Re: snort mixes multiple (unrelated) payloads into one alert

[Jason Brvenik <jasonb () sourcefire com>]

Frank Knobbe wrote:
> On Sun, 2006-07-23 at 23:34 -0400, Jason wrote:
> > \"This will cause Stream4 to zero out the memory of the
> > rebuilt packet before copying in the new data.  So, when
> > packets are missing from the middle of the rebuilt packet,
> > you'll get 0x00 in those bytes, rather than whatever was
> > there from the previous rebuild."
> >
> > The problem is packet loss. A single buffer is used for reassembly. If
> > you are missing packets when reassembly is done then the old data is
> > still in the gaps...
>
> Yeah, I remember that we've discussed that before now :) 
>
> But a question comes to mind: When Snort reassembles the stream,
> shouldn't it be able to tell which segments are reassembled and which
> not? Snort should be able to fill the first segment, the third segment,
> the fourth segment, and then realize that it never got the second
> segment, then null just that before sending the packet to the matching
> engine. 
>
> Nulling the whole buffer before reassembly seems like a waste of
> resources, which is probably why a lot of folks don't turn it on. I
> believe you guys even warned about the performance impact.
>
> To me, the solution would seem to write the stream4 such that is
> recognizes a missing segment and nulls just that. From a performance
> standpoint, it should require as much as copying an existing segment
> into the stream.

I doubt it would make enough difference to matter. Packet loss is packet
loss and the ultimate game has been lost at that point.
zero_flushed_packets only helps to remove erroneous events, not solve
the real problem.

> Thoughts?
>
> Cheers,
> Frank
>
>
>
> ------------------------------------------------------------------------
>
> -------------------------------------------------------------------------
> Take Surveys. Earn Cash. Influence the Future of IT
> Join SourceForge.net's Techsay panel and you'll get the chance to share your
> opinions on IT & business topics through brief surveys -- and earn cash
> http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV
>
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> Snort-users mailing list
> Snort-users () lists sourceforge net
> Go to this URL to change user options or unsubscribe:
> https://lists.sourceforge.net/lists/listinfo/snort-users
> Snort-users list archive:
> http://www.geocrawler.com/redir-sf.php3?list=snort-users

-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys -- and earn cash
http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV
_______________________________________________
Snort-users mailing list
Snort-users () lists sourceforge net
Go to this URL to change user options or unsubscribe:
https://lists.sourceforge.net/lists/listinfo/snort-users
Snort-users list archive:
http://www.geocrawler.com/redir-sf.php3?list=snort-users