Re: why the ethernet and ip header of packets, which are captured by libpcap function, are distorted

[Guy Harris <guy () alum mit edu>]

On Mar 15, 2013, at 2:45 PM, Michael Richardson <mcr () sandelman ca> wrote:

> > > > > > "wen" == wen lui <esolvepolito () gmail com> writes:
>    wen> I used libpcap function pcap_next() to capture some tcp packets
>    wen> I checked the bytes of the captured packets and notice that the
>    wen> ethernet and ip header of packets are distorted, in a mess with
>    wen> a lot 0's but the TCP header is fine
>
>    wen> what are potential reasons for this?
>
> if you capture on Linux with the cooked mode interface.

That probably won't happen if you're capturing on an Ethernet device, but it *will* happen if you capture on the "any" 
device.

However, yes, *NO* program using libpcap/WinPcap should simply *assume* it's getting Ethernet packets; if it's looking 
at the packets, not just blindly writing them to a file without examining the contents, then, if it doesn't need to 
handle 802.11 and PPP and so on, just Ethernet, it should at least call pcap_datalink() and fail if the return value 
isn't DLT_EN10MB.  (If it's writing them to a pcap file, pcap_dump_open() will call pcap_datalink() for you, to put the 
right link-layer header type in the file header.)

(Should we change libpcap so that if pcap_datalink() isn't called at least once before calling pcap_next(), 
pcap_next_ex(), pcap_dispatch(), or pcap_loop(), it prints a message to the standard error saying "you're probably 
assuming all the world is Ethernet, aren't you?" and calls abort(). :-))


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