[Snort-users] IDS Placement ideas for inside and outside a fi rewall.

Ponte, Paul F pponte at ...8780...
Thu Apr 3 19:04:25 EST 2003


Hi all -
  I'd like to ask your opinions on one part of this question.  When we talk
about a sensor on the inside of the firewall, I assume that means it can see
all traffic on the internal subnet.  But what do you give up if you monitor
just traffic passing on a VLAN between the firewall and the router sitting
between it and the rest of the network?  Is this a valid installation?
What's the danger in not monitoring all of the normal host to host traffic
on your network which doesn't need to cross the firewall?  I'm considering
this kind of deployment, so thanks for your opinions on this.
 
Paul
-----Original Message-----
From: Brian Laing [mailto:Brian.Laing at ...8609...] 
Sent: Thursday, April 03, 2003 5:58 PM
To: 'Brei, Matt'; 'David Glosser'; 'FWAdmin';
snort-users at lists.sourceforge.net
Subject: RE: [Snort-users] IDS Placement ideas for inside and outside a
firewall.


It can help, but I would not rely on it for prosecution the fact is the data
is too easy to spoof and is not collected in a forensically sound manager
either at the sensor or the management console.  By forensically sound I
mean certified to be free from tampering.  Not that this data wont help your
case, but its better to rely on it to see where and into what else the
attacker may have gotten into.
 
-------------------------------------------------------------------
Brian Laing
CTO
Blade Software
Cellphone: +1 650.280.2389
Telephone: +1 650 367.9376
eFax: +1 208.575.1374
Blade Software - Because Real Attacks Hurt
http://www.Blade-Software.com <http://www.Blade-Software.com> 
-------------------------------------------------------------------
-----Original Message-----
From: Brei, Matt [mailto:mbrei at ...8727...] 
Sent: Thursday, April 03, 2003 2:18 PM
To: brian.laing at ...8607...; David Glosser; FWAdmin;
snort-users at lists.sourceforge.net
Subject: RE: [Snort-users] IDS Placement ideas for inside and outside a
firewall.
 
That's exactly why I would want one outside of the firewall.  If I were to
find a successful break in, I could then review logs from the external IDS
and find that the same IP had done several scans or whatever that were
eventually blocked by the firewall and not picked up by the internal IDS.  I
would think that this would help build a better case if any type of legal
action were to be taken. 
 
Matt
 
-----Original Message-----
From: Brian Laing [mailto:Brian.Laing at ...8609...] 
Sent: Thursday, April 03, 2003 11:28 AM
To: 'David Glosser'; Brei, Matt; 'FWAdmin';
snort-users at lists.sourceforge.net
Subject: RE: [Snort-users] IDS Placement ideas for inside and outside a
firewall.
 
I would agree with this sort of implementation, in many of the installs I
have done I will setup the external sensors to do nothing but logging and
ignore the data till I see something worth looking at on one of the internal
servers.  I use this data to see what else that IP has been doing or what
other things have been attempted against a specific host
 
-------------------------------------------------------------------
Brian Laing
CTO
Blade Software
Cellphone: +1 650.280.2389
Telephone: +1 650 367.9376
eFax: +1 208.575.1374
Blade Software - Because Real Attacks Hurt
http://www.Blade-Software.com <http://www.Blade-Software.com> 
-------------------------------------------------------------------
-----Original Message-----
From: snort-users-admin at lists.sourceforge.net
[mailto:snort-users-admin at lists.sourceforge.net] On Behalf Of David Glosser
Sent: Wednesday, April 02, 2003 11:10 PM
To: Brei, Matt; FWAdmin; snort-users at lists.sourceforge.net
Subject: Re: [Snort-users] IDS Placement ideas for inside and outside a
firewall.
 
If you've never set up any IDS before, I'm not sure you would want to place
it outside your firewall immediately You'lll get overwhelmed with
probes,scans, script kiddies etc. 
First place the box (with the "snorting" NIC unnumbered). On the port
monitoring the *internal* interface of your firewall. Let it work on all of
the stuff your firewall lets through. Once you have that under control, then
place another box (or another NIC on the same box) to monitor your internal
servers (since breakins can come from internal users). 
Once you have these two under control, then you can worry monitoring stuff
outside the firewall,  which I believe is called *attack detection*. But do
you care that much about the stuff your firewall is successfully blocking?
 
--snip-
 I am trying to convince my company to implement IDS on our network but I
have a few questions. I know I would want one on both sides of the firewall,



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