RE: IDS evaluations procedures

["Nathan Davidson" <ndavidso () globix com>]

Hi Adam,

 

I am sure Tim can answer this one very well, but over the last 12 months I have spent a lot of time working with IPS in 
an IDS orientated company. So I thought I share my experiences. 

 

When we deploy an in-line IPS solution we define a number of parameters in the policy that should be present in ALL 
valid requests (White-listing). I use this to filter out all traffic that I know must be malicious. From my experience 
this is up to 95% of worm/scan traffic. We then apply IDS style signatures based on known attack vectors 
(Black-listing) but only on the remaining 5% of traffic. Thus we should have up to 95% less false positives (and 
generally we do). Additional benefits can be gained by dropping all subsequent packets from an abusing source IP 
address. 

 

An example would be to use an IPS to force all HTTP requests to have the host header www.xyz.com (your sites URL) this 
will stop a significant proportion of HTTP noise before signature matching.

 

Conversely with IDS you just don’t have the ability to white list traffic in this way, I guess you could RST any 
request that didn’t match the URL but I think fragmented buffer overflows and the like could sneak through - so it’s 
risky.

 

As you alluded to, the IPS signatures tend to be less aggressive than those on the IDS which I think reflects the much 
higher penalty of false positives on an in-line blocking device. For this reason I do still deploy NIDS/HIDS on the 
inside to collect forensic data, with the added benefit of having a second manufacturers signatures.

 

 

Internet 

     I

   IPS

     I

Firewall

     I

     I 

Switch=== NIDS

     I

     I

HIDS

Server

 

 

Hope that helps

 

Nathan Davidson

Senior Architect

Globix Corp.

London

 

-----Original Message----- 
From: Adam Powers [mailto:apowers () lancope com] 
Sent: Wed 13/07/2005 19:00 
To: THolman () toplayer com; David.Sames () sparta com; focus-ids () securityfocus com 
Cc: 
Subject: Re: IDS evaluations procedures

Tim, I hate to stir up this whole can of worms (pun alert) and yes I know
this is off topic but can you please qualify this seemingly non sequitur
statement?

"All IDS devices are subject to large numbers of false positives, which is
why if you're making a new investment you should consider IPS technology, as
this gives you a far lower TCO and real-world protection against zero-day
threats."

How so?

I really struggle with this whole "because it's inline it must be more
accurate" thing. Sure, if I turn off a bunch of sigs on the IPS that are
less reliable, accuracy will increase. But why not do the same thing on the
non-inline IDS?

Is there something magical about being inline that makes the system less
prone to false positives? If so, what?

----------

David, addressing your original question... (which, incidentally, was about
INTERNAL attack traffic, not Internet Storm Center quality stuff that's
randomly hitting the outside of your firewall), we'll need a few extra data
points.

1. What are you testing for? Traffic-based anomalies? Application level RFC
violations and anomalies? Relational-modeling anomalies?
Behavioral-anomalies?

2. What collection mechanism is employed? NetFlow? sFlow? Ethernet Frames?
Other?

3. Are you only interested in classic "attacks" (fire up Nessus, see what
happens) or other anomalies such as malfunctioning applications,
policy-driven anomalies, etc?





On 7/13/05 3:33 AM, "THolman () toplayer com" <THolman () toplayer com> wrote:

> Hi Dave,
>
> Take a peek at the Internet Storm Centre @ SANS -
>
> http://isc.sans.org/
>
> Gives you a good idea about what's going on.
> Which IDS devices are you looking at?  All IDS devices are subject to large
> numbers of false positives, which is why if you're making a new investment
> you should consider IPS technology, as this gives you a far lower TCO and
> real-world protection against zero-day threats.  It also saves you having to
> buy lots of IDS sensors, seeming a large proportion of the load will be
> absorbed and taken care of by the IPS.
> Just my 2 cents.. ;)
>
> Cheers,
>
> Tim
>
> -----Original Message-----
> From: Sames, David [mailto:David.Sames () sparta com]
> Sent: 13 July 2005 04:54
> To: THolman () toplayer com
> Subject: RE: IDS evaluations procedures
>
> Thanks for the info - those are exactly the kinds of characteristics I
> need to consider - at this point, I'm not evaluating a product per se -
> I'm evaluating some claims by some of our researchers :-) FP's are what
> I'm most concerned about. I'll check things out to see if I can get more
> stats - and of attempt to produce some data sets that may look like
> "anomalies" but are really traffic spikes and shouldn't be flagged.
>
> > >
>
> To specifically answer your question, look at current attack weather
> reports
> - you'll see approximately 15-20% of perimeter traffic is in fact worms
> trying to propagate.  Any evaluation should be designed with this in
> mind.
> ..but more importantly, make sure you're evaluating something that will
> do
> the job in hand and doesn't lead you up the garden path with inaccurate
> marketing collateral!  :)
> <<<
>
> That's exactly what I was looking for !
>
> Regards,
>
> Dave
>
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