Linux Basics: System Architecture

• SYS100 covers Linux Basics and Binary topics.

Learning Objectives

• Describe the basic components of a Linux System
• Describe the basic folder/path structure of Linux
• Describe the use of VI/VIM for file editing

Linux Architecture

• Hardware components include CPU, RAM, I/O Devices.
• The Kernel/Kernel Space is a special part of the OS which handles memory access and process scheduling and is protected, it can communicate directly to hardware.
• Device drivers, networking components, and other critical components run in the kernel.
• Kernels can be customized, with many devices running small, embedded kernels.
• A minimal Linux installation minimizes system libraries and device drivers, reducing exposure to critical systems.
• Servers may not have access to USB ports unless specific device drivers are installed and configured.
• GUIs can be memory and resource-intensive; critical servers may omit GUIs to conserve resources.
• User shells and processes in User Space communicate through system calls to the kernel.

Linux Kernel

• The Linux kernel is monolithic, written in C & Assembly.
• It is Free and open source.
• Released under the GNU General Public License version 2 (GPLv2).
• Commonly utilized in Android, Smart TVs, and webcams.
• Microkernels and hybrid kernels offer different security implications based on what runs in the OS protected space versus user space.

Linux Kernel Security

• Includes DAC and Superuser controls.
• Extensions include: Namespaces, Linux Security Modules, SELinux, Audit subsystem, Seccomp, and Integrity Management
• Linux has a comprehensive networking stack with support for security protocols, and can be used as a node and a router.
• It has cryptography support for encryption algorithms and operating modes.
• Security Enhanced Linux (SELinux) implements Mandatory Access Control (MAC) for government and military systems.
• Seccomp restricts access to system calls by processes.
• Integrity Management maintains the integrity of files using runtime integrity measurements that use cryptographic hashes.

User Space/Kernel Space

• The GNU C Lib (glibc) provides a mechanism to interact between kernel and user space.
• System calls offer basic read/write interface functions.
• Kernel services provide platform-independent kernel ops code.
• Device drivers are hardware-specific ops.

Daemon

• A daemon is a type of program that runs in the background.
• Daemons look like a "process".
• Daemons are assigned a unique process identification number (PID).

Software Libraries

• Software libraries consist of pre-written code, classes, procedures, scripts, and configuration data.
• Software libraries add functionality and automate functions in programs without requiring the developer to write additional code.

Linux Desktop Environments

• Options include Command Line (TTY/SSH Servers) and GUI.
• Command Line systems are more streamlined.
• GUIs offer point-and-click interaction.
• KDE, GNOME, Unity, and MATE are examples of Desktop Environments, which differ in their GUIs.

Basic Linux Commands

• ls (Directory listing)
• pwd (Print Working Directory) shows the current directory.
• cat concatenates files and view their contents.
• whoami displays the owner of the current login session (shells).
• Man accesses manual pages with details about other commands (help file).

LS Expanded

• ls -a include all files including hidden.
• ls -l display file RWX permissions, owner (yellow), group (red).
• ls -l (the letter, not the number 1) displays additional info including file/directory, size, date, time, owner and group permissions.
• ls -lh lists the subdirectories in human readable size (1.6K, etc).
• ls -r reverses order.
• ls -R list subdirectories recursively.
• ls --help displays the help information

File/Folder Commands

• touch creates a file or updates the timestamp.
• cd changes the directory.
• mkdir makes a directory.
• rmdir remove a directory.

Folders

• / is the Root files system.
• /bin is Binaries/executables
• /etc contains the configuration files.
• /home contains user folders; can be accessed via ~.
• /log contain the log files.
• /usr contains the user binaries.
• /mnt contains the mounts (devices, fileshares, etc.).

Path References

• The Current Folder is ~/sub1 or /home/student/sub1.
• Single dot (.) represents the current folder: such as ./sub2
• Double dot (..) moves up one level: such as ../sub3
• . and .. are called relative references because they are relative to the current folder.
• /home/student/sub1 is the absolute path, starting at the root folder.
• The tilde (~) refers to /home/<current user such as /home/student.
• Sub1 can be referenced as ~/sub1 (Relative Reference) or /home/student/sub1 (Absolute Reference) from the current working directory.

VI/VIM Basics

• VI/VIM is a command line editor.
• Command mode executes commands on a file.
• Insert mode allows text editing in a file.
• Pressing <Esc> will return to command mode.
• Use : (colon) to execute file commands.
• :x exits vi.
• :wq saves and exits.
• :q! exits without saving.

