Linux Fundamentals Part 2 — Cyber Security 101 — Linux Fundamentals — TryHackMe Walkthrough

Continue your learning Linux journey with part two. You will be learning how to log in to a Linux machine using SSH, how to advance your commands, file system interaction.

Room URL: https://tryhackme.com/r/room/linuxfundamentalspart2

YouTube Video: https://www.youtube.com/watch?v=7Zt2Mp2IeBI

Task 1 Introduction

Welcome to the second part of the reworked “Linux Fundamentals” series. We’ll be applying our knowledge from the first installment in this series, so I highly recommend you completing that room before proceeding further.

In part 2, we’ll be ditching the in-browser functionality and help you get started in what is a fundamental skill in being able to login to and control the terminals of remote machines. Not only this, but the room will also have you:

• Unlocking the potential of your first few commands by introducing you to using flags and arguments
• Advancing your knowledge of the filesystem to perform some more useful commands such as copying and moving files
• Discovering how access to files and folders is managed and how we can determine our access.
• Running your first few scripts and executables!

Answer the questions below

Let’s proceed!

Task 2 Accessing Your Linux Machine Using SSH (Deploy)

The in-browser functionality was used in Linux Fundamentals Part 1 to get you directly connected to your first ever Linux machine without any hassle.

In fact, the in-browser functionality uses the exact same protocol that we are going to be using today. This protocol is called Secure Shell or SSH for short and is the common means of connecting to and interacting with the command line of a remote Linux machine.

We will be deploying two machines in this room:

• Your Linux machine
• The TryHackMe AttackBox

What is SSH & how Does it Work?

Secure Shell or SSH simply is a protocol between devices in an encrypted form. Using cryptography, any input we send in a human-readable format is encrypted for travelling over a network — where it is then unencrypted once it reaches the remote machine, such as in the diagram below.

You can learn about the various types of encryption on a TryHackMe room. But for now, we only need to understand that:

• SSH allows us to remotely execute commands on another device remotely.
• Any data sent between the devices is encrypted when it is sent over a network such as the Internet

Deploying Your Linux Machine

Press the green “Start Machine” button on the top-right of this task and then scroll to the top of the page to see the deployment information like so:

The IP address displayed is the address of your Linux machine that you will be logging into using SSH. Take note of this for now.

Deploying the TryHackMe AttackBox

Looking at the top of the page, press the “Start AttackBox” button to deploy the TryHackMe AttackBox that we will be interacting with. The TryHackMe AttackBox is a Ubuntu Linux machine that is hosted online in the cloud and can be interacted with via your browser. You will be using this to interact with the machine that you deploy in this task.

Using SSH to Login to Your Linux Machine

The syntax to use SSH is very simple. We only need to provide two things:

1. The IP address of the remote machine

2. Correct credentials to a valid account to login with on the remote machine

For this room, we will be logging in as “tryhackme”, whose password is “tryhackme” without the quotation (“”) marks. Let’s use the IP address of the machine displayed in the card at the top of the room as the IP address and this user, to construct a command to log in to the remote machine using SSH. The command to do so is ssh and then the username of the account, @ the IP address of the machine.

But first, we need to open a terminal on the TryHackMe AttackBox. There is an icon placed on the desktop named “Terminal”. And now, we can proceed to input commands.

For example: ssh tryhackme@MACHINE_IP . Replacing the IP address with the IP address for your Linux target machine. Once executed, we will then be asked to trust the host and then provide a password for the "tryhackme" account, which is also "tryhackme".

Now that we are connected, any commands that we execute will now execute on the remote machine — not our own.

Note: When you type a password into an ssh login prompt there is no visible feedback — you will not be able to see any text or symbols appear as you type the password. It is still working, however, so just type the password and press enter to login.

Answer the questions below

I’ve logged into the Linux Fundamentals Part 2 machine using SSH!

Task 3 Introduction to Flags and Switches

A majority of commands allow for arguments to be provided. These arguments are identified by a hyphen and a certain keyword known as flags or switches.

We’ll later discuss how we can identify what commands allow for arguments to be provided and understanding what these do exactly.

