[Snort-users] Alert explanations

Martin Roesch roesch at ...1935...
Wed Aug 11 13:40:06 EDT 2004

Those are anomaly notifications coming from Snort's preprocessors, 
they're pretty self explanatory.  "Possible retransmission" means that 
the stream reassembler thinks it has seen the packet before and is 
letting you know (this can be someone trying to evade the stream 
reassembler/IDS).  "Overlapping fragment" messages are from the IP 
defragmenter, it's telling you that it's seeing fragments that overlap 
which is another anomaly and a possible indication that someone is 
trying to evade your IDS.

We should probably go through and document all the events that can be 
generated by the preprocessors one of these days...


On Aug 8, 2004, at 10:39 PM, Sean Brown wrote:

> Over the past few days, I'm seeing a lot of 'Possible RETRANSMISSION
> detection' and 'Overlapping new fragment (probable fragroute)' showing 
> up. I
> looked for a good explanation of what might be going on, but nothing 
> really
> jumped out, so are there any good sites that I could go to to read up 
> on
> these things?
> Thanks
> -Sean Brown
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Martin Roesch - Founder/CTO, Sourcefire Inc. - (410)290-1616
Sourcefire: Intelligent Security Monitoring
roesch at ...1935... - http://www.sourcefire.com
Snort: Open Source Network IDS - http://www.snort.org

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