plucky (5) pcap-savefile.5.gz

Provided by: libpcap0.8t64_1.10.5-2ubuntu1_amd64 bug

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(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(3PCAP)

                                                   16 Aug 2023                                  PCAP-SAVEFILE(5)