Re: BPF problem

[elof () sentor se]

Hi Mike!

Jumping in here even though I haven't read the full thread.

Is some of the sniffed traffic VLAN-tagged and some not? (This is a common 
and painful problem with SPAN on e.g. HP procurve switches.)

Perhaps you are filtering out all plain (untagged) traffic for net 
10.0.0.0/23, while tagged packets aren't filtered, and therefore processed 
by snort as usual?


You can test this theory by running:
  tcpdump -nl vlan and net 10.0.0.0/23 
If you see traffic, you have 10.0.0.0/23-traffic that is tagged.

To remove it, remove the vlan-tag from the SPAN (best solution) or 
extend your bpf filter to also filter out tagged net 10.0.0.0/23:
  not net 10.0.0.0/23 and not (vlan and not net 10.0.0.0/23)

(note that using bpf filters, and more so complex ones, will have 
negative performance impact)

/Elof


On Fri, 11 Jul 2014, Mike Patterson wrote:

> On Jul 11, 2014, at 2:14 PM, James Lay <jlay () slave-tothe-box net> wrote:
>
> > On 2014-07-11 12:05, Mike Patterson wrote:
> > > On Jul 11, 2014, at 2:00 PM, James Lay <jlay () slave-tothe-box net>
> > > wrote:
> > >
> > > > On 2014-07-11 11:55, Mike Patterson wrote:
> > > > > On Jul 11, 2014, at 1:49 PM, waldo kitty <wkitty42 () windstream net>
> > > > > wrote:
> > > > >
> > > > > > On 7/11/2014 1:34 PM, Mike Patterson wrote:
> > > > > > > Following up to myself: I’ve tried various permutations of my BPF
> > > > > > > filter to
> > > > > > > no avail. I tried Snort versions 2.9.5.3 (which is what’s on my
> > > > > > > old
> > > > > > > sensor),
> > > > > > > 2.9.6.0, and 2.9.6.1. Always, Snort says it’s reading my BPF
> > > > > > > filter, and
> > > > > > > always, it’s including alerts for IPs and networks that are in
> > > > > > > the
> > > > > > > filter.
> > > > > > >
> > > > > > > My current filter is of the form:
> > > > > > >
> > > > > > > not (net 1.2.3.4/8 or not net 10.0.0.0/24 or not 172.16.12.1)
> > > > > >
> > > > > > this may not be related to your problem but i can't help seeing
> > > > > > the
> > > > > > double
> > > > > > negatives in the above... are you wanting to include or exclude
> > > > > > traffic to/from
> > > > > > 10.0.0.0/24 and 172.16.12.1?
> > > > > >
> > > > > > if you want to exclude traffic from them, perhaps you mean to use
> > > > > >
> > > > > > not (net 1.2.3.4/8 or net 10.0.0.0/24 or 172.16.12.1)
> > > > >
> > > > > That’s actually what I’m using, I just can’t transcribe properly.
> > > > >
> > > > > Mike
> > > >
> > > > Give:
> > > >
> > > > not (net 1.2.3.4/8 or net 10.0.0.0/16 or 172.16.12.1)
> > >
> > > No joy.
> > >
> > > Mike
> >
> > This worked for me ok:
> >
> > sudo snort -i eth0 -c snort/snort.conf not net 10.0.0.0/8
> >
> > You can also test just BPF ability with:
> >
> > sudo snort -i eth0 not net 10.0.0.0/8
>
> Sadly, not for me:
>
> sudo /usr/local/bin/snort -i dna1@0 --daq-dir=/usr/local/lib/daq --daq pfring_dna --daq-mode passive not net (my entire 
> netblock)
>
> provides a firehose. But:
>
> Running in packet dump mode
>
>        --== Initializing Snort ==--
> Initializing Output Plugins!
> Snort BPF option: not net (my netblock)
> pfring_dna DAQ configured to passive.
> Acquiring network traffic from "dna1@0".
>
>        --== Initialization Complete ==--
>
>   ,,_     -*> Snort! <*-
>  o"  )~   Version 2.9.6.1 GRE (Build 56)
>   ''''    By Martin Roesch & The Snort Team: http://www.snort.org/snort/snort-team
>           Copyright (C) 2014 Cisco and/or its affiliates. All rights reserved.
>           Copyright (C) 1998-2013 Sourcefire, Inc., et al.
>           Using libpcap version 1.1.1
>           Using PCRE version: 7.8 2008-09-05
>           Using ZLIB version: 1.2.3
>
> I know that’s an older pcap version, but it’s what ships with PF_RING.
>
> Mike
>
>
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