tripwire report

Tripwire Intrusion System 1

1. INTRODUCTION

Tripwire is a reliable intrusion detection system. It is a software tool that checks to see

what has changed in your system. It mainly monitors the key attribute of your files; by

key attribute we mean the binary signature, size and other related data. Security and

operational stability must go hand in hand; if the user does not have control over the

various operations taking place, then naturally the security of the system is also

compromised. Tripwire has a powerful feature which pinpoints the changes that has taken

place, notifies the administrator of these changes, determines the nature of the changes

and provide you with information you need for deciding how to manage the change.

Tripwire Integrity management solutions monitor changes to vital system and

configuration files. Any changes that occur are compared to a snapshot of the established

good baseline. The software detects the changes, notifies the staff and enables rapid

recovery and remedy for changes. All Tripwire installation can be centrally managed.

Tripwire software’s cross platform functionality enables you to manage thousands of

devices across your infrastructure.

Security not only means protecting your system against various attacks but also

means taking quick and decisive actions when your system is attacked.

First of all we must find out whether our system is attacked or not, earlier system logs

are certainly handy. You can see evidences of password guessing and other suspicious

activities. Logs are ideal for tracing steps of the cracker as he tries to penetrate into the

system. But who has the time and the patience to examine the logs on a daily basis??


1.1 MOTIVATION

Penetration usually involves a change of some kind, like a new port has been opened

or a new service. The most common change you can see is that a file has changed. If

you can identify the key subsets of these files and monitor them on a daily basis, then

we will be able to detect whether any intrusion took place. Tripwire is an open source

program created to monitor the changes in a key subset of files identified by the user

and report on any changes in any of those files. When changes made are detected, the

system administrator is informed. Tripwire ‘s principle is very simple, the system

administrator identifies key files and causes tripwire to record checksum for those

files. He also puts in place a cron job, whose job is to scan those files at regular

intervals (daily or more frequently), comparing to the original checksum. Any changes,

addition or deletion, are reported to the administrator. The administrator will be able to

determine whether the changes were permitted or unauthorized changes. If it was the

earlier case the n the database will be updated so that in future the same violation

wouldn’t be repeated. In the latter case then proper recovery action would be taken

immediately.


2. BASIC PURPOSE OF TRIPWIRE

Almost the same principle is used in computers. If any change is met upon while

comparing the old values to the new ones, or if any data is being manipulated on the spot,

the logs are checked for intrusion and then detected, after which all the changes can be

undone.

Tripwire is a free and open-source1 software tool. It functions as a host-based intrusion

detection system. It does not concern itself directly with detecting intrusion attempts in

real time at the periphery of a computing system (as in network intrusion detection

systems), but rather looks for and reports on the resultant changes of state in the

computing system under observation

Intruders usually leave traces of their activities (changes in the system state). Tripwire

looks for these by monitoring key attributes of files that should not change—including

binary signatures, size, expected changes in size, etc.—and reporting its findings.

While useful for detecting intrusions after the event, it can also serve many other

purposes, such as integrity assurance, change management, policy compliance, and more.

A Host-based Intrusion Detection System (HIDS), as a special category of an

Intrusion-Detection System, focuses its monitoring and analysis on the internals of a

computing system rather than on its external interfaces (as a Network Intrusion Detection

System (NIDS) would do)


2.1 TRIPWIRE RELATED TOPICS

Open source describes practices in production and development that promote access to

the end product's source materials—typically, their source code. Some consider it as a

philosophy, and others consider it as a pragmatic methodology. Before open source

became widely adopted, developers and producers used a variety of phrases to describe

the concept; the term open source gained popularity with the rise of the Internet and its

enabling of diverse production models, communication paths, and interactive

communities. Subsequently, open source software became the most prominent face of

open source practices.

The open source model can allow for the concurrent use of different agendas and

approaches in production, in contrast with more centralized models of development such

as those typically used in commercial software companies. "Open source" as applied to

culture defines a culture in which fixations are made generally available. Participants in

such a culture are able to modify those products and redistribute them back into the

community.

Pragmatism, as a school of philosophy, is a collection of many different ways of

thinking. Given the diversity among thinkers and the variety among schools of thought

that have adopted this term over the years, the term pragmatism has become all but

meaningless in the absence of further qualification. Most of the thinkers who describe

themselves as pragmatists point to some connection with practical consequences or real

effects as vital components of both meaning and truth. The precise character of these

links to pragmata is, however, as diverse as the thinkers who do the pointing.


