Re: IDS deployment outside FW?

[Dr Bit Bucket <drbitbucket () comcast net>]

Chris,

If all possible, your IDS sensors should never be active on the 
network that they are monitoring (unless you're doing some sort of 
active response, such as with an IPS solution).  Have the management 
interface on a private network exclusive to security functions and 
not assign an IP address to the monitored interface(s).  By not 
having the management interface on this non-routable network, you 
mitigate the issues with dual-homed hosts.  If you use passive 
network taps, that's even better.  100TX taps are $355 a pop these 
days (netoptics, with send/receive split into two RJ-45 interfaces. 
Under Linux use channel bonding to recombine the traffic into a 
virtual interface and direct the IDS to that interface), while 1000SX 
taps are $550 each (the receiving NICs are equally expensive for 
fiber and a quad 100TX card runs about $400).  Pretty cheap and far 
better than spanning ports (in terms of packet drops during peak 
volume).

To deal with the extra noise, the main focus of a sensor out there is 
to monitor attacks against your perimeter devices.  You should be 
able to craft the monitoring rules to focus on those devices.  If 
there are other benefits to monitoring in that location, I'd like to 
hear from others on the list.

Jon Repaci
GCIA, CISSP

At 7:33 PM +0000 8/9/04, Chris Conacher wrote:
> Dear List
>
> I have moved into an organization that has two RealSecure Network 
> Sensors and a network architecture that is VLANd/DMZd to where 
> localized deployment to capture traffic would require 8 to 12 
> sensors to avoid bridging loops.
>
> The cheapest/simplest option (without deploying SNORT/Prelude, etc - 
> the organization wants to remain on a single application 
> architecture where possible) is to place the two sensors outside of 
> the firewall.
>
> I understand that this means:
> The sensors will be in hostile territory and need to be maintained 
> to a very high degree
> There will be an operations overhead of dealing with all of the 
> noise that would normally be filtered by a firewall
>
> Does anyone have experience of doing this?
> Are there any other issues that I have not considered?
>
> Chris
>
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