Automated Intrusion Monitoring with Snort

Snort is an excellent intrusion detection software package combining best-in-class technology with open-source configurability. Snort actually has a few different modes of operation, including a sniffer mode, a packet logger mode, an intrusion detection mode, and what is called inline mode. It is the intrusion detection mode that is of interest in this section. However, inline mode is also notable because it provides a way to configure Snort and iptables to work together to dynamically accept or drop packets based on Snort rules. For the purposes of this chapter, when referring to Snort I'm referring specifically to the intrusion detection mode.

When in intrusion detection mode, Snort works by using a number of rules that define anomalous traffic. Many of these rules come predefined for you by Sourcefire, the makers of Snort. Many other rules are available from the community, and of course you can also write your own rules as necessary. CERT has a good paper on writing rules, aptly titled "Writing Rules and Understanding Alerts for Snort," available at

In addition to rules, Snort has a number of preprocessors that enable modules to view and alter packets before they are handled by the intrusion detection engine of the software. Preprocessors can be developed to suit your needs, though the preprocessors already available are helpful. The preexisting preprocessors include two types of port scan detectors to help detect and take action when a port scan is detected. There are also preprocessors to reassemble TCP streams to provide stateful analysis and preprocessors to decode RPC traffic and inspect HTTP traffic. Other preprocessors are described in detail in the Snort documentation available with Snort or online at

Snort works by detecting and reporting on events. The actual process of reporting on events can be configured through event handling within Snort. Event handling calls for configuration based on thresholds. This highly configurable aspect of Snort helps to prevent being inundated with log entries and alerts.

Normally you'd want to be notified in some way when certain Snort rules are triggered. Snort uses output modules that can be configured to send the output to various locations. The primary or most commonly used output module is the alert_syslog module, which sends alerts to the local syslog facility. Other output modules exist, including alert_fast and alert_full. The former puts a fast entry into the file specified, and the latter sends the entire packet header along with the event message. Other output modules exist and more information on them can be found within the Snort documentation.

One interesting output module is the database output module. The database module enables Snort alerts to be sent to an SQL database. Using this output module enables you to leverage software called ACID, which can generate reports on the alerts and events in Snort.

Snort has numerous additional features and nuances that help make it one of the best, if not the best, intrusion detection software available.

Obtaining and Installing Snort

Snort is available with many Linux distributions as an additional package, and most distributions also include the Snort rules, either included with the Snort package or as an add-on package. You can also download Snort from

Installation of Snort and the default rules should nearly always be done by installing the package available with your distribution. If this isn't possible or if the available package doesn't include the options you'd like, you'll need to compile from source.

The Snort package comes in a gzip archive and therefore needs to be unzipped and unarchived prior to being compiled:

 tar -zxvf snort-<version>.tar.gz 

After it's unzipped and unarchived, you can cd into the Snort source directory and run the configure script:

 cd snort-<version> ./configure 

It is at the point of running the configure script where many compile-time options can be set. To obtain a list of some of these options, notably options to enable support for certain databases or enable certain other features, type the following:

 ./configure --help 

In addition, the INSTALL document and other documentation within the <snort-source>/doc directory explain these and other options available when compiling Snort from source.

Running the configure script, along with any options, will result in Snort looking for various prerequisites. For example, when compiling Snort from source, you might receive an error indicating that one or more prerequisites can't be found, such as this error:

 checking for pcre.h... no   ERROR! Libpcre header not found, go get it from 

With that error in mind, I was able to install the pcre development files, rerun the configure script, and continue.

After the configure script has run successfully, compile the software by typing this:


The software will now compile. Should you get any errors during this phase, consult the Snort documentation and mailing list archives to see whether you've received a known error for your architecture.

Finally, after the software is compiled, install it by typing the following:

 make install 

The software will now be installed and should be ready to use. By default, the software is installed into /usr/local/bin. You can test the basic Snort command by typing this:

 /usr/local/bin/snort -? 

You should see output with help options, similar to this:

   ,,_   -*> Snort! <*-  o" )~  Version 2.3.3 (Build 14)   ''''  By Martin Roesch & The Snort Team:       (C) Copyright 1998-2004 Sourcefire Inc., et al. USAGE: ./snort [-options] <filter options> Options:     -A     Set alert mode: fast, full, console, or none (alert file alerts only)           "unsock" enables UNIX socket logging (experimental). ... <output truncated> 

Configuring Snort

The source code for Snort includes a sample configuration file. If you've installed from your distribution's package, that too should include a sample Snort configuration file. Usually this file is called snort.conf. On most popular distributions, including SUSE and Debian, this file (along with a number of Snort rules) is placed in /etc/snort/.

If you're working with a source installation, the sample snort.conf configuration file is located in <snort-source>/etc/ and sample rules are located in <snort-source>/rules/. For those working with a source-code version, I recommend creating a directory in either /etc/ or /usr/local/etc/ called snort and placing the snort.conf configuration file and the Snort rules in that directory. In addition, the default Snort configuration file calls various map and extra configuration files as well. These files can also be found in the <snort-source>/etc/ directory. Creating the directory and copying all the files into it would look like this (again, this is applicable only to those who have compiled Snort from source):

 mkdir /etc/snort cp <snort-source>/etc/snort.conf /etc/snort/ cp <snort-source>/etc/*.map /etc/snort/ cp <snort-source>/etc/*.config /etc/snort/ cp <snort-source>/rules/*.rules /etc/snort/ 

One additional and important change is done to the snort.conf configuration file. After you've copied it to the /etc/snort directory, edit the file and change the RULE_PATH variable from its default of ../rules to /etc/snort. The line should look like this when you're done:

 var RULE_PATH /etc/snort 

Finally, create the Snort log directory with the following command:

 mkdir /var/log/snort 

With all the groundwork done, it's time to officially start Snort for the first time. If you've installed from your distribution's package, it's likely that you can start Snort through the normal run control mechanism, such as /etc/init.d/snort start. If you've compiled from source, you'll need to start Snort manually and point to the location of its configuration file:

 /usr/local/bin/snort -c /etc/snort/snort.conf 

If you receive any errors, chances are that there are missing files. Check the Snort source directory structure for the missing files and copy them to the appropriate location, based on the configuration file.

