OS: Linux Networking Basics

 `ping` – Test Connectivity

The ping command checks whether your system can reach another host (computer, server, or website).

• Example:

bash

ping google.com

This sends ICMP echo requests and shows replies, verifying network connectivity.

• Stop ping:

Press CTRL + C.

• Use -c to limit the number of packets:

bash

ping -c 4 8.8.8.8

`ifconfig` – Network Interface Information (Legacy)

ifconfig (from the net-tools package) shows and configures network interfaces. It’s considered deprecated, but still found on some systems.

• Show all interfaces:

bash

ifconfig

• Bring an interface up or down:

bash

sudo ifconfig eth0 up
sudo ifconfig eth0 down

 `ip` – Modern Networking Tool

The ip command (from the iproute2 package) is the modern replacement for ifconfig.

• Show IP addresses:

bash

ip addr

• Show network interfaces:

bash

ip link

• Show routing table:

bash

ip route

• Assign an IP address:

bash

sudo ip addr add 192.168.1.100/24 dev eth0

File Transfer (scp, rsync, wget, curl)

Linux provides several tools to copy files locally, transfer them across systems, and download from the internet. Each tool has its strengths.

`scp` – Secure Copy

scp copies files between local and remote systems over SSH.

• Copy from local to remote:

bash

scp file.txt user@server:/home/user/

• Copy from remote to local:

bash

scp user@server:/home/user/file.txt .

• Copy a directory (recursive):

bash

scp -r project/ user@server:/home/user/

 `rsync` – Efficient File Synchronization

rsync is more powerful than scp — it syncs files, only transferring changes.

• Copy local to remote:

bash

rsync -avz project/ user@server:/home/user/project/

• Copy remote to local:

bash

rsync -avz user@server:/home/user/project/ ./project/

• Key features:
• a → archive (preserve permissions, symlinks).
• v → verbose.
• z → compression.
• Can resume interrupted transfers.

 `wget` – Download from the Web

wget is a command-line downloader.

• Download a file:

bash

wget https://example.com/file.zip

• Resume a partial download:

bash

wget -c https://example.com/file.zip

• Download an entire website (mirroring):

bash

wget -r -np -k https://example.com

 `curl` – Transfer Data with URLs

curl is a versatile tool for fetching or sending data across protocols (HTTP, FTP, etc.).

• Download a file:

bash

curl -O https://example.com/file.zip

• Save output to a custom file:

bash

curl -o myfile.zip https://example.com/file.zip

• Test a website response:

bash

curl -I https://example.com

• Send POST data:

bash

curl -d "user=amr&pass=1234" https://example.com/login

• scp → secure file copy via SSH.
• rsync → sync directories efficiently.
• wget → simple web downloader.
• curl → advanced tool for HTTP/FTP requests and APIs.

SSH Basics (Secure Shell)

SSH (Secure Shell) is the standard way to securely connect to remote Linux systems over a network. It provides encrypted communication for running commands, transferring files, and managing servers.

What is SSH?

• A protocol for secure remote login and command execution.
• Replaces insecure tools like telnet and rlogin.
• Uses port 22 by default.
• Provides authentication via passwords or SSH keys.

Connecting to a Remote Server

Basic syntax:

ssh user@hostname

• user → the username on the remote system.
• hostname → IP address or domain of the server.

Example:

ssh amr@192.168.1.50

Using SSH Keys (Recommended)

SSH keys provide secure, passwordless authentication.

1. Generate a key pair:

bash

ssh-keygen -t rsa -b 4096

1. Copy public key to server:

bash

ssh-copy-id user@hostname

1. Connect without a password:

bash

ssh user@hostname

Common SSH Options

• Specify a custom port:

bash

ssh -p 2222 user@hostname

• Run a single command remotely:

bash

ssh user@hostname "ls -l /var/log"

• Use a config file (~/.ssh/config) for shortcuts:

bash

Host myserver
    HostName 192.168.1.50
    User amr
    Port 22

Then connect with:

ssh myserver