How to install Snort on Debian

Snort is a popular choice for running a network intrusion detection system or NIDS for short. It monitors the package data sent and received through a specific network interface. NIDS can catch threats targeting your system vulnerabilities using signature-based detection and protocol analysis technologies. NIDS software, when installed and configured appropriately, can identify the latest attacks, malware infections, compromised systems, and network policy violations. In this guide, you will find instructions on how to install Snort on Debian 9. The install guide is also available for cloud servers running CentOS 7 and Ubuntu 16.

Snort is one of the most commonly used network-based IDS. It is lightweight, open source, available on a multitude of platforms, and can be comfortably installed even on the smallest of cloud server instances. Although Snort is capable of much more than just network monitoring, this guide shows how to configure and run Snort in NIDS mode with a basic setup that you can later expand as needed.

Preparing your server

Setting up a basic configuration of Snort on Debian is fairly simple but takes a few steps to complete. You will first need to install all the prerequisite software to ready your cloud server for installing Snort itself. Install the required libraries with the following command.

sudo apt install -y gcc libpcre3-dev zlib1g-dev libluajit-5.1-dev 
libpcap-dev openssl libssl-dev libnghttp2-dev libdumbnet-dev 
bison flex libdnet autoconf libtool

With the prerequisites fulfilled, next up is how to install Snort on Debian 9. Snort can be downloaded and installed manually from the source. Below you will find instructions on how to get this done.

Installing from the source

Setting up Snort on Debian from the source code consists of a couple of steps: downloading the code, configuring it, compiling the code, installing it to an appropriate directory, and lastly configuring the detection rules.

Start by making a temporary download folder to your home directory and then changing into it with the command below.

mkdir ~/snort_src && cd ~/snort_src

Snort itself uses something called a Data Acquisition library (DAQ) to make abstract calls to packet capture libraries. Download the latest DAQ source package from the Snort website with the wget command underneath. Replace the version number in the command if a newer source is available.

wget https://www.snort.org/downloads/snort/daq-2.0.7.tar.gz

The download will only take a few seconds. When complete, extract the source code and jump into the new directory with the following commands.

tar -xvzf daq-2.0.7.tar.gz
cd daq-2.0.7

The latest version requires an additional step to auto-reconfigure DAQ before running the config. Use the command below which requires you need to have autoconf and libtool installed.

autoreconf -f -i

Afterwards, run the configuration script using its default values, then compile the program with make and finally install DAQ.

./configure && make && sudo make install

With the DAQ installed, you can get started with Snort, and change back to the download folder.

cd ~/snort_src

Next, download the Snort source code with wget. You can find the latest version number on the Snort downloads page. Replace it in the following command if necessary.

wget https://www.snort.org/downloads/snort/snort-2.9.16.tar.gz

Once the download is complete, extract the source and change into the new directory with these commands.

tar -xvzf snort-2.9.16.tar.gz
cd snort-2.9.16

Then configure the installation with sourcefire enabled, run make and make install.

./configure --enable-sourcefire && make && sudo make install

With that done, continue below on how to set up the configuration files.

Configuring Snort to run in NIDS mode

Next, you will need to configure Snort for your system. This includes editing some configuration files, downloading the rules that Snort will follow, and taking Snort for a test run.

Start with updating the shared libraries using the command underneath.

sudo ldconfig

Snort on Debian gets installed to /usr/local/bin/snort directory, it is good practice to create a symbolic link to /usr/sbin/snort.

sudo ln -s /usr/local/bin/snort /usr/sbin/snort

Setting up username and folder structure

To run Snort on Debian safely without root access, you should create a new unprivileged user and a new user group for the daemon to run under.

sudo groupadd snort
sudo useradd snort -r -s /sbin/nologin -c SNORT_IDS -g snort

Then create the folder structure to house the Snort configuration, just copy over the commands below.

sudo mkdir -p /etc/snort/rules
sudo mkdir /var/log/snort
sudo mkdir /usr/local/lib/snort_dynamicrules

Set the permissions for the new directories accordingly.

sudo chmod -R 5775 /etc/snort
sudo chmod -R 5775 /var/log/snort
sudo chmod -R 5775 /usr/local/lib/snort_dynamicrules
sudo chown -R snort:snort /etc/snort
sudo chown -R snort:snort /var/log/snort
sudo chown -R snort:snort /usr/local/lib/snort_dynamicrules

Create new files for the white and blacklists as well as the local rules.

sudo touch /etc/snort/rules/white_list.rules
sudo touch /etc/snort/rules/black_list.rules
sudo touch /etc/snort/rules/local.rules

Then copy the configuration files from the download folder.

sudo cp ~/snort_src/snort-2.9.16/etc/*.conf* /etc/snort
sudo cp ~/snort_src/snort-2.9.16/etc/*.map /etc/snort

Next up, you will need to download the detection rules Snort will follow to identify potential threats. Snort provides three tiers of rule sets, community, registered and subscriber rules.

