[Winpcap-users] Packet random access using file seek
clemens.gruber at pqgruber.com
Tue Apr 23 06:57:08 PDT 2013
> On Tue, Apr 23, 2013 at 12:10 AM, Gisle Vanem <gvanem at broadpark.no> wrote:
>> I forgot to ask how you compile and link with the WinPcap and Packet APIs.
>> And what compiler; MSVC or MingW or something else.
> Hi, thanks for having a look at my problem, it's very appreciated. I am
> currently using the 32 bit compiler on Visual Studio Express 2008 on
> Windows 7 x64, and later I will try the 64 bit compiler on Visual Studio
> 2008 Pro. My project is configured using CMake and built in Release mode.
> It links to wpcap.lib.
> My project does not have LIBPCAP_EXPORTS, so this means it is using:
> #define pcap_fopen_offline(f,b) \
> pcap_hopen_offline(_get_osfhandle(_fileno(f)), b)
> I took a look at the implementation of pcap_hopen_offline(). I see that it
> takes the input FILE* and creates a new FILE* using a series of function
> So, if I understand correctly, it is creating a new FILE* that is relative
> to its own CRT. I think that means I can no longer use any information I
> query about the original input FILE*, because winpcap has created its own
> FILE* stream to read from.
> So that leads me to think that my plan to use ftell() to record file
> positions of packets, and fseek() to jump to the begining of packets, is
> not going to work on Windows. What do you think? Is there a different way
> to use winpcap to seek to packets in a save file without reading each
> packet in sequential order starting at the beginning?
I had to do something similar on Windows, about a year ago.
What confused me in the beginning, was the non-availability of pcap_dump_fopen.
So, in the end, I wrote my own index file and used the pcap file seek routines according to Wireshark's implementation. Besides dumping the pcap packet in the pcap file with the Winpcap functions, I wrote an index struct to the seperate index file.
In that index file, I could jump to the packet number * my index structure's size and look up the offset for the pcap file, after that, I would call fseek and read the data from the pcap file.
Oh, in case you are also using Wireshark for inspiration, the wiretap subdirectory was helpful to me: http://anonsvn.wireshark.org/wireshark/trunk/wiretap/
There was also an interesting article: http://www.csg.ethz.ch/people/dimitroc/papers/pcapIndex.pdf
(But my home made indexing solution was not as impressive and professional as it is proposed in this paper ;-)
Bitmap indexing would be useful, because otherwise, in some cases the index files tend to become bigger than the pcap files..
What are you using for your index file?
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