Networking core concepts

L1 – Physical: Cables, NICs, bits. L2 – Data Link: MAC addresses, switches, VLANs, STP. L3 – Network: IP, routing, subnets, CIDR, ARP. L4 – Transport: TCP/UDP, ports, handshake, retransmission. L5 – Session: Connection management. Rare in practical ops. L6 – Presentation: Encryption, compression, TLS framing. L7 – Application: HTTP, DNS, SMTP, gRPC.

How to Install KVM Packages

egrep -c '(vmx|svm)' /proc/cpuinfo lsmod | grep kvm virt-host-validate Install KVM Packages on Rhel/Centos sudo dnf install -y qemu-kvm libvirt virt-install virt-manager Install KVM Packages Ubuntu sudo apt update sudo apt install -y qemu-kvm libvirt-daemon-system libvirt-clients virtinst Manage Libvirt Service systemctl status libvirtd;systemctl start libvirtd;systemctl enable libvirtd;systemctl restart libvirtd List Virtual Machines virsh list virsh list --all virsh start vmname ( To start the VM) virsh shutdown vmname ( To shutdown VM) virsh destroy vmname ( To Desntroy the VM) =======> Do not use this command virsh reboot vmname ( To reboot the VM) virsh suspend vmname ( To Suspend VM) virsh resume vmname ( To Resume VM) virsh dominfo vmname ( VM iminfo) virsh dumpxml vmname virsh domstats vmname (To Show CPU and memory usage) virsh domuuid vmname (To Get VM UUID) virsh snapshot-list vmname Create Virtual Machines Using ISO virt-install \ --name rhel9 \ --memory 4096 \ --vcpus 2 \ --disk size=50 \ --cdrom /iso/rhel9.iso \ --os-variant rhel9.0 \ --network bridge=br0 virsh snapshot-create-as vmname snapshot1 virsh snapshot-revert vmname snapshot1 virsh snapshot-delete vmname snapshot1 virsh net-list --all virsh net-start default virsh net-autostart default Storage Commands virsh pool-list --all virsh vol-list default virsh pool-info default Console Access virsh console vmname Ctrl + ] virsh migrate --live vmname qemu+ssh://destination-host/system virsh setmem vmname 8G --live virsh setvcpus vmname 4 --live journalctl -u libvirtd virsh domiflist vmname Daily KVM Administration Commands virsh list --all virsh dominfo vmname virsh start vmname virsh shutdown vmname virsh reboot vmname virsh console vmname virsh domifaddr vmname virsh snapshot-list vmname virsh net-list --all virsh pool-list --all

LVM Basics

PV (Physical Volume) – Physical disks/partitions VG (Volume Group) – Pool of storage LV (Logical Volume) – Logical partitions created from the pool pvs # List PVs vgs # List VGs lvs # List LVs pvcreate /dev/sdb vgcreate vg_data /dev/sdb lvcreate -L 10G -n lv_app vg_data mkfs.xfs /dev/vg_data/lv_app mount /dev/vg_data/lv_app /app lvextend -L +5G /dev/vg_data/lv_app xfs_growfs /app vgextend vg_data /dev/sdc lvremove /dev/vg_data/lv_app vgremove vg_data pvremove /dev/sdb # Add new disk pvcreate /dev/sdb # Add disk to VG vgextend vg_root /dev/sdb # Extend LV lvextend -l +100%FREE /dev/vg_root/lv_data # Grow filesystem xfs_growfs /data # XFS resize2fs /dev/vg_root/lv_data # EXT4

IPV4 Subneting

Access mode and Trunk mode.

Feature	Access Mode	Trunk Mode
VLANs carried	One VLAN	Multiple VLANs
VLAN Tagging	No	Yes
Used for	End devices	Network devices
Traffic type	Untagged	Tagged

Common Linux System Calls

 

Category	Examples	Purpose
File Operations	open(), read(), write(), close()	File handling
Process Control	fork(), exec(), wait()	Process creation
Memory Management	mmap(), brk()	Memory allocation
Device Management	ioctl()	Device communication
Networking	socket(), bind(), connect()	Network operations
Information	getpid(), uname()	System information

What is the Kernel?

User Space Where normal programs run (bash, nginx, bash, curl, docker python) Kernel Space Where the kernel runs with full hardware Scheduler, memory manager access syscalls API through which programs request open(), read(), fork() kernel services

PXE Boot Flow

Client PXE Boot
↓
DHCP assigns IP
↓
TFTP downloads pxelinux.0
↓
PXE menu appears
↓
Kernel + initrd loaded
↓
Installer downloads packages via HTTP

How to setup Automated PXE Server Setup Script

.

