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attackers to evade detection Given the disadvantages, this technique is not used widely in commercial systems Another technique for detecting malicious activity has been introduced recently although it is not commonly used yet in commercial systems This technique, called speci cation-based detection, assumes the existence of a precise protocol speci cation Malicious behavior is detected by comparing the protocol traf c with the protocol speci cation The detectors typically build precise models of expected behavior (eg by using state machines) based on the protocol speci cations and then compare the observed behavior in the network against the model The advantage of speci cation-based detection techniques is that, given accurate speci cations of normal behavior (eg protocols speci cations), malicious behavior can be detected with a high degree of certainty This eliminates the possibility that a detector will classify a normal behavior as malicious, which reduces false positives Such detectors may also spot novel attacks since these detectors do not depend on the existence of speci c attack signatures On the other hand, developing models of normal behavior for each protocol is often a fairly complex task Further, this approach requires models of normal behavior for all the protocols being used in the network in order to detect a wide range of attacks These models need to be executed for each node in the network As a result, these detection schemes require signi cant CPU resources for a large enterprise network Another limitation of speci cation-based detection is that detectors using this technique do not detect attacks that do not violate the speci cation, but exploit permitted behavior to launch an attack For example, in a ooding attack or a TCP SYN ood attack, a node s behavior is acceptable by the protocol speci cation, but the behavior is actually harmful to the operation of the enterprise and is therefore malicious Detection of attacks is typically not suf cient for protecting an enterprise because the attack may continue to cause harm to the network Therefore, intrusion detection systems are usually coupled with attack response systems Once an attack has been identi ed by the IDS, the response system is responsible for stopping the attack It could do this by isolating the malicious behavior or by cutting off the attacker from the network and if possible restoring the damage caused by the attacker Responses are of two types:
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In the case of active response, actions are taken automatically when certain types of intrusions are detected These might involve activities such as collecting additional information or changing the environment in order to stop the attack The purpose of collecting additional information could be to aid in the apprehension of the attacker Changing the environment on the other hand will result in halting an attack in progress and blocking subsequent access by the attacker We will discuss some potential approaches in this area in the next chapter This might involve actions such as injecting TCP reset packets, recon guring routers and rewalls, revoking the credentials of malicious users, patching vulnerable machines, and so on Another possible action is also to respond back towards the machine inferred to be the attacker Note, though, that there are legal issues associated with this line of action and hence this is not generally recommended In the case of passive response, the objective is to trigger additional monitoring that can more closely observe the malicious behavior and provide information to
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Although IDSs for enterprise environments have been studied extensively and several products/tools have been developed and are available today, these systems are not readily usable in a MANET environment In this section we will focus on discussing why existing IDSs are not appropriate for a MANET environment That discussion will also help us elucidate the unique characteristics that an IDS needs to have to operate in a MANET environment We will also discuss the potential intrusion detection approaches that may be effective in a MANET environment Defending MANET networks is much more challenging than defending traditional enterprise networks for a variety of reasons Characteristics such as volatility, mobility, as well as the ease of listening to wireless transmissions make the network inherently less secure Existing tools usually assume a well-structured and static network and therefore cannot be used as they are The nature of MANET networks makes it easier for malicious users to disrupt the network because by de nition MANETs are exible and lack a xed infrastructure It is possible for a malicious node to join the network and become responsible for providing key services This implies that, when securing the network we cannot assume that the threat is mostly from outside the network Therefore, the network needs to be protected from all nodes, both external and internal Another key difference between MANET and enterprise networks is that MANET networks do not have an established perimeter like traditional enterprise networks This plus the ability of nodes to move implies that there are no well-de ned chokepoints where the IDS systems can be placed to monitor the traf c This lack of chokepoints implies that nodes may only be able to observe a small portion of the network traf c and many attacks might escape detection by a single IDS Therefore, a much more cooperative approach to intrusion detection is necessary In this approach, multiple nodes have to work together to detect attacks Nodes monitor traf c around them and then exchange information with other nodes Nodes then use the exchanged information to detect attacks that cannot be detected by local information As a result more attacks can be detected MANET networks also introduce a large number of complex protocols New protocols for performing important functions such as routing, automated con guration, and mobility management, have been introduced These protocols create new opportunities for malicious users to identify protocol vulnerabilities and use them for launching attacks New techniques for detecting attacks exploiting the behavior of these protocols are needed MANET protocols by their nature depend on nodes cooperating with each other in performing the functions of the network One such critical function is routing In static enterprise networks, routers are usually speci c dedicated nodes Routers are usually also located in well-protected areas In a MANET, every node can be a router and therefore it is possible for a single malicious node to cause signi cant disruption in the operation of the network For example, node A may begin advertising that it has direct connectivity to a large number of nodes This may result in node A receiving a large number of packets which it can then inspect, drop, or loop around Such attacks against
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