Intrusion Detection Systems with Snort - DOC by lcK58e7A


									       Intrusion Detection Systems with Snort


Security is a big issue for all networks in today's enterprise environment. Many

methods have been developed to secure the network infrastructure and

communication over the Internet. One relatively new method is intrusion

detection methods, which started appearing in the last few years. Using intrusion

detection methods, you can collect and use information from known types of

attacks and find out if someone is trying to attack your network or particular hosts.

A comprehensive security system consists of multiple tools, including:

      Firewalls that are used to block unwanted incoming as well as outgoing

       traffic of data.

      Intrusion detection systems (IDS) that are used to find out if someone

       have gotten into or are trying to get into your network.

      Vulnerability assessment tools that are used to find and plug security

       holes present in your network.

These tools can work together and exchange information with each other.

Intrusion Detection System (IDS) is software, hardware or combination of both

used to detect intruder activity. There are two types of IDS. One is Network

Intrusion Detection System (NIDS) that capture data packets traveling on the

network media and match them to a database of signatures. The other is Host-

based Intrusion Detection System (HIDS) that are installed as agents on a host.

This system can look into system and application log files to detect any intruder

activity. Snort is an open source NIDS which is available free of cost.

In this report, I will discuss installation procedure for Snort as well as other

products that work with Snort, components of Snort, most frequently used

functions and testing of Snort/ACID.

In a Snort based Intrusion Detection System, first Snort captured and analyze

data. Then, it stores this data in the MySQL database using the database output

plug-in. Apache web server takes help from ACID, PHP, ADODB and JPGraph

packages to display the data in a browser window when a user connects to

Apache. A user can then make various type of operations to analyze, archive,

graph and delete data.

In essence, an IDS building on a single computer is shown in Figure 1

Figure1: A network intrusion detection system in a single machine

In the enterprise environment, multiple Snort sensors are used behind every

router or firewall. In that case, a single centralized database is used to collect

data from all of the sensors. Figure2 shows the architecture used in such a


Figure2: Multiple Snort sensors logging to a centralized database server

1 Installation

Snort can be downloaded from it’s official website at Snort

is supported on a number of hardware platforms and operating systems. These


      Linux

      OpenBSD

      FreeBSD

      NetBSD

      Solaris (both Sparc and i386)

      HP-UX

      AIX

      IRIX

      MacOS

      Windows

In this report, we downloaded the source file package and compiled from scratch,

however, Snort website also provide binary version for linux and windows


1.1 Softwares Installed Before Snort

Before compile and install snort, we have to first install the related software and

libraries, this includes:

      Zlib1.2.1 (

      LibPcap0.7.2 (

      MySQL4.0.15 (


      Apache2.0.52 (

      PHP4.3.3 (

For detailed description about the installation of these softwares, please read the

article: ”Snort, Apache, PHP, MySQL and Acid Install on RH9.0”


The following is the installation procedures on RedHat9.0 linux operating system:

shell> tar –xzvf snort-2.2.0 .tar.gz

shell> cd snort-2.2.0

shell> ./configure –with-mysql=/usr/local/mysql

shell> make

shell> make install

1.2 Installing Snort Rules And Configuration File

After successfully installed Snort, we then need to install the rules and configure

file used for Snort. This can be down like this:

shell> mkdir /etc/snort

shell> mkdir /var/log/snort

shell> cd rules

shell> cp * /etc/snort

shell> cd ../etc

shell> cp snort.conf /etc/snort

shell> cp *.config /etc/snort

1.3 Configure Snort System File

The Snort system file snort.conf is now located in /etc/snort, we need to make the

following changes to make it works well on our system.

var HOME_NET (make this whatever your internal network is)

var RULE_PATH /etc/snort/ (this is the location where the snort rules are stored)

output database: log, mysql, user=snort password=xxx dbname=snort

host=localhost (this is to tell Snort to log to the mysql database)

