[Snort-users] Very high amount of "TCP Small Segment Threshold Exceeded"

Russ Combs rcombs at ...1935...
Wed Feb 29 08:08:48 EST 2012


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 at ...11827...> 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 at ...14940...>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
>
>
>>
>>
>> ------------------------------------------------------------------------------
>> Keep Your Developer Skills Current with LearnDevNow!
>> The most comprehensive online learning library for Microsoft developers
>> is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3,
>> Metro Style Apps, more. Free future releases when you subscribe now!
>> http://p.sf.net/sfu/learndevnow-d2d
>> _______________________________________________
>> Snort-users mailing list
>> Snort-users at 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!
>>
>
>
> ------------------------------------------------------------------------------
> Keep Your Developer Skills Current with LearnDevNow!
> The most comprehensive online learning library for Microsoft developers
> is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3,
> Metro Style Apps, more. Free future releases when you subscribe now!
>
> http://p.sf.net/sfu/learndevnow-d2d_______________________________________________
> Snort-users mailing list
> Snort-users at 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!
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.snort.org/pipermail/snort-users/attachments/20120229/18675a01/attachment.html>


More information about the Snort-users mailing list