tcpdump mailing list archives
Re: why the ethernet and ip header of packets, which are captured by libpcap function, are distorted
From: Wesley Shields <wxs () FreeBSD org>
Date: Fri, 22 Mar 2013 00:22:25 -0400
On Thu, Mar 21, 2013 at 01:03:56PM -0400, Bill Fenner wrote:
On Mon, Mar 18, 2013 at 11:08 PM, Wesley Shields <wxs () freebsd org> wrote:On Fri, Mar 15, 2013 at 06:37:25PM -0700, Guy Harris wrote: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(). :-))As I'm not sure if you're serious or not I decided to look into this to satisfy my own curiosity. In case you are serious: https://github.com/wxsBSD/libpcap/commit/70cbe36e2bd12498ca1622349ecb1716a874c376 If you are serious and want this I'll submit a pull request.Since pcap_compile() calls pcap_datalink(), I don't think that this will have as much affect as Guy was imagining.
I noticed that. I think I mentioned it in commit.
(Now introduce an argument to pcap_datalink() that says "I'm calling you from pcap_compile()," and ... ;-)
That would be breaking a lot of existing applications. -- WXS _______________________________________________ tcpdump-workers mailing list tcpdump-workers () lists tcpdump org https://lists.sandelman.ca/mailman/listinfo/tcpdump-workers
Current thread:
- why the ethernet and ip header of packets, which are captured by libpcap function, are distorted wen lui (Mar 15)
- Re: why the ethernet and ip header of packets, which are captured by libpcap function, are distorted Guy Harris (Mar 15)
- Re: why the ethernet and ip header of packets, which are captured by libpcap function, are distorted Michael Richardson (Mar 15)
- Re: why the ethernet and ip header of packets, which are captured by libpcap function, are distorted Guy Harris (Mar 15)
- Re: why the ethernet and ip header of packets, which are captured by libpcap function, are distorted Wesley Shields (Mar 18)
- Re: why the ethernet and ip header of packets, which are captured by libpcap function, are distorted Bill Fenner (Mar 21)
- Re: why the ethernet and ip header of packets, which are captured by libpcap function, are distorted Wesley Shields (Mar 21)
- Re: why the ethernet and ip header of packets, which are captured by libpcap function, are distorted Guy Harris (Mar 15)