One of the first things I learned about computer security was logging [1]. If you don't have logs, then trying to reconstruct what happened when something breaks, or when you get broken into, is almost impossible. The second thing I learned was that you have to centralize your logging; this is the only way to get a complete picture and ensure that an attacker can't simply wipe the logs on a compromised host, leaving you nothing to work with. But none of this will alert you to an attacker or, even more importantly, stop an attacker from getting in. It will simply give you something to look at once you figure out you have been broken into. For this, you need a human being in the loop, right? Well, you either need a human being or some smart software.

Wouldn't it be great if you could monitor critical logfiles (like mail and web) and actually have something respond to attacks, notifying you and even blocking the attacker from further access if you so wished? Well you're not the only one. Daniel B. Cid is the lead developer of the OSSEC project, an effort to build an open source host-based intrusion detection system [2]. OSSEC uses a traditional server and agent approach: You install the agent software on each system you want to monitor, and a central server collects all the data and sends out alerts. Additionally, the OSSEC project has released a web-based interface; however, it is only capable of reporting. Unfortunately, it can't be used to configure the system.

Installing OSSEC

When installing OSSEC, you have three options. The server option allows you to have it monitor itself and collect alerts from other systems. The agent option simply monitors local events and fires anything interesting off to the server. The local option runs the monitoring locally and can send email alerts, but it does not listen for any remote agents (so if you have one server or want to test it, this is the option for you). Simply download the OSSEC package (ossec-hids-2.0.tar.gz") and unpack it to a directory:

# wget http://www.ossec.net/files/ossec-hids-2.0.tar.gz
# tar -zxf ossec-hids-2.0.tar.gz
# cd ossec-hids-2.0
# ./install.sh

Now you simply choose your language, your server type, and whether you want to run the integrity check daemon, run the rootkit detection engine, enable active response, and enable the firewall to block attacks. If you are setting the system up as an agent, you also need to point it to your server and paste in the agent key. The agent key is a long string used to secure communications between an agent and the server, preventing fake messages from being injected, and so on. Why is it important to prevent spoofed or fake messages from being sent to the server?

Beware

If an attacker can trigger fake or spoofed attacks and a system blocks IP addresses or users because of this, the attacker can easily block legitimate systems and lock users out. In a worst case scenario, you might have to break into your own system if your accounts are locked out, which is why most HIDS and NIDS support whitelisting (see the "HIDS vs. NIDS" box). Administrators simply create a list of hosts and networks that are critical. Of course, determining which hosts are critical depends on the exact setup (DNS, email, file servers, authentication servers, routers, etc. are all a good place to start). With OSSEC, the whitelist is held in the ossec.conf file (by default, this is kept in /var/ossec/etc/), and you can specify individual hosts or networks:

<global>
 <white_list>127.0.0.1</white_list>
 <white_list>1.2.3.4</white_list>
 <white_list>10.0.0.0/8</white_list>
 <white_list>192.168.0.0/16</white_list>
</global>

Running OSSEC

The OSSEC program comes with its own control program called ossec-control. Additionally, when installed on Red Hat Linux or CentOS, a standard set of rc.d/init scripts will be added, allowing the OSSEC services to be control through the standard chkconfig utility. When OSSEC is running, you should see a number of programs running.

The monitoring processes generally need to run as root:

USER PID COMMAND
ossecm 17381 /var/ossec/bin/ossec-maild
root   17385 /var/ossec/bin/ossec-execd
ossec  17389 /var/ossec/bin/ossec-analysisd
root   17393 /var/ossec/bin/ossec-logcollector
root   17405 /var/ossec/bin/ossec-syscheckd
ossec  17409 /var/ossec/bin/ossec-monitord