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Re: How to write rules for non-TCP (LLC) packets?
From: Joel Esler <jesler () sourcefire com>
Date: Tue, 23 Apr 2013 13:32:02 -0400

On Apr 23, 2013, at 1:11 PM, Eric Fowler <eric.fowler () gmail com> wrote:

I have a connection between two devices with fixed, known IP addresses bound to fixed, known MAC addresses, that are 
communicating on known IP ports. The traffic going both ways is UDP, but when I snort the packets, the adapter (or 
driver, not clear) is messing with the headers and confusing snort, wireshark, and all other pcap applications I can 
find. In particular,snort and wireshark are not able to detect these packets as being UDP, and can't see IP 
addresses, even though they are embedded in the packets (the interface adds 12 bytes of header upstream). 

I have tried configuring the interface *not* to do this but that has ultimately been fruitless. Now I am trying to 
work with what I have. 

I have noticed that the packets I need are flagged in Wireshark as protocol LLC. I am able to extract (in wireshark) 
a filter with the MAC addresses, and the MAC addresses look fine in the display. 

I wish to find a way to trap all traffic coming to/from these mac addresses that "looks like" LLC packets, and find 
and print the payload data.

I am using snort rules, but since snort only understands TCP, UDP, etc. as protocols (not LLC, too low level), none 
of the alerts fire. 

So the question at long last is: how can I write a snort rule that will alert on all packets (1) coming from a given 
mac address or (2) with certain bytes (IP addresses) at certain offsets or (3) that look like LLC packets? 

try "alert ip"

Joel Esler
Senior Research Engineer, VRT
OpenSource Community Manager
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