• Community rules are freely available although slightly limited.
• By registering for free on their website you get access to your Oink code, which lets you download the registered user’s rule sets.
• Lastly, subscriber rules are just that, available to users with an active subscription to Snort services.

Underneath you can find instructions for downloading both community rules and registered user rule sets.

Option 1. Using community rules

If you just want to quickly test out Snort, grab the community rules using wget with the command below.

wget https://www.snort.org/rules/community -O ~/community.tar.gz

Extract the rules and copy them to your configuration folder.

sudo tar -xvf ~/community.tar.gz -C ~/

sudo cp ~/community-rules/* /etc/snort/rules

By default, Snort on Debian expects to find a number of different rule files which are not included in the community rules. You can easily comment out the unnecessary lines using the sed command underneath.

sudo sed -i 's/include $RULE_PATH/#include $RULE_PATH/' /etc/snort/snort.conf

Option 2. Obtaining registered user rules

You can also take a moment and register on the Snort website. Registering gives you access to use their Oink code to download the registered user rules. You can find the code in the Snort user account details.

Replace the oinkcode in the following command with your personal code.

wget https://www.snort.org/rules/snortrules-snapshot-29160.tar.gz?oinkcode=oinkcode -O ~/registered.tar.gz

Once downloaded, extract the rules over to your configuration directory.

sudo tar -xvf ~/registered.tar.gz -C /etc/snort

The rule sets for the registered users include an extensive amount of useful preconfigured detection rules. If you tried out Snort with the community rules first, you can enable additional rules by uncommenting their inclusions towards the end of the snort.conf file.

Configuring the network and rule sets

With the configuration and rule files in place, edit the snort.conf to modify a few parameters. Open the configuration file in your favourite text editor, for example, using nano with the command below.

sudo nano /etc/snort/snort.conf

Find these sections shown below in the configuration file and change the parameters to reflect the examples here.

# Setup the network addresses you are protecting
ipvar HOME_NET server_public_IP/32

# Set up the external network addresses. Leave as "any" in most situations
ipvar EXTERNAL_NET !$HOME_NET

# Path to your rules files (this can be a relative path)
var RULE_PATH /etc/snort/rules
var SO_RULE_PATH /etc/snort/so_rules
var PREPROC_RULE_PATH /etc/snort/preproc_rules

# Set the absolute path appropriately
var WHITE_LIST_PATH /etc/snort/rules
var BLACK_LIST_PATH /etc/snort/rules

In the same snort.conf file, scroll down to section 6 and set the output for unified2 to log under the filename of snort.log like below.

# unified2
# Recommended for most installs
output unified2: filename snort.log, limit 128

Lastly, scroll down towards the bottom of the file to find the list of included rule sets. You will need to uncomment the local.rules to allow Snort to load any custom rules.

include $RULE_PATH/local.rules

If you are using the community rules, add the line underneath to your ruleset as well, for example just below your local.rules line.

include $RULE_PATH/community.rules

Once you are done with the configuration file, save the changes and exit the editor.

Validating settings

Your Snort should now be ready to run. Test the configuration using the parameter -T to enable test mode.

sudo snort -T -c /etc/snort/snort.conf

After running the Snort configuration test, you should get a message like this example below.

        --== Initialization Complete ==--

   ,,_     -*> Snort! <*-
  o"  )~   Version 2.9.16 GRE (Build 118) 
   ''''    By Martin Roesch & The Snort Team: http://www.snort.org/contact#team
           Copyright (C) 2014-2020 Cisco and/or its affiliates. All rights reserved.
           Copyright (C) 1998-2013 Sourcefire, Inc., et al.
           Using libpcap version 1.8.1
           Using PCRE version: 8.39 2016-06-14
           Using ZLIB version: 1.2.11

