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Re: Handling firewall rejected packets in SNort IPS
From: waldo kitty <wkitty42 () windstream net>
Date: Fri, 17 May 2013 09:56:04 -0400

On 5/17/2013 02:54, VES Education wrote:

This is very basic qtn on Snort IPS. Over last few days,I couldn't find answer
on net. Our intention is find packet flow in our application.

it would seem to be pretty basic, but i'm not so sure about that ;)

We would like to use Snort IPS( Currently we use Snort IDS). If we go for inline
mode, whether all incoming packets would be placed in NF queue by firewall.
Suppose a packet is getting rejected in firewall layer how Snort IPS will come
to know it.

As per current my understanding, if a packet is rejected in firewall, it will
not go to SNort IPS. Hence IDS feature is missing in Snort IPS mode.

this depends on where, in your firewall routing rules, you inject the rule to 
send the traffic to snort...

consider a firewall script that starts off as

iptables -A Input jmptosnort
iptables -A Input jmptogood


iptables -I Input jmptosnort
iptables -I Input jmptogood

in both cases, the actual ordering is not going to be what you expect it to 
be... jmptosnort will be last in line and everything else will be acted on first...

what you really want is to force/ensure that the jmptosnort rule is at the top 
of the chain so that it is entered first and then anything that passes snort 
will come back to traverse the rest of the rules...

NOTE: yes, this is a very simplistic and incomplete example... there's a reason 
for that ;) O:)

That means we need to use both Snort IDS mode and inline mode. Is that possible
to run two instances of snort in different modes in same machine.

you don't need to do this if you get your firewall rules in the proper order ;)

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