1.4 Setting Up The Database In MySQL

Open MySQL client, type the following commands:

mysql> set password for root@localhost=password(‘xxx’);

mysql> create database snort;

mysql > grant insert, select on root,.* to snort@localhost;

mysql> set password for snort@localhost=password(‘xxx’);

mysql> grant create, insert, select, delete, update on snort.* to snort@localhost;

mysql> grant create, insert, delete, select, update on snort.* to snort;

mysql> exit

From the Snort source directory execute the following command:

shell> /usr/local/mysql/bin/mysql –u root –p < ./contrib./create_mysql snort

Enter password: xxx (which is the password set above for root account)

Finally, install the extra DB tables using the following command from the contrib


shell> zcat snortdb-extra.gz | /usr/local/mysql/bin/mysql –p snort

Enter password: xxx

1.5 Softwares Installed After Snort

In order to display alert massages generated by Snort when capturing

prospective intrusion actions, we need to install Analysis Console for Intrusion

Detection (Acid) software package (

0.9.6b23.tar.gz) as well as related libraries JPGraph

( and ADODB

( and copy them to the htdocs directory

of Apache web server. For detailed description about the installation of these

softwares, please read the article: ”Snort, Apache, PHP, MySQL and Acid Install

on RH9.0” (

1.6 Check to See If Everything Is Working

To check if you have successfully installed the above softwares and build a

Snort-based IDS, you can execute the following commands:

shell> /usr/local/apache/bin/apachectl start (start Apache web server)

shell> /usr/local/mysql/bin/mysqld_safe & (start MySQL database in the


shell> /usr/local/bin/snort –c /etc/snort/snort.conf –D (star Snort in the


shell> ping

Then, type the following address in your favorite web browser: If you can see the main page of Acid, and it is displaying

the alerts from MySQL database generated by Snort, then congratulations, you

have successfully installed a Snort-based IDS on a single machine.

Figure3: Acid main page displaying the alerts from MySQL database

2 Components of Snort

A Snort-based IDS contains the following components:

      Packet Decoder

      Preprocessors

      Detection Engine

      Logging and Alerting System

      Output Modules

Figure4 shows how these components work together to detect particular attacks

and to generate output. Any data packet coming from the Internet enters the

packet decoder. On its way towards the output modules, it is either dropped,

logged or an alert is generated.

Figure4: Components of Snort

2.1 Packet Decoder

In an organization’s network, the network interface can be Ethernet, SLIP, or

PPP and so on. The packet decoder takes packets from different types of

network interfaces and prepares the packets to be preprocessed or to be sent to

the detection engine.

2.2 Preprocessors

Preprocessors are components that can be used to arrange or modify data

packets before the detection engine does some operation to find out if the packet

is being used by an intruder. Hackers use different techniques to fool an IDS in

different way, these may include:

      Exact match: For examples, you may have created a rule to find a

       signature “httpd/conf” in HTTP packets. If you are matching this string

       exactly, a hacker can easily fool you by modifying the string as

       “httpd/./conf” or “httpd../httpd/conf”. In addition, since the web servers

       usually understand hexadecimal characters or Unicode characters,

       hackers can also inset these kinds of characters into the web URI. This

       also makes the exact match rule void. A preprocessor can rearrange the

       string so that it is detectable by the IDS.

      Packets fragmentation: When a large data chunk is transferred to a host,

       the packet is usually fragmented into multiple data packets so that each

       packet fragment satisfies the requirement of default maximum length of

       some particular network interface. Later, the receiving systems need to

       reassemble these smaller units to form the original data packets. Hackers

       can use fragmentation to hide a signature into several small units to fool

       the IDS. A Preprocessor can reassemble these small units first and send

       the whole packet to the detection engine for signature testing.

2.3 The Detection Engine

The detection engine is the most important part of Snort. Its responsibility is to

detect if any intrusion activity exists in a packet. The detection engine employs

Snort rules for this purpose. The rules are read into internal data structures or

chains where they are matched against all packets. If a packet matches any rule,

Snort will send alerts to system logs or output plug-ins; otherwise the packet is

dropped. The detection engine is the time-critical part of Snort. The load on it

depends on the following factors:

      Number of rules

      Power of the machine on which Snort is running

      Speed of internal bus used in the Snort machine

      Load on the network

Note that the detection engine can dissert a packet and apply rules on different

parts of the packet. These parts may be:

      The IP header of the packet

      The Transport layer header: This header includes TCP, UDP or other

       transport layer headers. It may also work on the ICMP header.

