OSSEC edit

The guarantee of the security of a computer cluster is a crucial topic. Such a cluster may often have vulnerabilities that are easy to exploit, so that essential parts of it can be damaged. Therefore, a supervisory piece of software is needed which monitors the activities within the system and in the network, respectively. Moreover it should warn the system’s administrator and - more importantly - it should block an attack to protect the system.
OSSEC is such a software. It is a host based intrusion detection system (HIDS) which monitors all the internal and network activities of a computer system. This includes the monitoring of essential system files (file integrity check, section 7.1) and the analysis of log files which give information about the system’s activities (log monitoring, section 7.3). Log files include protocols of every program, service and the system itself, so that it is possible to trace what is happening. This can help to find implications of illegal operations executed on the system. Moreover, OSSEC can detect software that is installed secretly on the system to get information about system and user data (rootkit detection, section 7.2). Another feature of OSSEC is the active response utility (section 7.3). Whenever OSSEC recognizes an attack, it tries to block it (e.g. by blocking an IP address) and sends an alert to the system administrator.
To analyze the events occurring in the system, OSSEC needs a central manager. This is usually the master node of the computer cluster, where all programs and services are installed. The monitored systems are the agents, normally the workers of the computer cluster. They collect information in realtime and forward it to the manager for analysis and correlation.
In this chapter the capabilities of OSSEC - file integrity checking, rootkit detection, log analysis and active response - will be explained. The setup (section 7.6) of this intrusion detection system consists of two parts. First, the configuration of the OSSEC server - namely the main node of the computing system - and secondly the configuration of the agents. A web user interface (WUI) can be installed optionally (section 7.7); it helps the user to view all the statistics the OSSEC server has collected during the running time of the whole system. The WUI’s installation (section 7.7.1) and functionality (section 7.7.2) are explained. Finally, a summary accomplishes this chapter.

File Integrity Checking edit

A file has integrity when no illegal changes in form of alteration, addition or deletion have been made. Checking for file integrity maintains the consistency of files, the protection of them and the data they are related to. The check is typically performed by comparing the checksum of the file that has been modified against the checksum of a known file. OSSEC uses two algorithms to perform the integrity check: MD5 and SHA1. These are two widely used hash functions that produce a hash value for an arbitrary block of data.
The OSSEC server stores all the checksum values that have been calculated while the system was running. To check file integrity, OSSEC scans the system every few hours and calculates the checksums of the files on each server and agent. Then the newly calculated checksums are checked for modifications by comparing them with the checksums stored on the server. If there is a critical change, an alert is sent.
It is possible to specify the files that will be checked. The files that are inspected by default are located at /etc, /usr/bin, /usr/sbin, /bin, /sbin. These directories are important, because they contain files that are essential for the system. If the system is under attack, the files located at these directories will be probably changed first.

Rootkit Detection edit

We use all these system calls because some kernel-level rootkits hide files from some system calls. The more system calls we try, the better the detection

A rootkit is a set of malicious software, called malware, that tries to install programs on a system to watch its and the user’s activities secretly. The rootkit hides the existence of this foreign software and its processes to get privileged access to the system, especially root access. This kind of intrusion is hard to find because the malware can subvert the detection software.
To detect rootkits, OSSEC uses a file that contains a database of rootkits and files that are used by them. It searches for these files and tries to do system operations, e.g. fopen, on them, so that rootkits on a kernel level can be found.
A lot of rootkits use the /dev directory to hide their files. Normally, it contains only device files for all the devices of a system. So, OSSEC searches for files that are strange in this directory. Another indication for a rootkit attack are files with root access and write permissions to other files which are not owned by root. OSSEC’s rootkit detection scans the whole filesystem for these files.
Among these detection methods, there are a lot of other checks that the rootkit detection performs.[1]

