[Snort-users] Asymmetric Data

Matt Kettler mkettler at ...4108...
Fri Jul 18 12:26:03 EDT 2003

At 02:06 PM 7/18/2003 -0400, Michael L. Artz wrote:
>How well does snort handle asymmetric data, i.e. an incoming link but no 
>outgoing link?  I figure that most of the signatures should be fine, since 
>most of them are looking for content and/or packet flags, but what about 
>the preprocessors, such as stream4?  Are there certain preprocessors that 
>should be left turned off if snort is only seeing one side of the 
>traffic?  Any suggestions on how to best tune snort given only one side of 
>a link?

Actualy the signatures will break as well.. Any signature which uses flows 
will fail, which is most of them in 2.0.0.

Sorry, but the stateful ruleset helps avoid false positive and some 
avoidance cases, but requires snort to see the whole 3-way handshake of TCP 
establishment in order for it to know which side is the server, which is 
the client, and if the connection is established or not.

Stream4 will also be defunct without the whole traffic stream, as it will 
usually flush when data is sent in the "other direction" than the currently 
buffered part of a stream.

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