The Linux Command Line Interface (CLI)

Learning Goals

Goals for this week's lab

  • Get to know the Linux Command Line Interface (CLI)
    • Understand its importance
    • Learn basic commands for
      • Navigation
      • Remote access
      • Networking-related tasks
    • Understand basic virtualization principles

Overview of the Day

  • Connect to the Lab VM
  • Practice using basic commands for file system manipulation and file search in a Jeopardy-style Capture-the-Flag (CTF) scenario
  • Practice using text search and text aggregation commands to solve a whodunit-style crime mystery

Connecting to the Lab VM

After receiving the credentials for your team's VM, please refer to the preparation page for a guide on how to connect to it.

We suggest that you use one computer per group to connect to the VM and type in the commands.

Upon connecting to the VM, launch the Terminal Emulator application from the desktop.

Capture the Flag

Capture the Flag (CTF) is a special kind of information security competition. The goal is to collect passwords, or flags to progress. There are three common types of CTFs: Jeopardy, Attack-Defence, and mixed.

  • Jeopardy-style CTFs have a couple of questions (tasks) in a range of categories. For example, web, forensic, or crypto. Teams get points for every solved task. Generally, teams will be awarded more points for more difficult tasks. Consecutive tasks are unlocked after solving the previous one. The team with the most points when the time is over is the winner. A famous example of this kind of CTF is the DEFCON CTF Qualifier.
  • Attack-defence is another interesting kind of competition. Here, every team has their own network, with many machines, or a single computer, with vulnerable services. The teams are given an amount of time to patch services and secure the network or system, and to develop exploits to hack the systems of other teams. Then, organizers connect participants of the competition and the wargame starts. You should protect own services for defence points and hack opponents for attack points. Historically, this is the first type of CTFs. For instance, the DEFCON CTF is something like the World Cup of CTF competitions.

Today, we are going to do a relatively short Jeopardy-style CTF about basic Linux commands. It will help you become more familiar with the terminal and navigation around the Linux file structure. The game consists of 12 levels that are supposed to make you feel a little more comfortable using the Terminal as a tool for the Linux Command Line. The first level is used as an example to show how the game is run.

Preparing for the CTF

Note: this CTF is based on Halvor Groven's original implementation from 2020.

Step 1: Move to the CTF Directory

Open the terminal and check which folder you are in. We do that with the command pwd ("print working directory"):

pwd

pwd

Move into the teknostart_ctf folder inside your home directory, using the cd ("change directory") command:

cd /home/netlab/teknostart_ctf

cd /home/netlab/teknostart_ctf

You can also move to the home directory by writing cd ~. The wiggly line that looks like a snake is called tilde and an abbreviation for your home directory.

Step 2: The CTF and Levelcheck Programs

This is a terminal-based game and is written in the programming language Python. There are two files that are important for the CTF:

  • teknostart_ctf.py, which is the actual CTF program. Its usage is explained in the next section.
  • levelcheck is needed for some levels and checks if the tasks have been completed. It prints out the flag if the task has been done correctly. It is used to retrieve the flag (password) for some levels, where it has to be checked whether you completed the task correctly.

To run an executable file in the folder that you are currently in, we type ./ followed by the name of the program.

Example: First CTF Task

Step 1

Run the program for the first time.

./teknostart_ctf.py

./teknostart_ctf.py

You should get an output that looks like this.

CURRENT LEVEL: level 1

----------------------------------------

In which folder are you located when you open the terminal?

Example answer:      /folder/folder      
Answer given:        None                
Hash of answer:      dc937b59892604f5a86ac96936cd7ff09e25f18ae6b758e8014a24c7fa039e91
Correct hash:        1c4d3877bf4fe64f9d55cdbbf10d9b02c329b2a5a162ae3014b83da8c3024a71
Levelchecker:        NO                  

----------------------------------------


CURRENT LEVEL: level 1

----------------------------------------

In which folder are you located when you open the terminal?