Log Monitoring and Active Response edit

OSSEC’s log analysis uses log files as the primary source of information. It detects attacks on the network and/or system applications. This kind of analysis is done in realtime, so whenever an event occurs, OSSEC analyzes it. Generally, OSSEC monitors specified log files - that usually have syslog as a standard protocoll - and picks important information of log fields like user name, source IP address and the name of the program that has been called. The analysis process of log files will be described in more detail in section 7.5.
After the analysis of the log files, OSSEC can use the extracted information as a trigger to start an active response. These triggers can be an attack, a policy violation or an unauthorized access. OSSEC can then block specific hosts and/or services in order to stop the violation. For example, when an unauthorized user tries to get access via ssh, OSSEC determines the IP address of the user and blocks it by setting it on a black list. Determining the IP address of a user is a task of the OSSEC HIDS analysis process (see section 7.5).

OSSEC Infrastructure edit

Figure 7.1 depicts OSSEC’s important directories and the corresponding files. The directories contain the binary files, configuration files, the decoder for decoding the events (see section 7.5) and the log files. They are all located in /var/ossec or in the directory which was specified during the installation, respectively.
Some of the essential executables are listed in this figure. For example ossec_control is used for starting the OSSEC system on the server or on an agent. syscheckd is the program for performing the file integrity check (section 7.1). To generate and import keys for the agents (see section 7.6) manage_agents is used.
All main configurations of OSSEC, for example setting the email notification, are done in ossec.conf. OSSEC’s meta-information that has been specified during the installation (see section 7.6) are stored in the ossec-init.conf and in internal_options.conf.
The file ossec.log stores everything that happens inside OSSEC. For example, if an OSSEC service starts or is canceled, this is listed in the ossec.log file. In alert.log all critical events are logged.

Figure 7.1: Infrastructure of OSSEC. This is only an extract of the whole infrastructure.

The OSSEC HIDS Analysis Process edit

When changes are made within the system, the type of this changing event has to be classified. Figure 7.2 shows the analysis process of such an event. It is performed in two major steps called predecoding and decoding. These steps extract relevant information from the event and rate its severity. This rating is done by finding a predefined rules which matches the event. The rule stores the severity of an event in different levels (0 to 15). If the event has been classified as an attack to the system, an alert is sent to the administrator and OSSEC tries to block the attack (active response). In the next sections, the steps predecoding and decoding are explained in more detail.

Figure 7.2: The analysis process of an event in OSSEC [BC08].

Predecoding edit

The predecoding step only extracts static information like time, date, hostname, program name and the log message. These are well-known fields of several used protocols. There are a lot of standards for computer data logging - like the Apple System Log (ASL) and syslog - which use a different formatting to handle and to store log messages. However, OSSEC is able to distinguish between these types of protocols. As an example, the following log message shows an invalid login of an unknown user using the ssh command (syslog standard).

Feb 27 13:11:18 master sshd[13030]: Invalid user evil from


OSSEC now extracts these information and classify the fields of this message. Table 7.1 shows the fields that are picked by the predecoding process and their description.

Table 7.1: Example of a predecoded ssh event.

As mentioned before, there are several protocols that store log messages in different ways. The log messages have to be normalized so that the same rule can be applied for differently formatted log files. This is the task of the decoding phase that is described in the next section.

Decoding and Rule Matching edit

Following the predecoding phase, the decoding step extracts the nonstatic information of the events, e.g. the IP address, usernames and similar data, that can change from event to event. To get this data, a special XML file is used as a collection of predefined and user-defined decoders. They are matched with the current event using regular expressions. A decoder is specified by several options. These options define on which conditions a decoder will be executed. First of all, a decoder is delimited by a <decoder></decoder> tag, where the name of the decoder is specified. Within this tag all the possible options can be applied to fully refine the characteristics of the decoder. When working with syslog, the program name has to be set so that the decoder is only called if the program name appears in the log message. Another important option is the <prematch></prematch> tag. It can optionally be used jointly with the program name as an additional condition or as a single condition for non-syslog messages. To extract nonstatic information the decoder needs a <regex></regex> tag. This tag describes a regular expression which defines a pattern for matching the information included in the log message. As there are several fields in a message, the order of the fields also has to be specified, so that OSSEC knows how to parse out the fields of the message. This is done by the <order></order> tag.
Listing 7.1 gives an example of a decoder for an ssh event. It extracts the name and the source IP address of a user who tries to get access without any permissions via ssh corresponding to the given example.

