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 syslog and its ability to separate logged events by their severity (for example, minor printer messages versus major kernel warnings). 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 collect them in its own specialized log for administrator analysis.

Host-based IDSes 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.

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.

9.2.2. RPM as an IDS

The RPM Package Manager (RPM) is another program that can be used as a host-based IDS. RPM contains various options for querying packages and their contents. These verification options can be invaluable to an administrator who suspects that critical system files and executables have been modified.

The following list details some options for RPM that can verify file integrity on a Red Hat Enterprise Linux system. Refer to the Red Hat Enterprise Linux System Administration Guide for complete information about using RPM.


Some of the commands in the following list require the importation of the Red Hat GPG public key into the system's RPM keyring. This key verifies that packages installed on the system contain an Red Hat package signature, which ensures that the packages originated from Red Hat. The key can be imported by issuing the following command as root (substituting <version> with the version of RPM installed on the system):

rpm --import /usr/share/doc/rpm-<version>/RPM-GPG-KEY

rpm -V package_name

The -V option verifies the files in the installed package called package_name. If it shows no output and exits, this means that none of the files have been modified in anyway since the last time the RPM database was updated. If there is an error, such as the following

S.5....T c /bin/ps

then the file has been modified in some way and you need to assess whether to keep the file (such as with modified configuration files in the /etc/ directory) or delete the file and reinstall the package that contains it. The following list defines the elements of the 8-character string (S.5....T in the above example) that notifies of a verification failure.

  • . — The test has passed this phase of verification

  • ? — The test has found a file that could not be read, which is most likely a file permission issue

  • S — The test has encountered a file that that is smaller or larger than it was when originally installed on the system

  • 5 — The test has found a file whose md5 checksum does not match the original checksum of the file when first installed

  • M — The test has detected a file permission or file type error on the file

  • D — The test has encountered a device file mismatch in major/minor number

  • L — The test has found a symbolic link that has been changed to another file path

  • U — The test has found a file that had its user ownership changed

  • G — The test has found a file that had its group ownership changed

  • T — The test has encountered mtime verification errors on the file

rpm -Va

The -Va option verifies all installed packages and finds any failure in its verification tests (much like the -V option, but more verbose in its output since it is verifying every installed package).

rpm -Vf /bin/ls

The -Vf option verifies individual files in an installed package. This can be useful when performing a quick verification of a suspect file.

rpm -K application-1.0.i386.rpm

The -K option is useful for checking the md5 checksum and the GPG signature of an RPM package file. This is useful for checking whether a package about to be installed is signed by Red Hat or any organization for which you have the GPG public key imported into a GPG keyring. A package that has not been properly signed triggers an error message similar to the following:

application-1.0.i386.rpm (SHA1) DSA sha1 md5 (GPG) NOT OK
		  (MISSING KEYS: GPG#897da07a)

Exercise caution when installing packages that are unsigned as they are not approved by Red Hat, Inc. and could contain malicious code.

RPM can be a powerful tool, as evidenced by its many verification tools for installed packages and RPM package files. It is strongly recommended that the contents of the RPM database directory (/var/lib/rpm/) be backed up to read-only media, such as CD-ROM, after installation of Red Hat Enterprise Linux. Doing so allows verification of files and packages against the read-only database, rather than against the database on the system, as malicious users may corrupt the database and skew the results.

9.2.3. Other Host-based IDSes

The following list discusses some of the other popular host-based intrusion detection systems available. Refer to the websites of the respective utilities for more information about installing and configuring them.


These applications are not included with Red Hat Enterprise Linux and are not supported. They have been included in this document as a reference to users who may be interested in evaluating such applications.