Re: Very high amount of "TCP Small Segment Threshold Exceeded"

[Giacomo <lib.giacomo () gmail com>]

After changing small_segments to 0 (snort install default is 3) the events did not pop up anymore. I don't know why 
removing detect_anomalies (like Shane suggested) did not do the trick. small_segments is only enabled when 
detect_anomalies is defined (according to the docs). Anyway, thanks for the help guys.

On 01/03/2012, at 3:44 AM, waldo kitty wrote:

> On 2/29/2012 08:08, Russ Combs wrote:
> > If you can trigger the alerts, can you capture a pcap that reproduces the
> > problem?  Maybe we can tweak the settings based on that.
>
> +1
> that's exactly what i was just getting ready to write and then i saw your post 
> in the thread and read it first ;)
>
> > On Wed, Feb 29, 2012 at 3:40 AM, Giacomo <lib.giacomo () gmail com
> > <mailto:lib.giacomo () gmail com>> wrote:
> >
> >    Hi there,
> >
> >    Sorry I put it indeed in the subject but forgot to mention it in the email.
> >    The event that gets thrown is: "stream5: TCP Small Segment Threshold Exceeded"
> >    The configuration adjustments Shane Castle suggested don't really seem to do
> >    the trick.
> >    I did notice today though that the events seem to be thrown when I connect
> >    with the (default) ssh client for Mac OS X. Connecting with putty seems to
> >    go fine (no events are generated). This is a bit of a mystery to me why...
> >
> >    Cheers.
> >
> >    On 29/02/2012, at 7:00 AM, Russ Combs wrote:
> >
> > >    On Tue, Feb 28, 2012 at 2:52 PM, waldo kitty <wkitty42 () windstream net
> > >    <mailto:wkitty42 () windstream net>> wrote:
> > >
> > >        On 2/27/2012 03:39, Giacomo wrote:
> > > > Hi there,
> > > >
> > > > I recently started using Snort. After enabling the (default)
> > >        preprocessor configuration I started receiving very large amounts of
> > >        events regarding stream5.
> > > > Since it is a server that is not being used for anything I assume
> > >        this event is generated by my SSH connection. A couple of topics have
> > >        discussed this but none come with a very clear answer why this is
> > >        occurring and how you can solve it.
> > > > The only two suggestions I found was to change the max_tcp value in
> > >        stream5_global or increase the memcap. But both of these suggestions
> > >        don't work. So I am wondering if any one of you has an idea why this
> > >        is occurring and what I can do about it.
> > >
> > >        what, exactly, are the SIDs being reported? the items you saw are for
> > >        one or two
> > >        things but stream5 can alert on numerous items...
> > >
> > >        here's what the snort-2.9.2.1's README.stream5 has to say...
> > >
> > >        Alerts
> > >        ======
> > >        Stream5 uses generator ID 129. It is capable of alerting on 10
> > >        anomalies, all of
> > >        which relate to TCP anomalies. There are no anomaly detection
> > >        capabilities for
> > >        UDP or ICMP.
> > >
> > >        SID   Description
> > >        ---   -----------
> > >        1     SYN on established session
> > >        2     Data on SYN packet
> > >        3     Data sent on stream not accepting data
> > >        4     TCP Timestamp is outside of PAWS window
> > >        5     Bad segment, overlap adjusted size less than/equal 0
> > >        6     Window size (after scaling) larger than policy allows
> > >        7     Limit on number of overlapping TCP packets reached
> > >        8     Data after Reset packet
> > >        9     Possible Hijacked Client
> > >        10    Possible Hijacked Server
> > >        11    TCP packet with any control flags set
> > >        12    Limit on number of consecutive small segments reached
> > >        13    4-way handshake detected
> > >        14    Packet missing timestamp
> > >
> > >
> > >        [ yes, there's a typo up there where it says 10 anomalies and then
> > >        shows 14 of
> > >        them ;) ]
> > >
> > >
> > >    It's actually more than that:
> > >
> > >    $ grep "^129" ../etc/gen-msg.map
> > >    129 || 1 || stream5: SYN on established session
> > >    129 || 2 || stream5: Data on SYN packet
> > >    129 || 3 || stream5: Data sent on stream not accepting data
> > >    129 || 4 || stream5: TCP Timestamp is outside of PAWS window
> > >    129 || 5 || stream5: Bad segment, overlap adjusted size less than/equal 0
> > >    129 || 6 || stream5: Window size (after scaling) larger than policy allows
> > >    129 || 7 || stream5: Limit on number of overlapping TCP packets reached
> > >    129 || 8 || stream5: Data sent on stream after TCP Reset
> > >    129 || 9 || stream5: TCP Client possibly hijacked, different Ethernet Address
> > >    129 || 10 || stream5: TCP Server possibly hijacked, different Ethernet Address
> > >    129 || 11 || stream5: TCP Data with no TCP Flags set
> > >    129 || 12 || stream5: TCP Small Segment Threshold Exceeded
> > >    129 || 13 || stream5: TCP 4-way handshake detected
> > >    129 || 14 || stream5: TCP Timestamp is missing
> > >    129 || 15 || stream5: Reset outside window
> > >    129 || 16 || stream5: FIN number is greater than prior FIN
> > >    129 || 17 || stream5: ACK number is greater than prior FIN
> > >    129 || 18 || stream5: Data sent on stream after TCP Reset received
> > >    129 || 19 || stream5: TCP window closed before receiving data
>
>
>
> ------------------------------------------------------------------------------
> Virtualization & Cloud Management Using Capacity Planning
> Cloud computing makes use of virtualization - but cloud computing 
> also focuses on allowing computing to be delivered as a service.
> http://www.accelacomm.com/jaw/sfnl/114/51521223/
> _______________________________________________
> 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
>
> Please visit http://blog.snort.org to stay current on all the latest Snort news!


------------------------------------------------------------------------------
Virtualization & Cloud Management Using Capacity Planning
Cloud computing makes use of virtualization - but cloud computing 
also focuses on allowing computing to be delivered as a service.
http://www.accelacomm.com/jaw/sfnl/114/51521223/
_______________________________________________
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

Please visit http://blog.snort.org to stay current on all the latest Snort news!