Technical - Linux/OS

Question 1

Hard vs Soft/Sym Link (+how to create)

Answer 1

Hard Link:
Definition: Filesystem entry pointing directly to the inode of a file.
Deletion Behavior: No problem
Creation: ln <source></source> <target></target>

Soft/Symbolic Link:
Definition: File system entry that creates a new inode that references the path to the target file.
Deletion Behavior: Broken link
Creation: ln -s <source></source> <target></target>

Question 2

File Descriptor (Explain stdout, stderr, and stdin)

Answer 2

Definition: An integer that uniquely identifies an open file in a process.

Standard Descriptors:
stdin (0): Standard input.
stdout (1): Standard output.

Question 3

Process vs Thread

Answer 3

Process:
Definition: An independent program in execution, with its own memory space.
Resource Sharing: None, separate memory space and resources.

Thread:
Definition: A smaller unit of execution within a process.
Resource Sharing: Separate registers and stack, but shares memory space.

Question 4

Linux Boot Process (BIOS > MBR > GRUB/LILO > Kernel > Init System)

Answer 4

BIOS: Initializes hardware and loads bootloader from MBR (Master boot record).
GRUB/LILO: Bootloader; loads the Linux kernel.
Kernel: Initializes system hardware and mounts the root filesystem.
Init System: Starts user-space processes (e.g., systemd, SysVinit).

Question 5

Full Disk Encryption LUKS (TPM Integration)**

Answer 5

LUKS: Linux Unified Key Setup; provides full disk encryption.
TPM: Trusted Platform Module; stores encryption keys securely.
Integration: LUKS can be used with TPM to protect encryption keys against tampering.

Question 6

Secure Boot

Answer 6

Purpose: Ensures that only signed and authorized software is loaded during boot.
Mechanism: Checks digital signatures of bootloaders and kernels to prevent unauthorized modifications.

Question 7

Inode (How to check inode number and inode usage)

Answer 7

Index Node:
Definition: Data structure containing metadata about a file (e.g., size, permissions), but not file name or contents.
Check: ls -i <file> shows the inode number. df -i shows inode usage.</file>

Question 8

Zombie Process

Answer 8

Definition: A process that has completed execution but still has an entry in the process table.
Cause: Parent process has not yet read the exit status.
Check: ps aux | grep Z

Question 9

Iptables vs UFW**

Answer 9

Definition: User-space utility for configuring Linux kernel firewall.
Usage: iptables -A INPUT -p tcp –dport 22 -j ACCEPT (Allow SSH)

Question 10

DF vs DU

Answer 10

DF: Shows filesystem disk space usage.
Command: df -h
DU: Shows disk usage of files and directories.
Command: du -sh <directory></directory>

Question 11

SELinux**

Answer 11

Definition: Security-Enhanced Linux; provides mandatory access controls.
Commands:
Check Status: sestatus
Manage Policies: semanage, setenforce

Question 12

How to check disk space, memory, cpu, and io

Answer 12

Disk Space: df -h
Memory: free -h
CPU: top or htop
IO: iostat (requires sysstat package)

Question 13

How to check open files (+why significant)

Answer 13

Command: lsof
Significance: Identifies files and sockets opened by processes, useful for troubleshooting and security.

Question 14

How to check open ports on local system

Answer 14

netstat -tuln

Question 15

How to scan external system for open ports (+flags)

Answer 15

nmap -p <port-range> <target></target></port-range>

Question 16

Systemd (How to create, status, stop, start)

Answer 16

Create Service: Write a .service file in /etc/systemd/system/
Status: systemctl status <service>
Start: systemctl start <service>
Stop: systemctl stop <service></service></service></service>

Question 17

How to trace a network path from source to target

Answer 17

traceroute <destination></destination>

Question 18

How to lookup IP for DNS record and reverse

Answer 18

DNS Record: dig <domain> or nslookup <domain>
Reverse Lookup: dig -x <IP> or nslookup <IP></IP></IP></domain></domain>

Question 19

Linux Filesystem Architecture

Answer 19

Standard Dirs:
/ - root
/bin - binaries
/boot - kernel and ram disk
/dev - rep hardware devices
/etc - system-wide config
/home - user home dirs
/lib - shared libraries and kernel modules
/media - mount point for removable media
/mnt - mount point for temp mounted fs
/opt - optional application pkgs
/root - Home dir for root user
/tmp - temporary fs
/usr - user-related programs
/var - variable data files w/frequent change

Special FSs:
/proc - virtual fs providing process and kernel info
/sys - virtual fs providing info and control over kernels devices and modules
/dev/shm - Temp file storage for inter-process comms

Question 20

Linux Kernel Architecture**

Answer 20

Monolithic Kernel: A single large kernel that includes the core functionalities and all device drivers.

System Call Interface: Provides the interface between user-space applications and kernel services.

Question 21

How to configure basic LAN:

Answer 21

Use netplan or ip commands to configure interface, addresses, gateway, and nameservers.

Question 22

How to check running processes (+how to kill)

Answer 22

Running Processes: ps aux or top
Kill Process: kill <PID> or kill -9 <PID> (force kill)</PID></PID>

Question 23

/proc**

Question 24

/sys**

Question 25

/dev/shm**

Question 26

Linux Namespaces

Question 27

Linux Control Groups (cGroups)

Question 28

AppArmour

Question 29

RAM Disk