Listing 7.1 Example of a decoder for ssh.
<decoder name="ssh-invalid-user">
         <prematch>^Invalid user</prematch>
         <regex>(\S+) from (\S+)$</regex>

This decoder will only be called when the program name sshd has been extracted in the predecoding phase. Additionally, the prematch tag has to be satisfied. In this example the prematch option searches for the expression Invalid user in the log message. When this pattern will match what is in the log, the regular expression will be called to extract the user and source IP address. So only the decoder relevant for finding a rule associated with an invalid ssh success will be executed.
There are a lot of other options to specify the decoders. Here, the most important ones were explained. See OSSEC’s homepage[2] for more explanations on decoder options.
By now OSSEC has all the information it needs to find a rule that matches an event. Rules are typically stored as XML files in /var/ossec/rules. A rule is identified by a unique number and a severity level. Depending on this level, OSSEC decides if this event should be escalated to an alert.
Two types of rules exist: atomic and composite. Atomic rules are based on single events, whereas composite rules are related to multiple events. For example an ssh event can be escalated to an alert already after one single ssh authentication failure (atomic) or can be suspended until three authentications have failed (composite).

Setup and Configuration edit

Before starting the setup of OSSEC on the system, a C compiler and a makefile tool should be available on the system, because OSSEC is written in C and for compiling the sources, Makefiles are used. The sources of OSSEC are available at the OSSEC’s homepage and can simply be downloaded with the wget command (Listing 7.2).

Listing 7.2 Getting the OSSEC source code.

wget http://www.ossec.net/files/ossec-hids-2.6.tar.gz After extracting the archive, and changing to the extracted directory, the setup will start by executing the installer script install.sh (Listing 7.3). Subsequently, the user is guided through the setup.

Listing 7.3 Extracting the archive and starting the installation.

tar -xvf ossec-hids-2.6.tar.gz cd ossec-hids-2.6 ./install.sh OSSEC is a host-based intrusion detection system, that means if several computers are part of the cluster and all computers have to be monitored, OSSEC has to be installed on all of them. Furthermore, servers that manage all the data of the other computers - that means the agents - have to be declared. Depending on the intended type of usage of the computer (server or agent), OSSEC will be installed and configured in a different way. In the following, the setup of the server and the agents will be explained in separate sections. All installations were performed on Ubuntu Server 12.04.

Setup of the Server edit

The setup of the server is straight forward. One simply follows the instructions of the install script which include:

  1. setting the language (default is English)
  2. choosing Server as install type
  3. choosing the directory where OSSEC will be installed (default is /var/ossec)
  4. configuring the email notification (default is no email notification, see section 7.6.1 for how to set up the email notification)
  5. enabling/disabling the integrity check, the rootkit detection engine, active response and the firewall-drop response (enabled by default)
  6. adding IP addresses to the white list (localhost is the only IP address on the list by default)
  7. enabling/disabling remote syslog (allows redirecting the syslog messages from one host to another host)

When the setup is successful, the installer script provides some final information. If it was not successful, the script shows ”Building error. Unable to finish the installation” at the end of the compiler output. Maybe a prerequisite is missing; the OSSEC homepage might be a good help.
OSSEC is to be started with the command ossec-control located at the directory of OSSEC’s binary files (Listing 7.4). This command starts all the services as specified in the setup, e.g. rootkit detection and integrity check.

Listing 7.4 Starting OSSEC.

/var/ossec/bin/ossec-control start Whenever starting the server, OSSEC will be started automatically. OSSEC is active until the user stops it with ossec_control stop.
It recommended not to run any other intrusion detection systems in parallel with OSSEC. It is possible that they mutually influence each other. For example OSSEC could recognize the other intrusion detection and its actions as an “attack” to the system, when it modifies files that are monitored by OSSEC.

