Table of contents

  1. Introduction
  2. Learning objectives
  3. Logistics
    1. Generating your .env file
    2. Network topology
  4. Experiment 1: TCP connection establishment
    1. Question sheet
  5. Experiment 2: Reaching a down server
    1. Question sheet
  6. Experiment 3: Trying a non existing server
    1. Question sheet
  7. Experiment 4: Closing a connection
    1. Question sheet
  8. Experiment 5: Exchanging data
    1. Question sheet
  9. Experiment 6: Observing telnet
    1. Question sheet
  10. Food for thought
    1. Question Sheet

Introduction

In this concept lab, we will explore the basics of the Transport Control Protocol (TCP) and understand how it operates. We will run a few experiments using netcat and telnet and explore how TCP helps those two services operate. That would allow us to explore the possible vulnerabilities in TCP so that we could exploit them later on in the class.

Learning objectives

At the end of this session, you should be able to:

  • Identify the steps involved in the TCP protocol.
  • Explore the TCP session setup steps.
  • Identify vulnerabilities in the TCP protocol.

Logistics

For this lab, we will be using GitHub classroom to get the starter code. Please follow this link to accept the assignment and obtain your own fork of the lab repository.

The first time you accept an invite, you will be asked to link your account to your student email and name. Please be careful and choose your appropriate name/email combination so that I can grade appropriately.

Generating your .env file

Before we spin up our containers, there are some configuration variables that must be generated on the spot. To do so, please run the gen_env_file.sh script from the prelab repository directory as follows:

$ ./gen_env_file.sh

If run correctly, several files will be generated:

  1. .env (hidden file - use ls -al to see it) contains your UID and GID variables.

  2. connect_*.sh a utility script to quickly connect to each container in this lab.

Network topology

The topology for this minilab consists of only two machines, a server and a client. The client has the IPv4 address of 10.10.0.4 while the server has the IPv4 address of 10.10.0.5.

Experiment 1: TCP connection establishment

For this first experiment, we will capture traffic related to an established TCP session between a netcat client and server. To launch the experiment, bring up your environment and grab three terminals: 2 on the server and one on the client.

On the server end, start a packet capture for TCP traffic:

sudo tcpdump -i eth0 tcp -w /volumes/experiment_1.pcap

and in the other terminal, start a netcat server using:

nc -n -v -l 1234

On the client side, connect to the server using:

nc server 1234

In this experiment, we will not send any data through the netcat session, we will just open it and see what packets are exchanged between the client and the server. Just leave the client running but do not enter any data in the window.

Stop the packet capture on the server end using C-c in the tcpdump window, the capture file will be saved under volumes/experiment_1.pcap.

Download the pcap file to your local machine and open it using Wireshark, you should see exactly three TCP packets. So before any data is exchanged, the client and server have negotiated the setup of the connection using these three packets.

Question sheet

Observe the three packets that show up, and answer the following questions:

  1. How does a client initiate a connect request with the server?

  2. If the server is ready to accept a connection, how does it tell the client so?

  3. What does the client do when it receives the server’s confirmation?

  4. Open the TCP header fields, which part of the TCP header dictates the type of the packet?

  5. Why do you think the server expects the client to respond back to confirm the connection’s establishment (i.e., why do we need a third packet)?

Experiment 2: Reaching a down server

Let’s now repeat the experiment, but without having the server running. In other words, do not start the netcat server on the server container, simply try to connect to the server from the client.

Run a packet capture for TCP traffic on the server end:

sudo tcpdump -i eth0 tcp -w /volumes/experiment_2.pcap

On the client, attempt to connect to the server using:

nc server 1234

Obviously, the connection will fail, let’s see how the server informs the client of that. Stop the packet capture on the server, and download the generated experiment_2.pcap file.

Open the packet capture file and observe the behavior of the server.

Question sheet

Based on your observations, answer the following questions:

  1. What does the server container do when it receives a connection request for a service that it does not normally provide?

  2. What happens at the client when it receives that information?

  3. Do you notice any potential problems with this particular option in the TCP protocol?

Experiment 3: Trying a non existing server

Before we get to the meat with things, let’s try to see what happens if we try to reach a non-existing server. For this experiment, we will need two terminals at the client, the server containers won’t be involved. We would like to talk to the non-existing server 10.10.0.80.