3. THE ACTUAL WORKING OF THE TRIPWIRE SYSTEM

A HIDS will monitor all or part of the dynamic behavior and of the state of a computer

system. Much as a NIDS will dynamically inspect network packets, a HIDS might detect

which program accesses what resources and assure that (say) a word-processor hasn't

suddenly and inexplicably started modifying the system password-database. Similarly a

HIDS might look at the state of a system, its stored information, whether in RAM, in the

file-system, or elsewhere; and check that the contents of these appear as expected.

One can think of a HIDS as an agent that monitors whether anything/anyone - internal or

external - has circumvented the security policy that the operating system tries to enforce.

3.1 MONITORING DYNAMIC BEHAVIOUR

Many computer users have encountered tools that monitor dynamic system behavior in

the form of anti-virus (AV) packages. While AV programs often also monitor system

state, they do spend a lot of their time looking at who is doing what inside a computer -

and whether a given program should or should not access one or another system resource.

The lines become very blurred here, as many of the tools overlap in functionality.

3.2 MONITORING STATE

The principle of operation of a HIDS depends on the fact that successful intruders

(crackers) will generally leave a trace of their activities. (In fact, such intruders often

want to own the computer they have attacked, and will establish their "ownership" by

installing software that will grant the intruders future access to carry out whatever

activity (keyboard logging, identity theft, spamming, botnet activity, spyware-usage etc.)

they envisage.)

In theory, a computer user has the ability to detect any such modifications, and the HIDS

attempts to do just that and reports its findings. Ideally a HIDS works in conjunction with

a NIDS, such that a HIDS finds anything that slips past the NIDS.


Ironically, most successful intruders, on entering a target machine, immediately apply

best-practice security techniques to secure the system which they have infiltrated, leaving

only their own backdoor open, so that other intruders can not take over their computers.

(Crackers are a competitive bunch...) Again, one can detect (and learn from) such

changes.

3.3 TECHNIQUE

In general a HIDS uses a database (object-database) of system objects it should monitor -

usually (but not necessarily) file-system objects. A HIDS could also check that

appropriate regions of memory have not been modified, for example - the system-call

table comes to mind for Linux, and various vtable structures in Microsoft Windows.

For each object in question a HIDS will usually remember its attributes (permissions,

size, modifications dates) and perhaps create a checksum of some kind (an MD5 hash or

similar) for the contents, if any. This information gets stored in a database for later

comparison (checksum-database). Note that a matching MD5 hash does not provide a

complete guarantee that an intruder or other unauthorised user has not tampered with the

target file. Recent (2004) research has resulted in claims (still under debate) that the

probability of such tampering may exceed what one might hope.


4. OPERATION OF TRIPWIRE

At installation time - and whenever any of the monitored objects change legitimately - a

HIDS must initialise its checksum-database by scanning the relevant objects. Persons in

charge of computer security need to control this process tightly in order to prevent

intruders making un-authorized changes to the database(s). Such initialisation thus

generally takes a long time and involves cryptographically locking each monitored object

and the checksum databases or worse. Because of this, manufacturers of HIDS usually

construct the object-database in such a way that makes frequent updates to the checksum

database unnecessary.

Computer systems generally have many dynamic (frequently changing) objects which

intruders want to modify - and which a HIDS thus should monitor - but their dynamic

nature makes them unsuitable for the checksum technique. To overcome this problem,

HIDS employ various other detection techniques: monitoring changing file-attributes,

log-files that decreased in size since last checked, and a raft of other means to detect

unusual events.

Once a system administrator has constructed a suitable object-database - ideally with help

and advice from the HIDS installation tools - and initialized the checksum-database, the

HIDS has all it requires to scan the monitored objects regularly and to report on anything

that may appear to have gone wrong. Reports can take the form of logs, e-mails or

similar.

4.1 PROTECTING THE HIDS

A HIDS will usually go to great lengths to prevent the object-database, checksum-

database and its reports from any form of tampering. After all, if intruders succeed in

modifying any of the objects the HIDS monitors, nothing can stop such intruders from

modifying the HIDS itself - unless security administrators take appropriate precautions.

Many worms and viruses will try to disable anti-virus tools, for example. Sadly, a lot of


them succeed in doing so.

Apart from crypto-techniques, HIDS might allow administrators to store the databases on

a CD-ROM or on other read-only memory devices (another factor militating for

infrequent updates...) or storing them in some off-system memory. Similarly, a HIDS will

often send its logs off-system immediately - in some instances via one-way

communications channels, such as a serial port which only has "Transmit" connected, for

example.

