Firewalls do a good job of keeping the bad guys out.  But wouldn't it be nice to know when the bad guys are knocking on your door?  This article covers just that, how to determine when the bad guys are probing your network.  We discuss how you can use the IDS script alert.sh to track when you are being probed, and by whom. If you would like to see actual intrusion detection results, click here.

Downloads:
The alert.sh script has been developed for Unix. You can download the latest version here, ver 1.4.4.

The alert.sh script has also been ported to run on NT systems. I have not personally tested these scripts, so you are on your own.
James Oryszczyn has developed detect.pl, written in PERL
Oscar Wahlberg has developed fw1_alert.pl, written in PERL
Roberson Andrew has written alert.zip, written mainly in Windows Script Host 5.1

Features Include:
An installation script that builds, installs, and tests everything for you. Includes an optional feature that automatically determines the the admin of the scanning source and notifies them via email.  Also new with FW-1 ver 4.0, you have the option to automatically block systems that are scanning you.  You should also find this version much easier to read and understand.  You can add your own alerting modules, such as page alerts or snmp traps.

New with ver 1.4.4:
Added TOTAL SCAN LIMIT, which protects you against getting swamped by emails. The default maximum number of emails you can get is 500 a day (can be set to whatever you want).

The Problem
How do you know if someone is probing your network?  Logs are a great place to start, especially your FW-1 logs.  Unfortunately, those logs can be difficult and time consuming to sift through (especially if you are logging 400+ MB every day). Wouldn't it be great if there was an automated method of alerting you?  Well, this article does just that.

There are a variety of different probes and attacks black-hats will attempt. The type we will be focusing on is port scans. Port scans are where an individual attempts to connect to a variety of ports to identify what services a system is running.  The scans can be used on a specific target, or used to scan entire IP ranges, often chosen at random.  This is one of the most popular information gathering methods used by black-hats today as it identifies what ports and services are open. If you are interested in learning more about these techniques, check out Know Your Enemy.

The Goal
The goal is to receive an email alert whenever someone is attempting to scan or probe your network.  We want to automate the entire process, saving you time and energy.  We also want to archive this information in a database for future use.  Our definition of an email alert would look as follows:

Date: Wed, 31 Dec 1997 15:40:01 -0600 (CST)
From: ids@example.net
To: fwadmin@example.net
Subject: #### Firewall ALERT ####

You have received this message because someone is potentially scanning your systems.  The information below is the packet that was denied and logged by the Firewall. This is email alert number 3, with a limit of 5 from evil.example.org.

         ----- CRITICAL INFORMATION -----

Date:        31Dec1997
Time:        15:39:59
Source:      evil.example.org
Destination: ns1
Service:     domain-tcp

         ----- ACTUAL LOG ENTRY -----

31Dec1997 15:39:59 drop   fw1    >elx0 mail proto tcp src evil.example.org dst ns1 service domain-tcp s_port 37401 len 44 rule 6

As you see, this email is alerting the fwadmin that someone has attempted to conduct an un-authorized zone transfer from the nameserver.  However, the firewall denied the attempt, logged it, AND alerted the fwadmin.  In the email alert, we have parsed the critical information for easy reading, and also included the actual log entry for more data.  You can also creat your own alerts, such as text pages or snmp traps.

The Solution
To detect and monitor these scans, we will build a script that first logs whenever someone connects to a predetermined port, and then emails us an alert.  To do this, we will be using the User Defined Alert.  The User Defined Alert will be our script, alert.sh.  Whenever the User Defined Alert is activated, it will launch our script,  which parses the data and does all the work for us.  We set up our Firewall rule base as follows..  First, we identify the most commonly scanned ports we want to monitor. Then we select several systems to listen on these ports.  When an intruder scans our network, he will most likely hit our systems listening on these ports. When these ports are scanned, the Firewall logs the attempts then executes the User Defined Alert..

The end result is you receive an email for each port scanned. If you have 3 systems, each listening on 4 ports, then you may get up to 12 emails from a single network port scan. However, this is normally not the case. If black-hats are scanning an entire network, they are normally looking for a single vulnerability, such as imap (port 143). In this case, we would have received only three emails, one from each system. When they scan a single target, often they scan a range of ports, such as 1-1024. In that case, we would have received only 4 emails, one for each port on the system. Based on what emails you get, you can quickly determine what the intruder is interested in.  Also, the script alert.sh is configurable to limit the total amount of emails as a result of a scan (the default limit is 5).  All scans are also logged to a database.

For those of you with more then one firewall, the script should be installed on the Management Server. The Management Server receives all firewall logs and executes the User Defined Alert (our script) local on itself. So if you have 5 Firewall modules reporting to a single Managment Module, you would install the script on the single Management Module. All 5 Firewall modules can use the script(as defined in their rulebases), however the script is executed on the Management module (where all the logs are). This is extremely advantageous to you, as you can detect scan patterns happening across multiple firewalls.

