1PCAP-SAVEFILE(5) File Formats Manual PCAP-SAVEFILE(5)
2
6 pcap-savefile - libpcap savefile format
7
9 NOTE: applications and libraries should, if possible, use libpcap to
10 read savefiles, rather than having their own code to read savefiles.
11 If, in the future, a new file format is supported by libpcap, applica‐
12 tions and libraries using libpcap to read savefiles will be able to
13 read the new format of savefiles, but applications and libraries using
14 their own code to read savefiles will have to be changed to support the
15 new file format.
16 ``Savefiles'' read and written by libpcap and applications using libp‐
18 cap start with a per-file header. The format of the per-file header
19 is:
20 ┌──────────────────────────────┐
22 │ Magic number │
23 ├──────────────┬───────────────┤
24 │Major version │ Minor version │
25 ├──────────────┴───────────────┤
26 │ Time zone offset │
27 ├──────────────────────────────┤
28 │ Time stamp accuracy │
29 ├──────────────────────────────┤
30 │ Snapshot length │
31 ├──────────────────────────────┤
32 │ Link-layer header type │
33 └──────────────────────────────┘
34 The per-file header length is 24 octets.
35 All fields in the per-file header are in the byte order of the host
37 writing the file. Normally, the first field in the per-file header is
38 a 4-byte magic number, with the value 0xa1b2c3d4. The magic number,
39 when read by a host with the same byte order as the host that wrote the
40 file, will have the value 0xa1b2c3d4, and, when read by a host with the
41 opposite byte order as the host that wrote the file, will have the
42 value 0xd4c3b2a1. That allows software reading the file to determine
43 whether the byte order of the host that wrote the file is the same as
44 the byte order of the host on which the file is being read, and thus
45 whether the values in the per-file and per-packet headers need to be
46 byte-swapped.
47 If the magic number has the value 0xa1b23c4d (with the two nibbles of
49 the two lower-order bytes of the magic number swapped), which would be
50 read as 0xa1b23c4d by a host with the same byte order as the host that
51 wrote the file and as 0x4d3cb2a1 by a host with the opposite byte order
52 as the host that wrote the file, the file format is the same as for
53 regular files, except that the time stamps for packets are given in
54 seconds and nanoseconds rather than seconds and microseconds.
55 Following this are:
57 A 2-byte file format major version number; the current version
59 number is 2.
60 A 2-byte file format minor version number; the current version
62 number is 4.
63 A 4-byte time zone offset; this is always 0.
65 A 4-byte number giving the accuracy of time stamps in the file;
67 this is always 0.
68 A 4-byte number giving the "snapshot length" of the capture;
70 packets longer than the snapshot length are truncated to the
71 snapshot length, so that, if the snapshot length is N, only the
72 first N bytes of a packet longer than N bytes will be saved in
73 the capture.
74 a 4-byte number giving the link-layer header type for packets in
76 the capture; see pcap-linktype(7) for the LINKTYPE_ values that
77 can appear in this field.
78 Following the per-file header are zero or more packets; each packet
80 begins with a per-packet header, which is immediately followed by the
81 raw packet data. The format of the per-packet header is:
82 ┌──────────────────────────────────────────────┐
84 │ Time stamp, seconds value │
85 ├──────────────────────────────────────────────┤
86 │Time stamp, microseconds or nanoseconds value │
87 ├──────────────────────────────────────────────┤
88 │ Length of captured packet data │
89 ├──────────────────────────────────────────────┤
90 │ Un-truncated length of the packet data │
91 └──────────────────────────────────────────────┘
92 The per-packet header length is 16 octets.
93 All fields in the per-packet header are in the byte order of the host
95 writing the file. The per-packet header begins with a time stamp giv‐
96 ing the approximate time the packet was captured; the time stamp con‐
97 sists of a 4-byte value, giving the time in seconds since January 1,
98 1970, 00:00:00 UTC, followed by a 4-byte value, giving the time in
99 microseconds or nanoseconds since that second, depending on the magic
100 number in the file header. Following that are a 4-byte value giving
101 the number of bytes of captured data that follow the per-packet header
102 and a 4-byte value giving the number of bytes that would have been
103 present had the packet not been truncated by the snapshot length. The
104 two lengths will be equal if the number of bytes of packet data are
105 less than or equal to the snapshot length.
106
108 pcap(3PCAP)
109 24 April 2020 PCAP-SAVEFILE(5)