When using a command, unless otherwise specified, it will perform its default behaviour. For example, ls lists the contents of the working directory. However, hidden files are not shown. We can use flags and switches to extend the behaviour of commands.

Using our ls example, ls informs us that there is only one folder named "folder1" as highlighted in the screenshot below. Note that the contents in the screenshots below are only examples.

Using ls to view the contents of a directory

tryhackme@linux2:~$ ls
folder1
tryhackme@linux2:~$

However, after using the -a argument (short for --all), we now suddenly have an output with a few more files and folders such as ".hiddenfolder". Files and folders with "." are hidden files.

Using ls to view hidden folders

tryhackme@linux2:~$ ls -a 
.hiddenfolder folder1
tryhackme@linux2:~$

Commands that accept these will also have a --help option. This option will list the possible options that the command accepts, provide a brief description and example of how to use it.

Listing the options we can use with ls

tryhackme@linux2:~$ ls --help
Usage: ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).
Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.

Mandatory arguments to long options are mandatory for short options too.
  -a, --all                  do not ignore entries starting with .
  -A, --almost-all           do not list implied . and ..
      --author               with -l, print the author of each file
  -b, --escape               print C-style escapes for nongraphic characters
      --block-size=SIZE      with -l, scale sizes by SIZE when printing them;
                               e.g., '--block-size=M'; see SIZE format below
  -B, --ignore-backups       do not list implied entries ending with ~
  -c                         with -lt: sort by, and show, ctime (time of last
                               modification of file status information);
                               with -l: show ctime and sort by name;
                               otherwise: sort by ctime, newest first
  -C                         list entries by columns
      --color[=WHEN]         colorize the output; WHEN can be 'always' (default
                               if omitted), 'auto', or 'never'; more info below
  -d, --directory            list directories themselves, not their contents
  -D, --dired                generate output designed for Emacs' dired mode
  -f                         do not sort, enable -aU, disable -ls --color
  -F, --classify             append indicator (one of */=>@|) to entries
      --file-type            likewise, except do not append '*'
      --format=WORD          across -x, commas -m, horizontal -x, long -l,
                               single-column -1, verbose -l, vertical -C
      --full-time            like -l --time-style=full-iso
  -g                         like -l, but do not list owner
      --group-directories-first
tryhackme@linux2:~$

This option is, in fact, a formatted output of what is called the man page (short for manual), which contains documentation for Linux commands and applications.

The Man(ual) Page

The manual pages are a great source of information for both system commands and applications available on both a Linux machine, which is accessible on the machine itself and online.

To access this documentation, we can use the man command and then provide the command we want to read the documentation for. Using our ls example, we would use man ls to view the manual pages for ls like so:

Listing the options we can use with ls

tryhackme@linux2:~$ man ls
LS(1)                                               User Commands                                               LS(1)

NAME
       ls - list directory contents
SYNOPSIS
       ls [OPTION]... [FILE]...
DESCRIPTION
       List  information  about the FILEs (the current directory by default).  Sort entries alphabetically if none of
       -cftuvSUX nor --sort is specified.
       Mandatory arguments to long options are mandatory for short options too.
       -a, --all
              do not ignore entries starting with .
       -A, --almost-all
              do not list implied . and ..
       --author
              with -l, print the author of each file
       -b, --escape
              print C-style escapes for nongraphic characters
       --block-size=SIZE
              with -l, scale sizes by SIZE when printing them; e.g., '--block-size=M'; see SIZE format below
 Manual page ls(1) line 1 (press h for help or q to quit)

Answer the questions below

3.1 Explore the manual page of the ls command

man ls

Answer: No answer needed

3.2 What directional arrow key would we use to navigate down the manual page?

Answer: down

3.3 What flag would we use to display the output in a “human-readable” way?

Answer: -h

Task 4 Filesystem Interaction Continued

We covered some of the most fundamental commands when interacting with the filesystem on the Linux machine. For example, we covered how to list and find the contents of folders using ls and find and navigating the filesystem using cd.