      The application layer level header: Application layer headers include, but

       are not limited to, DNS header, FTP header, SNMP header, and SMTP

       header. You may have to use some indirect methods for application layer

       headers, like offset of data to be looked for.

      Packet payload. This means that you can create a rule that is used by the

       detection engine to find a string inside the data that is present inside the


2.4 Logging and Alerting System

Depending on what the detection engine finds inside a packet, the packet may be

used to log the activity or generate an alert. Logs are kept in simple text files,

tcpdump-style files or some other form. All of the log files are stored under

/var/log/snort folder by default. You can use –l command line options to modify

the location of generating logs and alerts. Many command line options discussed

in the next chapter can modify the type and detail of information that is logged by

the logging and alerting system.

2.5 Output Modules

Output modules or plug-ins can do different operations depending on how you

want to save output generated by the logging and alerting system of Snort.

Basically these modules control the type of output generated by the logging and

alerting system. Depending on the configuration, output modules can do things

like the following:

      Simply logging to /var/log/snort/alerts file or some other file

      Sending SNMP traps

      Sending messages to syslog facility

      Logging to a database like MySQL or Oracle.

      Generating eXtensible Markup Language (XML) output

      Modifying configuration on routers and firewalls

      Sending Server Message Block (SMB) messages to Microsoft Windows-

       based machines

Other tools can also be used to send alerts in other formats such as e-mail

messages or viewing alerts using a web interface.

3 Snort Modes

Snort operates in two basic modes:

      Sniffer mode

      NIDS mode

It can be used as a packet sniffer, like tcpdump or snoop. When sniffing packets, Snort

can also log these packets to a log file. The file can be viewed later on using Snort or

tcpdump. No intrusion detection activity is done by Snort in this mode of operation.

Using Snort for this purpose is not very useful as there are many other tools available for

packet logging. For example, all Linux distributions come with the tcpdump program

which is very efficient.

When you use Snort in network intrusion detection (NIDS) mode, it uses its rules to find

out if there is any network intrusion detection activity.

3.1 Network Sniffer Mode

In the network sniffer mode, Snort acts like the commonly used program tcpdump. You

don’t need a configuration file to run Snort in the sniffing mode. With the capacity of

parameters, you can capture and display network packets with different levels of detail.

Figure5 is a network sniffing mode test. In this test, I start Snort server that is running as

a network sniffer by typing the following command:

Shell> snort –v

Shell> ping

Snort will continue to display captured packets on the screen until you break using Ctrl-

C. At the time Snort terminates, it will display statistical information.

Figure5: packets captured by Snort using sniffing mode with parameter v

Figure6: statistical information generated when terminating Snort

To get the layer 2 information as well as the headers, you can start Snort by using

parameter dv. The output is like this:

Figure7: packets captured by Snort using sniffing mode with parameter dv

Note the above information is not all the packets information on the network. In order to

capture all of them, showing the data link layer headers and application layer payload,

you have to use parameter dev. This command displays captured data in hexadecimal as

well as ASCII format.

Figure8: packets captured by Snort using sniffing mode with parameter dev

3.2 Logging Snort Data in Text Format

You can log Snort data in text mode by adding -l <directory name> on the

command line. The following command logs all Snort data in /var/log/snort

directory in addition to displaying it on the console.

snort -dev -l /var/log/snort

When you go to the /var/log/snort directory, you will find multiple directories

under it. Each of these directories corresponds to one host and contains multiple

files. The name of the directory is usually the same as the IP address of host.

These files contain logs for different connections and different types of network

data. For example, files containing TCP data will start with TCP. A typical name

for a file containing TCP data is TCP:2489-23. A typical file containing ICMP data

may be ICMP_ECHO. The format of data logged in these files is the same as the

data displayed on the screen when you run Snort in the network sniffer mode.

