Amoroso, Ed. Intrusion Detection. Sparta, NJ: Intrustion.Net Books, 1999.
Excellent introduction to the subject.
http://web.mit.edu/tytso/www/linux/ext2intro.html
Card, Rémy, Theodore Ts'o, and Stephen Tweedie. "Design and Implementation of the Second Extended Filesystem."
Excellent paper on the LinuxEXT2 filesystem; the section entitled "Basic File System Concepts" is of particular interest to Tripwire users.
Northcutt, Stephen and Judy Novak. Network Intrusion Detection: An Analyst's Handbook. Indianapolis: New Riders Publishing, 2001.
A very practical book with many examples showing system log excerpts and configurations of popular IDS tools.
http://www.chkrootkit.org/
Home of the chkrootkit shell script and an excellent source of information about how to detect and defend against rootkits.
http://sourceforge.net/projects/tripwire
Project pages for Tripwire Open Source. The place to obtain the very latest Tripwire Open Source code and documentation
http://prdownloads.sourceforge.net/tripwire/tripwire-2.3.0-docs-pdf.tar.gz
Tripwire Open Source Manual and the Tripwire Open Source Reference Card in PDF format. Required reading! (If this link doesn't work, try http://sourceforge.net/project/showfiles.php?group_id=3130).
http://www.tripwire.org
Home page for Tripwire Open Source. Binaries for Linux available here.
http://www.tripwire.com/downloads/tripwire_asr/index.cfml?
Tripwire Academic Source Release download site.
http://securityportal.com/topnews/tripwire20000711.html
Article on using Tripwire Academic Source Release, by Jay Beale (principal developer of Bastille Linux).
http://www.cs.tut.fi/~rammer/aide.html
Official web site for the Advanced Intrusion Detection Environment (AIDE).
http://www.geocities.com/fcheck2000/
Official web site for FCheck, an extremely portable integrity checker written entirely in Perl.
Ranum, Marcus J. "Intrusion Detection & Network Forensics."
Presentation E1/E2 at the Computer Security Institute's 26th Annual Computer Security Conference and Exhibition, Washington, D.C., 17-19 Nov 1999.
http://www.snort.org
Official Snort web site: source, binaries, documentation, discussion forums, and amusing graphics.
http://www.cert.org/kb/acid
The Analysis Console for Intrusion Databases (ACID) is a PHP application that analyzes IDS data in real time. ACID is a popular companion to Snort because it helps make sense of large Snort data sets; this is its official home page.
http://www.algonet.se/~nitzer/oinkmaster
Home of the Oinkmaster auto-Snort rules update script.
http://www.whitehats.com
Security news, tools, and the arachNIDS attack signature database (which can be used to update your SNORT rules automatically as new attacks are discovered).
http://www.lids.org
The Linux Intrusion Detection System (LIDS) web site. LIDS is a kernel patch and administrative tool that provides granular logging and access controls for processes and for the filesystem.