PCAP-SAVEFILE(5) FreeBSD 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, 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 (big-endian 0x00 0x02 or little-endian 0x02 0x00).
A 2-byte file format minor version number; the current version
number is 4 (big-endian 0x00 0x04 or little-endian 0x04 0x00).
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(7) 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(3)
6 Jan 2025 PCAP-SAVEFILE(5)