Figure9: log files generated inside the /var/log/snort directory

3.3 Network Intrusion Detection Mode

In intrusion detection mode, Snort does not log each captured packet as it does in the

network sniffer mode. Instead, it applies rules on all captured packets. A packet is

captured only if it matches a rule. If it is not match any rule, the packet is dropped silently

and no log entry is created. The following command starts Snort in the NID mode:

Shell> snort –c /etc/snort/snort.conf

When you start this command, Snort will read the configuration file /etc/snort/snort.conf

and all other files included in this file. Typically these files contain Snort rules and

configuration data. After reading this files, all captured packets will be matched against

these rules and appropriate action will be taken. These include sending alerts to database,

Syslog, SNMP traps and MS Windows etc.

4 Working With Rules

Most intruder activity will leave some kind of footprint, this footprint is called signature

in the Snort jargon. Snort rules are created based on these signatures. These signatures

may be present in the header parts of a packet or in the payload. Snort rules can be used

to check various parts of a data packet including both network layer protocols and

application layer protocols.

4.1 Structure of a Rule

A Snort rule is divided into two parts: rule header and rule options. The rule header

contains information about what action a rule takes. It also contains criteria for matching

a rule against data packets. The general structure of a Snort rule header is shown in


Figure10: structure of Snort rule header

The options part contains additional criteria for matching a rule against data

packets. A rule may detect one type or multiple types of intrusion activity.

Intelligent rules should be able to apply to multiple intrusion signatures.

Rule options are enclosed inside a pair of parentheses. There may be one or

many and the options are separated with a semicolon. In general, an option may

have two parts: a keyword and an argument. Arguments are separated from the

option keyword by a colon.

4.2 Using ACID With Snort

Analysis Console for Intrusion Database (ACID) is a tool used to analyze and

present Snort data using a web interface. It is written in PHP. It works with Snort

and databases like MySQL. A user will use a web browser to interact with ACID.

The following test is to show you how user customized rules are used by Snort and how

we use ACID to analysis Snort alerts from MySQL database. The first test is as follow:

First you need to delete all the build-in rules used by Snort in the snort.conf configuration

file, and then add the following rule to the snort.conf file rules part:

alert ip any any -> any any (msg: “IP Packet detected”;)

This rule will generate an alert message for every captured IP packet. The ACID main

page provides an overview of currently available data. It has different sections to display

information in groups. You can view traffic profiles by different protocols, get a snapshot

of sensors, search data and so on.

Figure11: ACID main page when using user customized rule

From the main page, you can click on a protocol to get information about packets logged

fro that particular protocol. Figure12 shows a screen shot for TCP protocol. The bottom

part of the screen shows the last fifteen individual packets that have been logged into the


Figure12: TCP protocol data

You can click on any one of these lines at the bottom to find out more details about a

particular packet. Figrue13 shows details about a particular TCP packet.

Figure13: TCP alert detail

One important feature of ACID is that it can be used to search the captured alert data.

Figure14 is the screen shot for some searching procedure. All the criteria that you specify

in this screen are translated to a SQL statement that is passed to the MySQL database

server. Figure15 is the result of this query.

Figure14: Searching database using ACID

Figure15: Searching result

Finally, you can go to the ACID main page where a link is provided to generate graphs.

When generating graph, you can select data and type of graph. Figure13 and 14 are the

line graph and bar graph generated for the IP…

Figure16: ACID Pie graph

Figure17: ACID Bar graph


Intrusion Detection System look for attack signatures, which are specific patterns

that usually include malicious or suspicious intent. Network-based Intrusion

Detection Systems (IDS) use raw network packets as the data source. Snort is a

light weight IDS, which uses a bunch of build-in rules and user customized rules

to prevent prospective intruder from intruding in a network system. This report

discussed installation procedure for Snort as well as other products that work

with Snort, components of Snort, and most frequently used functions. Finally, we

gave a comprehensive test in RedHat9 Linux system about how to use ACID to

analyze alerts generated by Snort based on some specific rules.


   1. Snort User Manual:

   2. Snort, Apache, PHP, MySQL, ACID on Redhat 9.0 Installation Guide:

   3. Network Intrusion Detection Using Snort:

   4. Intrusion Detection with SNORT: Advanced IDS Techniques Using

      SNORT, Apache, MySQL, PHP, and ACID:


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