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

[Russ Combs <rcombs () sourcefire com>]

If you can trigger the alerts, can you capture a pcap that reproduces the
problem?  Maybe we can tweak the settings based on that.

On Wed, Feb 29, 2012 at 3:40 AM, Giacomo <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>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
>
>
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