Intrusion Detection Systems21
Another element to securing networks complementing firewall implementation is an intrusion detection system
(IDS). An IDS works in conjunction with routers and firewalls by monitoring network usage anomalies. It protects
a company’s IS resources from external as well as internal misuse. An IDS operates continuously on the system,
running in the background and notifying administrators when it detects a perceived threat.
Broad categories of IDSs include:
• Network-based IDSs—These identify attacks within the monitored network and issue a warning to the operator. If
a network-based IDS is placed between the Internet and the firewall, it will detect all the attack attempts, regardless
of whether they enter the firewall. If the IDS is placed between a firewall and the corporate network, it will detect
those attacks that enter the firewall (i.e., it will detect intruders). The IDS is not a substitute for a firewall, but rather
it complements the function of a firewall.
• Host-based IDSs—These are configured for a specific environment and will monitor various internal resources of
the operating system to warn of a possible attack. They can detect the modification of executable programs, detect
the deletion of files and issue a warning when an attempt is made to use a privileged command.
Components of an IDS are:
• Sensors responsible for collecting data in the form of network packets, log files, system call traces, etc.
• Analyzers that receive input from sensors and determine intrusive activity
• An administration console
• A user interface