If all goes well, you should see a message such as this, near the end of the output:

 --== Initialization Complete ==-- 

As you can see, the shell prompt didn't return. This is because Snort was not told to fork into daemon mode. Press Ctrl+C to kill Snort and add a -D to the command line. It should now look like this:

 /usr/local/bin/snort -c /etc/snort/snort.conf -D 

Snort will start again and this time fork off into the background, returning you to the shell prompt.

There is, of course, much more to Snort configuration than merely getting it running with the default options and rulesets. For more information on specific Snort configurations, refer to the Snort documentation.

Testing Snort

With Snort now running in the background, you could assume that it's running perfectly fine and that log entries will be placed into /var/log/snort for you. However, I'm not one to assume things, especially about computer security. Therefore, to test the Snort installation I'll use the handy hping2 tool to craft a packet or two and fire them toward the host running Snort.

In this case, I'm just looking for verification that Snort is running and monitoring something. The default rules look for bad packets, so crafting one of those should be trivial with hping2. From another host on the network (, I ran the following hping2 command toward the host running Snort (

 hping2 -X 

The -X option causes an Xmas scan to be run. Looking in /var/log/snort on the host running Snort reveals that the alert file has received some information and there is also a new directory called, which was the source of the test packets. Inside that directory are files corresponding to the packets I sent, the contents of which are shown here:

 [**] BAD-TRAFFIC tcp port 0 traffic [**] 06/07-16:19:00.712543 -> TCP TTL:64 TOS:0x0 ID:48557 IpLen:20 DgmLen:40 *2****** Seq: 0xED1609B Ack: 0x13E893C5 Win: 0x200 TcpLen: 20 =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ [**] BAD-TRAFFIC tcp port 0 traffic [**] 06/07-16:19:00.712610 -> TCP TTL:64 TOS:0x0 ID:10034 IpLen:20 DgmLen:40 DF ***A*R** Seq: 0x0 Ack: 0xED1609B Win: 0x0 TcpLen: 20 =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 

As you can see from the output, Snort has captured what it believes to be (correctly so) bad TCP packets. The alert log file /var/log/snort/alert also contains information that is especially useful for sorting the alerts. Here are the log entries from the alert log file that correspond to the previously shown entries from the specific host file:

 [**] [1:524:8] BAD-TRAFFIC tcp port 0 traffic [**] [Classification: Misc activity] [Priority: 3] 06/07-16:19:00.712543 -> TCP TTL:64 TOS:0x0 ID:48557 IpLen:20 DgmLen:40 *2****** Seq: 0xED1609B Ack: 0x13E893C5 Win: 0x200 TcpLen: 20 [**] [1:524:8] BAD-TRAFFIC tcp port 0 traffic [**] [Classification: Misc activity] [Priority: 3] 06/07-16:19:00.712610 -> TCP TTL:64 TOS:0x0 ID:10034 IpLen:20 DgmLen:40 DF ***A*R** Seq: 0x0 Ack: 0xED1609B Win: 0x0 TcpLen: 20 

As you can see from these entries, there are some additional items such as Classification and Priority that can help to, well, classify and prioritize the alert. Both the classification and the priority can be configured within the alert log file.

Receiving Alerts

I recommend working with Snort to gain experience with rules and configuration options before configuring it to send alerts in email or via another means. You might easily find yourself overwhelmed by alerts with the default Snort rules, depending on your network layout.

Recall from Chapter 10 that log file monitoring software was introduced and a recipe was given for using Swatch to monitor log files for certain events, at which time it would send an email based on the alert. If you see where I'm going with this, then congratulations!


With its default configuration, Snort logs to /var/log/snort/alert. Therefore, creating a Swatch configuration to monitor this file is quite easy. Again, it would be easy to overwhelm yourself or the system with alerts and emails from Swatch, so you should use caution when configuring any actions based on Snort alerts until you've had a chance to configure Snort further.

Recall that Snort logs some prioritization data within /var/log/snort/alert. Therefore, you could set up a Swatch rule to watch for anything with a certain priority, say 3, for example, and send an email when that's seen. This would be placed within your Swatch configuration file which, by default, is ~/.swatchrc. Here's the configuration entry:

 watchfor /Priority: 3/   mail 

Starting Swatch and pointing it toward the Snort alert file, /var/log/snort/alert, looks like this:

 swatch --tail-file=/var/log/snort/alert 

Now when an alert with Priority: 3 is logged, an email will be sent by Swatch.

Final Thoughts on Snort

Snort comes highly recommended as a means to automate the task of intrusion detection. I've only been able to touch on the very basics of Snort here in hopes of giving you a starting point for working with it. From here, you can combine Snort with MySQL and ACID to create an enterprise-class intrusion detection system. Snort can be configured just as you need and extended to fit any size of organization.

Linux Firewalls
Linux Firewalls: Attack Detection and Response with iptables, psad, and fwsnort
ISBN: 1593271417
EAN: 2147483647
Year: 2005
Pages: 163
Authors: Michael Rash

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