Editing Text

• Arrow keys (or hjkl) move the cursor around the file.
• i inserts text before the cursor.
• a appends text after the cursor.
• o inserts a line below the cursor.
• O inserts a line above the cursor.
• <Del> or x deletes a single character.

Searching

• /<text> searches forward for text.
• ?<text> searches backward for text.
• n finds the next occurrence.
• N finds the previous occurrence.
• More commands exist, so finding a VI/VIM cheat sheet is recommended.
• VI is typically monochrome, while VIM is colorized (but not always installed).

Communication Basics

• Humans exchange information using letters and words.
• Computers use 0s and 1s, encoded into electronic signals, light, or radio waves to communicate.

Binary Communication

• Letters (and other data) are converted into 0s and 1s for computer communication.
• The process involves turning a letter into a number and then representing that number in 0s and 1s.

Numbering Systems

• Decimal (base 10) is the most familiar numbering system.
• Example: 1,234 is interpreted as 1 thousand (10³), 2 hundreds (10²), 3 tens (10¹), and 4 ones (10⁰).

Numbering Systems: Binary (Base 2)

• Binary uses powers of 2.
• From right to left, columns represent powers of 2.
• 1001 in binary equals 1 eight (2³), 0 fours (2²), 0 twos (2¹), and 1 one (2⁰).
• The binary number may sound like "one-zero-zero-one", and is the number nine.
• Binary uses two digits: 0 and 1.
• A "bit" is a "Blnary digiT".

Digits

• Decimal (base 10) requires ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9.
• Binary (base 2) requires two digits: 0 and 1.

Numbering Systems: Hex

• Hexadecimal (base 16) would need sixteen digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F.
• A=10, B=11, C=12, D=13, E=14, and F=15

Numbering Systems: HEX

• A hexadecimal number such as 7F can be expressed 0x7F to explicitly show it is in hex.
• From right to left, the columns represent powers of 16.
• 7F is 7 sixteens (16¹) plus F ones (16⁰) or 716 + 151 = 112 + 15 = 127.

Numbering Systems

• Hexadecimal condenses four binary bits into one digit.
• Example: 0b1101 = 0xD (hex) = 13 (Decimal), making binary data more digestible for humans.

Binary to Hex Relationship

• 0000 = 0
• 0001 = 1
• 0010 = 2
• 0011 = 3
• 0100 = 4
• 0101 = 5
• 0110 = 6
• 0111 = 7
• 1000 = 8
• 1001 = 9
• 1010 = A
• 1011 = B
• 1100 = C
• 1101 = D
• 1110 = E
• 1111 = F

Octal

• Octal (Base 8) uses the digits 0-7.
• 3 bits of binary map directly to Octal.
• Effectively it is a subset of Hex.
• 000 = 0
• 001 = 1
• 010 = 2
• 011 = 3
• 100 = 4
• 101 = 5
• 110 = 6
• 111 = 7

Summary: Numbering Systems

• Decimal Numbering System (Base 10) are best for humans
• Binary Numbering Systems (Base 2) are best for computers
• Hexadecimal Numbering Systems(Base 16) are used to view binary data in a more compact way.

Communication: Encoding

• Any number can be turned into binary.
• Encoding is the method of encoding letters into a number.
• Many different encodings exist.

Encoding: ASCII

• Arbitrarily, the character-to-number map is 'A' = 65, 't' = 116, 'a' = 97, 'c' = 99, 'k' = 107.
• This is done referencing the ASCII Table.

ASCII Encoding

• ASCII is the American Standard Code for Information Interchange.
• A standard character encoding that assigns numbers to the most common typed characters.
• Consists of the numbers (0-255)
• Does not include less-common characters like those found in the Cyrillic, Greek, Russian, or Chinese alphabets
• Type "man ascii" in a bash session to see the ASCII table

ASCII Chart

• Some key pairings: 64=40=@, 65=41=A, 97=61=a

Possible Confusion

• The letter 'A' maps to the number 65 (0x41).
• The letter 'Z' maps to the number 90 (0x5A).

Putting it Together

• To send the message "Attack at dawn," turn every letter into a number.
• Then, represent every number with 0s and 1s: "A" → 65 or 0100 0001 (0x41), "t"→116 or 0111 0100 (0x74), "a"→ 97 or 0110 0001 (0x61).