134 lines
5.1 KiB
Plaintext
134 lines
5.1 KiB
Plaintext
.\" Copyright (c) 1994, 1996, 1997
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that: (1) source code distributions
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.\" retain the above copyright notice and this paragraph in its entirety, (2)
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.\" distributions including binary code include the above copyright notice and
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.\" this paragraph in its entirety in the documentation or other materials
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.\" provided with the distribution, and (3) all advertising materials mentioning
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.\" features or use of this software display the following acknowledgement:
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.\" ``This product includes software developed by the University of California,
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.\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
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.\" the University nor the names of its contributors may be used to endorse
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.\" or promote products derived from this software without specific prior
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.\" written permission.
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.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
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.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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.\"
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.TH PCAP-SAVEFILE 5 "29 July 2013"
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.SH NAME
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pcap-savefile \- libpcap savefile format
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.SH DESCRIPTION
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NOTE: applications and libraries should, if possible, use libpcap to
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read savefiles, rather than having their own code to read savefiles.
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If, in the future, a new file format is supported by libpcap,
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applications and libraries using libpcap to read savefiles will be able
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to read the new format of savefiles, but applications and libraries
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using their own code to read savefiles will have to be changed to
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support the new file format.
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.PP
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``Savefiles'' read and written by libpcap and applications using libpcap
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start with a per-file header. The format of the per-file header is:
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.RS
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.TS
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box;
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c s
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c | c
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c s.
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Magic number
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_
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Major version Minor version
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_
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Time zone offset
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_
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Time stamp accuracy
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_
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Snapshot length
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_
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Link-layer header type
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.TE
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.RE
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.PP
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All fields in the per-file header are in the byte order of the host
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writing the file. Normally, the first field in the per-file header is a
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4-byte magic number, with the value 0xa1b2c3d4. The magic number, when
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read by a host with the same byte order as the host that wrote the file,
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will have the value 0xa1b2c3d4, and, when read by a host with the
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opposite byte order as the host that wrote the file, will have the value
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0xd4c3b2a1. That allows software reading the file to determine whether
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the byte order of the host that wrote the file is the same as the byte
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order of the host on which the file is being read, and thus whether the
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values in the per-file and per-packet headers need to be byte-swapped.
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.PP
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If the magic number has the value 0xa1b23c4d (with the two nibbles of
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the two lower-order bytes of the magic number swapped), which would be
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read as 0xa1b23c4d by a host with the same byte order as the host that
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wrote the file and as 0x4d3cb2a1 by a host with the opposite byte order
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as the host that wrote the file, the file format is the same as for
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regular files, except that the time stamps for packets are given in
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seconds and nanoseconds rather than seconds and microseconds.
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.PP
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Following this are:
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.IP
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A 2-byte file format major version number; the current version number is
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2.
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.IP
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A 2-byte file format minor version number; the current version number is
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4.
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.IP
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A 4-byte time zone offset; this is always 0.
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.IP
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A 4-byte number giving the accuracy of time stamps in the file; this is
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always 0.
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.IP
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A 4-byte number giving the "snapshot length" of the capture; packets
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longer than the snapshot length are truncated to the snapshot length, so
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that, if the snapshot length is
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.IR N ,
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only the first
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.I N
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bytes of a packet longer than
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.I N
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bytes will be saved in the capture.
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.IP
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a 4-byte number giving the link-layer header type for packets in the
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capture; see
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.BR pcap-linktype (7)
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for the
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.B LINKTYPE_
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values that can appear in this field.
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.PP
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Following the per-file header are zero or more packets; each packet
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begins with a per-packet header, which is immediately followed by the
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raw packet data. The format of the per-packet header is:
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.RS
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.TS
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box;
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c.
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Time stamp, seconds value
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_
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Time stamp, microseconds or nanoseconds value
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_
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Length of captured packet data
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_
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Un-truncated length of the packet data
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.TE
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.RE
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.PP
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All fields in the per-packet header are in the byte order of the host
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writing the file. The per-packet header begins with a time stamp giving
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the approximate time the packet was captured; the time stamp consists of
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a 4-byte value, giving the time in seconds since January 1, 1970,
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00:00:00 UTC, followed by a 4-byte value, giving the time in
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microseconds or nanoseconds since that second, depending on the magic
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number in the file header. Following that are a 4-byte value giving the
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number of bytes of captured data that follow the per-packet header and a
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4-byte value giving the number of bytes that would have been present had
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the packet not been truncated by the snapshot length. The two lengths
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will be equal if the number of bytes of packet data are less than or
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equal to the snapshot length.
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.SH SEE ALSO
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pcap(3PCAP), pcap-linktype(7)
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