tcpdump mailing list archives
Re: vlan tagged packets and libpcap breakage
From: Bill Fenner <fenner () gmail com>
Date: Fri, 2 Nov 2012 12:13:18 -0400
On Wed, Oct 31, 2012 at 6:20 PM, Guy Harris <guy () alum mit edu> wrote:
On Oct 31, 2012, at 2:50 PM, Ani Sinha <ani () aristanetworks com> wrote:pcap files that already have the tags reinsrted should work with current filter code. However for live traffic, one has to get the tags from CMSG() and then reinsert it back to the packet for the current filter to work.*Somebody* has to do that, at least to packets that pass the filter, before they're handed to a libpcap-based application, for programs that expect to see packets as they arrived from/were transmitted to the wire to work. I.e., the tags *should* be reinserted by libpcap, and, as I understand it, that's what the #if defined(HAVE_PACKET_AUXDATA) && defined(HAVE_LINUX_TPACKET_AUXDATA_TP_VLAN_TCI) ... #endif blocks of code in pcap-linux.c in libpcap are doing. Now, if filtering is being done in the *kernel*, and the tags aren't being reinserted by the kernel, then filter code stuffed into the kernel would need to differ from filter code run in userland. There's already precedent for that on Linux, with the "cooked mode" headers; those are synthesized by libpcap from the metadata returned for PF_PACKET sockets, and the code that attempts to hand the kernel a filter goes through the filter code, which was generated under the assumption that the packet begins with a "cooked mode" header, and modifies (a copy of) the code to, instead, use the special Linux-BPF-interpreter offsets to access the metadata. The right thing to do here would be to, if possible, do the same, so that the kernel doesn't have to reinsert VLAN tags for packets that aren't going to be handed to userland.
In this case, it would be incredibly complicated to do this just postprocessing a set of bpf instructions. The problem is that when running the filter in the kernel, the IP header, etc. are not offset, so "off_macpl" and "off_linktype" would be zero, not 4, while generating the rest of the expression. We would also have to insert code when comparing the ethertype to 0x8100 to instead load the vlan-tagged metadata, so all jumps crossing that point would have to be adjusted, and if the "if-false" instruction was also testing the ethertype, then the ethertype would have to be reloaded (again inserting another instruction). Basically, take a look at the output of "tcpdump -d tcp port 22 or (vlan and tcp port 22)". Are the IPv4 tcp ports at x+14/x+16, or at x+18/x+20? If we're filtering in the kernel, they're at x+14/x+16 whether the packet is vlan tagged or not. If we're filtering on the actual packet contents (from a savefile, for example), they're at x+18/x+20 if the packet is vlan tagged. Also, an expression such as 'tcp port 22' would have to have some instructions added at the beginning, for "vlan-tagged == false", or it would match both tagged and untagged packets. This would be much more straightforward to deal with in the code generation phase, except until now the code generation phase hasn't known whether the filter is headed for the kernel or not. Bill _______________________________________________ tcpdump-workers mailing list tcpdump-workers () lists tcpdump org https://lists.sandelman.ca/mailman/listinfo/tcpdump-workers
Current thread:
- vlan tagged packets and libpcap breakage Ani Sinha (Oct 31)
- Message not available
- Re: vlan tagged packets and libpcap breakage Ani Sinha (Oct 31)
- Re: vlan tagged packets and libpcap breakage Guy Harris (Nov 01)
- Re: vlan tagged packets and libpcap breakage Ani Sinha (Oct 31)
- Re: vlan tagged packets and libpcap breakage Guy Harris (Nov 01)
- Re: vlan tagged packets and libpcap breakage Ani Sinha (Oct 31)
- Re: vlan tagged packets and libpcap breakage Ani Sinha (Dec 06)
- Re: vlan tagged packets and libpcap breakage Ani Sinha (Oct 31)
- Re: vlan tagged packets and libpcap breakage Bill Fenner (Nov 11)
- Re: vlan tagged packets and libpcap breakage Ani Sinha (Nov 13)
- Re: vlan tagged packets and libpcap breakage Ani Sinha (Nov 13)
- Re: vlan tagged packets and libpcap breakage Michael Richardson (Nov 14)
- Message not available
- Re: vlan tagged packets and libpcap breakage Eric W. Biederman (Nov 18)
- Message not available
- Re: vlan tagged packets and libpcap breakage Daniel Borkmann (Nov 18)
- Re: vlan tagged packets and libpcap breakage Eric W. Biederman (Nov 18)
- Message not available
- Re: vlan tagged packets and libpcap breakage Eric W. Biederman (Nov 18)
- Re: vlan tagged packets and libpcap breakage Ani Sinha (Dec 06)
- Re: vlan tagged packets and libpcap breakage Eric W. Biederman (Dec 06)
- Re: vlan tagged packets and libpcap breakage Ani Sinha (Dec 06)