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Implementing Snort: A Lightweight Intrusion Detection System

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Implementing Snort can be a bit daunting for those unfamiliar with Intrusion Detection Systems. This second article on Snort walks you through the process of obtaining, installing, configuring, testing, and managing Snort in a multiplatform computing environment.

Snort is a self-described "Lightweight Intrusion Detection System" that can easily be incorporated into any network security architecture.

Installing Snort

Depending on your OS and CPU architecture, installing Snort may be as simple as typing in three words and hitting Enter, or it can be as fun as having oral surgery without the benefit of anesthesia. Snort does not come with an installer application; nor is there a 250-page installation, administration, and user's guide in a spiral-bound book with glossy pages. The next few sections will help educate and expand on the installation process to help you negotiate the briars and thickets of Snort sensor installation.

What Version Should I Install?

Snort version 1.7 became available on January 5, 2001. By the time you read this article, version 1.8 should have been released. There are many new exciting features and bug-fixes that were included in the 1.8 release, some of which are as follows:

  • Most command-line configuration switches can now be set via the config file.

  • Prioritization plugin, which allows you to assign priorities to alerts. This feature provides the capability to assign critical priorities to known attacks; and lower priorities to informational, reconnaissance or probe type of traffic.

  • Classification and prioritization to existing full, fast, syslog, and SMB alerting methods.

  • RPC decoding preprocessor.

  • Improved Unicode preprocessor.

  • Content inspection to catch Back Orifice traffic that is not on the default port of 31337.

  • IP protocol inspection, so that IP protocols other than the ones Snort has knowledge of can be checked.

  • Improved HTTP preprocessor speeds content-checking by checking a subset of the URI content.

  • Several new command-line switches, one of which is the most useful –T "test" command for validating setup and configuration.

However, if you are a bit conservative and want to only deploy tried-and-true software in your production environment, there is nothing inherently wrong with deploying the older 1.7. Everything in this article has been written around the deployment of Snort 1.7.

I strongly recommend against installing any versions older than 1.7. Prior versions are not only missing such important features as the newer preprocessor modules, but many of the updated rulesets on the Snort homepage or the arachNIDS database require at least Snort 1.7.

The port of Snort to Win32 usually lags behind the UNIX release, so if you are running servers on Win32 platforms, the latest release may not yet be available.

Snort Dependencies

Snort depends on a few libraries to be present on your system before it will operate properly. libpcap is a library responsible for actually interfacing with the operating system's device drivers to do the packet capture. Most BSD-derived systems and Linux distributions already provide precompiled versions of libpcap. All you need to do is install the RPM, or deb.

If you are using another UNIX, or want to compile the library from source, you can get libpcap from its Web site, http://www.tcpdump.org/.

libpcap has been ported to Windows NT and 2000 systems, and is known as winpcap. winpcap can be found in several places. The winpcap homepage is http://netgroup-serv.polito.it/winpcap/, although you can also find it linked off the Snort and Snort-Win32 home pages.

There are additional dependencies for additional features of Snort, but those will be covered in the "Compiling from Source" section because they are not necessary for the installation of Snort.

Precompiled Ready-to-Use Snort Packages

The Snort homepage has binary packages linked from the main page. There are precompiled versions available for a number of operating systems and CPU architectures.

UNIX/Linux

It may be more desirable to install a preconfigured or precompiled version of Snort from your UNIX or Linux distribution.

In Debian GNU/Linux, installing and configuring Snort is as simple as issuing the following command as the root user:

apt-get install snort

Part of the Debian packaging philosophy is policy, which means that your software is not only installed, but also configured to provide a minimum working system. It is certainly worthwhile to review your Snort configuration as prescribed later in this article.

RedHat or other RPM-based GNU/Linux distributions can install directly from your CD media, or you can FTP the files from the distribution's FTP site. After you download the file, you simply issue the following command as the root user:

rpm –Uvh snort*rpm

This will install Snort onto your system. You will still need to configure Snort installed from RPM packages.

BSD derived systems, such as OpenBSD, provide Snort in the form of a "port," which is a piece of software that is useful, but not part of the base operating system. Issue the following commands as root to install Snort on a recent BSD system:

cd /usr/ports/net/snort
make install

This will compile and install a copy of Snort specific to your BSD system. You will still need to configure Snort, and also make sure that Snort is launched after a boot.

