SSH Learning

Installing SSH

Topics

Level: Beginner → Intermediate

Before learning authentication, keys, or tunneling, you must know how to properly install, verify, manage, and troubleshoot SSH on different operating systems.

What is OpenSSH?
SSH Client vs SSH Server
OpenSSH Components
Linux Installation
Ubuntu/Debian Installation
RHEL/CentOS/Rocky/AlmaLinux Installation
Arch Linux Installation
Windows Installation
macOS Installation
Systemd Services
SSH Configuration Files
Verification and Testing
Internal Working After Installation
Architecture Diagram
Commands
Code Examples
Real-World Examples
Production Examples
Common Mistakes
Troubleshooting
Security Notes
Interview Questions
Hands-On Labs
Exercises
Quiz

1. What is OpenSSH?

OpenSSH is the most widely used implementation of SSH.

Project:

OpenSSH

Provides:

SSH Client

SSH Server

SFTP

SCP

Key Management

Port Forwarding

Secure Remote Access

Why OpenSSH?

Benefits:

Open Source

Secure

Cross Platform

Industry Standard

Actively Maintained

OpenSSH in Modern Systems

Used in:

Linux Servers

AWS EC2

Azure VMs

Google Cloud

Docker Hosts

Kubernetes Nodes

Enterprise Data Centers

2. SSH Client vs SSH Server

Many beginners confuse these.

SSH Client

Used to initiate connections.

Command:

ssh

Example:

ssh user@server

Installed on:

Your Laptop

Your Workstation

Admin Computer

SSH Server

Accepts incoming connections.

Process:

sshd

Installed on:

Linux Server

Cloud VM

Remote Machine

Diagram

+-------------+
| SSH Client  |
+-------------+
       |
       |
Encrypted Connection
       |
       |
+-------------+
| SSH Server  |
|   sshd      |
+-------------+

Simple Rule

Client = Connects

Server = Accepts Connections

3. OpenSSH Components

OpenSSH installs multiple tools.

ssh

Client program.

Example:

ssh user@server

sshd

Server daemon.

Example:

sudo systemctl status ssh

ssh-keygen

Generates keys.

Example:

ssh-keygen

ssh-agent

Stores private keys in memory.

Example:

ssh-agent

ssh-add

Loads keys into agent.

Example:

ssh-add ~/.ssh/id_ed25519

scp

Secure file copy.

Example:

scp file.txt server:/tmp

sftp

Secure file transfer.

Example:

sftp user@server

Component Overview

Component	Purpose
ssh	Client
sshd	Server
ssh-keygen	Key generation
ssh-agent	Key storage
ssh-add	Add keys
scp	Secure copy
sftp	Secure transfer

4. Linux Installation

Most Linux distributions use OpenSSH.

Two packages:

openssh-client

openssh-server

Client Package

Provides:

ssh

scp

sftp

ssh-keygen

Server Package

Provides:

sshd

Verify Existing Installation

Check client:

ssh -V

Example:

OpenSSH_9.8

Check server:

which sshd

5. Ubuntu / Debian Installation

Update package list:

sudo apt update

Install client:

sudo apt install openssh-client

Install server:

sudo apt install openssh-server

Verify installation:

ssh -V

Check service:

sudo systemctl status ssh

Enable service:

sudo systemctl enable ssh

Start service:

sudo systemctl start ssh

Verify Listening Port

ss -tlnp | grep ssh

Expected:

LISTEN 0 128 *:22

6. RHEL / CentOS / Rocky / AlmaLinux

Install package:

sudo dnf install openssh-server

sudo yum install openssh-server

Enable service:

sudo systemctl enable sshd

Start service:

sudo systemctl start sshd

Check status:

sudo systemctl status sshd

Verify Port

ss -tlnp | grep sshd

7. Arch Linux Installation

Install package:

sudo pacman -S openssh

Enable service:

sudo systemctl enable sshd

Start service:

sudo systemctl start sshd

Verify:

systemctl status sshd

8. Windows Installation

Modern Windows includes OpenSSH.

Check Existing Installation

PowerShell:

ssh -V

Example:

OpenSSH_for_Windows

Install via Settings

Navigate:

Settings
 → Apps
 → Optional Features
 → Add Feature
 → OpenSSH Client

Install Server:

OpenSSH Server

PowerShell Installation

Check capability:

Get-WindowsCapability -Online | Findstr OpenSSH

Install Client:

Add-WindowsCapability -Online -Name OpenSSH.Client~~~~0.0.1.0

Install Server:

Add-WindowsCapability -Online -Name OpenSSH.Server~~~~0.0.1.0

Start Service

Start-Service sshd

Enable Auto Start

Set-Service -Name sshd -StartupType Automatic

Verify

Get-Service sshd

9. macOS Installation

OpenSSH is preinstalled.