Email Notification

Some SMTP servers permit emails to be relayed from the host running the OSSEC HIDS. Because of that, it is better to install a local mail server. For this, all the settings are done on the OSSEC server. First a mail server is needed, for example postfix.[3] To process the emails, an email program has to be installed. The bsd-mailx, a command-line based email program is a possible choice for it (Listing 7.5).

Listing 7.5 Installing the mail server and an email program.

sudo apt-get install postfix sudo apt-get install bsd-mailx Finally, OSSEC needs to know where to send the emails to. In the main configuration file ossec.conf, the following entry has to be added:

Listing 7.6 Configuring the email notification in OSSEC.

This enables the email notification. All emails will be send to the user root who has an account on the localhost (the OSSEC server master in this example), where the mail server has been installed. The <email_from></email_from> tag specifies the sender of the email notification.
Subsequent to the base configuration, it is possible to personalize the email notification. At default, OSSEC will send an email for every alert. This can be customized by setting the email alert level. Events are rated by their severity ranging from 0 to 15. Listing 7.7 shows how to set the email alert level, that means the minimum level an event must have so that an email will be sent. In the above example (Listing 7.6) there will only be sent an email if the event has an severity level greater than 7. The same option can be specified for writing the log files. In the below example (Listing 7.7) every event will be logged and saved in a log file, because the log alert level is set on the smallest possible severity level 0. Both options are located in the <alert></alert> tag in the main configuration file ossec.conf.

Listing 7.7 Setting the email alert level.

Setup of the Agent edit

Before installing the agents, the server needs to know the agents from which it has to collect the data. The agents must be able to identify themselves to the server and the server must be able to validate the identity of an agent.
The communication between server and agent is handled by key management. Keys are generated on the server and are then imported on each agent. OSSEC offers a tool, to perform the key management: manage_agents (Listing 7.8). First this tool has to be executed on the server to add the agents.

Listing 7.8 Starting the key management.

/var/ossec/bin/manage_agents After starting the tool, there are five options to choose:

for adding an agent
for extracting the key for an agent
for listing already added agents
for removing an agent and
to quit the service

When adding an agent (option (A)), the user is prompted for host details and identifier for the agent. This includes a name, the IP address of the agent and a unique identifier which is a three-digit number (Listing 7.9]).

Listing 7.9 Adding an agent.
- Adding a new agent (use '\q' to return to the main menu).
  Please provide the following:
  * A name for the new agent: worker1
  * The IP address of the new agent:
  * An ID for the new agent[001]: 001
Agent information:
  ID: 001
  Name: worker1
  IP Address:
Confirm adding it? (y/n): y
Agent added.

Then OSSEC has to be installed on each agent separately using the installer script. The setup is very similar to the setup of the server (see section 7.6.1), the only difference is that the agents need to know the IP address of the server (Listing 7.10). Here the server’s IP address in the subnetwork is needed (Hint: ifconfig eth1).

Listing 7.10 Setup of OSSEC on the agent’s side. The only difference is the setting of the server’s IP address.
3 - Configuring the OSSEC HIDS.
    3.1 - What's the IP Address of the OSSEC HIDS server?:
        - Adding Server IP

After the installation has finished, a key for each agent has to be generated. This is done by starting the manage_agents tool again on the server’s side and then choosing option (E) for extracting a key for the agent. By selecting an agent by its three-digit identification number, a key will be generated (Listing 7.11). This key can be copied to the clipboard.

Listing 7.11 Generating the key for an agent (worker1).
Available agents:
    ID: 001, Name: worker1, IP:
Provide the ID of the agent to extract the key: 001
Agent key information for '001' is:
** Press ENTER to return to the main menu.

To import the key, the manage_agents tool has to be executed on the corresponding agent. There are only two options to choose: (I) for importing a key from the server and (Q) to quit the service. After choosing the option (I), the key value is pasted from the clipboard into the terminal behind the prompt Paste it here. The tool provides some information about the agent for verifying the agent.

