PCAP-SAVEFILE(5) File Formats Manual PCAP-SAVEFILE(5)
NAME
pcap-savefile - libpcap savefile format
DESCRIPTION
NOTE: applications and libraries should, if possible, use libpcap to
read savefiles, rather than having their own code to read savefiles.
If, in the future, a new file format is supported by libpcap, applica-
tions and libraries using libpcap to read savefiles will be able to
read the new format of savefiles, but applications and libraries using
their own code to read savefiles will have to be changed to support the
new file format.
``Savefiles'' read and written by libpcap and applications using libp-
cap start with a per-file header. The format of the per-file header
is:
┌──────────────────────────────┐
│ Magic number │
├──────────────┬───────────────┤
│Major version │ Minor version │
├──────────────┴───────────────┤
│ Time zone offset │
├──────────────────────────────┤
│ Time stamp accuracy │
├──────────────────────────────┤
│ Snapshot length │
├──────────────────────────────┤
│ Link-layer header type │
└──────────────────────────────┘
The per-file header length is 24 octets.
All fields in the per-file header are in the byte order of the host
writing the file. Normally, the first field in the per-file header is
a 4-byte magic number, with the value 0xa1b2c3d4. The magic number,
when read by a host with the same byte order as the host that wrote the
file, will have the value 0xa1b2c3d4, and, when read by a host with the
opposite byte order as the host that wrote the file, will have the
value 0xd4c3b2a1. That allows software reading the file to determine
whether the byte order of the host that wrote the file is the same as
the byte order of the host on which the file is being read, and thus
whether the values in the per-file and per-packet headers need to be
byte-swapped.
If the magic number has the value 0xa1b23c4d (with the two nibbles of
the two lower-order bytes of the magic number swapped), which would be
read as 0xa1b23c4d by a host with the same byte order as the host that
wrote the file and as 0x4d3cb2a1 by a host with the opposite byte order
as the host that wrote the file, the file format is the same as for
regular files, except that the time stamps for packets are given in
seconds and nanoseconds rather than seconds and microseconds.
Following this are:
A 2-byte file format major version number; the current version
number is 2.
A 2-byte file format minor version number; the current version
number is 4.
A 4-byte time zone offset; this is always 0.
A 4-byte number giving the accuracy of time stamps in the file;
this is always 0.
A 4-byte number giving the "snapshot length" of the capture;
packets longer than the snapshot length are truncated to the
snapshot length, so that, if the snapshot length is N, only the
first N bytes of a packet longer than N bytes will be saved in
the capture.
a 4-byte number giving the link-layer header type for packets in
the capture; see pcap-linktype(7) for the LINKTYPE_ values that
can appear in this field.
Following the per-file header are zero or more packets; each packet be-
gins with a per-packet header, which is immediately followed by the raw
packet data. The format of the per-packet header is:
┌──────────────────────────────────────────────┐
│ Time stamp, seconds value │
├──────────────────────────────────────────────┤
│Time stamp, microseconds or nanoseconds value │
├──────────────────────────────────────────────┤
│ Length of captured packet data │
├──────────────────────────────────────────────┤
│ Un-truncated length of the packet data │
└──────────────────────────────────────────────┘
The per-packet header length is 16 octets.
All fields in the per-packet header are in the byte order of the host
writing the file. The per-packet header begins with a time stamp giv-
ing the approximate time the packet was captured; the time stamp con-
sists of a 4-byte value, giving the time in seconds since January 1,
1970, 00:00:00 UTC, followed by a 4-byte value, giving the time in mi-
croseconds or nanoseconds since that second, depending on the magic
number in the file header. Following that are a 4-byte value giving
the number of bytes of captured data that follow the per-packet header
and a 4-byte value giving the number of bytes that would have been
present had the packet not been truncated by the snapshot length. The
two lengths will be equal if the number of bytes of packet data are
less than or equal to the snapshot length.
SEE ALSO
pcap(3PCAP)
24 April 2020 PCAP-SAVEFILE(5)
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