#!/bin/bash

# ============================================================
# Automated PXE Server Setup
# Supported OS : RHEL / Rocky / AlmaLinux 8 & 9
# Author       : ChatGPT
# ============================================================

set -e

# -----------------------------
# Variables
# -----------------------------

PXE_SERVER_IP="192.168.1.10"
SUBNET="192.168.1.0"
NETMASK="255.255.255.0"
RANGE_START="192.168.1.100"
RANGE_END="192.168.1.200"
GATEWAY="192.168.1.1"

OS_NAME="rhel9"
ISO_MOUNT="/mnt/rhel9"
HTTP_ROOT="/var/www/html/${OS_NAME}"

TFTP_ROOT="/var/lib/tftpboot"
PXE_CFG="${TFTP_ROOT}/pxelinux.cfg"

# -----------------------------
# Install Required Packages
# -----------------------------

echo "Installing required packages..."

dnf install -y \
dhcp-server \
tftp-server \
syslinux \
httpd \
xinetd \
wget \
firewalld

# -----------------------------
# Enable Services
# -----------------------------

systemctl enable --now firewalld
systemctl enable --now httpd
systemctl enable --now tftp.socket
systemctl enable --now dhcpd

# -----------------------------
# Configure Firewall
# -----------------------------

echo "Configuring firewall..."

firewall-cmd --permanent --add-service=http
firewall-cmd --permanent --add-service=dhcp
firewall-cmd --permanent --add-service=tftp
firewall-cmd --reload

# -----------------------------
# Create TFTP Structure
# -----------------------------

echo "Creating PXE boot directories..."

mkdir -p ${PXE_CFG}
mkdir -p ${TFTP_ROOT}/images/${OS_NAME}

# -----------------------------
# Copy PXE Boot Files
# -----------------------------

echo "Copying PXE boot files..."

cp /usr/share/syslinux/pxelinux.0 ${TFTP_ROOT}/
cp /usr/share/syslinux/menu.c32 ${TFTP_ROOT}/
cp /usr/share/syslinux/libutil.c32 ${TFTP_ROOT}/
cp /usr/share/syslinux/libcom32.c32 ${TFTP_ROOT}/

# -----------------------------
# Mount ISO
# -----------------------------

echo "Mounting ISO..."

mkdir -p ${ISO_MOUNT}

echo "Please place RHEL ISO at /root/rhel9.iso"

mount -o loop /root/rhel9.iso ${ISO_MOUNT}

# -----------------------------
# Copy Installation Files
# -----------------------------

echo "Copying installation files..."

mkdir -p ${HTTP_ROOT}

cp -r ${ISO_MOUNT}/* ${HTTP_ROOT}/

# -----------------------------
# Copy Kernel and Initrd
# -----------------------------

echo "Copying kernel and initrd..."

cp ${ISO_MOUNT}/images/pxeboot/vmlinuz \
${TFTP_ROOT}/images/${OS_NAME}/

cp ${ISO_MOUNT}/images/pxeboot/initrd.img \
${TFTP_ROOT}/images/${OS_NAME}/

# -----------------------------
# Create PXE Menu
# -----------------------------

echo "Creating PXE menu..."

cat <<EOF > ${PXE_CFG}/default

DEFAULT menu.c32
PROMPT 0
TIMEOUT 100
ONTIMEOUT local

MENU TITLE Automated PXE Boot Menu

LABEL local
    MENU LABEL Boot from Local Disk
    LOCALBOOT 0

LABEL ${OS_NAME}
    MENU LABEL Install RHEL 9
    KERNEL images/${OS_NAME}/vmlinuz
    APPEND initrd=images/${OS_NAME}/initrd.img \
    ip=dhcp \
    inst.repo=http://${PXE_SERVER_IP}/${OS_NAME}

LABEL rescue
    MENU LABEL Rescue Installed System
    KERNEL images/${OS_NAME}/vmlinuz
    APPEND initrd=images/${OS_NAME}/initrd.img rescue

EOF

# -----------------------------
# Configure DHCP
# -----------------------------

echo "Creating DHCP configuration..."