One could argue that the trusted platform module comprises a type of HIDS. Although its

scope differs in many ways from that of a HIDS, fundamentally it provides a means to

identify whether anything/anyone has tampered with a portion of a computer.

Architecturally this provides the ultimate (at least at this point in time) host-based

intrusion detection, as depends on hardware external to the CPU itself, thus making it that

much harder for an intruder to corrupt its object and checksum databases.


FIG.1: FLOW CHART SHOWING THE WORKING OF TRIPWIRE


1. Install Tripwire and customize the policy file

Install the Tripwire software into the system and then specify the files to be checked

by writing the policy files. Using the version 4.0 writing the policy file is made very

easy.

2. Initialize the Tripwire database

The database is initialized with the important key attribute in the file to be checked.

Build database of critical system files to monitor based on the contents of the new,

signed Tripwire policy file.

3. Run the integrity check

Compare the newly created Tripwire database with the actual system files, looking for

missing or altered files, according to the integrity check timing specified by in the

policy file for different files that are to be monitored.

4. Examine the Tripwire report file

View the Tripwire report file to note any integrity violations.

5. If unauthorized integrity violations occur, take appropriate security measures

If monitored files have been altered inappropriately, the system administrator have to

take immediate action, you can either replace the original files from backup copies

reinstall the program, or completely reinstall the operating system.

6. If the file alterations were valid, verify and update the Tripwire database file.

If the changes made to monitor files are intentional, edit Tripwire’s database file to

ignore those changes in subsequent report.

7. If the policy file fails verification, update the Tripwire policy file


To change the list of files Tripwire monitors or how it treats integrity violations,

update the supplied policy file, regenerate a signed copy, and update the Tripwire

database.


5. TRIPWIRE MANAGER

Tripwire Manger is a fully functional, cross platform management console that allows

system and security professionals to easily manage all installations of Tripwire for

Servers software across an enterprise network. Tripwire Manager eliminates the need

to manually monitor multiple discrete network platforms and point solutions. Instead,

IT professionals have a comprehensive view of data integrity status from a single

centralized console. Tripwire Manager also enables you to view and analyze reports

from installations of Tripwire for Servers. With Tripwire Manager you can retrieve an

integrity system, which is made up of the configuration, database, policy, local and site

key, from a single “golden” machine which can then be distributed to as many servers

that need to be compared against this snapshot. In version 4.0 of the Tripwire Manager

you can create and modify policy files by using graphical policy editor. This GUI will

scan the remote file system of a Tripwire for Servers installation and provide you with

an easy mechanism for editing or creating a policy file without having to know the

policy file syntax. Tripwire Manager can manage the functions of Tripwire for Servers

on up to 2500 machines.

Adding or removing recognition of Tripwire for Servers is easy to do from within the

Tripwire Manager console. All you need to know is host name, IP address and a port

number. The Tripwire for Servers database can be updated by using the database

update mode within Tripwire Manager. All communication between Tripwire Manager

and installation of Tripwire for Servers takes place using Secured Socket Layer (SSL)

technology with 168-bit Triple DES encryption. To protect against unauthorized

modification, important files on each Tripwire for Servers installation are stored in a

binary-encoded and signed form. Database, policy, configuration, and report files

generated by the integrity assessment are protected by using El Gamal asymmetric

cryptography with a 1024-bit signature.


There are mainly two types of Tripwire Manager

Active Tripwire Manager

Passive Tripwire Manager

A user can have more than one Tripwire Manager managing the same set of Tripwire for

Servers machines. However, only one can be in active mode and have complete

management control of Tripwire for Servers machines. This active Tripwire Manager

gives a user the ability to update the database, schedule integrity checks, update and

distribute policy and configuration files and view integrity reports. The other Tripwire

Manager is in a passive mode. The passive mode only allows these Tripwire Manager to

view the status of the machines and integrity reports. Once the active Tripwire Manager

shuts down, the next time the passive Tripwire Manager pings the Tripwire for Servers

machine it connects as an active Tripwire Manager. If more than two passive Tripwire

Managers, the one that connects first to the Tripwire for Servers machine after the active

Manager has hut down becomes the active Manager.


6. TRIPWIRE FOR SERVERS

Tripwire for Servers is software that is exclusively used by servers. This software can

be installed on any server that needs to be monitored for any changes. Typical servers

include mail servers, web servers, firewalls, transaction server, development server etc.