In this task, we’re going to learn some more commands for interacting with the filesystem to allow us to:

• create files and folders
• move files and folders
• delete files and folders

More specifically, the following commands:

CommandFull NamePurposetouchtouchCreate filemkdirmake directoryCreate a foldercpcopyCopy a file or foldermvmoveMove a file or folderrmremoveRemove a file or folderfilefileDetermine the type of a file

Protip: Similarly to using cat, we can provide full file paths, i.e. directory1/directory2/note for all of these commands

Creating Files and Folders (touch, mkdir)

Creating files and folders on Linux is a simple process. First, we’ll cover creating a file. The touch command takes exactly one argument — the name we want to give the file we create. For example, we can create the file “note” by using touch note. It's worth noting that touch simply creates a blank file. You would need to use commands like echo or text editors such as nano to add content to the blank file.

Using touch to create a new file

tryhackme@linux2:~$ touch note
tryhackme@linux2:~$ ls           
folder1 note

This is a similar process for making a folder, which just involves using the mkdir command and again providing the name that we want to assign to the directory. For example, creating the directory "mydirectory" using mkdir mydirectory.

Creating a new directory with mkdir

tryhackme@linux2:~$ mkdir mydirectory
tryhackme@linux2:~$ ls           
folder1 mydirectory note

Removing Files and Folders (rm)

rm is extraordinary out of the commands that we've covered so far. You can simply remove files by using rm. However, you need to provide the -R switch alongside the name of the directory you wish to remove.

Using rm to remove a file

tryhackme@linux2:~$ rm note
tryhackme@linux2:~$ ls           
folder1 mydirectory

Using rm recursively to remove a directory

tryhackme@linux2:~$ rm -R mydirectory
tryhackme@linux2:~$ ls           
folder1

Copying and Moving Files and Folders (cp, mv)

Copying and moving files is an important functionality on a Linux machine. Starting with cp, this command takes two arguments:

1. the name of the existing file

2. the name we wish to assign to the new file when copying

cp copies the entire contents of the existing file into the new file. In the screenshot below, we are copying "note" to "note2".

Using cp to copy a file

tryhackme@linux2:~$ cp note note2
tryhackme@linux2:~$ ls           
folder1 note note2

Moving a file takes two arguments, just like the cp command. However, rather than copying and/or creating a new file, mv will merge or modify the second file that we provide as an argument. Not only can you use mv to move a file to a new folder, but you can also use mv to rename a file or folder. For example, in the screenshot below, we are renaming the file "note2" to be named "note3". "note3" will now have the contents of "note2".

Using mv to move a file

tryhackme@linux2:~$ mv note2 note3
tryhackme@linux2:~$ ls           
folder1 note note3

Determining File Type

What is often misleading and often catches people out is making presumptions from files as to what their purpose or contents may be. Files usually have what’s known as an extension to make this easier. For example, text files usually have an extension of “.txt”. But this is not necessary.

So far, the files we have used in our examples haven’t had an extension. Without knowing the context of why the file is there — we don’t really know its purpose. Enter the file command. This command takes one argument. For example, we'll use file to confirm whether or not the "note" file in our examples is indeed a text file, like so file note.

Using file to determine the contents of a file

tryhackme@linux2:~$ file note
note: ASCII text

Answer the questions below

4.1 How would you create the file named “newnote”? (Question Hint Look at the answer formatting — we’re not expecting quotation marks for this)

Answer: touch newnote

4.2 On the deployable machine, what is the file type of “unknown1” in “tryhackme’s” home directory?

In the terminal window type the commend: file <File_Name> to discover the file type

Answer: ASCII text

4.3 How would we move the file “myfile” to the directory “myfolder”

Answer: mv myfile myfolder

4.4 What are the contents of this file?

Answer: THM{FILESYSTEM}

4.5 Continue to apply your knowledge and practice the commands from this task.

Answer: No answer neesed

Task 5 Permissions 101

As you would have already found out by now, certain users cannot access certain files or folders. We’ve previously explored some commands that can be used to determine what access we have and where it leads us.

