BPF Berkeley Packet Filter Question

[Julian Elischer]

On 30/12/2015 8:11 PM, Daniel Janzon wrote:
> Hello Julian,
It's not me that was asking, but Juan
I'm sure that he's reading though..

>
> I'm not sure I follow what you want to do but maybe I can help you 
> get in the right direction.
>
> You can define a BPF program with macros, like
>
> struct bpf_insn instructions[] = {
>     ...
>     BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, proto, 0, 1),
>     BPF_STMT(BPF_RET + BPF_K, (uint16_t)-1),
>     BPF_STMT(BPF_RET + BPF_K, 0)
> };
>
> struct bpf_program bpf_program = { 4, (struct bpf_insn*)&instructions };
> ioctl(fd, BIOCSETF, (struct bpf_program*)&bpf_program);
>
> etc, google for a complete example.
>
> Then you can use the -d option of tcpdump to get some help to find 
> the right instructions, for instance
>
> tcpdump -i em0 -d host 10.10.10.1 and greater 150  # capture packets 
> greater than 150
>
> You will probably have to modify the output a bit to get what you 
> want so you will have to learn a bit how it works. See the section 
> Filter machine in the bpf manual (man 4 bpf).
>
> Hope that helps.
>
> All the best,
> Daniel Janzon
>
>
> On Wed, Dec 30, 2015 at 9:58 AM Julian Elischer <julian at freebsd.org 
> <mailto:julian at freebsd.org>> wrote:
>
>     On 30/12/2015 12:46 PM, Juan Herrera wrote:
>     > Hello BSD folks,
>     >
>     > I am developing a networking application in C and I have a
>     question
>     > regarding BPF (Berkeley Packet Filters), I will give you an
>     idea of the app
>     > first,  I need to send a packet from machine A to machine B
>     (any kind of
>     > packet) so for this I wrote a packet generator application
>     which will send
>     > a packet to machine B, but before sending the packet I need to
>     append some
>     > metadata values at the end of the packet, already done, so in
>     machine B I
>     > have a raw socket listener app ready to receive incoming
>     packets from
>     > machine A, however I want to implement filtering with BPF on
>     machine B, but
>     > as my metadata was appended at the end of the packet (have to
>     be at the
>     > end), I need to read the packet length with(using) Berkeley
>     Packet Filter
>     > to match a specific field to filter one of the bytes at the
>     end of my
>     > packet (metadata appended), in other words I need to know the
>     incoming
>     > packet length to filtered against one of the metadatas fields
>     and be able
>     > to drop the packet before reaching user space
>     applications(drop it in
>     > kernel space).
>     >
>     > So my question is, Can I use BPF to read the packet length ?
>     to continue on my previous mail.
>
>     you can also use netgraph to do this in several ways as well.
>     But I'd need more information to be able to explain what to do.
>
>     >
>     > TIA!
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