to compromise or otherwise do harm to other network devices. There are two kinds of
Intrusion Detection Systems, the first analyzes log files from routers, firewalls, servers
and other network devices to detect previously known attack signatures, which have
been stored beforehand in the IDS’s database ([1]). This system, called rule-based IDS,
is fast and very secure in detecting older attacks, however it cannot protect a system
against attacks that occur for the first time. The second type of IDS, the behavioral
based systems, store a “normal” system behavior into a database and analyzing log files
to determine an abnormal system state and certain behavior according to the state. This
method allows for the detection of new unknown attacks but is susceptible to false pos-
itive and false negative alerts thus dampening the trust in the system. An IDS can, upon
detection of an intrusion, issue alarms or alerts and take various kinds of automatic
action, ranging from shutting down Internet links or specific servers to launching back-
traces. The implementation of an Intrusion Detection System can be software based,
hardware based or combined in preinstalled and preconfigured stand-alone IDS boxes.
There are three basic types of Intrusion Detection Architectures implementing Intrusion
Detection Systems: Network based IDS, Host based IDS and Distributed IDS.
In practice a combination of all three approaches can be implemented to allow for
a higher state of security. An IDS can not only serve as an intrusion detector alone, but
can also monitor database access, DNS functionality and it can also protect the e-mail
servers and be applied as a company policy watch to provide the enterprise with the
possibility to enforce copyright and electronic laws.
2.3 SNORT
An example for an intrusion detection system is SNORT ([1]), an efficient, stable, free-
ware, and open source implementation. It is considered a lightweight intrusion detection
system, leaving a small footprint on the system with the possibility to run on various
platforms, including Windows and Unix. Providing a real-time IP traffic analysis there
are three ways to configure SNORT’s core packet sniffer. In the first configuration, it is
unaltered set up as a network sniffer, reading packages of the network and displaying
them to the screen. The second way is a setup as a package logger, writing transmit-
ted packages to the disks. The last way is a setup up as a network intrusion detection
system, analyzing network traffic for matches against a user-definable rule-set, making
SNORT a rule-based IDS with the ability to identify:
– CGI scans
– Buffer overflows
– SMB probes
– Unauthorized server services
– OS fingerprinting attemps
– Obfuscation (camouflaging the source code)
Identifying an intrusion, SNORT can record, ignore or alert a system administrator
about the unhealthy traffic activities. The logging or alerting methods can be set up
using Syslog, XML, plain text or WinPopUps.
The Snort Architecture as shown in Figure 2 consists of a Sniffer, a Preprocessor,
a Detection Engine, and an alert and logging module, responsible for the output. The
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