TCP SYN flood - USC/ISI

Created by: Jelena Mirkovic, USC/ISI, sunshine@isi.edu.
Modified by Erin Chambers.
Contents
  1. Overview
  2. Required Reading
  3. Introduction
  4. Assignment Instructions
    1. Setup
    2. Tasks
      1. Generating legitimate traffic
      2. Turning off SYN cookies
      3. Generating attack traffic
      4. Collecting statistics
    3. What Can Go Wrong
  5. Extra Credit
  6. Submission Instructions

Overview

This exercise demonstrates a well-known denial-of-service attack, called TCP SYN flood. Students will be able to create a real attack using DETER tools, and to observe its effect on legitimate traffic. Afterwards, they will be asked to apply a known defense against SYN flood known as SYN cookies, repeat the attack and observe the protection.

This exercise helps students learn the following concepts: (1) How TCP/IP works and how its design can be misused for attacks, (2) How easy it is to perpetrate a DoS attack, with fully legitimate traffic and at a low rate, (3) How easy it is to protect machines from this type of attacks via built-in OS mechanisms. Additionally, extra credit questions improve a student's understanding of how networks and TCP/IP work.

Required Reading

All students should have completed an introductory networking course with grade B or better.

Introduction

Denial of service attacks deny service to legitimate clients by tying up resources at the server with a flood of legiitmate-looking service requests or junk traffic. Before proceeding to the assignment instructions make sure that you understand how TCP SYN flood attack works, which resource it ties up and how, and how syncookies help mitigate this attack.

Assignment Instructions

Setup

Each student should load the supplied topology file synf.ns into the DETER testbed to create a new experiment. Do not modify the topology file but read it through and identify what each directive does.

Especially important are the lines in the NS file: 

   #Add SEER support to each node
   tb-set-node-startcmd $node "sudo python /share/seer/v160/experiment-setup.py Basic"
that add support for traffic generation and visualization via SEER. Once an experiment is swapped in use SEER GUI by clicking at "Launch SEER now" option from here. The code should work on any computer supporting a reasonably recent version of Java. Once the code starts, choose the "Emulab" interface in the first dialog.

Then, the experiment must be "attached" to the GUI. This is done by choosing Emulab Interface->Attach to Experiment from the GUI and then input Project and Experiment Name. Note: capitalization matters here. Next, you will need to input your username on DETER (for example smith@users.deterlab.net) and the same password you use to log on to DETER. You may want to input your public key into the dialog (SSH Key File) if you have already set up passwordless access to DETER. Otherwise delete the information in the SSH Key File input field and you will be asked for your DETER password.

Once attached, experiment topology is visible in the Topology tab. Right clicking on nodes should open a menu that allows users to open graphs in the Graph tab. Once traffic starts flowing, legitimate traffic will show as green and attack traffic will show as red on these graphs. Notice that you can change the counters to show packets or bytes (selection option on top of a graph). If the Src option in the Graph tab is set to "forward" the attack traffic will show as black. Change it to the IP address option and you should be able to see incoming and outgoing traffic for any node, both legitimate and attack. The rest of this assignment assumes that you have opened the graph that shows traffic reaching and leaving the server node.

Tasks

Generating legitimate traffic

Create a Web traffic stream between the client and the server nodes by following these steps from Controls tab in SEER:

  1. Click on Traffic item on the left sidebar
  2. Click on Web
  3. Click on New Web group
  4. Choose some name for the group
  5. Choose client node as a client, server node as a server. Choose exponential distribution of arrival times with lambda 5, scale 1 and max 5. Choose minmax distribution of file sizes with min 1,000 and max 10,000.
  6. Click Start. You should be able to see some green traffic now on your graph. Use Print Screen button on your keyboard to capture this graph, paste it into a Word document, and include it in your project submission.

Turning off SYN cookies

SYN cookies are often on by default in Linux and FreeBSD. To check if they are on do the following: 
  ssh yourusername@users.deterlab.net
  ssh server.YourExperiment.YourProject
  sudo sysctl net.ipv4.tcp_syncookies
If you see 1 as the result, SYN cookies must be set to zero for the demo to work. Type the following on the server machine: 
sudo sysctl -w net.ipv4.tcp_syncookies=0
Verify that SYN cookies are now off by typing on the server machine: 
  sudo sysctl net.ipv4.tcp_syncookies

Generating attack traffic

Create a SYN flood between the attacker and the server nodes.