Windows NT/2000

Michael Davis has been maintaining the Win32 port of Snort, allowing those users in the Windows NT and 2000 world to enjoy the power and flexibility that UNIX users have enjoyed.

The Snort-Win32 homepage is http://www.datanerds.net/~mike/snort.html. The Win32 packaged binaries are available in a number of "flavors." Each flavor has a different set of features enabled that would otherwise create unnecessary overhead if you were to not use them.

Basic Static flavor is just Snort without any additional features enabled. The "static" description implies that the required libraries were linked to the Snort application when it was compiled, rather than expecting you to provide the necessary DLL files at run-time. All prepackaged Win32 flavors are statically compiled.

FlexResp is the Flexible Response mechanism of Snort. This allows you to define how your system will react to a specific alert condition. FlexResp is not enabled, by default, and is not enabled in any of the rulesets available on the Web. It is up to you, the administrator, to determine what rules to respond to, and in what form that response should take.

MySQL is a free, Open Source database. It is very popular, and is used by many high-visibility Web sites on the Internet today. With the MySQL functionality, you can log to a MySQL database rather than just a local log file, syslog server, or local Eventlog.

Select which flavor of Snort-Win32 you want to use, and download it. In this example, we will log to a MySQL database, so the following examples make that assumption.

First, create a directory structure that looks like this:

C:\Snort\bin
C:\Snort\logs
C:\Snort\rules

Extract everything from the snort-1.7-win32-MySQL-static.zip and place the contents in C:\Snort\bin.

Compiling from Source

If there are no prepackaged binaries available for your platform, if you prefer to build a custom binary with only the appropriate features you want enabled, or if you're just paranoid about prepackaged applications (an otherwise healthy attitude for a network security engineer), you can compile Snort from source.

The released Snort source archives can be found at the main Snort site, http://www.snort.org/. If you are looking for something fresher, you can use anonymous CVS to retrieve up-to-date as the developers finish changes releases. Finally, for those who don't want to tangle with CVS, daily source snapshots can be found at http://snort.sourceforge.net/.

Those of you on Win32 platforms don't need to feel left out because you can install the Cygnus cygwin development environment, which provides a UNIX wrapper on the Win32 API, easing the porting of UNIX derived programs. The Cygnus cygwin environment can be found at http://www.redhat.com/.

A complete walk-through on compiling Snort from source is unfortunately beyond the scope of this article. The instructions in the FAQ and the INSTALL file, found inside the source distribution, are quite easy to follow. However, I will discuss a few of the more important compile-time configuration options in the next section.

Important Compile-time Configuration Options

  • enable-smbalerts—Compiles in support to send alert messages via the WinPopUp NETBIOS service. This is not supported on Win32 platforms, and isn't really recommended on UNIX systems, either. It requires smbclient from the SAMBA package to be installed.

  • --enable-flexresp—Enables the FlexResp feature. If you want to write or modify rules to allow your server to react to alerts via TCP RST or ICMP messages, enable FlexResp. FlexResp depends on the libnet library, which can be found at http://www.packetfactory.net/Projects/Libnet/. The FlexResp feature is intended for advanced users only!

  • --with-mysql—Compiles in support for sending log or alert messages to a MySQL database. If the configure program does not find the MySQL client libraries, you can specify the path like so: --with-mysql=/path/to/libs.

  • --with-postgresql—Compiles in support for sending log or alert messages to a PostgreSQL database. If the configure program does not find the PostgreSQL client libraries, you can specify the path like so: --with-postgresql=/path/to/libs.

  • --with-odbc—Compiles in support for sending log or alert messages to a database via ODBC. If the configure program does not find the ODBC client libraries, you can specify the path like so: --with-odbc=/path/to/libs.

  • --with-oracle—Compiles in support for sending log or alert messages to an Oracle database. If the configure program does not find the Oracle client libraries, you can specify the path like so: --with-oracle=/path/to/libs.

There are many patches to be found in the snort-devel mailing list archives. If you are having problems, it would be beneficial to search and read through these archives prior to posting your problem to the list. Many useful patches have been posted there as well. For example, if you are looking to log to a centralized syslog server from both UNIX and Win32 platforms, you may wish to investigate this patch:

http://www.geocrawler.com/archives/3/5344/2001/4/100/5503898/

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