pcap-savefile - libpcap savefile format


   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

   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.


   pcap(3PCAP), pcap-linktype(7)

                             8 March 2015                 PCAP-SAVEFILE(5)


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