In our previous tasks, we learned how to extend the use of commands through flags and switches. Take, for example, the ls command, which lists the contents of the current directory. When using the -l switch, we can see ten columns such as in the screenshot below. However, we're only interested in the first three columns:

Using ls -lh to list the permissions of all files in the directory

tryhackme@linux2:~$ ls -lh
-rw-r--r-- 1 cmnatic cmnatic 0 Feb 19 10:37 file1
-rw-r--r-- 8 cmnatic cmnatic 0 Feb 19 10:37 file2

Although intimidating, these three columns are very important in determining certain characteristics of a file or folder and whether or not we have access to it. A file or folder can have a couple of characteristics that determine both what actions are allowed and what user or group has the ability to perform the given action — such as the following:

• Read
• Write
• Execute

Using su to switch to user2

tryhackme@linux2:~$ su user2
Password:
user2@linux2:/home/tryhackme$

Let’s use the “cmnatic.pem” file in our initial screenshot at the top of this task. It has the “-” indicator highlighting that it is a file and then “rw” followed after. This means that only the owner of the file can read and write to this”cmnatic.pem” file but cannot execute it.

Briefly: The Differences Between Users & Groups

The great thing about Linux is that permissions can be so granular, that whilst a user technically owns a file, if the permissions have been set, then a group of users can also have either the same or a different set of permissions to the exact same file without affecting the file owner itself.

Let’s put this into a real-world context; the system user that runs a web server must have permissions to read and write files for an effective web application. However, companies such as web hosting companies will have to want to allow their customers to upload their own files for their website without being the webserver system user — compromising the security of every other customer.

We’ll learn the commands necessary to switch between users below.

Switching Between Users

Switching between users on a Linux install is easy work thanks to the su command. Unless you are the root user (or using root permissions through sudo), then you are required to know two things to facilitate this transition of user accounts:

• The user we wish to switch to
• The user’s password

The su command takes a couple of switches that may be of relevance to you. For example, executing a command once you log in or specifying a specific shell to use. I encourage you to read the man page for su to find out more. However, I will cover the -l or --login switch.

Simply, by providing the -l switch to su, we start a shell that is much more similar to the actual user logging into the system - we inherit a lot more properties of the new user, i.e., environment variables and the likes.

Using su to switch to user2 interactively

tryhackme@linux2:~$ su user2
Password:
user2@linux2:/home/tryhackme$

For example, when using su to switch to "user2", our new session drops us into our previous user's home directory.

Using su to switch to user2 interactively

tryhackme@linux2:~$ su -l user2
Password:
user2@linux2:~$ pwd
user2@:/home/user2$

Where now, after using -l, our new session has dropped us into the home directory of "user" automatically.

Answer the questions below

5.1 On the deployable machine, who is the owner of “important”?

In the terminal window type the commend: ls -lh important to discover the owner of “important

Answer: user2

5.2 What would the command be to switch to the user “user2”?

Answer: su user2

5.3 Now switch to this user “user2” using the password “user2”

Answer: No answer needed

5.4 Output the contents of “important”, what is the flag?

In the terminal window type the commend: cat important to output the contents of “important”

Answer: THM{SU_USER2}

Task 6 Common Directories

/etc

This root directory is one of the most important root directories on your system. The etc folder (short for etcetera) is a commonplace location to store system files that are used by your operating system.

For example, the sudoers file highlighted in the screenshot below contains a list of the users & groups that have permission to run sudo or a set of commands as the root user.

Also highlighted below are the “passwd” and “shadow” files. These two files are special for Linux as they show how your system stores the passwords for each user in encrypted formatting called sha512.

Some notable contents of the /etc directory

tryhackme@linux2:/etc$ ls
shadow passwd sudoers sudoers.d

/var

The “/var” directory, with “var” being short for variable data, is one of the main root folders found on a Linux install. This folder stores data that is frequently accessed or written by services or applications running on the system. For example, log files from running services and applications are written here (/var/log), or other data that is not necessarily associated with a specific user (i.e., databases and the like).

Some notable contents of the /var directory

tryhackme@linux2:/var$ ls
backups log opt tmp

/root

Unlike the /home directory, the /root folder is actually the home for the “root” system user. There isn’t anything more to this folder other than just understanding that this is the home directory for the “root” user. But, it is worth a mention as the logical presumption is that this user would have their data in a directory such as “/home/root” by default.

