[Snort-users] How to write rules for non-TCP (LLC) packets?

Russ Combs rcombs at ...1935...
Tue Apr 23 16:08:18 EDT 2013

The datalink type in the pcap is ethernet but that doesn't match the outer
layer encapsulation.  You need some way to skip over those first 12 bytes.
 It is fairly easy to patch Snort to do that, but having a different DLT to
key off of would be best.

On Tue, Apr 23, 2013 at 1:49 PM, Eric Fowler <eric.fowler at ...11827...> wrote:

> Tried that. It misses 'em.
> The LLC designation is wrong, I have seen other packets marked as IPX and
> containing valid data, which is totally impossible. So the headers are
> getting munged and wireshark is getting confused.
> Maybe I need a lower level tool.
> Eric
> On Tue, Apr 23, 2013 at 10:32 AM, Joel Esler <jesler at ...1935...>wrote:
>> On Apr 23, 2013, at 1:11 PM, Eric Fowler <eric.fowler at ...11827...> 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
>> Sourcefire
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