Check version:

ssh -V

Example:

OpenSSH_9.x

Verify client:

which ssh

Example:

/usr/bin/ssh

Navigate:

System Settings

→ General

→ Sharing

→ Remote Login

Or command line:

sudo systemsetup -setremotelogin on

Verify:

sudo systemsetup -getremotelogin

10. Systemd Services

Linux uses:

systemd

to manage SSH service.

Common Commands

Status:

sudo systemctl status ssh

sudo systemctl status sshd

Start:

sudo systemctl start ssh

Stop:

sudo systemctl stop ssh

Restart:

sudo systemctl restart ssh

Reload:

sudo systemctl reload ssh

Enable at Boot:

sudo systemctl enable ssh

Disable:

sudo systemctl disable ssh

Service Lifecycle Diagram

Installed
    |
Enable
    |
Boot
    |
Start
    |
Running
    |
Restart
    |
Stop

11. SSH Configuration Files

Understanding configuration files is critical.

Client Configuration

Location:

~/.ssh/config

Example

Host prod
    HostName 10.0.0.5
    User ubuntu

Global Client Config

Location:

/etc/ssh/ssh_config

Server Configuration

Location:

/etc/ssh/sshd_config

Example

Port 22

PermitRootLogin no

PasswordAuthentication yes

PubkeyAuthentication yes

Host Keys

Directory:

/etc/ssh/

Files:

ssh_host_rsa_key

ssh_host_ed25519_key

ssh_host_ecdsa_key

User Keys

Directory:

~/.ssh/

Examples:

id_ed25519

id_ed25519.pub

authorized_keys

known_hosts

Configuration Layout

/etc/ssh/
│
├── ssh_config
├── sshd_config
├── ssh_host_rsa_key
├── ssh_host_ed25519_key
└── ssh_host_ecdsa_key

12. Verification and Testing

Verify Client

ssh -V

Verify Server

which sshd

Verify Service

sudo systemctl status ssh

Verify Listening Port

ss -tlnp | grep ssh

Test Local Connection

ssh localhost

Expected:

Successful Login

Verify Process

ps aux | grep sshd

Verify Firewall

sudo ufw status

Verify Remote Connectivity

ssh user@server-ip

13. Internal Working After Installation

When SSH starts:

Step 1

OS launches:

sshd

Step 2

sshd loads:

Host Keys

Configuration

Authentication Rules

Step 3

sshd listens on:

Port 22

Step 4

Client connects.

Step 5

Secure tunnel created.

Step 6

Authentication occurs.

Step 7

Session established.

Internal Diagram

OS
 |
sshd
 |
Port 22 Listening
 |
Client Connects
 |
Encryption
 |
Authentication
 |
Shell Session

14. Architecture Diagram

+----------------------+
|      SSH Client      |
|       ssh            |
+----------------------+
            |
            |
Encrypted Channel
            |
            |
+----------------------+
|      SSH Server      |
|       sshd           |
+----------------------+
            |
            |
Configuration Files
            |
            |
+----------------------+
| /etc/ssh/sshd_config |
+----------------------+

15. Commands

Check version:

ssh -V

Generate key:

ssh-keygen

Check daemon:

ps aux | grep sshd

Check service:

systemctl status ssh

Check ports:

ss -tlnp

Test localhost:

ssh localhost

16. Code Examples

Install Ubuntu:

sudo apt update
sudo apt install openssh-server openssh-client

Enable service:

sudo systemctl enable ssh
sudo systemctl start ssh

Verify:

ssh localhost

17. Real-World Examples

Developer Laptop:

SSH Client Installed

Production Server:

OpenSSH Server Installed

Workflow:

ssh dev@server

18. Production Examples

AWS EC2

Laptop
   |
SSH Client
   |
Internet
   |
EC2 Instance
   |
sshd

Enterprise Data Center

Admin Workstation
     |
     SSH
     |
1000+ Linux Servers

19. Common Mistakes

Mistake 1

Installing only client.

Cannot accept incoming connections.

Mistake 2

Forgetting to start sshd.

Mistake 3

Firewall blocks port 22.

Mistake 4

Editing config without restart.

Mistake 5

Wrong service name.

