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 │
              ├──────────────┴───────────────┤
              │      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 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)

                                          24 April 2020                          PCAP-SAVEFILE(5)