Some notable contents of the /root directory

root@linux2:~# ls
myfile myfolder passwords.xlsx

/tmp

This is a unique root directory found on a Linux install. Short for “temporary”, the /tmp directory is volatile and is used to store data that is only needed to be accessed once or twice. Similar to the memory on your computer, once the computer is restarted, the contents of this folder are cleared out.

What’s useful for us in pentesting is that any user can write to this folder by default. Meaning once we have access to a machine, it serves as a good place to store things like our enumeration scripts.

Some notable contents of the /tmp directory

root@linux2:/tmp# ls
todelete trash.txt rubbish.bin

Answer the questions below

6.1 Read me!

Answer: No answer needed

6.2 What is the directory path that would we expect logs to be stored in?

Answer: /var/log

6.3 What root directory is similar to how RAM on a computer works? (Question Hint The contents of this root directory do not persist after reboot)

Answer: /tmp

6.4 Name the home directory of the root user

Answer: /root

6.5 Now apply your learning and navigate through these directories on the deployed Linux machine.

Answer: No answer needed

Task 7 Conclusions and Summaries

Nice work! This room was quite theory-heavy and covered quite a range of the fundamentals in getting you familiar with Linux. To quickly recap, this room taught you:

• How to connect to a Linux machine remotely using SSH
• Advancing your use of commands by providing flags, switches and where you can go to learn about these for each command (man pages)
• Some more commands that you’ll frequently be using to interact with the filesystem and its contents
• A brief introduction to file permissions & switching users
• A summary paragraph of the important root directories on a Ubuntu Linux install and how we may be able to use the data stored within these.

I encourage you to go through this room again once or twice to gain some familiarity with the concepts. After all, practice makes perfect!

Answer the questions below

Proceed to the next task to continue your learning

Task 8 Linux Fundamentals Part 3

Visit part three of the Linux fundamentals series here! https://tryhackme.com/room/linuxfundamentalspart3

Answer the questions below

Terminate the machine from task 2!

Summary of Linux Fundamentals Part 1

The Linux Fundamentals Part 2 room on TryHackMe is designed to build upon the knowledge gained in Part 1 and dive deeper into essential Linux concepts.

Part 2 solidifies your foundational Linux skills and prepares you for more advanced interactions with Linux machines. Mastering these commands and concepts is a critical step for roles in cybersecurity, IT administration, and ethical hacking.

Here’s a concise overview for those looking to understand what was covered and the value of this lesson:

1. Accessing Linux Remotely Using SSH:

• Learned to use the Secure Shell (SSH) protocol for securely logging into remote Linux machines.
• Gained an understanding of encrypted communication between devices over a network.

2. Advancing Command Usage:

• Introduced to flags and switches that extend the functionality of basic commands.
• Practiced exploring command documentation using — help and man pages to unlock advanced usage.

3. Filesystem Interaction:

• Practiced creating, moving, copying, and deleting files and directories using commands like touch, mkdir, cp, mv, and rm.
• Learned to identify file types using the file command and manage directory structures more effectively.

4. File Permissions:

• Gained insights into how Linux manages access control with permissions for reading, writing, and executing files.
• Learned to switch between users with su and navigate permissions to interact with specific files.

5. Common Directories:

• Explored essential Linux directories like /etc, /var, /tmp, and /root.
• Understood their purpose, such as storing system configurations, logs, temporary data, and root user files.

Key Takeaways:

1. Practical Skills: This room focused on hands-on learning, equipping you with the ability to log into remote systems, interact with the file system, and manage files efficiently.

2. Command Mastery: By using flags, switches, and man pages, you’re now better equipped to extend the functionality of basic Linux commands.

3. System Understanding: Familiarity with Linux permissions and directory structure helps you confidently navigate and manage Linux systems.

When ready, you can proceed to Linux Fundamentals Part 3!

Insights &Walkthrough For Linux Fundamentals Part 3 : https://medium.com/@iritt/linux-fundamentals-part-3-jcyber-security-101-linux-fundamentals-tryhackme-walkthrough-92265883c489

Continue building your expertise.