  1. Click on the Attack item on the left sidebar
  2. Click on Packet Flooder
  3. Click on New Packet Flooder
  4. Choose some name for the group
  5. Choose attacker node as a participating node. Choose flat attack rate, with high rate of 1,000 packets per second (this is the unit of measure for rate fields in SEER). The protocol type for traffic should be TCP. Source field should be 1.1.2.0 and mask 255.255.255.0 to ensure subnet spoofing. The server node is the target node. Leave the length range unspecified, set the destination ports to min 80 and max 80 and set the TCP flags field to SYN. Do not specify any other options - you can either delete default values or leave them as they are.
  6. Click Start. You should be able to see some red traffic now on your graph. Make sure you are looking at a graph of pps not Bps (selection option on top of the graph). Use Print Screen button on your keyboard to capture this graph, paste it into a Word document, and include it in your project submission.

Collecting statistics

You will now collect tcpdump statistics on client machine with and without syncookies, calculate connection duration and draw graphs of connection duration on y-axis and connection start time on x-axis. Perform the following steps:
  1. Stop all traffic using Stop button for each traffic group in SEER
  2. Start tcpdump on the client 
      ssh yourusername@users.deterlab.net
  ssh client.YourExperiment.YourProject
  ip route get 5.6.7.8
    You should see something like this as a result: 
            5.6.7.8 via 1.1.2.2 dev eth2  src 1.1.2.3
           cache  mtu 1500 advmss 1460 metric 10 64
    Thus the interface name leading to 5.6.7.8 is eth2. To see the traffic flowing type: 
      sudo tcpdump -nn -i eth2
    then generate some traffic, e.g. by starting SEER's Web traffic again. You will need to discover proper tcpdump options to see only IP traffic and to save recorded traffic into a file. Start tcpdump with these options.
  3. Using a stopwatch perform the following scenario:
    1. Start legitimate traffic
    2. After 30 seconds start the attack
    3. After 120 seconds stop the attack
    4. After 30 seconds stop the legitimate traffic
    5. Stop the tcpdump on the client and save the file
  4. Turn the SYN cookies on and repeat the above steps.
  5. Using the recorded traffic files and tcpdump to read them, process the output and calculate connection duration for each TCP connection seen in the files. Connection duration is the difference between the time of the first SYN and of the ACK following a FIN-ACK (or between the first SYN and the first RESET) on a connection. Remind yourself what uniquely identifies a TCP connection, i.e. how to detect packets that belong to the same connection? If a connection did not end with an ACK following a FIN-ACK assign to it the duration of 200 s. Include two graphs in your submission, showing connection duration vs connection start time for the case without and with SYN cookies. Label the graphs so they can be distinguished and indicate on each graph using vertical lines or arrows the start and the end of the attack.

What can go wrong

Extra Credit

There are two extra-credit questions:
  1. Remove spoofing from the attack. Repeat the exercise without SYN cookies and observe and explain the effect. What happens? Can you explain why this happens? For hints run a tcpdump on the server node and look for traffic patterns. Can you modify the attack so that it is effective without spoofing and how would you do this?
  2. Modify the NS file to introduce point-to-point routes, using the Modify Experiment option. Hint, you need to remove the server's route to lan1 and to add routes from the server to the attacker, and from the server to the client. Then click on Submit. It will take several minutes for the experiment to be restarted and you will receive an email notification once this is done. Now repeat the exercise without SYN cookies and observe and explain the effect. What happens? Can you explain why this happens? For hints run a tcpdump on the server node and look for traffic patterns.

Submission Instructions

You should submit a document (in Word, Open Office, or as a pdf) with the following items (label each section):
  1. Brief explanation how the TCP SYN flood attack works.
  2. Brief explanation how SYN cookies work to prevent denial-of-service effect from SYN flood attack
  3. Picture of your topology in DETER. Copy this directly from the DETER Web page using high level of detail (clicking on the small picture will open a larger one with detail selection option) so IP addresses are visible.
  4. Screen shot showing the legitimate traffic flowing to and from the server node in packets per second.
  5. Screen shot showing the attack traffic flowing to and from the server node in packets per second. Explain why the amount of legitimate traffic increases during the attack (hint, look at tcpdump at the server node).
  6. Graphs of distribution of client's connection duration for no-SYN-cookies and SYN-cookies cases. Each graph must be labeled and have vertical lines or arrows show the start and the stop of the attack. Explain what happens in each case. Is the attack effective? How can you tell this from the graphs?
  7. Answers to extra credit questions if any.