[Snort-users] Help with noisy alerts for known application

Geoff Sweet geoff.sweet at ...15218...
Sun Apr 10 22:27:19 EDT 2011


So I added a filter that blocked out our game ports.  I can see in the process list that snort is running with the -F option and I know it's loading the file since I put a little error in the bpf file and it errored out on it.  The filter that I put is:

not port 15779 or not port 7000 or not port 7100 or not port 7101 or not port 7200 or not port 7201 or not port 7202 or not port 7203 or not port 7204 or not port 7205

Seems pretty simple and straight forward.  But still I see TONS of alerts on these ports as "(portscan) Open Port".  I don't get it.

Help?

-Geoff

From: Joel Esler [mailto:jesler at ...1935...] 
Sent: Friday, April 08, 2011 12:05 PM
To: Geoff Sweet
Cc: snort-users at lists.sourceforge.net
Subject: Re: [Snort-users] Help with noisy alerts for known application

I would try a bpf statement instead ignoring those ports.  The pass rules will make the rules engine not process traffic.  But the portscan preprocessor is further up the Snort stack, so pass rules don't cover those.  

Check out a bpf.

Joel
On Fri, Apr 8, 2011 at 2:26 PM, Geoff Sweet <geoff.sweet at ...15218...> wrote:
When we first implemented Snort we found that we were generating tons of alerts from our games. That was to be expected and so we started digging in to try to quiet down the alerts.  The very first thing that we trimmed was the "COMMUNITY SIP TCP/IP message flooding directed to SIP proxy" alert that was thrown for basically every single connection to our game.  A bit of reading in the old snort forum said that getting rid of that rule was ok so I commented it out of the rule file.  So after a bit of reading online I came up with two rule files that describe our two primary games, and from the reading set them to "pass" so that Snort would recognize the traffic and quietly pass it.  The rules look like this:
 
/etc/snort/rules$ cat wemade-mir3.rules
pass tcp $EXTERNAL_NET any -> any 7000 (msg:"MIR3 Application";)
pass udp $EXTERNAL_NET any -> any 7000 (msg:"MIR3 Application";)
pass tcp $EXTERNAL_NET any -> any 7100 (msg:"MIR3 Application";)
pass udp $EXTERNAL_NET any -> any 7100 (msg:"MIR3 Application";)
pass tcp $EXTERNAL_NET any -> any 7101 (msg:"MIR3 Application";)
pass udp $EXTERNAL_NET any -> any 7101 (msg:"MIR3 Application";)
pass tcp $EXTERNAL_NET any -> any 7200 (msg:"MIR3 Application";)
pass tcp $EXTERNAL_NET any -> any 7201 (msg:"MIR3 Application";)
pass tcp $EXTERNAL_NET any -> any 7202 (msg:"MIR3 Application";)
pass tcp $EXTERNAL_NET any -> any 7203 (msg:"MIR3 Application";)
pass tcp $EXTERNAL_NET any -> any 7204 (msg:"MIR3 Application";)
pass tcp $EXTERNAL_NET any -> any 7205 (msg:"MIR3 Application";)
pass udp $EXTERNAL_NET any -> any 7200 (msg:"MIR3 Application";)
pass udp $EXTERNAL_NET any -> any 7201 (msg:"MIR3 Application";)
pass udp $EXTERNAL_NET any -> any 7202 (msg:"MIR3 Application";)
pass udp $EXTERNAL_NET any -> any 7203 (msg:"MIR3 Application";)
pass udp $EXTERNAL_NET any -> any 7204 (msg:"MIR3 Application";)
pass udp $EXTERNAL_NET any -> any 7205 (msg:"MIR3 Application";)
 
/etc/snort/rules$ cat joymax-silkroads.rules
pass tcp $EXTERNAL_NET any -> any 15779 (msg:"Silkroads Online";)
pass tcp $EXTERNAL_NET any -> any 12989 (msg:"Silkroads Online";)
pass tcp $EXTERNAL_NET any -> any 15021 (msg:"Silkroads Online";)
pass tcp $EXTERNAL_NET any -> any 15020 (msg:"Silkroads Online";)
 
The problem at this point is that every connection to the games generates a portscan alert.  I have over 220K of them in a 12 hour period.  I was under the assumption from the documentation that by creating this rule with the specific ports listed and the action as "pass" that snort wouldn't raise an alert.  Am I doing something wrong with this rule?  All the alerts are marked with the signature "(portscan) Open Port: [whatever game port from above]" and links to http://www.snortid.com/snortid.asp?QueryId=122-27
 
Any help would be greatly appreciated.
 
-Geoff
 

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Joel Esler | http://blog.snort.org | http://vrt-blog.snort.org | http://blog.clamav.net
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