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14.4. Network-Intrusion Detection Systems

Network-intrusion detection systems (NIDS) are becoming an important part of network security. NIDS sniff the network traffic and inspect both the traffic flow and its content.

There are two basic kinds of NIDS: network signatures-based and those based on network flow and protocol anomaly analysis. Some NIDS combine both methods.

  1. The signature analyzer module matches signatures in the network data. Signatures can be written to analyze network protocol headers5 or to match a sequence of bytes in the data within the network packets. For example, signatures are matched in particular network traffic, such as HTTP, port 80 traffic only.

  2. The network flow and protocol analyzer functionality of NIDS is practically a heuristics engine. For example, a giant protocol analyzer module can have knowledge of the most relevant protocols, such as HTTP, FTP, SMTP, and so on, and match any anomalies in the protocols. For example, a protocol analyzer can detect the CodeRed worm as an overly long URL. Similarly, a protocol anomaly analyzer can alert the user any time a particular field, in any part of a known protocol, is overly long. This allows NIDS to detect generically many possible exploitation techniques that are based on overflowing some field of network protocol structures, causing a buffer overflow condition on the target.

If a firewall comes with a performance penalty on your network, so does a NIDS. A good NIDS must use a packet reassembler, and this process can be very performance intensive.

For example, the intrusion detection system, Snort (, authored by Marty Roesh, has the following major components6:

  • Packet decoder: This module picks the packets coming in from the various interfaces and passes them to the preprocessor.

  • Preprocessor: This module is very important because it handles some of the common attacks that can be executed using simple signature insertion attacks7. This module also handles the reassembling of network packets, which is important because signatures can overlap between packets, fragmenting network traffic. In addition, packets can come out of order, and the reassembler must put the puzzle together using the sequence numbers in the packets. Because reassembling is very costly, some intrusion-detection systems try to become faster by simply pretending that they only need to analyze normal traffic. This obviously reduces IDS's capability to detect advanced attacks precisely. Although normal traffic rarely gets fragmented, attackers can force fragmentation to bypass NIDS systems.

  • Detection engine: This component matches the rules against the reassembled network stream. It is vital to have a fast matching engine to allow more signatures to be matched. If this component of the IDS is slow, the IDS might start to drop packets when there are too many signatures to match. When the detection engine finds a known signature, it calls the alerting and logging module.

  • Altering and logging module: This module generates an alert and places it in the appropriate output, such as a log file. Because intrusion detection systems might produce many alerts, it is becoming increasingly popular to outsource IDS monitoring, in case a corporation does not have enough trained resources to do the monitoring 24/7 in house.

An IDS can be placed in as many places on your network as you desire, but keep in mind whether or not you have the available resources to process all the alerts that will be generated. Several IDS products are capable of producing reports that can be imported into a database and can correlate the IDS alerts further with other security events on your networkto help eliminate duplicate alerts or to escalate lower-level alerts to higher levels.

A common place for a NIDS is the perimeter, somewhere close to your firewall, as shown in Figure 14.1.

Another important decision is how to hook up an IDS. There are two different basic modes for IDS: logging and blocking. In logging mode, an IDS might be hooked up on a port of a network switch that receives replicated traffic. In this mode, the IDS will generate an alert but will not be able to drop the packet to prevent the attack. In this mode, the malicious packet might hit the target, but at least your "smoke detector" might alert you about it so that you can respond appropriately at once. Needless to say, an IDS works much faster in logging mode.

In blocking mode, the IDS will stall the network traffic and inspect it before the malicious traffic can arrive at the target. This solution allows the malicious traffic to be dropped, but it is usually much more performance-intensive than logging mode. Performance can be much more effective for IDS solutions that deploy anomaly detection engines; there will be less need for signatures to match malicious traffic. However, specific IDS signatures can help to refine an attack and provide better security for protocols not yet supported by the anomaly detection engine. A hybrid solution is usually the best, combining the two techniques for increased security.

Later in this chapter, I will introduce several network captures of computer worms and discuss IDS signature development in both threat-specific and generic forms.

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