Re: Fast [SYN], [FIN, ACK] to port 80 with no data

[Gulfie <gulfie () grotto-group com>]

On Tue, Jan 10, 2006 at 09:23:46AM -0500, Marc Cozzi wrote:
> I am observing IP hosts on a network here that are opening
> connections [SYN], to port 80(http) on remote systems then
> immediately closing [FIN,ACK] the connection or usually within
> about 90ms. Then again within a min or so, repeat. It
> also seems there is zero data exchanged between the connected
> systems. I'm guessing it is a status polling application
> that is determining if a remote system is up or down.
> Probably not a necessary activity on the network and possibly
> interpreted by remote network managers as scans/probes/mapping
> attempts.
>
> Question is:
> Is there a way to detect, within snort, this rapid opening and
> closing of remote port 80 and fire off an alert. Also, maybe
> fire off a block of the remote system?
>
> Thanks for any ideas.
 
1) Use flow bits, to locate empty connections.
        set a this_stream_isempty bit on session construction
        unset it whenever there is server bound data. 
        this_stream_isempty and you see fin packets, alert. 
        

2) Optionaly use thresholding to clamp the number of events. 
or trigger some blackholeing.
 

        This may not be the best way, but it is a way. 

                                -gulfie


>   --marc
>
>
>
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