RangerIDS - Intrusion Detection System (IDS)



Network IPS (NIPS)


The Network IPS combines features of a standard IDS, an IPS and a firewall, and is sometimes known as an In-line IDS or Gateway IDS (GIDS)


As with a typical firewall, the NIPS has two network interfaces, one designated as internal and one as external. As packets appear at the internal interface they are passed to the detection engine, at which point the IDS device functions much as any IDS would in determining whether or not the packet being examined poses a threat. However, if it should detect a malicious packet, in addition to raising an alert, it will discard the packet and mark that flow as bad. As the remaining packets that make up that flow arrive at the IPS device, they are discarded immediately.  


Legitimate packets are passed through to the internal interface and on to their intended destination. A useful side effect of some NIPS products is that as a matter of course - in fact as part of the initial detection process - they will provide “packet scrubbing” functionality to remove protocol inconsistencies resulting from varying interpretations of the TCP/IP specification (or intentional packet manipulation). Thus any fragmented packets or packets with obfuscated URLs will be “cleaned up” before being passed to the destination host. 


Seems too good to be true, doesn’t it? Why bother with an IDS at all, since the NIPS device does everything an IDS can do as well as provide true prevention capabilities? Well, when something seems to good to be true, it usually is


Firstly, there is the aforementioned potential for an in-line device to introduce a Denial of Service condition by accidentally blocking a legitimate packet flow. If the NIPS is at the gateway to your DMZ containing your e-commerce servers, you had better be sure that none of your attack signatures are susceptible to false positives.  


For this reason, most adopters of this technology would probably run the NIPS with a reduced signature set that contained only those signatures that had a very low or zero propensity for triggering accidentally on benign traffic. They would then run a standard IDS product on the protected network behind the NIPS which utilised a more complete signature set in order to deal with the alerts that were raised by traffic not covered by the NIPS itself. 


The second potential problem is that given the amount of work this device has to do, it can act as a serious bottleneck when installed at the gateway to your network. Most NIPS vendors have recognised this and have sought to get around it by using custom hardware, populated with expensive FPGAs and ASICs. At the time of writing, therefore, adopters of this technology tend to require deeper pockets than those buying plain old NIDS products. 


One thing to watch out for - don’t let the “reactive” IDS vendors kid you into believing that they have intrusion prevention capabilities just because they can send TCP reset commands when they detect an attack (a worrying piece of FUD that we have noticed in some IDS marketing literature recently). That does not count as prevention because by the time the IDS has detected it, the attack payload may already been delivered. Certainly the initial packet that caused the alert will have reached its target, and if the payload is contained in a single packet…. game over! On high speed networks, it would also be possible to deliver an entire payload spread across many packets before the TCP reset command took effect. Only an in-line device (or Host IPS, of course) is capable of real prevention