Listing 7.12 Importing the key of the agent (worker1) generated on the server.
Paste it here:
Agent information:
    IP Address:
    Confirm adding it?(y/n): y

Then the service is quitted and OSSEC is restarted with ossec_control restart. When the restart and the connection to the server are successful, you can find a similar entry - seen in Listing 7.13 - in the log file ossec.log of the agent.

Listing 7.13 Success of the setup of the agent.

2007/10/10 23:25:48 ossec-agentd: Connecting to server (<IP-address-server>:1514). 2007/10/10 23:25:48 ossec-agentd(4102): Connected to the server.

The OSSEC Web User Interface edit

The web user interface (WUI) visualizes all the statistics that are collected during the running time of OSSEC. This includes the events, alerts, statistics about firewall drops and log file entries in a tabular form. OSSEC’s web-based interface makes the access to the statistics more comfortable.
The following sections show how to setup the WUI on an OSSEC HIDS server and the main functionality of the WUI.

Setup edit

The WUI runs on a fixed OSSEC HIDS server which collects and manages the agent’s data. The WUI cannot be installed on an agent, because all collected alerts are sent back to an OSSEC HIDS server for further processing. Before installing the WUI, there are some basic prerequisites that have to be met. The OSSEC HIDS server needs to be set as an HTTP web server - choosing apache2 here - and a package including the interpreter for the server-side scripting language PHP has to be installed (Listing 7.14). With this, the code containing the web page with php code can be interpreted by the web server.

Listing 7.14 Installing the apache server and php5.

sudo apt-get install apache2 php5 After downloading and extracting the archive containing the sources (Listing 7.15), the extracted directory is moved to /var/www, so as the web server can access the files.

Listing 7.15 Getting the WUI sources.

wget http://www.ossec.net/files/ossec-wui-0.3.tar.gz tar -xvf http://www.ossec.net/files/ossec-wui-0.3.tar.gz The directory contains a installer script guiding the user through the setup (Listing 7.16).

Listing 7.16 Extracting the wui files, moving it to /var/www and starting the setup.

mv ossec-wui-0.3 /var/www cd /var/www/ossec-wui-0.3 ./setup.sh The installation prompts for a username and a password (Listing 7.17). After this, the installation starts and the installer reports whether the setup was successful.

Listing 7.17 Installing the WUI.

Setting up ossec ui... Username: schroeder New password: Re-type new password: Adding password for user schroeder ...#Setup output Setup completed successfully. To complete the setup, the web server user has to be added to the ossec group (Listing 7.18). This user is typically Apache, www or www-data.

Listing 7.18 Adding user www-data to ossec group.

adduser www-data ossec After restarting the whole system, the WUI is available at http://<ip-address-of-webserver>/ossec-wui-0.3/.

Functionality edit

The WUI offers a powerful functionality that helps the user to view the events that are happening within the system. Figure [fig:overview] shows the main window. It gives an overview about the available server(s) and agents, the latest modified files and the latest events. The available agents are described by their name and the associated IP address. This information is shown when clicking on the corresponding agent. If the agent is inactive or unable to connect to the server, the word Inactive is displayed beside the agent’s name. In the figure, the agent named worker1 is currently set inactive. Only the OSSEC server is available.

Figure 7.3: The WUI’s main window.

Moreover the main window contains four tabs, where each of them provides a specific functionality. These are

  • Main, shows the main window
  • Search, allows searching through collected alerts
  • Integrity Checking, allows searching through collected syscheck alerts and
  • Stats, shows aggregated statistics about collected alerts.

The fifth tab About does not provide a special service and just shows information about the WUI version and its contributors.

