Re: Handling firewall rejected packets in SNort IPS

[VES Education <veseducation () yahoo com>]

Thanks for the reply.

point 1

"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..."

Would it result in performance loss as packet is coming to layer 2 then layer 7 then again layer 2 then layer 7 for 
actual application.

If packet is allowed by Snort I am not sure how it will come back to firewall again.

It seems to be tricky thing. Would you mind you to clarify. 
 
Thanks,
B.Vijayakumar Athithan


--- On Fri, 17/5/13, waldo kitty <wkitty42 () windstream net> wrote:

From: waldo kitty <wkitty42 () windstream net>
Subject: Re: [Snort-users] Handling  firewall rejected packets in SNort IPS
To: snort-users () lists sourceforge net
Date: Friday, 17 May, 2013, 1:56 PM

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

OR

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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