Re: frag3 Fragmentation overlap Alert

[Joel Esler <eslerj () gmail com>]

On Jan 8, 2009, at 5:13 PM, Michael Green allegedly wrote:

> Hi
>
> I'm running snort 2.8.3.1 on CentOS 5. I'm getting a lot of alerts  
> from frag3 for a Fragmentation overlap. After doing a packet capture  
> I can confirm that there is indeed Fragmentation overlap or at least  
> repeats of the same fragment. Would snort count that as an overlap?
>
> Anyway it's one of my Cisco ASAs doing a large OSPF LS update and  
> while I'd like to stop the firewall sending multiple fragments I  
> also want to stop snort alerting on this!
>
> In my frag3 config I have the following specifically for my Cisco  
> devices, I also have other policies for my other devices:
>                 preprocessor frag3_engine: policy last bind_to  
> [<list of cisco ip's>]
>
> Note I don't have "detect_anomalies" for this particular policy. I  
> thought that this would stop the alerts, but apparently not.
>
> I would appreciate any suggestions on how to stop this? Also if  
> "detect_anomalies" doesn't work as I thought what is it for?


Two things to remember, are you seeing fragmentation overlap when you  
look at the packets?  If so, of course Snort is going to alert.

Second thing is, remember that if you have multiple policy lines  
within the frag3_engine piece of the frag3 configuration, you are  
going to want to go from "most specific" to "least specific".  Please  
see the README.frag3 on the doc/ directory of the Snort tarball.

--
Joel Esler
  http://www.joelesler.net
  http://www.twitter.com/joelesler
[m]

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