[Snort-users] high count, long time in threshold

Marc Norton marc.norton at ...1935...
Wed Aug 11 07:04:58 EDT 2004

The rule to catch 1500 syns in 60 seconds is applied to each individual
host that the rule covers - hence the 'track by_src' it does not track
1500 syns in 60 seconds for 'any-ip -> any-ip' in a cumulative fashion.
So unless a specific IP is generating 1500 syns in 60 seconds, this
threshold definition won't allow the rule to fire and be logged. I am
not sure if this helps but I wanted to be clear about the difference in
behavior that you may be seeing. 

> -----Original Message-----
> From: snort-users-admin at lists.sourceforge.net [mailto:snort-users-
> admin at lists.sourceforge.net] On Behalf Of sekure
> Sent: Wednesday, August 11, 2004 8:42 AM
> To: Snort Users
> Subject: [Snort-users] high count, long time in threshold
> Hi all,
> For the past few days i've been trying to figure out a rule to alert
> me whenever there is a large # of SYNs going by the sensor.  This
> traffic is specific to something on my network and is usually directed
> to one particular port, so using portscan or flow_portscan
> preprocessors is out of the question, at least based on my
> understanding.
> Normally i see about 10-15 SYN's/second on my network, but
> occasionally it gets to 40, 80, even 100.  Definitely abnormal.  At
> first I tried this:
> alert tcp $HOME_NET any -> any any (msg: "High SYN Traffic"; flags:S;
> threshold: type threshold, track by_src, seconds 1, count 40;
> classtype:misc-activity; sid: 1000035; rev:1;)  So 40 SYNs in one
> second and I'd get an alert.  This worked flawlessly.  The problem
> though was that this traffic would be sustained for about 4-5 hours,
> so in the morning i'd end up with hundreds of alerts.  So I tried a
> variation:
> alert tcp $HOME_NET any -> any any (msg: "High SYN Traffic"; flags:S;
> threshold: type both, track by_src, seconds 60, count 1500;
> classtype:misc-activity; sid: 1000035; rev:1;)  Alert once per 60
> seconds if there are more than 1500 SYNs in that time interval.
> That's an average of 25 SYNs/second.  Definitely abnormal on my
> network, so I'd like to catch it.
> HOWEVER, this rule doesn't really work.  For a few days it was
> occasionally alerting me to portscans that scanned 5 hosts for 1 port,
> so at most maybe 30 SYNs (after all the retries, etc). And then this
> morning, when i looked at my perfmon preprocessor statistics I saw a
> sustained SYN rate of about 80 SYNs/sec for 5 hours overnight, but NO
> alerts.
> Help?
> Is there a limit to how high i can set a count or a time in a
> threshold rule?  Is snort running out of memory trying to keep track
> of the number of SYNs send by EVERY host in a given time period?
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