           Rules Engine: SF_SNORT_DETECTION_ENGINE  Version 3.1  
           Preprocessor Object: SF_DCERPC2  Version 1.0  
           Preprocessor Object: SF_SSH  Version 1.1  
           Preprocessor Object: SF_FTPTELNET  Version 1.2  
           Preprocessor Object: SF_SDF  Version 1.1  
           Preprocessor Object: SF_DNP3  Version 1.1  
           Preprocessor Object: SF_REPUTATION  Version 1.1  
           Preprocessor Object: SF_IMAP  Version 1.0  
           Preprocessor Object: SF_SMTP  Version 1.1  
           Preprocessor Object: SF_GTP  Version 1.1  
           Preprocessor Object: appid  Version 1.1  
           Preprocessor Object: SF_MODBUS  Version 1.1  
           Preprocessor Object: SF_POP  Version 1.0  
           Preprocessor Object: SF_DNS  Version 1.1  
           Preprocessor Object: SF_SSLPP  Version 1.1  
           Preprocessor Object: SF_SIP  Version 1.1  

Snort successfully validated the configuration!
Snort exiting

In case you get an error, the printout should tell you what the problem was and where to fix it. Most likely problems are missing files or folders, which you can usually resolve by either adding any you might have missed in the setup above or by commenting out unnecessary inclusion lines in the snort.conf file. Check the configuration part and try again.

Testing the configuration

To test if Snort is logging alerts as intended, add a custom detection rule alert on incoming ICMP connections to the local.rules file. Open your local rules in a text editor.

sudo nano /etc/snort/rules/local.rules

Then add the following line to the file.

alert icmp any any -> $HOME_NET any (msg:"ICMP test"; sid:10000001; rev:001;)

The rule consists of the following parts:

• action for traffic matching the rule, alert in this case
• traffic protocol like TCP, UDP or ICMP like here
• the source address and port, simply marked as any to include all addresses and ports
• the destination address and port, $HOME_NET as declared in the configuration and any port
• some additional bits
  • log message
  • unique rule identifier (sid) which for local rules needs to be 1000001 or higher
  • rule version number.

Save the local.rules and exit the editor.

Start Snort with -A console option to print the alerts to stdout. You will need to select the correct network interface with the public IP address of your server, for example, eth0.

sudo snort -A console -i eth0 -u snort -g snort -c /etc/snort/snort.conf

If you are not sure which interface to use, check your UpCloud control panel for the public IPv4 address of your server in the Network settings. You can also use the following command on your server.

ip addr

The output will list all of your currently configured network interfaces. Find the one with the same public IP address as shown in the Network settings, commonly eth0.

With Snort up and running, ping your cloud server from any other computer. You should see a notice for each ICMP call in the terminal running Snort.

07/12-11:20:33.501624  [**] [1:10000001:1] ICMP test [**] [Priority: 0] {ICMP} 83.136.252.118 -> 80.69.173.202

After the alerts show up you can stop Snort with ctrl+C.

Snort records the alerts to a log under /var/log/snort/snort.log.timestamp, where the timestamp is the point in time when Snort was started marked in Unix time. You can read the logs with the command underneath. Since you have only run Snort once, there is only one log, complete your command by pressing TAB.

snort -r /var/log/snort/snort.log.

The log shows a warning for each ICMP call with source and destination IPs, time and date, plus some additional info as shown in the example below.

WARNING: No preprocessors configured for policy 0.
07/12-11:20:33.501624 83.136.252.118 -> 80.69.173.202
ICMP TTL:63 TOS:0x0 ID:20187 IpLen:20 DgmLen:84 DF
Type:8 Code:0 ID:13891 Seq:1 ECHO

Running Snort in the background

To run Snort on Debian as a service in the background you will need to add a startup script for Snort. Open a new file in a text editor for example with the next command.

sudo nano /lib/systemd/system/snort.service

Enter the following to the file, save and exit the editor.

[Unit]
Description=Snort NIDS Daemon
After=syslog.target network.target

[Service]
Type=simple
ExecStart=/usr/local/bin/snort -q -u snort -g snort -c /etc/snort/snort.conf -i eth0

[Install]
WantedBy=multi-user.target

With the service defined, reload the systemctl daemon.

sudo systemctl daemon-reload

Snort can then be run with the configuration you set up using the command below.

sudo systemctl start snort

The startup script also includes other usual systemctl commands: stop, restart, and status. For example, you can check the status of the service with the following command.

sudo systemctl status snort

Conclusions

Congratulations, you should have now successfully configured and tested a network-based intrusion detection system. This guide however only covers the very basics with an introduction to Snort and NIDS in general. To get more out of your installation, check out the deployment guides over at the Snort documents page, or jump right into writing your own detection rules with their helpful Snort rules info graph.