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9.2. Host-based IDS
A host-based IDS analyzes several areas to determine misuse (malicious or abusive activity inside the network) or intrusion (breaches from the outside). Host-based IDSes consult several types of log files (kernel, system, server, network, firewall, and more), and compare the logs against an internal database of common signatures for known attacks. UNIX and Linux host-based IDSes make heavy use of
syslogand its ability to separate logged events by their severity (for example, minor printer messages versus major kernel warnings). The
syslogcommand is available when installing the
sysklogdpackage, which is included with Red Hat Enterprise Linux. This package provides system logging and kernel message trapping. The host-based IDS filters logs (which, in the case of some network and kernel event logs, can be quite verbose), analyzes them, re-tags the anomalous messages with its own system of severity rating, and collects them in its own specialized log for administrator analysis.
A host-based IDS can also verify the data integrity of important files and executables. It checks a database of sensitive files (and any files added by the administrator) and creates a checksum of each file with a message-file digest utility such as
md5sum(128-bit algorithm) or
sha1sum(160-bit algorithm). The host-based IDS then stores the sums in a plain text file and periodically compares the file checksums against the values in the text file. If any of the file checksums do not match, the IDS alerts the administrator by email or cellular pager. This is the process used by Tripwire, which is discussed in Section 9.2.1, “Tripwire”.
Tripwire is the most popular host-based IDS for Linux. Tripwire, Inc., the developers of Tripwire, opened the software source code for the Linux version and licensed it under the terms of the GNU General Public License. Tripwire is available from http://www.tripwire.org/.
Tripwire is not included with Red Hat Enterprise Linux and is not supported. It has been included in this document as a reference to users who may be interested in using this popular application.