Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 151

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Chapter 12. Installing and Configuring Tripwire 151
12.4. Tripwire Components
The Tripwire policy file is a text file containing comments, rules, directives, and variables. This file
dictates the way Tripwire checks your system. Each rule in the policy file specifies a system object to
be monitored. Rules also describe which changes to the object to report and which to ignore.
System objects are the files and directories you wish to monitor. Each object is identified by an object
name. A property refers to a single characteristic of an object that Tripwire software can monitor.
Directives control conditional processing of sets of rules in a policy file. During installation, the text
policy file (/etc/tripwire/twpol.txt) is encrypted and renamed, becoming the active policy file
(/etc/tripwire/tw.pol).
When first initialized, Tripwire uses the signed policy file rules to create the database file
(/var/lib/tripwire/host_name.twd). The database file is a baseline snapshot of the system in
a known secure state. Tripwire compares this baseline against the current system to determine what
changes have occurred. This comparison is called an integrity check.
When you perform an integrity check, Tripwire produces report files in the
/var/lib/tripwire/report directory. The report files summarize any file changes that violated
the policy file rules during the integrity check.
The Tripwire configuration file (/etc/tripwire/tw.cfg) stores system-specific information, such
as the location of Tripwire data files. Tripwire generates the necessary configuration file information
during installation, but the system administrator can change parameters in the configuration file at any
time after that point. Note that the altered configuration file must be signed in the same way as the
policy file in order for it to be used by default.
The configuration file variables POLFILE, DBFILE, REPORTFILE, SITEKEYFILE, and
LOCALKEYFILE specify the locations of the policy file, database file, report files, and site and
local key files. These variables are defined by default at the time of installation. If you edit the
configuration file and leave any of them undefined, the configuration file will be considered invalid
by Tripwire. This causes an error on the execution of tripwire, making the program exit.
Note that the altered configuration file must be signed in the same way as the policy file in order for it
to be used by Tripwire. See Section 12.11.1 for instructions on signing the configuration file.
12.5. Modifying the Policy File
You can specify how Tripwire checks your system by modifying the Tripwire policy file
(twpol.txt). Modifying the policy file to your particular system configuration increases the
usefulness of Tripwire reports by minimizing false alerts for files or programs you are not using but
Tripwire is still reporting as altered or missing.
Locate the default policy file at /etc/tripwire/twpol.txt. An example policy file (located at
/usr/share/doc/tripwire-
version-number /policyguide.txt) is included to help you
learn the policy language. Read the example policy file for instructions on how to edit the default
policy file.
If you modify the policy file immediately after installing the tripwire package, be sure to type
/etc/tripwire/twinstall.sh to run the configuration script. This script signs the modified pol-
icy file and renames it to tw.pol. This is the active policy file used by the tripwire program when
it executes.
If you modify the sample policy file after running the configuration script, see Section 12.11 for
instructions on signing it to make the required tw.pol file.
Note
If you modify the sample policy file, it will not be used by Tripwire until it is signed, encrypted and
made into the new /etc/tripwire/tw.pol file (see Section 12.11).
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