On the client, start a packet capture for TCP packets using:

sudo tcpdump -i eth0 tcp -w /volumes/experiment_3.pcap

Before we can actually try this experiment, we need to do a little trick to see the behavior desired. If the server we are trying to access (say 10.10.0.80) is not on the network, then the ARP requests from the client to that IP address will fail, which means that no TCP packets will ever leave the client.

In this experiment, we will inject an entry in the ARP cache of the client to convince it that that particular server exists, even if it does not. Fortunately, we won’t write an exploit for that, we can do it from the client using:

sudo arp -s 10.10.0.80 02:32:0a:0a:00:06

Then from the other terminal window, try to reach a server that is not on the network, let’s try for 10.10.0.80.

nc 10.10.0.80 1234

The netcat client would hang for a bit (give it a minute or two), then it will exit. Store the packet capture once the process dies, download it, and open it using Wireshark.

Question sheet

Observe the behavior of the client when trying to reach the non-existing server and answer the following questions:

  1. By default, what does the client do when it does not hear a response from the server to its SYN packets?

  2. How many times does the client try to connect before giving up (in addition to the first one)?

  3. On the client machine, check out the value in cat /proc/sys/net/ipv4/tcp_syn_retries, what do you notice?

  4. Observe the timestamps at which the packets are sent, what can you say about the intervals between packet retries (approximately)?

  5. Based on the above three experiments, draw a finite state machine diagram that represents the TCP connection establishment phase. We refer to this phase as the TCP three-way-handshake.

Experiment 4: Closing a connection

Next, we would like to explore how a TCP connection is closed by either ends. We have already seen one way to do so in experiment 2, namely by sending a TCP Reset packet. But that is abnormal conditions, let’s see how a TCP connection is normally closed.

We will repeat the setup for experiment 1, but this time, we will close the connection normally before stopping the packet capture.

On the server, start a tcpdump packet capture using:

sudo tcpdump -i eth0 tcp -w /volumes/experiment_4.pcap

Then start a netcat server using:

nc -n -v -l 1234

At the client end, connect to the server using:

nc server 1234

Do not send any data over the connection, simply kill it using C-c on the client. This will cause by the server and the client to stop the netcat program and drop back into the shell. Once that happens, stop the packet capture on the server side and download the packet capture to open in using Wireshark.

Question sheet

After the connection has been established using the regular three-way-handshake that we discussed before, the connection is terminated by the client. Observe the packet capture and answer the following questions:

  1. How does the client signal to the server that it wishes to end the connection?

  2. What does the server do when it receive a connection termination request from the client?

  3. Why do you think the server waits for the client to confirm that it has received its acknowledgment of connection termination packet?

  4. Assume that the server did not receive the client’s acknowledgment, what do you think it will do at that point?

Experiment 5: Exchanging data

Now it is time to start looking at the data exchange between the client and the server. We will pretty much repeat the setup for experiment 4 all the while exchanging data between the client and the server now.

On the server, start a tcpdump packet capture using:

sudo tcpdump -i eth0 tcp -w /volumes/experiment_5.pcap

Then start a netcat server using:

nc -n -v -l 1234

On the client’s side, start a connection to the server using:

nc server 1234

Now, anything you type on the client’s side will show up at the server’s side (it will be simply echoed on the standard output at the server’s end). To keep things consistent, let’s try to send the following three messages through the connection (hit Enter after each one to make sure it is sent separately).

  1. Send Hello, make sure it shows up on the server’s side.

  2. Send the paragraph below, you can copy and paste it from here, make sure it shows up on the server’s side.

    Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Purus semper eget duis
at. Nulla malesuada pellentesque elit eget gravida cum sociis natoque
penatibus. Senectus et netus et malesuada fames ac turpis egestas sed. Purus
in mollis nunc sed. Nam at lectus urna duis convallis. Sit amet consectetur
adipiscing elit duis. Dui faucibus in ornare quam. Lobortis elementum nibh
tellus molestie nunc non blandit massa. Pellentesque nec nam aliquam sem et
tortor. Augue lacus viverra vitae congue. Viverra vitae congue eu consequat.
  3. Send Goodbye and end the connection using C-c.

After sending these three messages, stop the packet capture at the server end and download the packet capture file to open it with Wireshark.

Question sheet

Please note that the number of packets you will see might vary, so it’s okay if it doesn’t always appear the same. We are only interested here in the packets between the end of the three-way-handshake and the start of the connection termination step. For me those were packet 4 through 15, but the end number might be different for you.