Any server where it is imperative to identity if and when a file system change has

occurred should be monitored with tripwire for servers. For the tripwire for server’s

software to work two important things should be present – the policy file and the

database.

The tripwire for Servers software conducts subsequent file checks, automatically

comparing the state of the system with the baseline database. Any inconsistencies are

reported to the Tripwire Manager and to the host system log file. Reports can also be

emailed to an administrator. If a violation is an authorized change, a user can update

the database so changes no longer show up as violations.

6.1 FLEXIBLE POLICY LANGUAGE

The power behind Tripwire technology lies in its highly configurable policy language.

The policy file is how a user directs Tripwire for Servers to monitor specific files or

directories. The flexible policy tool can be customized to fit the needs of each and

every server. With the release of version 4.0, policy file creation has become even

easier. From Tripwire Manager 4.0, a graphical policy editor allows users to select the

files and directories, along with the scanning options that need to be monitored in each

integrity check. Included in the products are default policy files for each supported

operating system to make it easy for the user to set up which files should be monitored.

In the latest version, wildcard application is also supported which enables users to add

objects to the policy file by specifying the file type. In 4.0, objects listed in the policy

file but not present on the user’s machine will no longer be categorized as violations.


By only showing violations caused by added, deleted or changed files, report noise is

greatly reduced. Tripwire policy languages also allow you to group objects around

easy-to-understand rule names and then prioritize them.

The snapshot and the policy file are cryptographically signed with 168-bit Triple DES

encryption algorithm that detects any unauthorized tampering. The default policy file

also monitors the tripwire binary files, in short, it uses tripwire itself to monitor the

tripwire.

In the latest version 4.0 in addition to reporting the administrator which file has

changed, when the change occurred and where the change took place it also to some

extend determines who made these changes. Tripwire for Servers track the identity of

who made the change by correlating the information from the operating system’s event

and audit log with the integrity information that is detected by Tripwire for Servers. It

uses this information to provide the identity of who made a certain change. Since we

rely on the operating system to gather this information, the product only captures the

“who” information from the operating system that track this. Linux and FreeBSD do

not track this information. This feature is called Event Log Correlation.

Each Tripwire for Servers report details when the database was last updated, providing

a quick benchmark of if or when detailing if the data files have been replaced. In order

to replace these files, an attacker requires root or administrator level privileges and

must know where Tripwire for Servers has been installed. On a properly secured

system, gaining this level of access takes time and leaves physical evidence behind for

Tripwire for Servers to detect prior to the system being compromised. Methods for

reducing the risk of an intruder being able to replace a Tripwire for Servers installation

include:

Hiding the application by renaming configuration, data, and binary files and

installing to a hidden location.

Installing Tripwire for Servers to a read-only partition such as a CD-ROM.


7. TRIPWIRE FOR NETWORK DEVICES

Router, switch, and firewall configurations are critical to overall network operation.

Unwanted changes to configuration files can result in downtime and security issues

and waste hours of staff time searching for the cause. Tripwire for Network Devices

monitors the integrity of routers, switches and firewalls-network devices that

communicate network traffic within and between networks. It helps network

administrator answer the question, “Has the state of my network devices changed from

a known, trusted state? If so, how?” Problem’s with one network device can seriously

disable an organization’s entire network. Network downtime can result in lost revenue

and lost customer confidence. Manual processes to secure your network devices are

available and important. Tripwire for Network Devices augments and helps guarantee

that the security of your network devices remains in tact. With Tripwire for Network

Devices, downtime is minimized. Network administrators can use Tripwire for

Network Devices to quickly investigate and isolate changes and restore changed

configuration files within minutes of an alert.

Tripwire for Network Device includes six primary functions:

Automatic notification of changes to your routers, switches and firewalls

Automatic restoration of critical network devices

Audit trail from log files and change reports – ideal for internal/external network

audits.

Baseline archiving and configuration file “Hot Back-up” solution

Heterogeneous support for today’s most commonly used network devices

Sets a framework for autonomic recovery


Tripwire for Network Devices does not provide real time monitoring. It checks your

network devices for change according to schedule you set. Device passwords stored by

the software are protected by robust 1024-bit Blowfish cryptography. The software has

four user authorization levels:

“Monitors” are allowed only to monitor the application. They cannot make

changes to Tripwire for Network Devices or to the devices that the software

monitors.

“Users” can make changes to Tripwire for Network Devices, such as add

routers, switches. Groups, tasks, etc., but they cannot make changes to the

devices it monitors.

