[Snort-users] Snort frustration

Humes, David G. David.Humes at ...383...
Fri Jun 2 12:32:38 EDT 2006

I added this rule to look for Google Desktop traffic.

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"Google
Desktop User-Agent detected"; flow:established,to_server; content:"GET
"; offset:0; depth:4; content:"|0d 0a|User-Agent\: Mozilla/4.0
(compatible\; Google Desktop)"; nocase; threshold:type limit,track
by_src, count 1,seconds 300; classtype:policy-violation; sid:8001018;

And it appears to have fired on this packet.

Generated by BASE v1.2.4 (melissa) on Fri,  2 Jun 2006 12:14:38 -0400

#(1 - 3031457) [2006-06-02 11:46:34] [local/8001018] [snort/8001018]
Google Desktop User-Agent detected
IPv4: ->
      hlen=5 TOS=0 dlen=83 ID=9124 flags=2 offset=0 TTL=126 chksum=65404
TCP:  port=2181 -> dport: 80  flags=***AP*** seq=395991789
      ack=781565658 off=5 res=0 win=65535 urp=0 chksum=64289
Payload:  length = 43

000 : 47 45 54 20 2F 64 73 6E 65 77 73 3F 6A 3D 36 26   GET /dsnews?j=6&
010 : 68 6C 3D 65 6E 26 65 64 3D 63 6F 6D 26 76 3D 32   hl=en&ed=com&v=2
020 : 20 48 54 54 50 2F 31 2E 31 0D 0A                   HTTP/1.1..

This is a continual frustration, where a rule fires and the payload does
not match.  Most of the time, the payload does match.  But, it's not
hard to find instances like the one above.  Can anyone offer some
reasons why this may be occurring and what can be done to correct the
problem.  Was there some payload that really did match and it's just
showing the wrong payload?  Or did the detection engine just mess up?  

We're running Version 2.4.3 (Build 26), but this problem has been with
us through two generations of Snort senors and numerous versions.  We
use Barnyard and unified logging, but the problem was seen before we
implemented Baryard.  Any help would be greatly appreciated.


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