Cover V10, I08



Distributed Intrusion Detection with Open Source Tools

Jason Chan

Over the past decade, businesses have rushed to connect and create a presence on the Internet. Unfortunately, the pace of corporations embracing information security as a benefit rather than an expense has only recently begun to quicken. Perhaps partially because of this reluctant corporate buy-in, the open-source security community has flourished. This growing community has created and improved the tools and resources to build and manage an entire security infrastructure based on open-source tools.

As businesses have begun to understand the importance of investing in security, intrusion detection has been one of the fastest advancing fields in both open- and closed-source security development. Without jumping into the semi-religious debate over the pros and cons of "free" software, I will present some open-source tools that can be used to introduce intrusion detection capabilities into your network.

Although some people argue the merits of Network Intrusion Detection Systems (NIDS/IDS), a properly designed and implemented NIDS can improve an organization's security posture while helping to enforce its security policy. An effective IDS will give visibility of a network's entire traffic flow and will provide data for informed decision making. This point leads to two of the most important issues in intrusion detection: distribution and correlation.

Distribution in an IDS refers to the effective placement of IDS sensors within an environment. An IDS must have full visibility of the environment it is monitoring to be of maximum utility. The most common example of a distributed IDS is seen in an organization with multiple external connections. If a network has both a dedicated connection to the Internet and a modem pool or other remote access solution for employees, IDS components should be positioned to observe traffic traveling to and from both access points. Key sensor locations for effective and comprehensive IDS distribution include traffic decryption points, remote access points, and internal networks.

The success of a distributed IDS relies heavily on data correlation. If an IDS can properly sense and report on data from multiple points within an infrastructure, the system must also be able to properly correlate this data. Have different IDS sensors detected the same IP address probing the network? What is the history of suspicious activity from a given network or network block? When an IDS is properly distributed and can effectively correlate information from different data sources, it can be a truly useful tool for securing a network and enforcing security policy.

The Design

Our distributed IDS example consists of IDS sensors logging data to a remote, centralized database. The communications from sensors to database will be encrypted. All components of the system are open-source tools.

The Tools


Snort ( is the excellent and widely used open-source IDS created by Martin Roesch. Snort requires libpcap ( and runs on a number of platforms, including Win32. This compact tool can be used as a simple network sniffer, as part of a distributed IDS, or just about anything in between. Snort has a flexible language for writing IDS rules that allows a user to generate customized rules. This flexibility has allowed the Snort user community to react quickly to the most recent exploits and vulnerabilities.

OpenSSL and Stunnel

One issue regarding the security of the IDS itself is the management of the data that the IDS captures and the alerts that it creates. An IDS can be configured to capture almost any data traversing the network, much of which may be sensitive. Also, the alerts and logging that the IDS itself generates should be considered sensitive data. As such, it is desirable to have a secure method of transferring data from the IDS sensors to the data correlation point. Some organizations handle this data transfer over a non-payload network secured from external access. Others handle the transfer manually with some form of removable media. My example system uses open-source encryption tools to secure the transfer of IDS data from sensors to the data aggregation point. OpenSSL ( is a toolkit that implements SSL version 2 and 3 as well as TLS (Transport Layer Security). Stunnel ( is a wrapper utility that allows for encryption of various TCP sessions via SSL.


Snort allows for multiple output options for alerts and logged data. One of these output options (and the most useful for IDS data correlation) is the database output plug-in written by Jed Pickel. The database output plug-in allows multiple Snort sensors to write Snort data to a single database and is available in Snort versions since 1.6.3. A robust database is the ideal method for managing information gathered from an IDS and is a key factor in leveraging an IDS for long-term traffic analysis, event correlation, and informed security policy decision making. The Snort database plug-in currently supports MySQL, Postgresql, Oracle, and UnixODBC. The example system uses MySQL (, the most popular open-source RDBMS (Relational DataBase Management System), for IDS data storage.