“Powerusers” can make changes to the software and to the devices it monitors.

“Administrator” can perform all actions, plus delete violations and log messages

as well as add, delete, or modify user accounts.

Tripwire for Network Devices maintains a log of all significant actions, including

adding and deleting nodes, rules, tasks, and user accounts. All log entries include a

time and date, and identify the user who initiated the process. The log entries cannot be

modified by anyone other than the administrator and can be copied and pasted into a

text file so you can create a library of log activities that are ideal for network audits.

Device password are stored by the software are protected by robust 1024-bit Blowfish

cryptography. Tripwire for Network Devices has been tested and can monitor

thousands of network devices. Tripwire for Network Devices software has been tested

up to 6,000 network devices running integrity checks every 10 minutes. With correct

configuration, the software can monitor more than 6,000 devices at one time.


8. HOW TO INSTALL AND USE THE TRIPWIRE SYSTEM

The following steps should be taken to properly install, use and maintain Tripwire:

Install Tripwire and customize the policy file — If not already done, install the

tripwire RPM. Then, customize the sample configuration

(/etc/tripwire/twcfg.txt) and policy (/etc/tripwire/twpol.txt) files and run the

configuration script (/etc/tripwire/twinstall.sh).

Initialize the Tripwire database — Build a database of critical system files to

monitor based on the contents of the new, signed Tripwire policy file

(/etc/tripwire/tw.pol).

Run a Tripwire integrity check — Compare the newly-created Tripwire database

with the actual system files, looking for missing or altered files.

Examine the Tripwire report file — View the Tripwire report file using twprint to

note integrity violations.

Take appropriate security measures — If monitored files have been altered

inappropriately, you can either replace the originals from backups or reinstall

the program.

Update the Tripwire database file — If the integrity violations are intentional and

valid, such as if you intentionally edited a file or replaced a particular program,

you should tell Tripwire's database file to not report them as violations in future

reports.

Update the Tripwire policy file — If you need to change the list of files Tripwire

monitors or how it treats integrity violations, you should update your sample

policy file (/etc/tripwire/twpol.txt), regenerate a signed copy

(/etc/tripwire/tw.pol), and update your Tripwire database.


9. HOW TO USE TRIPWIRE

Here is an experience of a web adminstrator:

A little over two years ago I was hacked. Someone broke into a web server I was

administrating that had only Apache and OpenSSH running publically, and all packages

were up-to-date. The hacker replaced my ps binary with his own to hide his processes,

added a new service that was executed from the binary "/bin/crond " (the space is

intentional - it makes it look like a normal and an expected process in a running-

processes listing and a normal binary in a directory listing). The "crond " process

gathered usernames and passwords and stored them in a text file in the directory

"/dev/pf0 / /", (5 and 2 spaces respectively), which also contained a root shell. The

chances of me finding and identifying this intrusion would have been extremely remote if

I had not been running Tripwire.

Tripwire is a file integrity checker for UNIX/Linux based operating systems and works as

an excellent intrusion detection system. It will not prevent an intrusion; for this see my

previous articles on setting up firewalls and securing a Linux distribution for help.

The idea behind Tripwire is quite simple: it first creates a "baseline" database of the state

of the files and directories on your system and then on subsequent runs it compares the

current state of the files and directories against this baseline identifying any deletions,

additions or changes. The files and directories to be checked are decided by a "policy"

file. This file also defines what attributes to compare; this can include access, inode and

modification timestamps, owner and group IDs, permissions, file size and type, MD5 and

SHA hash values, etc.

In this article I will guide you through the process of getting and installing Tripwire,

configuring it and setting it up to run on a daily basis. In the final section I will mention a

few additional steps you can take to ensure the integrity of your Tripwire database and

thus your file system.


2. Acquiring and Installing Tripwire

The easiest method of installing Tripwire is to use a vendor supplied package (I have

checked and these are available for RedHat/Fedora Core, SuSE, Mandrakesoft and

Debian). The advantages of using these are that the policy file will be already created and

configured for the system you are using. Make sure to use official packages for your

distribution to ensure they have not been trojaned.

If you cannot locate a precompiled package for your distribution, then you can download

the latest source code from http://sourceforge.net/projects/tripwire/. The version available

at time of going to press was 2.3.1-2. This version is dated March 2001 and when I tried

to compile it on my system I got a myriad of errors. The sources do not use the

autoconf/automake build system and this may be the main cause of the errors. I have

decided to place the resolution of these problems outside the scope of this article given

the availability of precompiled packages for many distributions.

