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), analyze
them, re-tag 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 that you may want to add) 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 will alert 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, recently opened the software
source code for the Linux version and licensed it under the terms of
the GNU General Public License. Red Hat Linux includes
Tripwire, which is available in RPM package
format for easy installation and upgrade.
Detailed information on the installation and configuration of
Tripwire can be found in the
Red Hat Linux Reference Guide.
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 you can use
to verify file integrity on your Red Hat Linux system. Refer to the
Red Hat Linux Customization Guide for complete information about using
RPM.
 | Important |
|---|
| | Some of the commands in the list that follows requirethat you
import the Red Hat GPG public key into your RPM keyring. This
key verifies that packages installed on your system contain an
Red Hat package signature, which ensures that your packages
originated from Red Hat. The key can be imported with the following
command (substituting <version>
with the version of RPM installed on your 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 all of the files
have not been modified in anyway since the last time the RPM
database was updated. If there is an error, such as
then the file has been modified in some way and you need to
assess whether to keep the file (such is the case with modified
configuration files in /etc) 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 if you wish to perform
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 you want to install is
signed by Red Hat or any organization for which you have the GPG
public key imported into your GPG keyring. A package that has
not been properly signed will emit 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 you backup the contents of your RPM database directory
(/var/lib/rpm/) to read-only media, such as CD-ROM,
after you install Red Hat Linux. Doing so allows you to safely verify 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 your 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 in your environment.
 | Note |
|---|
| | These applications are not included with Red Hat 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.
|
SWATCH http://www.stanford.edu/~atkins/swatch/
— The Simple WATCHer (SWATCH) uses log files generated by
syslog to alert administrators of anomalies based
on user configuration files. SWATCH was designed to log any event
that the user wants to add into the configuration file; however, it
has been adopted widely as a host-based IDS.
LIDS http://www.lids.org — The
Linux Intrusion Detection System (LIDS) is a kernel patch and
administration tool that can also control file modification with
access control lists (ACLs) and protect processes and files, even
from the root user.