Ubuntu:

ssh

RHEL:

sshd

20. Troubleshooting

ssh Command Missing

Check:

which ssh

Install client package.

sshd Missing

Check:

which sshd

Install server package.

Connection Refused

Check:

systemctl status ssh

Port Closed

Check:

ss -tlnp

Firewall Issue

Check:

ufw status

Configuration Error

Validate:

sudo sshd -t

No output means valid.

21. Security Notes

Install only required components.

Keep OpenSSH updated.

Verify:

sudo sshd -t

before restarting.

Use:

Public Key Authentication

instead of passwords.

Disable root login when possible.

22. Interview Questions

Difference between SSH Client and SSH Server?

Answer:

Client initiates connection.
Server accepts connection.

What process provides SSH service?

Answer:

sshd

Where is sshd configuration stored?

Answer:

/etc/ssh/sshd_config

How do you verify OpenSSH installation?

Answer:

ssh -V

How do you test SSH locally?

Answer:

ssh localhost

23. Hands-On Labs

Lab 1

Verify SSH Client

ssh -V

Lab 2

Install OpenSSH Server

Ubuntu:

sudo apt install openssh-server

Lab 3

Start Service

sudo systemctl start ssh

Lab 4

Verify Listening Port

ss -tlnp | grep ssh

Lab 5

Connect to Local Machine

ssh localhost

Lab 6

Inspect Configuration

cat /etc/ssh/sshd_config

24. Exercises

Exercise 1

Explain the difference between
openssh-client and openssh-server.

Exercise 2

Draw SSH installation architecture.

Exercise 3

List important OpenSSH tools.

Exercise 4

Identify all SSH configuration files.

Exercise 5

Describe service startup flow.

25. Quiz

Question 1

OpenSSH Server process?

A. ssh
B. sshd
C. ssh-agent
D. scp

Answer:

Question 2

SSH server configuration file?

A. ssh.conf
B. ssh.cfg
C. sshd_config
D. sshd.conf

Answer:

Question 3

Command to verify SSH version?

A. sshd -v
B. ssh version
C. ssh -V
D. ssh --check

Answer:

Question 4

Which package accepts incoming SSH connections?

A. openssh-client
B. openssh-server
C. ssh-agent
D. scp

Answer:

Question 5

Command to validate sshd configuration?

sudo sshd -t

Answer:

Correct

Summary

You should now understand:

✓ OpenSSH

✓ Client vs Server Packages

✓ Linux Installation

✓ Windows Installation

✓ macOS Installation

✓ Systemd Services

✓ SSH Configuration Files

✓ Host Keys

✓ User Keys

✓ Verification and Testing

✓ Service Management

✓ Troubleshooting Installation Problems

Next Topic:

Topic #4 — SSH Connection Lifecycle

DNS Resolution
TCP Handshake
Protocol Version Exchange
Algorithm Negotiation
Host Verification
Key Exchange
Encryption Setup
Authentication
Session Creation
Channel Creation
Interactive Shell

Topics

Table of Contents

1. What is OpenSSH?

Why OpenSSH?

OpenSSH in Modern Systems

2. SSH Client vs SSH Server

SSH Client

SSH Server

Diagram

Simple Rule

3. OpenSSH Components

ssh

sshd

ssh-keygen

ssh-agent

ssh-add

scp

sftp

Component Overview

4. Linux Installation

Client Package

Server Package

Verify Existing Installation

5. Ubuntu / Debian Installation

Verify Listening Port

6. RHEL / CentOS / Rocky / AlmaLinux

Verify Port

7. Arch Linux Installation

8. Windows Installation

Check Existing Installation

Install via Settings

PowerShell Installation

Start Service

Verify

9. macOS Installation

Enable Remote Login

10. Systemd Services

Common Commands

Service Lifecycle Diagram

11. SSH Configuration Files

Client Configuration

Global Client Config

Server Configuration

Host Keys

User Keys

Configuration Layout

12. Verification and Testing

Verify Client

Verify Server

Verify Service

Verify Listening Port

Test Local Connection

Verify Process

Verify Firewall

Verify Remote Connectivity

13. Internal Working After Installation

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7

Internal Diagram

14. Architecture Diagram

15. Commands

16. Code Examples

17. Real-World Examples

18. Production Examples

19. Common Mistakes

Mistake 1

Mistake 2

Mistake 3

Mistake 4

Mistake 5

20. Troubleshooting

ssh Command Missing

sshd Missing

Connection Refused

Port Closed