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)