Re: How snort handles several copies of the same packet?

[Russ Combs <rcombs () sourcefire com>]

On Wed, Oct 24, 2012 at 11:35 AM, <elof () sentor se> wrote:

> Open Source Snort.
> No global threshold.
>
> I'm logging directly to ascii from snort and to unified2 where barnyard2
> then take over.
> In both my ascii alert log and in my postgres I only get one alert.
>
> I think this is great! ...but I'm curious and would like to know how and
> where in the process this filtering/aggregation is done.

There are a couple things going on here.  Stream5 basically processes data
the same way a receiving host would.  For example, retransmitted or
duplicated data that falls to the left of the window (ie it was already
acknowledged) will just be discarded.

And Snort will not queue events that have already fired on the same session
(or fragment).  These non-events are counted and output at shutdown as
Limits::Alert:

Limits:
      Match:            0
      Queue:            0
        Log:            0
      Event:            0
      Alert:            0

Hope that helps.

> /Elof
>
>
> On Wed, 24 Oct 2012, Joel Esler wrote:
>
> > Are you talking about Open Source Snort?  Or Sourcefire product?
> >
> > Do you have a global threshold in place?
> >
> >
> > On Oct 24, 2012, at 10:42 AM, elof () sentor se wrote:
> >
> > > Yes, there's an performance impact. That is expected.
> > >
> > > But what about the alerting? Somewhere snort must be
> filtering/aggregating the packets, understanding that the "duplicates" are
> actually the same packet, and only generate ONE alert for its bad payload
> data.
> > > I'm asking for a description of this part.
> > >
> > > How does snort detect and filter out these "duplicates"?
> > > Which packets are disregarded and which are kept?
> > >
> > > Like if the packet in my example contain malicious code, will the
> logged packet be
> > > routing)
> > > The first packet with TTL 60?
> > >
> > > retransmission)
> > > The first packet with ipid 3333?
> > >
> > > duplicate SPAN)
> > > Simply the first packet?
> > > Another question: Are true duplicates seen as retransmissions and
> processed as such?
> > > Perhaps the answer is that the logging system simply detects that the
> next received, analyzed and logged packet is the same as the one just
> logged, and silently supresses it.
> > > I don't think this filtering/aggregation happen this late in the
> process though.
> > > Some clarification of how this works would be appreciated.
> > >
> > > /Elof
> > >
> > >
> > > On Wed, 24 Oct 2012, Joel Esler wrote:
> > >
> > > > On Oct 24, 2012, at 4:48 AM, elof () sentor se wrote:
> > > > > I know that snort only generates ONE alert even if the mirrored
> traffic
> > > > > see the same packet twice or more:
> > > > >
> > > > > ...like before and after a router:
> > > > > x:x:x:x:x:x y:y:y:y:y:y 1.1.1.1:1234 -> 2.2.2.2:80 ipid 3333, TTL 60
> > > > > y:y:y:y:y:y z:z:z:z:z:z 1.1.1.1:1234 -> 2.2.2.2:80 ipid 3333, TTL 59
> > > > > ^^^^^^^^^^^^^^^^^^^^^^^                                           ^^
> > > > >
> > > > > ...or tcp retransmissions:
> > > > > x:x:x:x:x:x y:y:y:y:y:y 1.1.1.1:1234 -> 2.2.2.2:80 ipid 3333, TTL 60
> > > > > x:x:x:x:x:x y:y:y:y:y:y 1.1.1.1:1234 -> 2.2.2.2:80 ipid 3334, TTL 60
> > > > > x:x:x:x:x:x y:y:y:y:y:y 1.1.1.1:1234 -> 2.2.2.2:80 ipid 3335, TTL 60
> > > > >                                                        ^^^^
> > > > >
> > > > > ...or two *exact* duplicates of every packet due to faulty SPAN:
> > > > > x:x:x:x:x:x y:y:y:y:y:y 1.1.1.1:1234 -> 2.2.2.2:80 ipid 3333, TTL 60
> > > > > x:x:x:x:x:x y:y:y:y:y:y 1.1.1.1:1234 -> 2.2.2.2:80 ipid 3333, TTL 60
> > > > >
> > > > >
> > > > > Only having one alert in the above cases is really nice, but I wonder:
> > > > >
> > > > > Can someone describe how this is done and what is happening in snort,
> both
> > > > > on the individual packet level, and in stream5?
> > > > >
> > > > > How does snort detect and filter out these "duplicates"?
> > > > > Which packets are disregarded and which are kept?
> > > >
> > > > Everything is analyzed independently.  I've seen the problem commonly
> at many sites.  Filtering out the duplicate traffic on a span is important
> for optimum performance.
> > > > --
> > > > Joel Esler
> > > > Senior Research Engineer, VRT
> > > > OpenSource Community Manager
> > > > Sourcefire
>
>
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