3. An Overview of Tripwire's Files

The operation of Tripwire is controlled by a configuration file and a policy file; both of

these files are encoded and signed before use for security reasons. These files usually

reside in /etc/tripwire. The plain text versions are called twcfg.txt and twpol.txt, and the

encoded and signed versions are called tw.cfg and tw.pol. The plain-text version of the

configuration file contains key-value pairs including the following required variables

(default values for my distribution shown):

POLFILE = /etc/tripwire/tw.pol

DBFILE = /var/lib/tripwire/$HOSTNAME.twd

REPORTFILE = /var/lib/tripwire/report/$HOSTNAME-$DATE.twr

SITEKEYFILE = /etc/tripwire/site.key

LOCALKEYFILE = /etc/tripwire/$HOSTNAME-local.key

The POLFILE, DBFILE and REPORTFILE dictate the locations of the policy file, the

database file and the report file respectively. A report file is generated each time Tripwire


is used to check the integrity of the file system and its name is determined by both the

hostname and current date. The SITEKEYFILE and LOCALKEYFILE variables hold the

locations of the two key files; site keys are used for signing files that can be used on

multiple systems within an organisation such as the policy and configuration files, while

the local key is used for files specific to this system such as the database file.

Ensure that the $HOSTNAME environment variable is correctly set to your system's

hostname before using any of Tripwire's commands. Also, the HOSTNAME variable in

twpol.txt must be set correctly so that it matches the system's hostname. Other

configuration file values we will use are shown here followed by a description of each:

EDITOR =/bin/vi

MAILNOVIOLATIONS =true

EMAILREPORTLEVEL =3

REPORTLEVEL =3

MAILMETHOD =SENDMAIL

MAILPROGRAM =/usr/sbin/sendmail -oi -t

When updating the database after files have been added, removed or altered, a "ballot-

box" styled form must be completed by placing an 'x' opposite the files which we have

changed and do not indicate an intrusion; this variable sets the editor to use for this

process.

MAILNOVIOLATIONS

Tripwire e-mails a report whenever a violation was found. This option tells Tripwire to

always e-mail a report whether a violation was found or not. This is useful as it shows the

system administrator that Tripwire is running as expected.

EMAILREPORTLEVEL and REPORTLEVEL

The amount of information Tripwire includes in its report file and e-mail. Valid values

range from 0 to 4 with the default being 3.

MAILMETHOD and MAILPROGRAM


The mail method can either be SMTP (in which case additional variables have to be set to

indicate the SMTP host and port) or SENDMAIL (in which case we include the

MAILPROGRAM variable).

There are a number of other options and these are explained in the man page:

TWCONFIG(4).

Creating your own policy file is a long and tedious task that is also outside the scope of

this article. If you get a packaged version of Tripwire for your distribution then the policy

file should already be created. The policy file is essentially a list of rules and associated

files which should be checked by Tripwire; the rules indicate the severity of a violation.

The text version of the file itself is quite readable and is worth a look to fully understand

how Tripwire works. Also, irrespective of your distribution, you will find that Tripwire

generates a lot of the following errors when checking the filesystem: File system error.

Filename: XXXXXXXXXXXXX

No such file or directory

For each of these errors there is an entry for the named file in the policy file but this file

does not exist on your system. You will have to edit the policy file and comment out

these lines.

Tripwire comes with four binary files:

tripwire :The main file; used for initialising the database, checking the integrity of the file

system, updating the database and updating the policy.

twadmin :Tripwire's administrative and utility tool; used for creating and printing

configuration files, replacing and printing a policy file, generating site and local keys and

other encryption related functions.

twprint :Used to print the reports and database in human-readable format.


siggen :Generates the various hashes that Tripwire supports for checking the integrity of

files.

4. Initialising the Keys and Database

In this section we will set Tripwire up so that you can use it on a daily basis to check your

systems integrity. I am assuming that the current working directory is /etc/tripwire and

that the following files exist in the specified paths:

/etc/tripwire/twcfg.txt: plain-text version of the configuration file

/etc/tripwire/twpol.txt: plain-text version of the policy file

The first step is to generate the keys to be used when signing the database, policy file and

configuration file. You will be asked for a passphrase for each of the local and site keys;

it should be greater than 8 characters and include punctuation symbols as well as

alphanumeric characters.

[root@home /etc/tripwire]# twadmin --generate-keys --site-keyfile ./site.key

(When selecting a passphrase, keep in mind that good passphrases typically have upper

and lower case letters, digits and punctuation marks, and are at least 8 characters in

length.)