Example answer:      /folder/folder      
Answer given:        None                
Hash of answer:      dc937b59892604f5a86ac96936cd7ff09e25f18ae6b758e8014a24c7fa039e91
Correct hash:        1c4d3877bf4fe64f9d55cdbbf10d9b02c329b2a5a162ae3014b83da8c3024a71
Levelchecker:        NO                  

----------------------------------------
  • CURRENT LEVEL is the level that you are currently on, this will increase as you progress.
  • Question is the task for this level, read it carefully.
  • Example answer is an example answer to give you an idea of what format the answer should be. For instance, when asked about a file name, this example tries to show that we are looking for the full filename, including the file ending .txt.
  • Answer given is the answer you gave, which is "None" by default.
  • Hash of answer is a hashed version of your answer. The game takes the answer that you put in, does some math on it, and generates a string which it will try to compare to the hashed version of the correct answer.
  • Correct hash is the hashed version of the correct answer. If this hash matches the hash of your answer, the answer is correct.
  • levelcheck tells you whether you have to use the levelcheck program to check that you have done the task correctly. -- The value is either YES or NO for each level. If this says YES, it means you have to navigate to /home/netlab/teknostart_ctf and run ./levelcheck to check that you have completed the task correctly. If you have, you will get the password (flag) to progress to the next level.

Step 2: Solve the Task

The first you will see is what level you are on, level 1 in this case. As you progress, this will increase. Next is the challenge for this level. This question asks for the folder path that you are in each time you launch a new terminal.

First, we open a new terminal and check which folder we are in by typing pwd.

As we can see, we are located in /home/netlab.

Step 3: Check Your Answer

To test our answer, we run the program again and input our answer, /home/netlab.

First we change the directory back to where the program teknostart_ctf.py is located.

Because we are already inside the folder /home/netlab, we can simply type cd teknostart_ctf, which will put us in /home/netlab/teknostart_ctf.

You can confirm the presence of the program by typing ls to list the files in the directory, and checking for teknostart_ctf.py among the outputs.

We can now run it, and input our answer, /home/netlab.

Success! We have provided the correct flag and the next task is given to us. Solve the remaining tasks. Level 12 is the final level.

If you would like to reset the CTF to its original state and restart from level 1, you can navigate to your home folder /home/netlab and run ./reset_ctf.sh.

Notes on the Report for the CTF Part

For the CTF part, you can structure your report into two parts. First, provide a table with the columns Level, Commands, and Flag. In this table, you should include the commands you ended up using to solve the challenge as well as the resulting password / flag. Second, provide answers to the following questions:

  • In level 7, can you create both folders in a single command? The man page of the command you used could be helpful.
  • In level 10, how could you figure out the location of the file helloworld.txt if it was not provided in the question?
  • In level 11, what was your strategy to locate the file?

CLI Mystery

The second part of this lab deals with the Command Line Mystery. This is a whodunit-style crime mystery scenario that will help you internalize commands related to handling, searching, and analyzing text files. To get started, navigate into the clmystery folder inside your user folder and view the instructions via cat instructions.

As noted on the GitHub page, you can consult the cheatsheet.md or cheatsheet.pdf for an overview of the most relevant commands and options that might come in handy when solving the mystery. Additionally, there are hint files that can serve as inspiration if you're stuck.

It can be helpful to create temporary files for storing intermediate results or filtered versions of files. Recall the options for redirecting outputs using >, >>, and | that we discussed in the lecture. You can find more on this in the Command Output section of the cheatsheet.

We suggest taking notes on commands, evidence, and the overall process from the beginning, so that you'll have an easier time compiling the report.

If you would like to reset the task to its original state and restart, you can navigate to your home folder /home/netlab and run ./reset_clmystery.sh.

Notes on the Report for the CLI Mystery Part

You can use the following basic structure to create your report.

  • Tools and commands: outline and explain which commands you used to solve the mystery. Try to categorize the tools into the types of tasks they helped you with.
  • Key evidence and insights: for each piece of evidence, explain how you found it (specific commands and explanation of what they do, hints, ..) and elaborate on how it helped guide the investigation and narrow down the potential suspects. Finally, state the name of the murderer and how you arrived at that conclusion.
  • Reflections: discuss particular challenges that you encountered while solving the task and how you managed to overcome them. You can also mention commands or options that you discovered during the investigation and how they helped you.

Optional Task(s) - OverTheWire Wargames

If you're done with both the CTF and the mystery, you can check out the OverTheWire Wargames for more advanced CTF-style challenges that go beyond the scope of this course and help you to further explore the capabilities of the command line.

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