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INFA 630–Lab #3

Lab Assignment #3

Our third and final lab assignment builds on the “unacceptable site” detection we worked on in

assignment #2. In this lab we will attempt to accomplish the same goal using the new reputation

preprocessor in Snort. The documentation on the reputation preprocessor and the available

configuration options are in section 2.2.19 (starting on p. 119) of the Snort Manual, which is

posted under General Information under Course Content for your reference. The basic function

of the reputation preprocessor is similar in many ways to basic firewall operation: the

preprocessor evaluates source and destination IP addresses in network packets to see if they

appear on either a “whitelist” of approved/acceptable addresses or a “blacklist” of prohibited

addresses. Packets containing IP addresses on the blacklist are dropped. The overall intent for

this assignment is to block access to the “bad” site you selected for Lab #2 by adding the site to a

blacklist and enabling the reputation preprocessor in snort.conf.

To complete this assignment successfully, you will need to first edit the snort.conf file as


 At the end of Step #1, either set the path to the reputation preprocessor file location or

comment out these two lines (you can declare the blacklist file directly in the

preprocessor configuration settings if you don’t want to use a variable reference).

 At the end of Step #5, configure the reputation preprocessor. Look at the first

configuration example on page 119 of the Snort Manual as a guide, which simply

includes the preprocessor declaration and the specification of the blacklist and whitelist

files. You can run the preprocessor with either or both of these files, so for our purposes

you might just specify a blacklist file. The configuration could be as simple as:

“preprocessor reputation: blacklist /etc/snort/black.list”

 Save the snort.conf file.

Now, create a blacklist file and put it in the proper directory (such as /etc/snort/rules on Linux or

C:Snortetcrules on Windows). A blacklist file is just a plain text file with one IP address (or

address range, using CIDR notation) per line. The blacklist file name and file location should of

course match what you specified in the preprocessor configuration in snort.conf. Then startup

Snort as you would normally, open a browser, and visit the site corresponding to the IP

address(es) in the blacklist file.

For this assignment, compose a short writeup for submission to your Assignments folder that

includes the following:

1. The “unacceptable” site you selected in Lab #2 (you can pick a new one for this assignment if you prefer).

2. The IP address (individual, multiple, or a range) associated with that site. If you don’t know the IP address, you can either open a command shell and ping the site (e.g. “ping”), which will return the primary IP address on screen, or you can

look up the site on to find one or more IP addresses used by the site.

3. The contents of the blacklist file the reputation preprocessor references. 4. A brief summary comparing the rule-based and preprocessor-based approaches used in

Lab #2 and #3, with an emphasis on identifying any strengths or weaknesses associated

with each approach.

5. If you are able to get Snort to run successfully with the reputation preprocessor active, include the output produced (a copy of the ASCII log file is sufficient).

As in Lab Assignment #2, the successful completion of this exercise does not require you to use

an actual inappropriate site. The primary purpose of this exercise is not to make you an expert in

the reputation preprocessor, but to illustrate the point that there are often multiple viable

approaches to accomplishing the same intrusion detection objectives.

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