Date: Wed, 29 Aug 2001 17:00:42 -0400 (EDT) From: Robert Watson <rwatson@freebsd.org> To: Mike Silbersack <silby@silby.com> Cc: freebsd-net@freebsd.org Subject: Re: BPF question Message-ID: <Pine.NEB.3.96L.1010829165726.72843B-100000@fledge.watson.org> In-Reply-To: <Pine.BSF.4.30.0108291454041.45090-100000@niwun.pair.com>
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Depends on the SIMPLEX flag I believe, but for most ethernet interfaces,
there's a software loopback. I actually take advantage of that to support
the following BPF ioctl which I added:
BIOCSSEESENT
BIOCGSEESENT (u_int) Set or get the flag determining whether locally
generated packets on the interface should be returned by
BPF. Set to zero to see only incoming packets on the
interface. Set to one to see packets originating locally
and remotely on the interface. This flag is initialized
to one by default.
This is determined by checking that the mbuf pointer is non-NULL,
indicating it originated in the interface code, rather than NULL, which
signifies that it descended from a socket. Not 100% accurate with weird
netgraph/etc cases, but sufficient for my needs, which permitted userland
bridging using bpf.
Robert N M Watson FreeBSD Core Team, TrustedBSD Project
robert@fledge.watson.org NAI Labs, Safeport Network Services
On Wed, 29 Aug 2001, Mike Silbersack wrote:
> Silly question I could probably figure out myself, but thought it would be
> quicker to ask here...
>
> When listening on an ethernet interface, are the packets you see from
> yourself intercepted inside the network stack, or are you actually
> sniffing them off the wire? I'm helping someone track down corrupted
> packets he's seeing in tcpdump, and I'd like to know if I can locate where
> the corruption may be occuring more exactly.
>
> Thanks,
>
> Mike "Silby" Silbersack
>
>
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