This article will cover the setup and configuration of the end system including the centralized database and a remote network sensor. For the sake of brevity, you'll need the following to continue:

On the Database Server:

  • A properly installed and secured MySQL database server
  • A properly installed OpenSSL distribution
  • A properly installed Stunnel
  • A user to access the Snort database (e.g., user=snortdb)
  • A user and group to run stunnel (e.g., user=stunnel, group=stunnel)
  • A copy of the create_mysql file from the contrib directory of the Snort source distribution
  • A copy of the snortdb_extra.gz file from:
On the IDS Sensor:

  • A running Snort compiled with MySQL support (the --with-mysql configure script option)
  • A properly installed OpenSSL distribution
  • A properly installed Stunnel
  • A user and group to run Snort (e.g., user=runsnort, group=runsnort)
  • A user and group to run Stunnel (user=stunnel, group=stunnel)
Setting Up the Database

Once the MySQL daemon is running on the database server, a database must be created and configured for the Snort data. To do so, log into MySQL as root or an appropriate user:

and enter the command:

mysql>CREATE DATABASE snortdata;
where "snortdata" is the name of the database that will house the Snort data. Next, grant appropriate rights on the database for the user that the sensors will use to add information to the database:

mysql>grant INSERT, SELECT on snortdata.* to snortdb;
where "snortdb" is the name of the local user account that the sensors will be configured to use.

Next, the Snort database structure must be created. This is done by using the create_mysql file included with the Snort distribution in the contrib directory. Exit the database and issue the command:

#mysql snortdata < create_mysql
The snordb_extra file from adds additional useful tables to the Snort database. These tables are created in the database with the following command:

#zcat snortdb-extra.gz | mysql snortdata
or if the file has already been decompressed:

#mysql snortdata < snortdb-extra
At this point, the database is correctly configured and is ready to accept data.

Server Stunnel Configuration

Rather than allowing IDS sensors to communicate with the database over the network with unencrypted communications, my example system uses Stunnel to encrypt communications with OpenSSL. Future versions of MySQL will handle native SSL communications directly, but support is limited as of this writing.

By default, Stunnel will allow communications to configured TCP ports from all hosts. However, Stunnel can be used in conjunction with TCP Wrappers ( to limit communications by IP address. To enable this functionality, we'll create a service name for the encrypted MySQL communications:

#echo "mysqls  3307/tcp" >> /etc/services
where "mysqls" is the chosen name and "3307" is the chosen TCP port over which the encrypted communications will occur. The chosen service name and TCP port should not conflict with existing service entries.

Add sensor IP addresses to the hosts.allow file in /etc:

echo "mysqls:" >> /etc/hosts.allow
where "" is the IP address of our IDS sensor. /etc/hosts.allow will need to be edited further to account for multiple sensors.

Next, block all other access to the encrypted MySQL port in the /etc/hosts.deny file:

echo "mysqls: ALL" >> /etc/hosts.deny
Kick off Stunnel so that connections from authorized hosts to port 3307 are forwarded to the MySQL listener on port 3306 listening on the loopback address. The following command will do:

stunnel -f -d mysqls -r -p /home/stunnel/stunnel.pem -N mysqls -s stunnel -g stunnel
Here is a summary of the command-line options used:

-f -- Keeps the process in the foreground, with connection and debugging information sent to the console (stderr)

-d mysqls -- Starts Stunnel in daemon mode for the mysqls service (TCP port 3307, configured above)

-r -- Specifies the remote service to which connections to the daemon port (mysqls) will be forwarded (TCP port 3306, the default MySQL port)

-p /home/stunnel/stunnel.pem -- Specifies the location of the Stunnel private key/certificate

-N mysqls -- Specifies the service name for TCP wrapper checking

-s stunnel -- setuid() to user 'stunnel'

-g stunnel -- setgid() to group 'stunnel'

At this point, only the IDS sensor at is allowed access to the mysqls service (TCP port 3307) on our database server.

On the Sensor(s)

Configuring the Encrypted Client Connection

Now that I've shown how Stunnel works, the client configuration for the sensor Stunnel configuration should be intuitive. The sensor must be configured with a listener on TCP port 3306. The Snort sensor will direct logging information to the local listener, and Stunnel will forward the data to the database server over the encrypted channel.

The Stunnel command we'll use on the sensor (the MySQL client) is:

stunnel -f -c -d  -r -s stunnel -g stunnel
In this example, "" is the address of the remote MySQL server. One flag we have not seen yet is the -c option, which configures Stunnel to act as a client. Note that I have not configured TCP Wrappers on the client, because I specified the client-side listener to listen on the loopback address. Configure such controls as described above if desired.

With this configuration, any connections that the sensor creates to the local listener on TCP 3306 will be forwarded by Stunnel to the SSL-encrypted MySQL listener at

We can now configure Snort to log data to the MySQL database.


