Intrusion Detection with Tripwire
Details to the Tripwire Policies can be found in the Policy Guide
Tripwire software can help to ensure the integrity of critical system files and directories by identifying all changes made to them. Tripwire configuration options include the ability to receive alerts via email if particular files are altered and automated integrity checking via a cron job. Using Tripwire for intrusion detection and damage assessment helps you keep track of system changes and can speed the recovery from a break-in by reducing the number of files you must restore to repair the system.
Tripwire compares files and directories against a baseline database of file locations, dates modified, and other data. It generates the baseline by taking a snapshot of specified files and directories in a known secure state. (For maximum security, Tripwire should be installed and the baseline created before the system is at risk from intrusion.) After creating the baseline database, Tripwire compares the current system to the baseline and reports any modifications, additions, or deletions.
When initializing its database, Tripwire builds a collection of filesystem objects based on the rules in the policy file. This database serves as the baseline for integrity checks.
To initialize the Tripwire database, use the following command:
# tripwire --init
The The /var/lib/tripwire directory contains the Tripwire database of your system's files (*.twd) and a report directory where Tripwire reports are stored. The Tripwire reports, named host_name-date_of_report-time_of_report.twr, detail the differences between the Tripwire database and your actual system files.
When running an integrity check, Tripwire compares the current, actual filesystem objects with their properties as recorded in its database. Violations are printed to standard output and saved in a report file that can be accessed later by twprint.
An email configuration option in the policy file even allows particular email addresses to be sent notices when certain integrity violations occur.
To run an integrity check, use the following command:
# tripwire --check
The twprint -m r command will display the contents of a Tripwire report in clear text. You must tell twprint which report file to display.
A twprint command for printing Tripwire reports looks similar to the following (all on one line):
# twprint -m r --twrfile /var/lib/tripwire/report/<name>.twr
The -m r option in the command tells twprint to decode a Tripwire report. The --twrfile option tells twprint to use a specific Tripwire report file.
The name of the Tripwire report that you want to see includes the name of the host that Tripwire checked to generate the report, plus the creation date and time. You can review previously saved reports at any time. Simply type ls /var/lib/tripwire/report to see a list of Tripwire reports.
If monitored files have been altered inappropriately, you can either replace the originals from backups or reinstall the program.
If you run an integrity check and Tripwire finds violations, you will first need to determine whether the violations discovered are actual security breaches or the product of authorized modifications. If you recently installed an application or edited critical system files, Tripwire will (correctly) report integrity check violations. In this case, you should update your Tripwire database so those changes are no longer reported as violations. However, if unauthorized changes are made to system files that generate integrity check violations, then you should restore the original file from a backup or reinstall the program.
To update your Tripwire database to accept the violations found in a report, you must specify the report you wish to use to update the database. When issuing the command to integrate those valid violations into your database, be sure to use the most recent report.
Type the following command (all on one line), where name is the name of the report to be used:
# tripwire --update --twrfile /var/lib/tripwire/report/<name>.twr
Tripwire will show you the particular report using the default text editor (specified in the Tripwire configuration file on the EDITOR line). This is your chance to deselect files that you do not wish to be updated in the Tripwire database. It is important that you only allow authorized integrity violations to be changed in the database.
All proposed updates to the Tripwire database start with a [x] before the file name. If you want to specifically exclude a valid violation from being added to the Tripwire database, remove the x from the box. To accept any files with an x beside them as changes, write the file in the editor and quit the text editor. This signals to Tripwire to alter its database and not report these files as violations.
For example, the default text editor for Tripwire is vi. To write the file with vi and make the changes to the Tripwire database when updating with a specific report, type :wq in vi's command mode and press [Enter]. You will be asked to enter your local passphrase. Then, a new database file will be written to include the valid violations.
After a new Tripwire database is written, the newly authorized integrity violations will no longer show up as warnings when the next integrity check is run.
If you want to actually change the files Tripwire records in its database or modify the severity in which violations are reported, you need to edit your Tripwire policy file.
First, make whatever changes are necessary to the sample policy file (/etc/tripwire/twpol.txt). A common change to this policy file is to comment out any files that do not exist on your system so that they will not generate a file not found error in your Tripwire reports. For example, if your system does not have a /etc/smb.conf file, you can tell Tripwire not to try to look for it by commenting out its line in twpol.txt:
# /etc/smb.conf -> $(SEC_CONFIG) ;
Next, you must tell Tripwire to generate a new /etc/tripwire/tw.pol signed file and then generate an updated database file based on this policy information. Assuming /etc/tripwire/twpol.txt is the edited policy file, use this command:
# twadmin --create-polfile -S site.key /etc/tripwire/twpol.txt
You will be asked for the site passphrase. Then, the twpol.txt file will be parsed and signed.
It is important that you update the Tripwire database after creating a new /etc/tripwire/tw.pol file. The most reliable way to accomplish this is to delete your current Tripwire database and create a new database using the new policy file.
If your Tripwire database file is named /var/lib/tripwire/paragon.twd, type this command:
# rm /var/lib/tripwire/paragon.twd
Then type the command to create a new database:
# tripwire --init
A new database will be created according to the instructions in the new policy file. To make sure the database was correctly changed, run the first integrity check manually and view the contents of the resulting report. See the section called Running an Integrity Check and the section called Printing Reports for specific instructions on these points.
The text file with the configuration file changes (commonly /etc/tripwire/twcfg.txt) must be signed to replace the /etc/tripwire/tw.cfg and be used by Tripwire when it runs its integrity check. Tripwire will not recognize any configuration changes until the configuration text file is correctly signed and used to replace the /etc/tripwire/tw.pol file.
If your altered configuration text file is /etc/tripwire/twcfg.txt, type this command to sign it, replacing the current /etc/tripwire/tw.pol file:
# twadmin --create-cfgfile -S site.key /etc/tripwire/twcfg.txt
Since the configuration file does not not alter any Tripwire policies or files tracked by the application, it is not necessary to regenerate the database of monitored system files.
You can run a Tripwire integrity check daily by inserting the following commands in the script /etc/cron.daily/tripwire-check:
Tripwire can email someone if a specific type of rule in the policy file is violated. To
configure Tripwire to do this, you first have to know the email address of the person to be
contacted if a particular integrity violation occurs, plus the name of the rule you would
like to monitor. Note that on large systems with multiple administrators, you can have
different sets of people notified for certain violations and no one notified for minor
Sending Test Email Messages
To make sure that Tripwire's email notification configuration can actually send email correctly, use the following command:
/usr/sbin/tripwire --test --email firstname.lastname@example.org
A test email will immediately be sent to the email address by the tripwire program.