Re: Pcap files

[Rayne <hjazz6 () ymail com>]

So am I right to say that if I were to capture a packet, that packet should only consist of the 16-byte packet header 
and the L2-L7 content. But if I were to write that packet to a file of the libpcap format, then the 24-byte "header" 
will be prepended to the file?

Thank you.

Regards,
Rayne

--- On Sat, 10/17/09, Guy Harris <guy () alum mit edu> wrote:

> From: Guy Harris <guy () alum mit edu>
> Subject: Re: [Wireshark-users] Pcap files
> To: "Community support list for Wireshark" <wireshark-users () wireshark org>
> Date: Saturday, October 17, 2009, 1:23 AM
>
> On Oct 16, 2009, at 6:10 PM, Rayne wrote:
>
> > I noticed that every pcap file, even the empty ones
> without any  
> > packets, contain a 24-byte "header" at the beginning
> of the file. At  
> > least 3 of the bytes vary from file to file, and the
> rest appears to  
> > be the same, at least from the files I've seen. If I
> were to omit  
> > these 24 bytes from the file, Wireshark doesn't
> recognize the file  
> > as a pcap anymore.
> >
> > So I guess these 24 bytes are to indicate that the
> file is of  
> > libpcap format, but does anyone know what these 24
> bytes are in  
> > details, i.e. what they represent?
>
> On a machine with an OS that includes libpcap 1.0.0 or
> later (OS X  
> Snow Leopard, in this case, although recent versions of
> some Linux  
> distributions might also have it now, and recent versions
> of some  
> *BSDs might as well)
>
> $ man 5 pcap-savefile
> 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:
>
>            
>    +------------------------------+
>            
>    |        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:
>
>            
>    +---------------------------------------+
>            
>    |      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(3PCAP), pcap-linktype(7)
>
> (Note: before writing code to read this, PLEASE pay
> attention to the  
> first paragraph on the man page.  Unless you have a
> compelling reason  
> to read this file format yourself, rather than letting some
> existing  
> code read it, and unless you either promise not to complain
> if tools  
> start writing pcap-NG files or are willing to update your
> code to read  
> pcap-NG files at some point, leave it up to libpcap/WinPcap
> or  
> Wireshark's Wiretap library to do the reading.  If
> you're reading in a  
> program written in Perl/Python/Ruby/Java/some .Net
> language/etc., see
>
>     http://en.wikipedia.org/wiki/Pcap#Wrappers_for_use_of_libpcap.2FWinPcap_in_languages_other_than_C_and_C.2B.2B
>
> and
>
>     http://en.wikipedia.org/wiki/Pcap#External_links
>
> for information on wrappers for libpcap/WinPcap for your
> favorite  
> language.)
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