Enter the site keyfile passphrase: XXXXXXXXXXXXXXXXXX

Verify the site keyfile passphrase: XXXXXXXXXXXXXXXXXX

Generating key (this may take several minutes)...Key generation complete.

[root@home /etc/tripwire]# twadmin --generate-keys --local-keyfile ./$HOSTNAME-

local.key

Enter the local keyfile passphrase: XXXXXXXXXXXXXXXXXX


Verify the local keyfile passphrase: XXXXXXXXXXXXXXXXXX

Generating key (this may take several minutes)...Key generation complete.

[root@home /etc/tripwire]#

Now that we have generated our keys, we need to sign the configuration and policy files

(after editing them as required):

[root@home /etc/tripwire]# twadmin --create-cfgfile --cfgfile ./tw.cfg --site-keyfile

./site.key \

twcfg.txt

Please enter your site passphrase: XXXXXXXXXXXXXXXXXX

Wrote configuration file: /etc/tripwire/tw.cfg

[root@home /etc/tripwire]# twadmin --create-polfile --cfgfile tw.cfg --site-keyfile

site.key twpol.txt

Please enter your site passphrase: XXXXXXXXXXXXXXXXXX

Wrote policy file: /etc/tripwire/tw.pol


Do not leave the plain-text versions of the configuration and policy files on your hard

drive.

Move them onto a floppy disk or encrypt them using a utility such as GPG. Also ensure

that the permissions of the signed files are set such that they are only readable/writable by

root:

[root@home /etc/tripwire]# chmod 0600 tw.cfg tw.pol

The last job we must do to complete the set-up is create the baseline database:

[root@home /etc/tripwire]# tripwire --init --cfgfile ./tw.cfg --polfile ./tw.pol \

--site-keyfile ./site.key --local-keyfile ./home.barryodonovan.com-local.key

Please enter your local passphrase:


Parsing policy file: /etc/tripwire/tw.pol

Generating the database...

*** Processing Unix File System ***

Wrote database file: /var/lib/tripwire/$HOSTNAME.twd

The database was successfully generated.


5. Integrity Checking

Now that Tripwire is installed, configured and the baseline database has been created, we

can get on with the business of checking the integrity of the file system:

[root@home /etc/tripwire]# tripwire --check

Parsing policy file: /etc/tripwire/tw.pol

*** Processing Unix File System ***

Performing integrity check...

Wrote report file: /var/lib/tripwire/report/$HOSTNAME-20040823-210750.twr

Total objects scanned: 52387

Total violations found: 0

Each violation (an addition, removal or change) is reported to stdout and written to the

report file as indicated. On this occasion I have assumed the default locations of the

configuration and policy files. I could have specified these explicitly on the command

line as I have been doing with switches such as --cfgfile, etc.

Your goal should be to set this up to run on a daily basis. This can be done as a cron or an

Anacron job; Anacron is the better choice when the computer is not on 24/7. Using either

cron or Anacron, the output should be e-mailed to the root user on each run of Tripwire.

In the case of Anacron, create a file in /etc/cron.daily/ called (for example) tripwire-check

containing:

#!/bin/bash


/usr/sbin/tripwire --check

and ensure that it is executable (chmod u+x /etc/cron.daily/tripwire-check). If you want to

use a cron job, then add the following line to root's crontab to perform the check every

day at 3am (crontab -e):

00 03 * * * /usr/sbin/tripwire --check

6. Updating the Database

When any file that Tripwire checks changes, you must update the Tripwire database so

that it reflects the new information for the file. This can be done as part of the integrity

checking process by using the interactive switch ('-I') or by using the database update

mode of the tripwire command:

[root@home /etc/tripwire]# tripwire --update --twrfile

/var/lib/tripwire/report/$HOSTNAME-20040823-210750.twr

< At this point you will be asked to choose which file records to update in the >

< database via the ballot-box mechanism. Unless you specified otherwise, vi >

< will be the editor chosen. If you have not used vi before then I suggest you >

< change it to a pico, nedit or whatever you prefer. Add/remove the x's from >

< the ballot boxes, save and exit >

Please enter your local passphrase: XXXXXXXXXXXXXXX

Wrote database file: /var/lib/tripwire/home.barryodonovan.com.twd


As you can see from the command line above, you must specify a report file to be used

when updating the database. Choose the most recently generated report file. If you find

yourself having to constantly update the same non-critical files, then feel free to update

the policy so as to exclude those files.

