PCAP-SAVEFILE(4) Device and Network Interfaces PCAP-SAVEFILE(4)

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,
applications 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
libpcap start with a per-file header. The format of the per-file
header is:

+--------------------------------------------------+
| Magic number |
+------------------------+-------------------------+
| Major version | Minor version |
+------------------------+-------------------------+
| Reserved1 |
+--------------------------------------------------+
| Reserved2 |
+--------------------------------------------------+
| Snapshot length |
+--------------------------------------------------+
|Link-layer header type and additional information |
+--------------------------------------------------+
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 not used - SHOULD be filled with 0 by pcap file
writers, and MUST be ignored by pcap file readers. This value
was documented by some older implementations as "gmt to local
correction" or "time zone offset". Some older pcap file
writers stored non-zero values in this field.

A 4-byte not used - SHOULD be filled with 0 by pcap file
writers, and MUST be ignored by pcap file readers. This value
was documented by some older implementations as "accuracy of
timestamps". Some older pcap file writers stored non-zero
values in this field.

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 and optional additional information.

This format of this field is:

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|FCS len|R|P| Reserved3 | Link-layer type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The field is shown as if it were in the byte order of the host
reading or writing the file, with bit 0 being the most-
significant bit of the field and bit 31 being the least-
significant bit of the field.

Link-layer type (16 bits): A 16-bit value giving the link-
layer header type for packets in the file; see
pcap-linktype(5) for the LINKTYPE_ values that can appear in
this field.

Reserved3 (10 bits): not used - MUST be set to zero by pcap
writers, and MUST NOT be interpreted by pcap readers; a reader
SHOULD treat a non-zero value as an error.

P (1 bit): A bit that, if set, indicates that the Frame Check
Sequence (FCS) length value is present and, if not set,
indicates that the FCS value is not present.

R (1 bit): not used - MUST be set to zero by pcap writers, and
MUST NOT be interpreted by pcap readers; a reader SHOULD treat
a non-zero value as an error.

FCS len (4 bits): A 4-bit unsigned value giving the number of
16-bit (2-octet) words of FCS that are appended to each
packet, if the P bit is set; if the P bit is not set, and the
FCS length is not indicated by the link-layer type value, the
FCS length is unknown. The valid values of the FCS len field
are between 0 and 15; Ethernet, for example, would have an FCS
length value of 2, corresponding to a 4-octet FCS.

Following the per-file header are zero or more packets; each packet
begins 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
giving the approximate time the packet was captured; the time stamp
consists 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
microseconds 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)

16 Aug 2023 PCAP-SAVEFILE(4)