Re: Variable to specify 2 networks and exclude 1 host - How?

[Matt Kettler <mkettler () evi-inc com>]

Simon Yip wrote:
> Thanks PaulM.
>
> That could be a solution while I hope to have something like
> ![IP=155.200.2.10,!$HOME_NET]
> I have tried my above expression in a rule but it just give me a fatal
> error.
> where HOME_NET= [155.200.2.0/24,155.200.3.0/24]
>
> My purpose is to filter out a particular type of false positives alerts
> created by a host (its cron job generated the alert)  but I do not want
> to ignore all this type of alert.


Usually you wind up having to create a pass rule to ignore traffic to the single
host.

AFAIK there is no way to create a "range with an arbitrary hole in it" as a
single specifier in snort, but your idea is a good one.

You could try to rephrase the above declaration as this:
![155.200.2.10/32,!$HOME_NET]

Note the removal of the "IP=" syntax and instead I'm using /32 to limit it to
one IP. Snort's parser might be happier with that.

For those who wonder how this could work, In theory this parses as:

NOT ( 155.200.2.10/32 OR NOT HOME_NET)

Which if you do a DeMorgan's theorem conversion on it you get:

(NOT 155.200.2.10/32) AND (HOME_NET)

However, You might wind up having snort's parser barf with that, it might not
want to do anything that complex, I've never tried...

Try it and see.





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