In this assignment you will be investigating Snort, which is a tool to help monitor networks. For an introduction to Snort, read the very short Wikipedia article on Snort.
Snort is already installed on the docker containers. We will be running Snort on gateway, and our network connections will be running from inner.
You will be submitting an edited version of snort.py (src).
Note: this assignment is for Snort 2.9.15.1, which is the version installed via apt on Ubuntu 22.04. While Snort 3.x is out, it is not as easily installed, and thus we will not be looking at Snort 3. Some of the tutorial aspects in this assignment were taken from the Rapid7: How to Install Snort NIDS on Ubuntu Linux webpage.
If you are looking for the Snort documentation, make sure to use the 2.9 documentation, which can be found online in HTML and PDF.
Changelog
Any changes to this page will be put here for easy reference. Typo fixes and minor clarifications are not listed here.
Thu, Apr 18: you only have to write Snort rules for two of the “attacks” in the writing rules part
Introduction
Recall our network setup, which is shown to the right. In this assignment, we will always run Snort on gateway, listening to eth0 (the green link). All of our communications with the Internet will come from inner, and as it is routed through gateway, we will see the results in Snort.
Running Snort
On gateway, run snort via the following command:
snort -A console -q -c /etc/snort/snort.conf -k none -i eth0
It will appear to hang – it’s waiting for suspicious network packets to display. If you want to exit Snort, Ctrl-C may not work; if not, hit Ctrl-Z, an then kill the process (kill -9 %1).
The options we used are:
-A console: print the alerts to the console (the screen)
-q: suppress some extra output that we don’t care about
-c /etc/snort/snort.conf: what configuration file to use; we’ll see this more shortly
-k none: do not compute checksums, which is necessary in our particular Docker environment
-i eth0: what interface to listen on
Currently Snort will only display output if there is something suspicious. To show this, enter each of these commands from inner (some will not work; that’s intentional):
ping -c 1 firewall
ftp firewall # and then exit the ftp terminal
telnet firewall
On gateway, you should have seen no output.
Using Snort
Let’s see what Snort can output. From inner, enter nmap firewall. This is caught by a rule, and there should be output on gateway:
Try running the run_malware program that you used in the Wireshark assignment (md); recall that it is only installed on inner. While there are multiple things that the run_malware program does, only one of them is caught by the current set of Snort rules – sending a large amount of data through an ICMP packet, like you did in the Ping Shell Commands assignment (md). You should have seen output such as:
04/16-12:11:13.817900 [**] [1:499:4] ICMP Large ICMP Packet [**] [Classification: Potentially Bad Traffic] [Priority: 2] {ICMP} 192.168.200.3 -> 192.168.100.1
04/16-12:11:13.818019 [**] [1:499:4] ICMP Large ICMP Packet [**] [Classification: Potentially Bad Traffic] [Priority: 2] {ICMP} 192.168.100.1 -> 192.168.200.3
Recall that you should type “quit” to exit the run_malware program.
Snort rules
If Snort is still running from the previous section, you should stop it (hit Ctrl-Z, then enter kill -9 %1).
Snort Rule Configuration
Snort has a series of rules that it uses to analyze network traffic. The set of rules we are using is the “community” set, which means it is free. There are also various sets of rules that one has to pay for; we won’t look into those in this assignment.
The configuration file that contains the rules is in /etc/snort/snort.conf. We told Snort to use that file when we launched Snort, above. That file is long and complicated, and not something we need to look at – but you should know it exists. Among other things, it tells Snort that the rule sets are in /etc/snort/rules/. If you look in that directory, you will see a number of files, all containing many Snort rules.
We are going to look at local.rules – this file is for rules specific to that machine or network, as opposed to rules that everybody will want to use. Currently the file looks like this:
# $Id: local.rules,v 1.11 2004/07/23 20:15:44 bmc Exp $
# ----------------
# LOCAL RULES
# ----------------
# This file intentionally does not come with signatures. Put your local
# additions here.
<action>: what Snort should do if this rule matches. There are 5 total actions, the other four being dynamic, pass, log, and activate. We are only going to look at alert actions in this assignment.
<proto>: the protocol for the rule; supported protocols are tcp, udp, icmp, or ip. One can also use services as well, such as ssl, http, etc.
<src_addr>: the IP address that this originates from; can also be any. This can also be $HOME_NET, which is the current network Snort is being run from. This is either obtained from the snort.conf file or auto-detected.
<src_port>: the port that this originates from; can also be any.
<dir>: what direction the packet is moving. Options are -> (source to destination) and <> (bi-directional). If one wanted to use the equivalent of <-, one would just swap the source and destination fields and use ->.
<dest_addr>: the IP address that this is going to; can also be any.
<dest_port>: the port that this is going to; can also be any.