If any changes are found you will be presented with a "ballot-box" styled form that must

be completed by placing an 'x' opposite the violations that are safe to be updated in the


database (for example you updated the Apache web server yesterday and Tripwire is

reporting a change in the binary /usr/sbin/httpd as would be expected). If anything has

changed that you cannot directly account for then you should check it out as it may

indicate that someone has broken into your system.

7. Updating the Policy

The tripwire command has a policy update mode which means that a change in policy

does not require us to reinitialise the database. The policy update mode simply

synchronises the existing database with the new policy file. The new policy file expected

is the plain-text version - Tripwire will then ask for the local and site passphrases,

synchronise the database and sign both the new policy file and the database.

tripwire --update-policy --cfgfile ./tw.cfg --polfile ./tw.pol --site-keyfile ./site.key \

--local-keyfile ./$HOSTNAME-local.key new_policy_file.txt

Again, you should not leave the plain-text version of the policy file on the system.

8. Securing Tripwire

Using Tripwire as an intrusion detection system is only as effective as the security of

Tripwire itself. There are a number of procedures you can follow to ensure maximum

security:

ensure that no plain text copies of the configuration file or the policy file exist on

the system

ensure the access modes of the Tripwire binaries are 0500 (i.e. -r-x------); as root

execute: chmod 0500 /usr/sbin/tripwire /usr/sbin/twadmin /usr/sbin/twprint

/usr/sbin/siggen

ensure the access modes of the configuration directory and the database and report

directory are 0700 (i.e. -rwx------) and similarly for its contents; as root execute:

chmod -R u=rwX,go-rwx /var/lib/tripwire - the capital 'X' in the permissions sets


the execute (or access if a directory) bit if the file already has it set or if it is a

directory.

The last procedure is something that I would consider a 'must' rather than a 'should'.

Tripwire's database must be secure for an integrity check to be sufficiently trustworthy. If

you are not updating the database on a regular occasion (such as on a server, etc) then

you can keep the database on removable media without too much inconvenience. This

can be as simple as leaving a write-protected floppy cantaining the database in the floppy

drive, or a re-writable CD in a CD-ROM drive (read-only drive). If the database changes

then you can update the database on these mediums by write-enabling the floppy or

erasing and burning the new database to the CD-RW; but an attacker will be unable to

remove or alter the database in anyway.

A second solution would be to keep the database on another machine and download it as

required. This could be as simple as using wget to fetch the database from a web server

just prior to running the integrity check and removing it afterwards. For example, change

the Anacron script to:

#!/bin/bash

# switch to the database directory as specified by the Tripwire configuration file

cd /var/lib/tripwire

# download the database from a password protected directory (.htaccess)

wget http://www.someserver.com/private/$HOSTNAME.twd --http-user=username --

http-passwd=password

# perform the integrity check

/usr/sbin/tripwire --check

# remove the database


rm -f $HOSTNAME.twd

You can use scp, rsync, etc in a similar fashion.

. Further Resources

A standard installation of Tripwire comes with many man pages which provide all the

information you should need:

TRIPWIRE(8) - (i.e. execute man 8 tripwire to view this man page)

TWINTRO(8) - an introduction to Tripwire

TWADMIN(8)

TWPRINT(8)

SIGGEN(8)

TWCONFIG(8) - information on the Tripwire configuration file

TWPOLICY(8) - information on the Tripwire policy file

TWFILES(8) - information on the various Tripwire files


10. ADVANTAGES OF TRIPWIRE

Tripwire Integrity Management solutions give organizations visibility into service

affecting changes and, in the process, increase security, instill process accountability,

and improve system availability.

1. Increase security

Tripwire software immediately detects and pinpoints unauthorized change-whether

malicious or accidental, initiated externally or internally. Tripwire provides the only

way to know, with certainty, that systems remains uncompromised.

2. Instill Accountability

Tripwire identifies and reports the sources of change, enabling IT to “manage by fact.”

It also captures an audit trail of changes to servers and network devices.

3. Gain Visibility

Tripwire software provides a centralized view of changes across the enterprise

infrastructure and support multiple devices from multiple vendors.

4. Ensure Availability

Tripwire software reduces troubleshooting time, enabling rapid discovery and

recovery. Immediate detection of change enables the fastest possible restoration back

to a desired, good state.

tripwire report

Documents

tripwire intrusion system

intrusiondetection system

system administrator

computing system

system state

vital system

earlier system logs

activities changes