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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,
       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         |
              +---------------------------------------+
       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     |
              +----------------------------------------------+
       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), pcap-linktype(7)



                                21 October 2008               PCAP-SAVEFILE(5)