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Re: Snort only partially alerting
From: James Lay <jlay () slave-tothe-box net>
Date: Tue, 18 Jun 2013 16:13:15 -0600

On 2013-06-18 15:14, Frank Calone wrote:
I still dont have a fix yet to the problem of Snort only alerting
occasionally.  I have it setup to look for exe downloads using just 2
rules.  I have a web site setup to download (not https) an exe
file.  I decided to run snort in full packet logger mode to see what
was coming in (/usr/sbin/snort -dev -i p1p1 -l /var/log/snort -h
x.x.x.x/16).  I immediately started getting the following warning
messages:
 
(snort_decoder) WARNING: IP dgm len > captured len
 
I then ran the binary capture thru the snort playback (-dvr option). 
Looking at the packets tied to my PC, I can see that almost all of
them have a datagram length of 40.  Very few packets showed up with a
real payload, certainly not enough to amount to the size of the file 
I
downloaded during the testing.  Im not sure if there is a config
setting or something else going wrong here such that very few packets
have any real data.  Here is a sample of what I am seeing:
 

=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

06/18-16:20:19.229724 15.0.0.18:62287 [1] -> 212.13.197.229:80 [2]
TCP TTL:127 TOS:0x0 ID:7467 IpLen:20 DgmLen:40 DF
***A**** Seq: 0x3279955A  Ack: 0xEF27E0F7  Win: 0x4029  TcpLen: 20

=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

Frank


Frank,

Try capturing with tshark or tcpdump (use -s 0 for tcpdump to capture 
the full packet.  Then, after capturing, run it through snort with 
something like:

sudo snort -c snort.conf -r bleh.pcap -k none

James

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