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SubjectRe: BPF in the kernel
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kjh63 <kjh63@cantva.canterbury.ac.nz> writes:

> > How Linux Kernel and BPF relate to each other:
> >
> > a) linux has BPF (I don't think so).

It has LSF, the Linux Socket Filter.

> > b) Linux has own equivalent of BPF (part of NAT?)

Yes, the LSF.

> > c) linux does not have anything like BPF

BPF opcodes works on LSF. LSF has some extensions to BSF, like
fetching which interface the packet came from.

> > d) something else (if so, then what?)
>
> a) The Documentation/networking/filters.txt may say so but i dont think so
> either:
>
> [root@localhost networking]# ls -al /dev/bpf0
> ls: /dev/bpf0: No such file or directory
> [root@localhost networking]# cd /dev/
> [root@localhost /dev]# sh MAKEDEV bpf0
> MAKEDEV: don't know how to make device "bpf0"

LSF does not work with devices nodes, it works with setsockopt:

struct bpf_program bpfp;
/* Fill bpfp */
setsockopt(SOL_SOCKET, SO_ATTACH_FILTER, &bpfp, sizeof(bpfp));

> How can I make ethereal (or libpcap) work with LSF?

Last time I checked libpcap was emulating BPF in user space. Which is
bad of course because all packets are copied...

Ideally, libpcap should be extended to support the LPFisms. A LSF
filtering some ethernet traffic does not have to be bound to an
interface. You can filter all the packets coming in.

I can send you an example if you need so.

Phil.
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