The search window (Figure 7.4) offers a query interface with a lot of possible options to specify the kind of alerts the user is searching for. Options of interest may be the minimum severity level of an alert, the category (e.g. authentication control) and the log formats (e.g. sshd). A specific search period can be set. Alternatively, by choosing the real time monitoring, the WUI shows all the upcoming events.
There are some more options to reduce the search results. A rule ID can be specified, so that only alerts and events are shown that are allocated to this rule number. The field Pattern searches for a pattern in an event described by regular expressions. To show only events from a particular agent (for example worker1) or from an user account (for example root), the fields Location and User are used. When there are too many search results, the maximal number of alerts to display can be restricted with the field Max Alerts.
When results are found, the WUI divides the types of alerts into three classes:

Severity breakdown
Alerts are sorted by their severity level
Rules breakdown
Informs about the alert’s corresponding rule ID
Src IP breakdown
Informs about the alert’s corresponding source ID, i.e. the user who triggered this alert.
Figure 7.4: Searching for alerts in a period from 16th February 2013 to 25th march 2013. Only alerts with a minimum severity level of seven are shown.

The WUI also shows files that have been modified on the server and on all agents. Clicking on a specific file, for example /etc/resolv.conf in Figure 7.5, gives further information about where the file is located - specified by the server’s or agent’s name and a directory path - and when it has been modified.

Figure 7.5: Latest modified files for all agents and servers sorted by date. Figure

Figure 7.6 shows a detailed view of modified files for a particular agent. This view can be selected by choosing an agent via the drop down menu offering all the agents that are part of the OSSEC system. Clicking Dump database gives the overview of all files that have changed for this agent.

Figure 7.6: Modified files for a particular agent (worker1). The configuration files ossec.conf and internal_options.conf have changed.

Figure 7.7 shows the statistics of a selected date. It shows values aggregated over all agents sorted by severity level of events and by rules (described by their rule ID), respectively. Moreover it counts the total and the averaged number of events and how many events are classified as alerts.

Figure 7.7: Statistics for the 25th March 2013. It shows which and how many rules were found for specific events and how much rules were found with a certain severity level.

An event is represented as in Figure 7.8. This representation is used in the main window and when showing the search results. It contains the following information: The date and time when the OSSEC agent has recognized the event are shown. The date and time stamps are displayed at the beginning of every event. The rule ID (Rule Id) that is associated with the event has a link to pieces of information about the rule. Clicking on it will redirect the user to the web page containing rule specific information. The severity level of an event is also shown in this representation (level). The Location field gives information on the agent that reported the event and what file is associated with the event. Description summarizes the event in a short description.

Figure 7.8: Representation of an event with in the OSSEC WUI.

Summary edit

In this chapter OSSEC was introduced. OSSEC is meant to be a powerful tool to monitor a computer cluster. It includes features as file integrity checking, log monitoring, active response for blocking attacks and rootkit detection. OSSEC uses a special analysis to inspect log files. Log messages are processed in two steps. First the predecoding (section 7.5.1) was presented and then the decoding step (section 7.5.2) was explained. In the decoding step, an XML file is used to extract specific information from events.
The setup of OSSEC was explained in two parts, the server installation (section 7.6.1) and the agent installation (section 7.6.2). For the agent installation it was important to generate keys to make the communication between server and agents possible. The server installation included the configuration of a local email notification (section 7.6.1). This was necessary, because many SMTP servers do not accept messages that have been sent from the OSSEC server.
OSSEC offers an optional feature: the web user interface. Its installation - including the configuration of the OSSEC server as an apache web server - and functionality were explained in section [sec:wui].
The OSSEC basic configuration with all its rules and decoders actually offers a useful detection of well-known events that can occur in a system. However, it is possible to refine OSSEC’s functionality with self-defined rules. This was not described here, because it would go beyond the scope of this report. For further studies it would be interesting to adjust OSSEC’s functionality to the characteristics of the current computer cluster by defining own rules. For this, one has to analyze events that typically occur in the actual computer cluster. The main aim is to create useful and appropriate decoders and rules.

References edit

  1. http://www.ossec.net/doc/manual/rootcheck/manual-rootcheck.html
  2. http://www.ossec.net/
  3. http://www.postfix.org/