<rule_options>: how to identify the item, and other actions Snort should perform. It is a semi-colon separated list of key-value pairs, all enclosed in parentheses. You can see all of the rule options here (in the various sub-pages therein) or in the Snort documentation PDF staring in section 3.3 (page 185). The relevant options used above are:
msg: print the message out to the console (and, possibly, the log).
sid: the signature ID of the rule, which must be unique. This is so both Snort and various plug-ins can identify rules easily.
rev: the revision of the rule, as one often has to update the rules. This needs to be used with the sid option.
Writing Snort Rules
Recall that before, when we run a number of non-suspicious commands (FTP, ping, and telnet), Snort did not report anything to the console. We are going to change that by entering the following rules into that file:
alert tcp any any -> $HOME_NET 21 (msg:"FTP connection attempt"; sid:1000001; rev:1;)
alert icmp any any -> $HOME_NET any (msg:"ICMP connection attempt"; sid:1000002; rev:1;)
alert tcp any any -> $HOME_NET 23 (msg:"TELNET connection attempt"; sid:1000003; rev:1;)
To check your rules, run snort -T -i eth0 -c /etc/snort/snort.conf. There will be a lot of output, but the last two lines should look like the following:
Snort successfully validated the configuration!
Snort exiting
If you see that, then your rules are all valid.
Now start Snort on gateway, and run the commands from above on inner:
ping -c 1 firewall
ftp firewall # and then logout
telnet firewall
Note that we see the ICMP packet twice, as the first is the ping, and the second is the ping response. Unlike the other two rules, any ICMP packet – in either direction – qualifies for that rule, so both are printed.
Pcap Analysis
How to analyze pcap files
You can also run Snort on a saved pcap file using the -r option.
To test this, we are going to analyze two pcaps of malware from wireshark.org: the Slammer worm (slammer.pcap) and a DNS remote shell (dns-remoteshell.pcap). To download them onto your Docker container, run:
We are going to use the same Snort command as before, but instead of specifying an interface (-i eth0), we are going to specify a pcap file (-r filename.pcap).
Slammer Worm
The SQL Slammer Worm was from 2003, and exploited a buffer overflow in Microsoft’s SQL Server, which it used to propogate to other machines.
The command to analyze slammer.pcap with Snort is:
snort -A console -q -c /etc/snort/snort.conf -k none -r slammer.pcap
You should do the analysis on the dns-remoteshell.pcap on your own.
In the Ping Shell Commands assignment (md), you sent shell commands, and their responses, through ICMP (ping) packets. A similar concept can happen with DNS packets. This particular pcap is showing a past example of obtaining a remote shell via a DNS packet.
Analysis with Snort
In Canvas Files are two pcaps, named snort-uva-attack-1.pcap and snort-uva-attack-2.pcap. Both of these were actual malware attacks against UVA. Using Snort and Wireshark, you need to analyze these two pcaps. You will probably want to use Snort to figure out some basic information, and then investigate further using Wireshark.
The questions you should answer are:
What was the attack that is in the pcap? If the attack has an actual name (and CVE number), then use that. Otherwise, a description of how the attack worked is sufficient.
What was the result of the attack, if known? Specifically, was it successful? If so, what were the results of the attack? If not, what would have been the result of the attack had it been successful? If it’s not possible to determine if it was successful or not, state that.
The answer to this is to be in prose (a normal paragraph), and is limited to 250 characters. The answer will go in the appropriate field of snort.py (src). Note that if you cut-and-paste it from another editor, it may paste in smart quotes, which will cause the Gradescope submission checker to report an error (that it’s a utf-8 file, not an ascii file).
Writing Rules
The run_malware program has five different “things” it will transmit over the network. Four are various types of suspicious “attacks” (they aren’t really attacks, but look like they could be). The ICMP exfiltration was already caught by the default Snort rules, which you saw above, so there are three more. There is also a red herring network transmission (something that is not an attack, but meant to distract you from the actual attacks).
Using Wireshark, you should figure out what the other three attacks are, and write a Snort rule that will catch it for TWO of the attacks. You can search the web for example Snort rules to help you as you develop these rules. But, obviously, you have to write the rules yourself (no copying from friends, no generative AI). You do not need to write a rule for the red herring.
For each of the three other types of attacks, you will have to enter a brief description of the attack, as well as the full Snort rule, into the appropriate fields of snort.py (src). It does not matter the order that you enter the rules, although the description must obviously match the Snort rule.
NOTE: Your rule HAS to print out “CS4760” in the message, as that is what the auto-grader will look for. For example, “web attack in CS4760”. Note the case and lack of space!
We are going to test your rules by checking if they output a message (containing the required string) when tested on a pcap containing that attack. We are looking for a rule that finds just that attack – if there is an alert for all packets, or for packets that are not the attack, then you will receive zero credit. Specifically, given the five different types of “suspicious” network traffic, your rule should print out the required string (“CS4760”) for exactly one of them. We are also going to test each one on legitimate traffic of the same type, and that should not cause an alert.
Submission
You will be submitting an edited version of snort.py (src).
