
119 .PP
120 All fields in the per-packet header are in the byte order of the host
121 writing the file. The per-packet header begins with a time stamp giving
122 the approximate time the packet was captured; the time stamp consists of
123 a 4-byte value, giving the time in seconds since January 1, 1970,
124 00:00:00 UTC, followed by a 4-byte value, giving the time in
125 microseconds or nanoseconds since that second, depending on the magic
126 number in the file header. Following that are a 4-byte value giving the
127 number of bytes of captured data that follow the per-packet header and a
128 4-byte value giving the number of bytes that would have been present had
129 the packet not been truncated by the snapshot length. The two lengths
130 will be equal if the number of bytes of packet data are less than or
131 equal to the snapshot length.
132 .SH SEE ALSO
133 pcap(3PCAP), pcap-linktype(@MAN_MISC_INFO@)