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Intrusion detection - an explanation

Detection of break-ins or break-in attempts, manually or via software expert systems that operate on logs or other information available on network.The inadequacies inherent in current defences has driven the development of a new breed of security products known as Intrusion Prevention Systems (IPS). This is a term which has provoked some controversy in the industry since some firewall and IDS vendors think it has been “hijacked” and used as a marketing term rather than as a description for any kind of new technology.

Whilst it is true that firewalls, routers, IDS devices and even AV gateways all have intrusion prevention technology included in some form, we believe that there are sufficient grounds to create a new market sector for true Intrusion Prevention Systems.

These systems are proactive defence mechanisms designed to detect malicious packets within normal network traffic (something that the current breed of firewalls do not actually do, for example) and stop intrusions dead, blocking the offending traffic automatically before it does any damage rather than simply raising an alert as, or after, the malicious payload has been delivered.

Within the IPS market place, there are two main categories of product: Host IPS and Network IPS.


As with Host IDS systems, the Host IPS relies on agents installed directly on the system being protected. It binds closely with the operating system kernel and services, monitoring and intercepting system calls to the kernel or APIs in order to prevent attacks as well as log them.

It may also monitor data streams and the environment specific to a particular application (file locations and Registry settings for a Web server, for example) in order to protect that application from generic attacks for which no “signature” yet exists.

One potential disadvantage with this approach is that, given the necessarily tight integration with the host operating system, future OS upgrades could cause problems.

Since a Host IPS agent intercepts all requests to the system it protects, it has certain prerequisites - it must be very reliable, must not negatively impact performance, and must not block legitimate traffic. Any HIPS that does not meet these minimum requirements should never be installed in a host, no matter how effectively it blocks attacks.

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). The next-generation firewall - the deep inspection firewall - also exhibits a similar feature set, though we do not believe that the deep inspection firewall is ready for mainstream deployment just yet.

As with a typical firewall, the NIPS has at least two network interfaces, one designated as internal and one as external. As packets appear at the either interface they are passed to the detection engine, at which point the IPS 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 particular TCP session arrive at the IPS device, they are discarded immediately.

Legitimate packets are passed through to the second 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, out-of-order packets, or packets with overlapping IP fragments will be re-ordered and “cleaned up” before being passed to the destination host, and illegal packets can be dropped completely.

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 or re-configure a firewall when they detect an attack (a worrying piece of FUD that we have noticed in some IDS marketing literature recently).

The problem here is that unless the attacker is operating on a 2400 baud modem, the likelihood is that by the time the IDS has detected the offending packet, raised an alert, and transmitted the TCP Resets - and especially by the time the two ends of the connection have received the Reset packets and acted on them (or the firewall or router has had time to activate new rules to block the remainder of the flow) - the payload of the exploit has long since been delivered….. game over! Our guess is that there are not many crackers using 2400 baud modems these days….

A true IPS device, however, is sitting in-line - all the packets have to pass through it. Therefore, as soon as a suspicious packet has been detected - and before it is passed to the internal interface and on to the protected network, it can be dropped. Not only that, but now that flow has been flagged as suspicious, all subsequent packets that are part of that session can also be dropped with very little additional processing. Oh, and for good measure, some products are also capable of sending TCP Resets or ICMP Unreachable messages to the attacking host.