Observe the packet capture and answer the following questions:

  1. What flags are set in the packets that contain data in netcat?

  2. What does the server do when it receives a data packet from the client?

  3. For the packets between the connection establishment and tear-down, fill out the following table with the following fields:

    • The sequence number that you can obtain from the TCP header (use the relative number printed out by Wireshark).
    • The acknowledgment number that you can obtain from the TCP header (also use the relative one shown by Wireshark).
    • The TCP segment length that you can find in the TCP header or in the packet summary in Wireshark.
    • In the table, C -> S represents a packet sent from client to server, while S -> C represents a packet sent from the server to the client.
    • If you have less packets that the rows here, that’s okay, fill out the ones you have.
    Packet NumberSequence NumberAcknowledgment NumberTCP Segment Len
    4 C -> S   
    5 S -> C   
    6 C -> S   
    7 S -> C   
    8 C -> S   
    9 S -> C   
    10 C -> S   
    11 S -> C   
    12 C -> S   
    13 S -> C   
    14 C -> S   
    15 S -> C   

  4. Based on the content of the table above, what is relationship between the sequence number, the acknowledgment number, and the segment length?

  5. Assume now that we have a network that is very unstable, where packets can be delayed in the way, or even lost. How can the client and the server use the sequence and ack numbers to still communicate even if the medium is unreliable?

  6. Examine any packet containing data, you can see that Wireshark is printing a relative sequence number, while the real sequence number starts off very weird. Why do you think we need the sequence number to start at a very weird random location?

Experiment 6: Observing telnet

Finally, we will explore a different application that runs over TCP, namely the telnet service. telnet is kind of the predecessor for ssh where a remote client can login to a server and access a shell to issue commands. For good reason, telnet should generally not be used as it does everything in plain text, including authentication, so we normally just use it for debugging and testing.

The server has already been configured to host a telnet service; you can login to it with the username root and the password netsec. Our goal here is to examine a telnet session and see how authentication and data exchange happen.

On the server end, start a packet capture session, just like the previous experiments:

sudo tcpdump -i eth0 tcp -w /volumes/experiment_6.pcap

On the client side, start a telnet session as follows:

telnet server

You will be prompted for the username and password, once you enter those, you will be dropped into a root shell session that you can use to issue commands. You can use root as the username and netsec as the password.

For simplicity, from the created shell, let’s just print the current working directory using pwd.

When you issue a command, it will be executed on the server, but the output will show up on the client’s side. To avoid bloating the packet capture, let’s just exit the shell (using C-d or exit). This will close the connection by default.

Once done, stop the packet capture session at the server and download the packet capture file.

Question sheet

Let’s examine the packet capture and see how telnet operates. First, there will be a configuration or negotiation phase; we will not bother ourselves with it, we will simply skip over it. Our first point of interest is when the user enters the username they wish to use for the session.

To help us find the packet, we can use the search option in Wireshark to find the server login: prompt sent by the server. To do so, start a search using C-f in the Wireshark window. This will open small sub-menu.

To find the string let’s set the options, from the first drop down menu, select Packet bytes, leave the second as is, then in the third one, select string and enter server login: as the search key. If all goes well, you should be dropped to the packet that contains that string, for me, that was packet number 24.

Observe the next few packets after it, and answer the following questions:

  1. How does the client send its username to the server?

  2. How does the server respond to each packet sent by the client when entering the username? Who does the acknowledgments?

Next, use the search bar to find the Password: prompt sent by the server. As we mentioned, you will find the user’s password in plaintext in the next few packets. Examine those packets and answer the following questions:

  1. How does sending the password differ (in terms of the communication) between the client and the server?

Finally, find the pwd command in the packets. You’ll have to look for it a bit, and hint it will not be contained in a single packet. Once you find it, answer the following questions:

  1. How are commands sent from the client to the server?

Food for thought

Consider now a scenario where you are on a network and can sniff packets going over this network. However, the network admin, being sneaky, has configured static ARP mappings, i.e., you cannot perform an ARP cache poisoning attack. However, you’d still like to grab a root shell on a target server on the network.

Luckily, while listening on the network, you find yourself in the middle of an already established telnet between an admin machine and your target server. By observing the network packets, what can you do to be able to run arbitrary commands, without interruption, from your malicious machine on the target server?

Think about this for a while, that would be the topic of our next lab.

Question Sheet

Please summarize what you have learned about the TCP protocol. Make sure to mention who connections are established, how they are terminated, and how is data arranged.

When it comes to the Telnet protocol, list out any potential vulnerabilities you might think of if you were able to sit in the middle between the server and the client.