Implementation
To implement this methodology, we first identify two to three systems for each firewall to use for monitoring. I often select DNS servers, as these are primary targets, many scanning tools start by scanning these systems. Once you have identified the systems you will monitor, create a workstation object in your Firewall 1 GUI for each of the systems.  The systems do not necessarily have to actually exist.  Create an object for a non-existing system, give it a valid IP that your are not using, and use this as one of your scannable objects.  Once you have created your workstation objects, then select the most commonly scanned ports. Ensure that your systems are not using these ports, or every time someone connects to it, you will be alerted. To identify commonly scanned ports, CERT alerts are a great place to start, you can find these alerts at http://www.cert.org. Some ports that you can use are:
 

dns zone transfer  53/TCP
portmapper         111/TCP
http               80/TCP
SMB                139/TCP
imap               143/TCP
BackOrrifce        31337/UDP  -->  Yes, people are STILL scanning for old BO.

I like these ports since black-hats commonly look for them, but most of your systems will not be using them. Make sure these ports are not already blocked by a screening router. We will then set several system to listen on these ports, alerting us when there is a connection.  Below is an example of a rule that monitors and detects the scans.  Notice that quite a few ports are being monitored for these systems.

alert.uniq - A single, permanent log is added to this file whenever a system scans you.  Regardless of how many ports are scanned, there will only be one entry per unique source.  This way you can easily track who scanned you when.

alert.log - This file temporarily stores every port scanned for that day.  This is how the alert.sh script tracks how many times/ports have been scanned from a single source.  Every time the User Defined Alert executes alert.sh, the first thing the script does is count how many times the source ($src) has scanned us for that day by looking in alert.log.  When the script alert.sh is done, it adds an entry to the alert.log file.  So, the first time a source scans a port, alert.sh will not find anything logged in the alert.log.  The second time the sources scans a port, it will find one, and so on.  You set a limit on how many ports are scanned before the alert.sh stops reacting (this protects against DOS attacks).  At the end of the day, a cronjob (rotate.sh) will clear this file out and add all the entries to archive.log.  This way if the same source scans you again at a later date, alert.sh will log the attempt.

alert.archive - This file permanently archives every port scanned and logged.  This is an archivial database that can be used for research.  This file stores the actual log entries, so it tends to be difficult to read.  I find the file alert.uniq to be much easier to read.

rotate.sh - This is a cronjob you need to run every day.  It clears the file alert.log, saving all the logged entries to archive.log.  This resets the counter for the alert.sh script.  If the same source scans you two days in a row, this allows you to log it.

1.  First, we need to customize our script, which is done in the beginning of the script.  This should be self explanatory.

#################################################################
#               BEGIN CUSTOMIZING SCRIPT HERE                   #
#################################################################

# INSTALL DIRECTORY
# Define the directory that this script is in.
# Do NOT put a slash at the end.
# EXAMPLE: dir=/home/fwadmin/alert
dir=

# FW ADMIN
# Define the name of who gets the email alerts
# EXAMPLE: user=fwadmin@example.com
user=

# SCAN LIMIT
# Define maixmum number of scans/email alerts
limit=5

# EMAIL REMOTE SYSTEM
# Define as "true" if you want to automatically email
# the remote admin when you reach your scan limit.
email=false

# SAM
# Define as "true" if you want to autotmatically block
# the source if you reach your scan limit.
sam=false

# SAM TIMEOUT
# How long do you want the source blocked
# Default is 3600 seconds (1 hour).
timeout=3600
 

#################################################################
#               FINISH CUSTOMIZING SCRIPT HERE                  #
#################################################################

2.  We then define our system variables.  Also, we check to see if we have met our scan limit.  If we have, bail now to save CPU cycles.  This is where the speed optimization happens.

### Script variables
message=/tmp/.message_$$
send=/tmp/.send_$$

### Good code is secure code
umask=177
PATH=/usr/bin:/sbin:/usr/sbin:/usr/local/bin
export PATH

if test -a $message
then
        rm $message
fi

if test -a $send
then
	rm $send
fi

### Set trap in case of abrupt exit
trap "rm $send $message ; exit 5" 1 2 15

### Grab User Defined Alert log, pipe to $message.
cat - | tail -1 > $message

### Determine number of scans.
ip=`awk '{print $10}' $message`
number=`grep -c $ip $dir/alert.log`
scan=`expr $number + 1`

### Check number of scans.  If we have reached our limit, lets bail
### now and save CPU cycles.
if [ $scan -gt $limit ];then
	cat $message >> $dir/alert.log
	rm $message
	exit 10
fi

### Parse log file
date=`awk '{print $1}' $message`
time=`awk '{print $2}' $message`
dst=`awk '{print $12}' $message`

### Determine service (check some variables first)
#Determine if "Valid Address" is in log files for NAT
nat_check=`grep -c "(Valid Address)" $message`

#Determine if protocol is icmp
icmp_check=`grep -c " icmp " $message`

if [ "$nat_check" -eq 0 ];then
	if [ "$icmp_check" -eq 0 ];then
		service=`awk '{print $14}' $message`
	else
		service=`awk '{print $15,$16,$17,$18}' $message`
	fi
else
	if [ "$icmp_check" -eq 0 ];then
		service=`awk '{print $16}' $message`
	else
		service=`awk '{print $17,$18,$19,$20}' $message`
	fi
fi

3.  We then define all functions / modules used in the script.  The script comes with three predifined modules, you can add more of your own.

• Resolve () - If the source is an IP, resolve it to a fully qualified host name.
• Block () - Blocks any remote system that reaches its scan limit.  Works on FW-1 ver 4.0 only.