PCAP-SAVEFILE(5)                                              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, applica-
       tions 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 libp-
       cap start with a per-file header.  The format of  the  per-file  header
       is:

              box; c s c | c c s.  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.  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 oppo-
       site 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.

       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:

              box; c.  Time stamp, seconds value _  Time  stamp,  microseconds
              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 giv-
       ing the approximate time the packet was captured; the time  stamp  con-
       sists  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 since that second.  Following that are a 4-byte value giv-
       ing 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)

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