Configure Snort IDS and Create Rules

This tutorial is a comprehensive introduction and practical guide to the Snort IDS configuration and rules creation.

Snort is an open-source Intrusion Detection System (IDS) for network monitoring. By reading this tutorial, you will learn how to install Snort both on Debian and CentOS and set up a custom Snort configuration and rules.

This document includes real scenario attack detection.

All explanations in this tutorial include real scenario example screenshots, making it easy for any Linux user to understand how Snort works independently from his expertise level.

Installing Snort (Debian)

This section explains how to install Snort on Debian-based systems first; after Debian installation instructions, you’ll find steps to install it on CentOS.

Before installing Snort on Debian-based Linux distributions, update your system repositories by running the following command:

sudo apt-get update

After updating repositories, install Snort using the following command:

sudo apt install snort -y

The installation process will inform you that the syntax to define network addresses in the configuration file is CIDR (Classless Inter-Domain Routing). Press ENTER to continue with the installation.

The installer will automatically detect your network structure. In this step, check if the detection is correct and fix it if necessary. Then, press ENTER.

After pressing ENTER, the installation will conclude.

Installing Snort (CentOS)

To install Snort on CentOS, download the last Snort rpm package for CentOS at

Then, run the following command, where <Version> must be replaced with the Snort version you downloaded from the previous link:

sudo yum snort-<Version>.rpm

Important for Debian Users

Debian Linux overwrites some options related to network settings in the Snort default configuration file. Rewriting options are fetched from the OS. Under the Snort directory’s settings, there is the /etc/snort/snort.debian.conf file where Debian network settings are imported.

Therefore, if you use Debian first, open the /etc/snort/snort.debian.conf file to check the configuration file and edit it if necessary, using the following command:

sudo nano /etc/snort/snort.debian.conf

As you can see, in my case, the default configuration fetched from the OS is correct.

Note: If network settings are not correct in your case, run sudo dpkg-reconfigure snort

If your settings are correct, press Ctrl+Q to quit.

Configuring Snort

This section includes instructions for the initial Snort configuration.

To configure Snort, open the /etc/snort/snort.conf using nano, vi, or any text editor.

sudo nano /etc/snort/snort.conf

Inside the configuration file, find the following line:

ipvar HOME_NET any

You can add your network or specific IP addresses. To add to your network, replace the line with the following, where x.x.x.x/x must be replaced with a CIDR address:

ipvar HOME_NET x.x.x.x/x

In my case, I replace that line with the following:

ipvar HOME_NET

But, if you want to add specific IP addresses, the syntax is shown below, where,, and must be replaced with the IP addresses to be monitored by Snort. Type all IP addresses separated by a comma between square brackets.

ipvar HOME_NET [,,]

Leave the line ipvar EXTERNAL_NET any as default; below, you can see my configuration:

If you go down, you will see options to monitor specific services and uncomment your enabled services.

When you finish editing the file, close it to save changes. If you don’t have open services, then simply close saving changes.

Testing Snort Configuration With Real Attacks

Now, let’s test Snort by running the command shown below. Replace the IP address or network with yours.

sudo snort -d -l /var/log/snort/ -h -A console -c /etc/snort/snort.conf

Where previously executed command flags mean:

-d= tells Snort to show data

-l= determines the logs directory

-h= specifies the network to monitor

-A= instructs Snort to print alerts in the console

-c= specifies Snort the configuration file

To test Snort, while it’s running, launch an aggressive fingerprint (Xmas) scan from another computer using Nmap, as shown below:

sudo nmap -v -sT -O

As you can see in the following screenshot, Snort detects the fingerprint attempt:

Now, let’s launch a DDOS attack using Nping3 from another computer.

hping3 -c 10000 -d 120 -S -w 64 -p 21 --flood --rand-source

As you can see below, Snort detects malicious traffic:

Now that we see how Snort works, let’s create custom rules.

Getting Started With Snort Rules

Snort default available rules are stored in the /etc/snort/rules directory. To see what rules are enabled or commented on, you need to read the /etc/snort/snort.conf file we previously edited.

Run the following command and scroll down to see disabled and enabled rules. Some rules are disabled for Debian users because they are not available in the stock Debian rules.

less /etc/snort/snort.conf

As said previously, rule files are stored in the /etc/snort/rules directory.

Let’s check the rules to detect and report backdoors traffic.

sudo less /etc/snort/rules/backdoor.rules

As you can see, there are several rules to prevent backdoor attacks. Surprisingly, there is a rule to detect and report NetBus, a trojan horse that became popular decades ago. Let’s explain how this rule works.

alert tcp $HOME_NET 12345:12346 -> $EXTERNAL_NET any (msg:"BACKDOOR netbus

; flow:from_server,established; content:"NetBus"; reference:arachnid

s,401; classtype:misc-activity; sid:109; rev:5;)

alert tcp $EXTERNAL_NET any -> $HOME_NET 12345:12346 (msg:"BACKDOOR netbus getinfo"; flow:to_server,established; content:"GetInfo|0D|"; reference:arachnids,403; classtype:misc-activity; sid:110; rev:4;)


-> = Specifies the traffic direction, in this case from our protected network to an external one

content = Look for specific content within the packet. It can include text if between quotation marks (“ ”) or binary data if between (| |).

depth = Intense analysis; in the rule above, we see two different parameters for two different contents.

offset = Instructs Snort the starting byte of each packet to start searching for the content.

classtype = Reports what kind of attack Snort is alerting about.

sid:115 = Rule identifier.

How To Create Your Own Snort Rule

Now, we’ll create a new rule to notify about incoming SSH connections.

Create a /etc/snort/rules/yourrule.rules file using a text editor. You can name the file as you want. That’s arbitrary, so respect the path.

sudo nano /etc/snort/rules/yourrule.rules

Paste the following rule within the file. As you can see, the rule will notify when a device tries to connect through SSH.

alert tcp $EXTERNAL_NET any -> $HOME_NET 22 (msg:"SSH incoming"; flow:stateless; flags:S+; sid:100006927; rev:1;)

Close and save the file.

Now, add the rule to the Snort configuration file, and run the following command:

sudo nano /etc/snort/snort.conf

Scroll down, and in the rules section, add the following line, where “yourrule.rules” must be replaced by your custom rule name.

include $RULE_PATH/yourrule.rules

Close the text editor; thus, saving changes.

Now, run Snort by running the following command as we did previously; if it was already open, that’s ok:

sudo snort -d -l /var/log/snort/ -h -A console -c /etc/snort/snort.conf

I will try to connect from another computer using SSH.


As you can see in the following image, the rule we created reports the connection attempt.

That’s all for this tutorial. If you want to learn more about Snort custom alerts, I recommend this tutorial to continue reading about Snort alerts.


As you can see, configuring and creating Snort rules is simple. Every Linux user can do it by understanding the content previously explained. It is important to remember exclusive configuration aspects for Debian users previously explained. There are some Snort alternatives you may want to try, like OSSEC, but Snort remains the most popular for Linux users. It is also important that Snort works for all operating systems within the network.

Thank you for reading this article explaining how to configure Snort IDS and how to create rules. Keep following LinuxHint for more professional Linux tutorials.

About the author

David Adams

David Adams is a System Admin and writer that is focused on open source technologies, security software, and computer systems.