cat <<EOF > /etc/dhcp/dhcpd.conf

default-lease-time 600;
max-lease-time 7200;
authoritative;

allow booting;
allow bootp;

subnet ${SUBNET} netmask ${NETMASK} {

    range ${RANGE_START} ${RANGE_END};

    option routers ${GATEWAY};
    option subnet-mask ${NETMASK};
    option domain-name-servers 8.8.8.8;

    next-server ${PXE_SERVER_IP};
    filename "pxelinux.0";
}

EOF

# -----------------------------
# SELinux Adjustments
# -----------------------------

echo "Configuring SELinux..."

setsebool -P tftp_home_dir on

restorecon -Rv ${TFTP_ROOT}

# -----------------------------
# Restart Services
# -----------------------------

echo "Restarting services..."

systemctl restart dhcpd
systemctl restart httpd
systemctl restart tftp.socket

# -----------------------------
# Display Status
# -----------------------------

echo
echo "========================================"
echo " PXE SERVER SETUP COMPLETED"
echo "========================================"
echo
echo "PXE Server IP : ${PXE_SERVER_IP}"
echo "HTTP Repo     : http://${PXE_SERVER_IP}/${OS_NAME}"
echo "TFTP Root     : ${TFTP_ROOT}"
echo
echo "Boot target systems using PXE network boot."
echo

Linux Interview Que.

1. What is Linux?

Answer:

Linux is an open-source, Unix-like operating system based on the Linux kernel. It manages hardware resources and provides services for applications.

🔹 2. Explain Linux boot process

Answer:

BIOS/UEFI initializes hardware

Bootloader (GRUB) loads kernel

Kernel initializes system

init / systemd starts services

System reaches target (multi-user/graphical)

🔹 3. What is the difference between hard link and soft link?

Answer:

Hard link: Points to same inode, survives file deletion

Soft link (symlink): Points to file path, breaks if original file is deleted

🔹 4. What is a process?

Answer:

A process is a running instance of a program with its own memory space and PID.

🔹 5. How do you check CPU and memory usage?

Answer:

CPU: top, htop, mpstat

Memory: free -m, vmstat

🔹 6. Difference between df and du?

Answer:

df: Shows filesystem disk usage

du: Shows directory/file usage

🔹 7. What is swap memory?

Answer:

Swap is disk space used as virtual memory when RAM is full.

🔹 8. What is /etc/fstab?

Answer:

A configuration file that defines filesystems to mount at boot time.

🔹 9. How do you find large files?

Answer:

find / -type f -size +500M

🔹 10. What is SSH?

Answer:

SSH (Secure Shell) is a secure protocol for remote login and command execution.

🔹 11. How do you secure SSH?

Answer:

Disable root login

Use key-based authentication

Change default port

Use firewall rules

🔹 12. What is cron?

Answer:

Cron is a job scheduler to run tasks automatically at scheduled times.

🔹 13. What is LVM?

Answer:

LVM (Logical Volume Manager) allows flexible disk management, resizing partitions without downtime.

🔹 14. What is RAID?

Answer:

RAID is data storage virtualization combining multiple disks for redundancy/performance.

RAID 0 → performance

RAID 1 → mirroring

RAID 5 → parity

🔹 15. How do you check open ports?

Answer:

ss -tuln

🔹 16. How do you check running processes?

Answer:

ps -ef

top

🔹 17. Server is slow — how do you troubleshoot?

Answer (structured approach):

Check CPU → top

Check memory → free -m

Check disk → df -h, iostat

Check logs → /var/log/messages

Check network → ss, netstat

🔹 18. Disk is full — what will you do?

Answer:

Check usage → df -h

Find large files → du -sh *

Clean logs → /var/log

Remove temp files → /tmp

Extend disk (LVM if available)

🔹 19. High CPU usage — how to identify?

Answer:

top

ps -eo pid,ppid,cmd,%cpu --sort=-%cpu

🔹 20. What is load average?

Answer:

Load average shows number of processes waiting for CPU.

Compare with CPU cores

Load > cores → system overloaded

🔹 21. Difference between process and thread?

Answer:

Process: Independent, own memory

Thread: Lightweight, shares memory

🔹 22. What are file permissions?

Answer:

Permissions control access:

Read (r), Write (w), Execute (x)

Owner, Group, Others

Example:

chmod 755 file

🔹 23. What is a zombie process?

Answer:

A zombie process is a completed process whose entry still exists in process table.

🔹 24. How do you monitor logs in real time?

Answer:

tail -f /var/log/syslog

🔹 25. What is systemd?

Answer:

Systemd is the init system and service manager in modern Linux.