...making Linux just a little more fun!
psbinary 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.
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.
/etc/tripwire. The plain text versions are called
twpol.txt, and the encoded and signed versions are called
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
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
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,
HOSTNAME variable in
must be set correctly so that it matches the system's
hostname. If you are unsure of what the system's hostname is set to, then
echo $HOSTNAME on the command line.
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
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
There are a number of other options and these are explained in the man
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 directoryFor 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:
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:
|plain-text version of the configuration file|
|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 [root@home /etc/tripwire]#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/$HOSTNAME.twd The database was successfully generated. [root@home /etc/tripwire]#
[root@home /etc/tripwire]# tripwire --check Parsing policy file: /etc/tripwire/tw.pol *** Processing Unix File System *** Performing integrity check... Wrote report file: /var/lib/tripwire/report/$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
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
called (for example)
#!/bin/bash /usr/sbin/tripwire --check
and ensure that it is executable (
/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
00 03 * * * /usr/sbin/tripwire --check
-I') or by using the database update mode of the
[root@home /etc/tripwire]# tripwire --update --twrfile /var/lib/tripwire/report/$HOSTNAME-20040823-210750.twr < At this point you will be asked to choose which file records to update in the > < database via the ballot-box mechanism. Unless you specified otherwise, vi > < will be the editor chosen. If you have not used vi before then I suggest you > < change it to a pico, nedit or whatever you prefer. Add/remove the x's from > < the ballot boxes, save and exit > Please enter your local passphrase: XXXXXXXXXXXXXXX Wrote database file: /var/lib/tripwire/home.barryodonovan.com.twd [root@home /etc/tripwire]#
As you can see from the command line above, you must specify a report file to be used when updating the database. Choose the most recently generated report file. If you find yourself having to constantly update the same non-critical files, then feel free to update the policy so as to exclude those files.
If any changes are found you will be presented with a "ballot-box"
styled form that must be completed by placing an 'x' opposite the
violations that are safe to be updated in the database (for example you
updated the Apache web server yesterday and Tripwire is reporting a change in
/usr/sbin/httpd as would be expected). If anything
has changed that you cannot directly account for then you should check it out
as it may indicate that someone has broken into your system.
tripwirecommand has a policy update mode which means that a change in policy does not require us to reinitialise the database. The policy update mode simply synchronises the existing database with the new policy file. The new policy file expected is the plain-text version - Tripwire will then ask for the local and site passphrases, synchronise the database and sign both the new policy file and the database.
tripwire --update-policy --cfgfile ./tw.cfg --polfile ./tw.pol --site-keyfile ./site.key \ --local-keyfile ./$HOSTNAME-local.key new_policy_file.txt
Again, you should not leave the plain-text version of the policy file on the system.
-r-x------); as root execute:
chmod 0500 /usr/sbin/tripwire /usr/sbin/twadmin /usr/sbin/twprint /usr/sbin/siggen
-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
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
rsync, etc in a similar fashion.
TRIPWIRE(8)- (i.e. execute
man 8 tripwireto view this man page)
TWINTRO(8)- an introduction to Tripwire
TWCONFIG(8)- information on the Tripwire configuration file
TWPOLICY(8)- information on the Tripwire policy file
TWFILES(8)- information on the various Tripwire files
Barry O'Donovan graduated from the National University of Ireland, Galway
with a B.Sc. (Hons) in computer science and mathematics. He is currently
completing a Ph.D. in computer science with the Information Hiding Laboratory, University
College Dublin, Ireland in the area of audio watermarking.
Barry has been using Linux since 1997 and his current flavor of choice
is Fedora Core. He is a member of the Irish
Linux Users Group. Whenever he's not doing his Ph.D. he can usually be
found supporting his finances by doing some work for Open Hosting, in the pub with friends or running in the local
Barry has been using Linux since 1997 and his current flavor of choice is Fedora Core. He is a member of the Irish Linux Users Group. Whenever he's not doing his Ph.D. he can usually be found supporting his finances by doing some work for Open Hosting, in the pub with friends or running in the local park.