First, ensure that Snort is able to capture traffic normally. Run:

snort -vi eth0
where "eth0" is the network interface on which Snort listens. Note that the sensor must be in position to sniff network traffic (i.e., on a hub or a switch with port mirroring enabled).

Ruleset Configuration Notes

Once the sensor is detecting traffic, the Snort configuration file should be customized for the sensor. There is an online rules database at that can fill in most of a sensor's signature set. You may want to create some rules of your own, which is quite simple with Snort's rules language. The process of creating an effective IDS rule base is subject to debate. Factors affecting the size and breadth of a ruleset include performance requirements, network activity, and other available security countermeasures.

Additionally, sensor placement will play a huge role in ruleset customization. Is the sensor inside a firewall or outside? Is it on a DNS subnet or a mail and Web DMZ? Some users will use Web-attack signatures on sensors that sit on a mail subnet, while others prefer to tune their attack signatures to the services that the Snort sensor is monitoring. Larger and more general rulesets will increase the number of alerts generated and can give more insight into the attacks being attempted, but these rulesets will also increase the likelihood of targeted attacks being "lost in the noise". Additionally, large rulesets will be more vulnerable to denial of service attacks aimed at the IDS itself, where an attacker creates a large amount of noticeable attack traffic designed to overwhelm the sensor detecting the traffic and the analyst interpreting the data. Conversely, very specific rulesets will generally produce more specific data, but it is unlikely that all suspect traffic will be noted. The size of any given ruleset often has a lot to do with the security administrator's relative level of paranoia.

Configuring Snort to Log to Mysql

The option for logging Snort data to our MySQL database is the output directive. To turn on logging to the configured MySQL database, add the following line to snort.conf (or your chosen Snort configuration file):

output log_database: log, mysql, host= dbname=snortdata \
  user=snortdb password=foo, encoding=ascii
Here is a summary of the directives used:

log -- Specifies that logging information should be sent to the database

mysql -- Specifies that the remote database is a MySQL database

host= -- Sends Snort log information to the MySQL listener on the sensor's loopback address. The sensor's Stunnel client will redirect the data to the remote server over the encrypted channel.

dbname=snortdata -- The remote database name

user=snortdb -- The user context with which to access the remote database

password=foo -- Specifies the password with which to access the remote database

encoding=ascii -- Log to the database using ASCII text to represent binary data

Because there is a password in plaintext in this configuration file, appropriate permissions should be applied to the Snort configuration file.

#chown runsnort:runsnort snort.conf; chmod 400 snort.conf
All that remains is to start Snort. A command similar to the following can be used:

snort -c snort.conf -d -i eth0 -u runsnort -g runsnort
Here is a summary of command-line options:

-c snort.conf -- Specifies the configuration file Snort should use

-d -- Dump application layer data

-i -- Use interface eth0

-u runsnort -- Run Snort as user "runsnort" after initialization

-g runsnort -- Run Snort under group "runsnort" after initialization

There are many other Snort options; check the README file included with the distribution for details.

At this point, the sensor will be logging to the remote MySQL database over an encrypted channel. Once the sensor has collected and logged some data, you can verify proper operation by logging into the database from the Snort sensor.

#mysql -h -u snortdb
mysql>use snortdata
mysql>SELECT * FROM event;
This query should display logged Snort events.


Once you have your Snort sensors logging to a remote database, you'll probably want to query and analyze the logged data. You can do this with native MySQL access methods and tools. ACID (Analysis Console for Intrusion Data -- is an excellent PHP application that provides a Web front-end for accessing Snort data in a central database. Many other tools developed by the Snort user community can assist in the efficient administration, maintenance, and analysis of Snort sensors and data. Check for the latest information.


To create a more complete IDS, other factors still must be considered. Host-based IDS (HIDS), near real-time alert analysis, and incident response policy are all components of an effective IDS. Although a distributed and correlated IDS is certainly a step in the right direction for any organization seeking to increase its security posture, countless other issues contribute to a sound and appropriate security infrastructure. Besides such visible items as firewalls, secured servers, and VPNs, security knowledge and policy will always be the most important factors. Understanding how business objectives relate to security policy and decisions will always come first, no matter the cost of your security tools.

Jason Chan is a Senior Security Architect for @stake, Inc. He has designed and supported large-scale network security infrastructures, including secure routing design, firewalls, VPNs, IDS, load-balancing, and content scanning and inspection. He's worked in database application support and served as a network/systems administrator. He also worked as a writing consultant for three years. Jason can be reached at: